Long Lenses & Low Light

[Bill Ferris]

... I am generally able to get as close to the subjects (within reason) I shoot with the 400 2.8 (deer/owl/fox) & therefore the 400 2.8 has a significant advantage vs the 600 towards the end of the day. However, I do agree that if one cannot get closer and/or is relying on cropping to the same FOV, then the 600 has a slight advantage vs the former. Not sure if I’m missing anything & more than happy to be corrected.

Thanks for your contributions to this thread.

Speaking of owls, I often hear great horned owls in pre-dawn when I'm out for elk or pronghorn. I've done several late afternoon walks through the forest looking for sign but have never seen one. If you've got any tips you'd care to share - or a link to a thread on which you've already shared some tips - I'd be appreciative.

 

[Bill Ferris]

This video explains the entrance pupil issue succinctly. At just after the six minute mark it explains why the entrance pupil has to be larger at a longer focal length, in order to compensate for the narrower cone of targeted light.

Anthony, thanks for sharing the Gerald Undone video.

 

[Bill Ferris]

To add: the statement :


Comparing the same sized subject in the same frame = the same magnification..... e.g. Yes or No?

For example, 400 f4.5S, 400 f2.8S; ....the lens with widest entrance pupil has the fastest f Stop and more optimal exposure and narrower DoF under the same illumination.

Alternatively we can compare with the same Framing and ~Magnification of 560-600mm on FX sensor [Is this a representative comparison, give/take the 40mm focal length difference? Practicably, I say yes for wildlife subjects]

600 f4S Ф150mm;

400 f2.8 Ф143mm +TC14 = 560 f4;

300 f2.8G Ф107mm +TC2 = 600 f5.6;

180-400 f4E +TC14 Ф100mm = 560 f5.6;

600 f6.3 PF Ф95mm;

400 f4.5S Ф89mm +TC14 = 560 f6.3;

300 f4 PF Ф75mm +TC2 = 600 f8;

100-400mm f4-5.6 Ф71mm +TC14 = 560 f8

100-400mm f5.6-6.3 Ф64mm +TC14 = 560 f9;

70-300mm f/4.5-5.6 Ф54mm +TC2 =600 f11;


Again the fastest fStop is causally informative as it is simple to compare across these lenses. It is the standardized variable to predict the scope of performance for exposure and DoF. [edited for clarity]

Great list! Thanks for preparing and sharing it.

I'll call out and expand upon this comment: For example, 400 f4.5S, 400 f2.8S; ....the lens with widest entrance pupil has the fastest f Stop and more optimal exposure and narrower DoF under the same illumination.

The widest entrance pupil will always collect more light from a subject in the frame. Cropping (in post or in camera) or TC'ing a lens doesn't alter its entrance pupil. The 400mm f/2.8 has the same size EP as a 560mm f/4. This holds going down through the list.

That means noise in the subject is also the same. If you flip the TC in place on the 400 to make it any 560mm f/4, you'll probably use a higher ISO at the same shutter speed because the exposure is weaker. But it's not exposure that determines noise; total light energy does. And the light energy collected from the subject at 560mm f/4 is the same as that collected from the subject at 400mm f/2.8.

People often talk about a "noise penalty" using a TC. Practically speaking, there is none. You would've cropped the photo anyway to get the same framing. If the TC doesn't introduce significant optical aberrations degrading image sharpness, it makes sense to use it: more pixels on the subject and zero noise penalty vs cropping.

 

[SCoombs]

If by the former you're referring to an exposure with A being greater than B, exposure is, by definition, light energy per unit area of the sensor. If by the latter you're referring to one image being made with greater total light energy than another, that is very possible.

Let's use a single exposure made with a smartphone camera at f/1.2 (A) vs single exposure made with a medium format camera at f/4 (B). While the smartphone camera works with a greater exposure - more light energy per unit area - the medium format camera collects far more total light energy from the scene. As a result, it's image will have less visible noise and be cleaner.

While exposure is, arguably, the most discussed aspect of photography, it's the total light energy used to make the photo that goes farther in determining image quality.

There are a few things I would want to debate a bit further about this reply, but in the interests of trying to stay a bit more focused I will just look at this particular piece because really it hits on something I typically find to be the biggest issue I take whenever discussions about equivalency come up.

Namely, I really don't think that from a practical standpoint total light - which is what equivalency discussions always focus on - is something a photographer generally needs to worry all that much about but rather that light per pixel is the thing that really matters, and as you're noting here these two are not going to be the same thing.

What I hope is an easy example: say I am shooting a subject with an APS-C camera on a 400mm lens at f2.8. Now say I can instead shoot from the same position with a full frame camera at the same 400mm at f4 (or if my math is wrong here just assume for some reason it is something weird like f3.2 to make the comparison work). The full frame camera will gather more light, but I vastly prefer the DX camera because it is giving more light per pixel, which is what I actually care about. On the full frame camera I am going to throw half the gathered light away.

Now, what if I keep the f4 lens on the full frame camera and move closer so that I have the same subject size in the frame as I had on the DX camera. In that case I am not only collecting more overall light, but also more light per pixel with the full frame camera and - leaving aside any other possible considerations between the two choices - I am going to prefer the f4 lens on the full frame camera.

In general I really don't care how much total light was captured. I care how much light was captured on the pixels that I care about.

Anthony, thanks for sharing the Gerald Undone video.

Doesn't this video somewhat disagree with the premise here? It specifically calls the math that's underlaid everything in this thread as a misunderstanding or a myth, and then later it explains that wider entrance pupil's don't necessarily result in gathering more light because they need to make up for the fact that longer focal lengths decrease how much of the gathered light actually makes it to the sensor.

 

[Lance B]

One thing that I like about this forum is that we discuss many things. And while we don’t always agree about everything, our discussions are pretty civil.

Yes, it is a very well run forum, thanks to Steve and the moderators. Much less combative than DPR where it seems people want to always prove themselves superior in any way they can.

 

[Bill Ferris]

What I hope is an easy example: say I am shooting a subject with an APS-C camera on a 400mm lens at f2.8. Now say I can instead shoot from the same position with a full frame camera at the same 400mm at f4 (or if my math is wrong here just assume for some reason it is something weird like f3.2 to make the comparison work). The full frame camera will gather more light, but I vastly prefer the DX camera because it is giving more light per pixel, which is what I actually care about. On the full frame camera I am going to throw half the gathered light away.

Actually, the APS-C system gathers more light from the subject. At 400mm f/2.8, it's working with a 143mm diameter entrance pupil. The full frame system is at 400mm f/4 and working with a 100mm entrance pupil. It captures less total light energy (a full stop less) from the subject.

The full frame system also frames the scene wider. It's image will need to be cropped to match the framing of the photo made with the APS-C body. So, the image will be noisier and potentially have less resultion.

Now, what if I keep the f4 lens on the full frame camera and move closer so that I have the same subject size in the frame as I had on the DX camera. In that case I am not only collecting more overall light, but also more light per pixel with the full frame camera and - leaving aside any other possible considerations between the two choices - I am going to prefer the f4 lens on the full frame camera.

You're not collecting more light because the APS-C shooter is moving closer right along with you.

In general I really don't care how much total light was captured. I care how much light was captured on the pixels that I care about.

Light per pixel does not determine the noise level or the quality of a photo. The total number of photons used to make the photo does.

Doesn't this video somewhat disagree with the premise here? It specifically calls the math that's underlaid everything in this thread as a misunderstanding or a myth, and then later it explains that wider entrance pupil's don't necessarily result in gathering more light because they need to make up for the fact that longer focal lengths decrease how much of the gathered light actually makes it to the sensor.

You've misunderstood the Gerald Undone video. It actually confirms what's been known to be a fact for more than 100 years: the lens's virtual entrance pupil diameter determines the total light collected from a subject in the frame.

 

[BillW]

There are a few things I would want to debate a bit further about this reply, but in the interests of trying to stay a bit more focused I will just look at this particular piece because really it hits on something I typically find to be the biggest issue I take whenever discussions about equivalency come up.

Namely, I really don't think that from a practical standpoint total light - which is what equivalency discussions always focus on - is something a photographer generally needs to worry all that much about but rather that light per pixel is the thing that really matters, and as you're noting here these two are not going to be the same thing.

What I hope is an easy example: say I am shooting a subject with an APS-C camera on a 400mm lens at f2.8. Now say I can instead shoot from the same position with a full frame camera at the same 400mm at f4 (or if my math is wrong here just assume for some reason it is something weird like f3.2 to make the comparison work). The full frame camera will gather more light, but I vastly prefer the DX camera because it is giving more light per pixel, which is what I actually care about. On the full frame camera I am going to throw half the gathered light away.

Now, what if I keep the f4 lens on the full frame camera and move closer so that I have the same subject size in the frame as I had on the DX camera. In that case I am not only collecting more overall light, but also more light per pixel with the full frame camera and - leaving aside any other possible considerations between the two choices - I am going to prefer the f4 lens on the full frame camera.

In general I really don't care how much total light was captured. I care how much light was captured on the pixels that I care about.


Doesn't this video somewhat disagree with the premise here? It specifically calls the math that's underlaid everything in this thread as a misunderstanding or a myth, and then later it explains that wider entrance pupil's don't necessarily result in gathering more light because they need to make up for the fact that longer focal lengths decrease how much of the gathered light actually makes it to the sensor.

If you want more light per pixel, why not also shoot the full frame camera at 400 mm f2.8? You throw away half of the light by going to f4 In full frame.

Is using f4 an adjustment for some form of equivalence?

 

[bleirer]

I thought the myth the Undone video sought to rebuke was that f number was focal length divided by aperture diameter. When he said really it is focal length divided by entrance pupil.

 

[fcotterill]

With reference to the posts, above, about ISO, here are 4 explanations I sifted out of the plethora on the internet: covering the relationships to exposure and noise, and Dual Gain sensors

Understanding How ISO Affects Images

Camera ISO is often taught as a part of the Exposure Triangle, along with Aperture and Shutter Speed. It basically determines how much…

medium.com

Is ISO Fake? | DSLRBodies | Thom Hogan

Is ISO a standard or something fake?

dslrbodies.com

Dual Base ISO and Shot Noise - Z Systems, inc.

A dive into how Dual Base ISO works, and why images shot at higher ISO appear noisier, even when using a Dual Base ISO camera system.

zsyst.com

ISO Invariance Explained

In this article, we will explore ISO invariance in detail and how you can use it to your advantage in your photography

photographylife.com

For those who prefer videos, this somewhat nerdy production is worth the time, even though it was made 6 years ago. He does a decent job of explaining the respective roles of Analogue and Digital gain, and also ISO invariance

 

[fcotterill]

Equivalence is a tool for determining which settings different format cameras can use to make the same photo.

This thread is about a method for simplifying a comparison of noise performance by different focal length and format systems. A comparison of entrance pupil sizes being used cuts through all the other variables when addressing the question of noise.

Comparing entrance pupil alone is insufficient, See below and the following post

If by use of, subsumed, you're suggesting that the total light used to make a photo is somehow irrelevant - the exposure is all that matters - I would disagree.

If your intent is to state that total light energy and exposure are different but related, I would agree.



As I wrote in the top post and in other responses, the subject(s) one plans to photograph, distance, size & weight of the kit, cost, and light conditions are all factors one typically considers when choosing a lens to bring in a shoot. I am 100% in agreement with you that these (and other factors) are well worth considering.

With respect to noise performance of the camera/lens system one brings on that shoot, I'm advocating that one can simplify the task of evaluating contenders by comparing the entrance pupils of the lenses under consideration...all of which will be used to make the photo one envisions.

One still has to evaluate how the relationships between crop factor [sensor area], and noise with respect to Magnification (focal length) and the Lens speed(s) of candidate lenses. Just comparing the entrance pupil ignores these critical variables

 

[fcotterill]

Hi David,

The usefulness for me of comparing entrance pupils is that it cuts through the fog. Many bird and wildlife photographers focus on f-stop and ISO. If you add different format cameras to the mix, that adds another level of complexity.

Entrance pupil simplifies the comparison. Take any 2 or more lenses, put them in the same blind, and use them to make photos with the same framing of the same subject, and the lens with the largest entrance pupil will make the cleanest image.

It's not the only factor one considers when choosing a lens but it does simplify the low light performance question.

To add: the statement :


Comparing the same sized subject in the same frame = the same magnification..... e.g. Yes or No?

For example, 400 f4.5S, 400 f2.8S; ....the lens with widest entrance pupil has the fastest f Stop and more optimal exposure and narrower DoF under the same illumination.

Alternatively we can compare with the same Framing and ~Magnification of 560-600mm on FX sensor [Is this a representative comparison, give/take the 40mm focal length difference? Practicably, I say yes for wildlife subjects]

600 f4S Ф150mm;

400 f2.8 Ф143mm +TC14 = 560 f4;

300 f2.8G Ф107mm +TC2 = 600 f5.6;

180-400 f4E +TC14 Ф100mm = 560 f5.6;

600 f6.3 PF Ф95mm;

400 f4.5S Ф89mm +TC14 = 560 f6.3;

300 f4 PF Ф75mm +TC2 = 600 f8;

100-400mm f4-5.6 Ф71mm +TC14 = 560 f8;

100-400mm f5.6-6.3 Ф64mm +TC14 = 560 f9;

70-300mm f/4.5-5.6 Ф54mm +TC2 =600 f11;


Again the fastest fStop is causally informative as it is simple to compare across these lenses. It is the standardized variable to predict the scope of performance for exposure and DoF. [edited for clarity]

Great list! Thanks for preparing and sharing it.

I'll call out and expand upon this comment: For example, 400 f4.5S, 400 f2.8S; ....the lens with widest entrance pupil has the fastest f Stop and more optimal exposure and narrower DoF under the same illumination.

The widest entrance pupil will always collect more light from a subject in the frame. Cropping (in post or in camera) or TC'ing a lens doesn't alter its entrance pupil. The 400mm f/2.8 has the same size EP as a 560mm f/4. This holds going down through the list.

That means noise in the subject is also the same. If you flip the TC in place on the 400 to make it any 560mm f/4, you'll probably use a higher ISO at the same shutter speed because the exposure is weaker. But it's not exposure that determines noise; total light energy does. And the light energy collected from the subject at 560mm f/4 is the same as that collected from the subject at 400mm f/2.8.

People often talk about a "noise penalty" using a TC. Practically speaking, there is none. You would've cropped the photo anyway to get the same framing. If the TC doesn't introduce significant optical aberrations degrading image sharpness, it makes sense to use it: more pixels on the subject and zero noise penalty vs cropping.

The list is my response to your statement quoted at the top of this post, namely, "Entrance pupil simplifies the comparison. Take any 2 or more lenses, put them in the same blind, and use them to make photos with the same framing of the same subject, and the lens with the largest entrance pupil will make the cleanest image." My emphasis

I compiled the list to achieve comparable framing using Teleconverters and listing the Lens Speed for a frequently discussed magnification category - 560/600mm on FX. According to the well established application of optical physics theory, please explain how only Entrance pupil improves over using fStop as the universal currency for Lens Speed? To say this another way, the Lens Speed (fastest fStop) of my list of 560/600 options is the simplest and most informative variable [now emboldened in the quoted list above].

Moreover, Lens Speed, Magnification and image size are as fundamental - critical- to compare different area of sensors [aka Camera Format] with respect to Exposure and image noise. Here, comparisons of Equivalence end up in a quagmire trying to compare solely Entrance Pupil diameter in place of fStop

Four pages into this thread and you have failed to explain the supreme advantage of comparing the Entrance Pupil as the dominant explanatory variable.

 

[Dinusaur]

I thought the myth the Undone video sought to rebuke was that f number was focal length divided by aperture diameter. When he said really it is focal length divided by entrance pupil.

That is correct. I find the simplest explanation of f-number, entrance pupil and brightness of an object on image plane in the section called 'Notation' in Wikepedia:

f-number - Wikipedia

en.wikipedia.org

This sentence sums it up brilliantly: "The brightness of the projected image (illuminance) relative to the brightness of the scene in the lens's field of view (luminance) decreases with the square of the f-number". A longer focal length lens will project an image wider and higher on the image plane (magnification). A 400/4 lens will have the same illuminance as a 600/4 lens on a same sized sensor (Z9) for an object of same brightness.

 

[BillW]

I’m trying to understand what all this means for my wildlife and other photography. I use full frame Nikon Z9 and Z8s. I have the 400 mm TC and a number of other Z telephotos, but not the 600 mm TC.

It seems to me that entrance pupil is relevant. But it also seems to me that f stop combines entrance pupil and the magnification of the lens (through focal length), so that is relevant too.

Without getting into all the equivalence issues and different sensor sizes, can we say this:

At the same focal length, a faster lens will get you more light wide open and hence may generally be better for low light, all other things being equal (which they rarely are). Of course it might be too large/heavy, too expensive or you may need to stop down for DOF.

At the same time, that may not be true in all cases when comparing a faster shorter lens to a longer slower lens. For example, a 400 mm f2.8 lens is not better for low light than a 600 mm f4 lens, both used wide open and at the same distance, if you have to crop the image from the 400 mm lens or use a TC with the 400 mm lens to get a FOV similar to that obtained with the 600 mm lens. If you can get closer with the 400 mm lens and hence avoid the need to crop or use a TC, again the faster lens would generally be better for low light.

I think this was what Steve was saying in his video comparing the 400 mm TC and 600 mm TC lenses.

Is there something more I am missing. If this is it, it seems simple enough.

 

[Bill Ferris]

Comparing entrance pupil alone is insufficient, See below and the following post



One still has to evaluate how the relationships between crop factor [sensor area], and noise with respect to Magnification (focal length) and the Lens speed(s) of candidate lenses. Just comparing the entrance pupil ignores these critical variables

Entrance pupil incorporates all those factors. Regardless of focal length, exposure or sensor format, two lenses working with the same size entrance pupil will collect the same number of photons from a subject in the frame.

 

[Bill Ferris]

The list is my response to your statement quoted at the top of this post, namely, "Entrance pupil simplifies the comparison. Take any 2 or more lenses, put them in the same blind, and use them to make photos with the same framing of the same subject, and the lens with the largest entrance pupil will make the cleanest image." My emphasis

I compiled the list to achieve comparable framing using Teleconverters and listing the Lens Speed for a frequently discussed magnification category - 560/600mm on FX. According to the well established application of optical physics theory, please explain how only Entrance pupil improves over using fStop as the universal currency for Lens Speed?

The reliance upon the smallest f-number as an indicator of low light noise performance has led many photographers to discount a 150-600mm f/5-6.3 zoom vs a 200-500mm f/5.6 zoom because the "slower" zoom needs a higher ISO with more noise to make a photo at the same shutter speed.

It's a conclusion one arrives at when one doesn't understand how lenses or noise work. I know; that was me 10 years ago

The longer lens has a skosh larger entrance pupil at 600mm f/6.3 than the 500mm at f/5.6. The shorter lens has no light-gathering or noise advantage. For all practical matters, they perform the same.

The same can be said of any comparison of lenses of different focal lengths and f-stops where the "slower" lens has an entrance pupil matching or exceeding that of the "faster" lens.

Your chart confirms this fact. A lens with a 95mm entrance pupil without a TC has a 95mm entrance pupil with a TC. With or without the TC, the lens collects the same number of photons from a subject in its frame. The resulting image of that subject has the same SNR and noise level with our without the TC.

I've explained this, logically in multiple posts. I've provided resources proving this. I've demonstrated it in practice.

Entrance Pupil

Explore this photo album by Bill Ferris on Flickr!

www.flickr.com

To say this another way, the Lens Speed (fastest fStop) of my list of 560/600 options is the simplest and most informative variable [now emboldened in the quoted list above].

As stated at the outset of this thread and as demonstrated in multiple subsequent posts, f-stop at best is a reliable indicator of light-gathering only under limited circumstances. Entrance pupil is always a reliable indicator because it's the feature that directly determines how much light a lens collects from a subject in the frame.

Moreover, Lens Speed, Magnification and image size are as fundamental - critical- to compare different area of sensors [aka Camera Format] with respect to Exposure and image noise. Here, comparisons of Equivalence end up in a quagmire trying to compare solely Entrance Pupil diameter in place of fStop

Four pages into this thread and you have failed to explain the supreme advantage of comparing the Entrance Pupil as the dominant explanatory variable.

I stand by the case made in the top post. If you choose to apply another method, that's your choice. You're free to make it. I'm offering a simpler way to compare low light noise performance by lenses of different focal lengths on cameras of different formats. Use it or don't; its your choice.

 

[Anthony]

Entrance pupil incorporates all those factors. Regardless of focal length, exposure or sensor format, two lenses working with the same size entrance pupil will collect the same number of photons from a subject in the frame.

I am not sure about this. The Gerald Undone video indicates the opposite - the longer lens has to have a larger entrance pupil in order to compensate for the narrower cone of targeted light. In other words, if the longer lens had the same entrance pupil as the shorter, the resulting image would be darker, ie it would have captured fewer photons,

You can test this by looking at the variable entrance pupil of a zoom lens. I have just tested this with my 24-70 f4 S. Set the aperture at f4 at 24 mm and then zoom to 70 mm without changing the aperture. You will see the entrance pupil grow larger. Shoot a uniform target at f4, first at 24 mm and then at 70 mm. The only difference between the two images is caused by uncorrected vignetting at 24 mm. Where the vignetting does not apply, there is no visible difference.

 

[SCoombs]

Actually, the APS-C system gathers more light from the subject. At 400mm f/2.8, it's working with a 143mm diameter entrance pupil. The full frame system is at 400mm f/4 and working with a 100mm entrance pupil. It captures less total light energy (a full stop less) from the subject.

The full frame system also frames the scene wider. It's image will need to be cropped to match the framing of the photo made with the APS-C body. So, the image will be noisier and potentially have less resultion.

This seems plainly incorrect. The LENS gathers the same light on both systems, but on the DX camera half of that light does not fall on the sensor.

You're not collecting more light because the APS-C shooter is moving closer right along with you.



Light per pixel does not determine the noise level or the quality of a photo. The total number of photons used to make the photo does.



You've misunderstood the Gerald Undone video. It actually confirms what's been known to be a fact for more than 100 years: the lens's virtual entrance pupil diameter determines the total light collected from a subject in the frame.

I've watched the video several times and the portion at the 6:05 mark seems to me to do the opposite: he explicitly says that the larger entrance pupil will NOT be brighter because even though there is more light entering the pupil, that more light has to make up for the narrower cone.

 

[SCoombs]

Ultimately, the biggest reason I am skeptical of the fundamental claim here is experience. I mentioned above that I have shot HS soccer games in mediocre to bad late afternoon October light and used both the 70-200/2.8 and the 180-600.

Now the entrance pupil as discussed here is 33% larger on the 600mm at 6.3 - but over hundreds or thousands of photos swapping between these I found fairly clearly that I got much less noisy images cropping from the 200 2.8 vs taking the full shot on the 600 at 6.3.

 

[fcotterill]

Entrance pupil incorporates all those factors. Regardless of focal length, exposure or sensor format, two lenses working with the same size entrance pupil will collect the same number of photons from a subject in the frame.

Okay, apologies for delayed reply.... from a different hemisphere and time zone. We know you have the best intentions in your posts to improve understanding and help esteemed members here arrive at expensive decisions..... but I'm not the only one who's unconvinced by your attempts at explanations.

As in a recent dpr thread about the 300-600 f4 Sigma, here we traverse a wide circle discussing several phenomena: including the water jump that is Equivalence, Exposure, ISO and Noise etc and the intricacies of the physics of cones of illumination, which are shaped by Entrance Pupil Ф and Magnification in an indivisible causal relationship that illuminates the sensor in the Image Circle.

To elaborate on your opening sentence, above:

• At least 2 telephotos with an entrance pupil Ф of 143mm
• So do 2 telephotos with an entrance pupil Ф of 100mm
• 2 more have a Ф of 71mm
• And not least 3 have a Ф of 89mm

All these are Nikkors, and dare I say over the past decade I've accumulated many thousands of exposures with each of the above, including with all 6 Nikkor teleconverters in their respective combinations. The majority of these teleophotos have been passed on and hopefully are serving their current owners very well.....

The issue is If we relied solely on Window Ф without incorporating 2 critical variables we would be in a quagmire of confusions

The trite conclusion follows. One has to know more about the respective attributes about the above 9 Nikkors before expensive purchase decisions.

Entrance Pupil alone is simply inadequate.

 

[Doug Herr]

Hasn't this been beaten to death?

 

[DRwyoming]

I've watched the video several times and the portion at the 6:05 mark seems to me to do the opposite: he explicitly says that the larger entrance pupil will NOT be brighter because even though there is more light entering the pupil, that more light has to make up for the narrower cone.

Exactly, we can't ignore focal length and just think about entrance pupil when it comes to illuminating the image plane as focal length determines how much light is captured from the object side of the lens.

Consider two lenses pointed at a large uniform target with uniform illumination such as a very large evenly lit gray card.

The longer focal length lens integrates photons across a smaller area than the shorter focal length lens aimed at the same target. The longer focal length lens with the larger entrance pupil may do a better job of delivering those captured photons to the image circle but fewer photons were captured to start with.

Consider two lenses, a 200mm lens with a 12 degree angle of view and a 600mm lens with a 4.1 degree angle of view (reference: https://av.jpn.support.panasonic.com/support/global/cs/dsc/knowhow/knowhow12.html)

At a fixed distance of 10 meters the 3:2 aspect ratio rectangular area captured by these two lenses are:

2.16 m^2 for the 200mm lens
0.24 m^2 for the 600mm lens

Field of View calculator: https://www.scantips.com/lights/fieldofview.html#top2

When both lens look at a large uniformly lit and uniformly toned target (e.g. large gray card under uniform lighting) the 200mm lens captures 9 times the number of photons due to its wider field of view. (2.16 m^2/0.24 m^2 = 9)

So yes, the 600mm may have a larger entrance pupil but that alone does not predict which lens will deliver more light to the image plane. We also have to take into account focal length and that's exactly what Relative Aperture or what photographers think of as f/ stop does. It takes into account BOTH entrance pupil and focal length to determine how much light will be delivered to the image plane.

FWIW Shane, I agree with the results of your field test comparing the two lenses. Yes, doing this test at a fixed ISO was valid just as doing this test using film with its fixed ISO would have shown the same relative results. Similarly if you had access to a lab grade incident light meter and used that instead of the camera you would also have seen the same relative brightness results as the f/2.8 lens did in fact deliver more light to the sensor than the 500mm f/5.6 lens.

Physicists studying optics figured this and standardized the use of Relative Aperture (f/ stop) nearly a century ago. Since then and even today photographers use calibrated incident light meters to accurately predict shutter speed and ISO for a correct exposure based on f/ stop used and incident light. These meters do not typically have any direct entry for entrance pupil nor focal length but accurately predict appropriate exposure settings based on f/ stop as that takes into account not only entrance pupil but the angle of view of the lens and what that means for light gathering and delivery to the image plane.

I agree with the basic premise that when comparing similar long telephoto lenses the entrance pupil is one indicator of which gathers more light when scaled but you can do the same relative scaling with f/ stop in a way that may be easier for experienced photographers to grasp.

For instance comparing the 400mm f/2.8 (approximate entrance pupil of 143mm) to a 600mm f/4 (entrance pupil of 150mm) we could as was done in this thread say yup, when the lenses are normalized to the same field of view via a suitable TC (if we could find a high quality 1.5x TC) or via cropping the 600mm f/4 has a light gathering edge.

But we could get there based on f/ stop and focal length and come to the same conclusion.

400mm f/2.8 plus a hypothetical 1.5x TC yields a 600mm f/4.2 lens vs the native 600mm f/4 lens.

Same thing if we decided 800mm would be a better comparison focal length:

400mm f/2.8 with a 2x TC = 800mm f/5.6

600mm f/4 with a hypothetical 1.333x TC ~ 800mm f/5.33

Again it's easy via this f/ stop based normalization to see that the 600mm f/4 gathers more light even when both lenses are normalized to the same longer focal length and it doesn't require direct knowledge of entrance pupil as that is implied via focal length and f/ stop.

 

[fcotterill]

The reliance upon the smallest f-number as an indicator of low light noise performance has led many photographers to discount a 150-600mm f/5-6.3 zoom vs a 200-500mm f/5.6 zoom because the "slower" zoom needs a higher ISO with more noise to make a photo at the same shutter speed.

It's a conclusion one arrives at when one doesn't understand how lenses or noise work. I know; that was me 10 years ago

The longer lens has a skosh larger entrance pupil at 600mm f/6.3 than the 500mm at f/5.6. The shorter lens has no light-gathering or noise advantage. For all practical matters, they perform the same.

The same can be said of any comparison of lenses of different focal lengths and f-stops where the "slower" lens has an entrance pupil matching or exceeding that of the "faster" lens.

Your chart confirms this fact. A lens with a 95mm entrance pupil without a TC has a 95mm entrance pupil with a TC. With or without the TC, the lens collects the same number of photons from a subject in its frame. The resulting image of that subject has the same SNR and noise level with our without the TC.

I've explained this, logically in multiple posts. I've provided resources proving this. I've demonstrated it in practice.

Entrance Pupil

Explore this photo album by Bill Ferris on Flickr!

www.flickr.com

As stated at the outset of this thread and as demonstrated in multiple subsequent posts, f-stop at best is a reliable indicator of light-gathering only under limited circumstances. Entrance pupil is always a reliable indicator because it's the feature that directly determines how much light a lens collects from a subject in the frame.



I stand by the case made in the top post. If you choose to apply another method, that's your choice. You're free to make it. I'm offering a simpler way to compare low light noise performance by lenses of different focal lengths on cameras of different formats. Use it or don't; its your choice.

To expand further on my list - which can be compared by 1 or 2 fStops lower exposure and 1.4x and 2x magnifications with respective teleconverters :

• At least 2 telephotos with an entrance pupil Ф of 143mm

400 f2.8 and 800 f5.6 with corresponding TC correlates

• So do 2 telephotos have an entrance pupil Ф of 100mm

200 f2 and 180-400 TC14 with corresponding TC correlates

• 2 more have a Ф of 71mm

70-200 f2.8 and 100-400mm f4/5.6 with corresponding TC correlates

• And not least 3 have a Ф of 89mm

400 f4.5S, 200-500 f5.6E, 500 f5.6 PF with corresponding TC correlates

How does one decide and build a lens system using only Window Ф ? It is obvious that Focal Length and fStop are the fundamental and most informative variables. As Dave explains in great detail just above - post #96 - fStop normalization continues to hold powerful explanatory scope.

They have always been and will continue to be 2 of key variables, which subsume Window Ф

 

[bleirer]

I think the OP supported his original thesis with facts and examples, and I'm convinced within my non math or optical engineering limits. If someone who disagrees could give an example where the lens with the smaller entrance pupil provided more total light or less noise after all the required equivalency, that would help me understand.

 

[Bill Ferris]

This seems plainly incorrect. The LENS gathers the same light on both systems, but on the DX camera half of that light does not fall on the sensor.

The lens entrance pupil diameter determines how much light is collected from the subject. An APS-C sensor paired with a lens having a 143m entrance pupil will capture a stop more light from a subject in the frame than a full frame camera paired with a lens having a 100mm entrance pupil. This is basic physics of light and optics that's been known for more than a hundred years.

I've watched the video several times and the portion at the 6:05 mark seems to me to do the opposite: he explicitly says that the larger entrance pupil will NOT be brighter because even though there is more light entering the pupil, that more light has to make up for the narrower cone.

He plainly states in that section that the entrance pupil diameter determines how much light is collected. He also plainly states that the image of a subject in the frame of two photos will be made with more light in the image produced by the lens with the larger entrance pupil. He then explains why both images have the same ligthness (he says, brightness) having been made at the same f-stop; with the same exposure. That is correct.

 

[DRwyoming]

The lens entrance pupil diameter determines how much light is collected from the subject.

The entrance pupil AND the angular field of view based on focal length determine how much light is collected from the subject. It's not just entrance pupil as two different focal length lenses with the same entrance pupil will gather different amounts of light from the same uniformly lit and uniformly toned subject based on their respective fields of view.

You can't just toss out the collection area on the object side of the lens when discussing captured photons it's an important part of the process.