Would cropping give advantage over using TC in low light environment?

[bcgnz]

I understand that using the 1.4 TC will effectively decrease aperature e.g 1/2.8->1/4. In low light conditions, we might not be able to afford to do, since the image will be too dark.

However, if I don’t use 1.4 TC and simply crop the image in post, would the image still be too dark?

Very confused about this…

 

[Archibald]

The TC increases the aperture number by one stop. If you are at f/8 without the TC, then it is effectively f/11. So open up a stop and you are back to an effective f/8. However, if you are wide open, then you won't be able to do that. You can still adjust the ISO and/or shutter speed to get the same exposure.

Cropping doesn't change the exposure.

 

[bcgnz]

sorry I’m still confused. Let’s say I have 45MP camera and I’m viewing my image on a monitor only ccapable for 20MP.

Would the image taken with 1.4 TC versus another image with the same view taken from the same distance but cropped instead, have the same image quality, assuming environment is very dark?

Would cropping give advantage over using TC in dark environment?

 

[DRwyoming]

I understand that using the 1.4 TC will effectively decrease aperature e.g 1/2.8->1/4. In low light conditions, we might not be able to afford to do, since the image will be too dark.

However, if I don’t use 1.4 TC and simply crop the image in post, would the image still be too dark?

Very confused about this…

It’s not that the image will look too dark but that we’ll have to compensate in some other way to maintain the exposure we’re after. Assuming you can’t risk dropping shutter speed that means raising ISO.

Thinking of it that way, generally speaking adding a 1.4x TC means raising ISO and that means increased shadow noise and with it reduced Dynamic Range. Those are the comparisons we typically make between adding a TC vs cropping a similar amount. Sure there are other considerations like AF performance and background rendering but from an exposure standpoint we typically look at ISO, noise and DR.

From that standpoint, adding a TC and taking the noise hit due to higher ISO is basically the same as cropping an equivalent amount and then resizing the cropped image to the output size of the uncropped TC image.

There’s no free lunch but in some cameras it can make sense to leave the TC off in low light conditions to help the AF system, especially DSLRs.

But from a noise standpoint adding the 1.4x TC and then bumping ISO up a stop is pretty much the same as cropping an equivalent amount and resizing the image for the same output size.

@Steve did some very good videos on this topic a while back that doesn't change with newer cameras on how cropping impacts noise assuming we want to the same output size images:

steve perry bcg cropping noise

He also did a good blog post and video on your basic question about cropping vs TCs:

Use A Teleconverter Or Crop The Image? - Backcountry Gallery

Think fast! You’re out searching for wildlife – your camera in hand – when you spot some critter you’ve always wanted to bag off in the distance. Sadly, you know that your lens is just not going to be quite long enough to get the shot you want on its own. So, here’s the question – […]

backcountrygallery.com

 

[bcgnz]

Oh got it!

I have a follow up question - sometimes I see people say that when the environment gets too dark, they will take off their TC to be able to shoot - why do they say that?

From what I understood from the explanation, as long as they need to crop, having the TC on or off shouldn’t have mattered anyway?

 

[beano]

Oh I see. Thanks so much!

I have a follow up question - sometimes I see people say that when the environment gets too dark, they will take off their TC to be able to shoot - why do they say that?

From what I understood from the explanation, as long as they need to crop, having the TC on or off shouldn’t have mattered anyway?

As per the previous post, leaving the TC off might support better auto-focus on some cameras. As per @DRwyoming ’s good post above, there’s no free lunch wrt noise, using TC to get a better FOV, vs cropping. AF potentially is the only benefit then.

 

[bcgnz]

Thanks. I think I understand completely.

It’s just that it confuses me I read people say they will take their teleconverter off when it becomes dark - it seems like from this discussion, it shouldn’t have mattered anyway?!

 

[bcgnz]

This also really helps -https://backcountrygallery.com/cropping-better-drop-your-iso/


I suppose when people say they take off their TC when it becomes dark, they imply that they will change the composition so that the subject still fills a reasonable amount of the frame….

 

[bleirer]

Thanks. I think I understand completely.

It’s just that it confuses me I read people say they will take their teleconverter off when it becomes dark - it seems like from this discussion, it shouldn’t have mattered anyway?!

Could it be dslr users saying this?

 

[cchuck]

I think it important to realize that 1 stop higher aperture halves the light reaching sensor. One stop lower aperture (smaller number) doubles light reaching sensor. To compensate for one stop higher aperture, you must either cut shutter speed in half, which may cause blur, or double ISO, which will increase noise, to achieve same exposure.

 

[beano]

Could it be dslr users saying this?

Like said, most likely for auto-focus reasons, some cameras are having issues with AF >F/8, and it is quite easy to exceed that with a TC, when it gets dark. Older cameras will most likely suffer more in this respect in anyway - not all, but most. There are a few low-light DSLR monsters, like the D5/D6, out there…

 

[bcgnz]

I’m pretty sure whoever said that had the z8/z9… it was some comparisons between 400 f4.5 and 600 f6.3

 

[bcgnz]

I’m honestly confused. Because I read somewhere that the 600mm has a larger entrance pupil than the 400mm, and supposedly 600 6.3 does better than 400 4.5 in low light…

Here https://www.dpreview.com/forums/post/67727194

 

[DRwyoming]

I’m pretty sure whoever said that had the z8/z9… it was some comparisons between 400 f4.5 and 600 f6.3

A lot of folks haven't thought this all the way through with the impact of image size normalization on final noise after cropping and resizing.

Steve did a very recent video on this topic that applies to the lens comparison above.

 

[bcgnz]

It seems to me if 400 4.5 is no better than 600 6.3 in a low light, then ignoring the cost the only advantage 400 4.5 has over the 600 6.3 is the slightly shorter MFD.
Is that a valid statement?

 

[bcgnz]

A lot of folks haven't thought this all the way through with the impact of image size normalization on final noise after cropping and resizing.

Steve did a very recent discussion of this that applies:

Thank you. Taking a look

 

[DRwyoming]

I’m honestly confused. Because I read somewhere that the 600mm has a larger entrance pupil than the 400mm, and supposedly 600 6.3 has better low light performance than 400 4.5…

You shouldn't look at entrance pupil in isolation, it's relative aperture (f/ stop) that matters and that takes into account both entrance pupil AND focal length. But yes, all things considered if the two lenses had focal length proportional maximum apertures the 600mm would be roughly an f/6.75 lens but in fact it's an f/6.3 lens so it does gather a bit more light when you consider the focal length scaling.

600mm/400mm = 1.5, f/(4.5*1.5) ~= f/6.75

Or viewed the other way around to gather the same amount of light proportional to focal length the 400mm would be an f/4.2 lens (6.3/1.5) instead of an f/4.5 lens.

That said, the video linked above discusses why just looking at maximum f/stop without considering focal length and what you really need for your subjects and working distances can be misleading.

 

[bcgnz]

I see. Very interesting. It seems like light gathering capability is directly linked to the minimum DOF achievable!

 

[ricardo00]

This also really helps -https://backcountrygallery.com/cropping-better-drop-your-iso/


I suppose when people say they take off their TC when it becomes dark, they imply that they will change the composition so that the subject still fills a reasonable amount of the frame….

I always take my TC off when light gets low. First, one never knows where the wildlife will end up, close or further (it could fly ). And when the light is too low, even with my Z8 and Z9 it struggles to get focus. I rather have an animal in focus than not. Even if the amount of noise is the same, to me there is a better chance of having the subject in focus.

 

[bcgnz]

I think for context, people were saying that the benefit of the 400 4.5 +1.4 TC over 600mm 6.3 is when it gets dark, they can take off TC off the 400mm and continue with the f4.(. But based on Steve’s video, it seems like the 600 would actually do better than 400 even though 600 is supposedly a slower lens..

 

[bcgnz]

Steve explained in this video

He talked about it 13min..

the equivalency only applies when the person is photographing from the same distance. If the person with the shorter FL faster aperature lens is closer to the subject, then the faster aperature actually provides benefits…………

 

[fcotterill]

Fundamentally, Exposure is Scene Luminance projected by the Effective Aperture clipped by Shutter Speed.
Post exposure the ISO amplifies the signal gain after the photon signals captured on the sensor are digitized in silico.

As explained a TC is equivalent to stopping down to a higher fStop, which reduces the light hitting the sensor plane. Cropping in camera has the parallel result e.g. from a FX to a DX frame.

This lecture series by John Hess is worth investing the effort to understand the deeper aspects of exposure (invoking the Path of Light as the exposure metaphor)

Science of Exposure Lectures...

....Dynamic Range, ISO, ISO Invariance.... Dual Gain Sensors I know I'm not alone in struggling to find a fully detailed, and above all accurate, explanation of the science of exposure, which in particular explains Dynamic Range and ISO Sensitivity. These lectures by John Hess take some stamina...

bcgforums.com