ISO Invariance

If you would like to post, you'll need to register. Note that if you have a BCG store account, you'll need a new, separate account here (we keep the two sites separate for security purposes).

tjphxaz

tjphxaz

Active member
Supporting Member
Marketplace
Member's gallery
A presenter on the topic of Astrophotography at my camera club Zoom meeting tonight spoke briefly about ISO Invariance which I understand is not a separate setting but rather is built in to my D750 and D850 along with other relatively newer DSLR cameras (perhaps mirrorless too but that is not my interest area as of now). As I understand, ISO invariance allows one to shoot dark scenes at the camera's native ISO and then bring the exposure up in post processing with the benefit of having less noise than one gets when shooting at a much higher ISO. Supposedly one can recover up to 5-stops of exposure this way. I have never heard of this and I plan to experiment but I would ask the forum, and Steve, to enlighten me with your knowledge and experience with ISO invariance.
 
Member's gallery
Essentially the idea is that you can set the aperture size and shutter speed, but ISO is a way to amplify or magnify the information the sensor receives, and that can be done in the camera or during post processing. That's true to a point, but the cameras also include some level of RAW processing to create your RAW file. One example of that is "dual gain" - where the sensor data is amplified in a non-linear manner. You might also see some noise reduction at a pre-RAW stage.

If you underexpose intentionally, keep in mind that recovery is across all adjustments and sliders. You are limited to the range of those sliders. So you can adjust exposure, raise shadows, and then locally adjust areas of your image.

One other thing to remember is your choice of software matters. Not all software programs create brightening the image in post processing the same way. You might have 4-5 stops in LR, but only 2-3 stops of latitude in some other programs. The old Nikon Capture NX2 program had a maximum recovery of less than 2 stops.

I would not under expose by a full 5 stops. Under exposure of a couple of stops maintains your latitude to make additional adjustments using the sliders and puts you closer to your final image. BUT - if you are struggling to freeze action, it's a tool to have in your tool kit. I would much rather push midtones where there is more data than in deep shadows.
 
Last edited:
Member's gallery
tjphxaz said:
A presenter on the topic of Astrophotography at my camera club Zoom meeting tonight spoke briefly about ISO Invariance which I understand is not a separate setting but rather is built in to my D750 and D850 along with other relatively newer DSLR cameras (perhaps mirrorless too but that is not my interest area as of now). As I understand, ISO invariance allows one to shoot dark scenes at the camera's native ISO and then bring the exposure up in post processing with the benefit of having less noise than one gets when shooting at a much higher ISO. Supposedly one can recover up to 5-stops of exposure this way. I have never heard of this and I plan to experiment but I would ask the forum, and Steve, to enlighten me with your knowledge and experience with ISO invariance.
Click to expand...
Eric makes some great points above.

I'd also add this - the idea that it will reduce noise is false.

Let's say you shoot at base ISO (100 for example) and it results in a three stop underexposure. Now, when you get the image back to Lightroom, you pull the exposure up by three stops. However, you now have the same noise level as if you'd shot ISO 800. (I explain all of this in depth in my exposure and metering book, but that's the upshot).

You also need to consider that your camera has an analog amplifier as well as a digital one (Lightroom is like a digital amp). Your camera manufacturer knows how to get the best ISO amplification using both the analog and digital amps in the camera and although you may not see a difference with just a little push here and there of a stop or two, once you get into the 5, and 6+ stops pushes, the camera often does better.

I leverage ISO invariance when I'm faced with a high dynamic scene where I'm trying to preserve highlights. However, if you have deep shadows that you're underexposing, you have to be careful to keep ISO low since, again, for every stop of brightness you add in post, you get an extra stop of noise.
 
Member's gallery
Steve said:
Eric makes some great points above.

I'd also add this - the idea that it will reduce noise is false.

Let's say you shoot at base ISO (100 for example) and it results in a three stop underexposure. Now, when you get the image back to Lightroom, you pull the exposure up by three stops. However, you now have the same noise level as if you'd shot ISO 800. (I explain all of this in depth in my exposure and metering book, but that's the upshot).

You also need to consider that your camera has an analog amplifier as well as a digital one (Lightroom is like a digital amp). Your camera manufacturer knows how to get the best ISO amplification using both the analog and digital amps in the camera and although you may not see a difference with just a little push here and there of a stop or two, once you get into the 5, and 6+ stops pushes, the camera often does better.

I leverage ISO invariance when I'm faced with a high dynamic scene where I'm trying to preserve highlights. However, if you have deep shadows that you're underexposing, you have to be careful to keep ISO low since, again, for every stop of brightness you add in post, you get an extra stop of noise.
Click to expand...

Steve,

As you point out in your excellent exposure and metering book, the degree to which a camera is relatively ISO invariant varies quite a bit between cameras. Bill Claff's Shadow Improvement chart is good way to evaluate ISO invariance as shown in the link. The D7200 is ISO invariant and the D750 is nearly so as you mention. The D850 becomes ISO invariant at ISO 400 and above and shadow improvement levels off at about 1 stop at this ISO. The D5 and D6 are not ISO invariant at all. The Canon 5D IV shows improvement over the 5D II. Of course, you know all this but I am posting for the benefit of others.

Bill
 
Some cameras are partially invariant, some fully so. If you check out photons to photons you can see how shadows improve with ISO on the d850 up to around 400, then plateau, so after 400 it wouldn't matter if you brightened in post or in camera. Where the Hassleblad stays level at every ISO so is fully invarient. It's better to capture as many photons as you can via shutter speed and aperture. Bottom line is ISO is not technically part of exposure, exposure is only shutter speed and aperture and maybe scene brightness. ISO is just gain applied in the camera after the exposure is made.


 
Member's gallery
bleirer said:
Some cameras are partially invariant, some fully so. If you check out photons to photons you can see how shadows improve with ISO on the d850 up to around 400, then plateau, so after 400 it wouldn't matter if you brightened in post or in camera. Where the Hassleblad stays level at every ISO so is fully invarient. It's better to capture as many photons as you can via shutter speed and aperture. Bottom line is ISO is not technically part of exposure, exposure is only shutter speed and aperture and maybe scene brightness. ISO is just gain applied in the camera after the exposure is made.

Click to expand...

Graham,

That is more or less what I said in my previous post in this thread. You are correct that ISO is not a part of exposure (measured in lux seconds), presuming that the illumination does not change and this is important. When we make use of ISO-invariance to preserve the highlights, we set the shutter speed to what is necessary to freeze motion and the aperture to get the depth of field we need. In manual mode, would note what ISO is needed for an ETTR exposure with these settings and then decrease that ISO by 3 stops, keeping the shutter speed and aperture the same. We then brighten the image in post.

In this post, Jim Kasson explains the theory of this strategy using the Nikon D850.

In a second post, he explains how to do this in manual exposure mode. Shadow improvement is not that significant below ISO 400 with the D850, so personally I don't bother to use this strategy below ISO 400 which is where the D850 becomes ISO-invariant. Jim recommends then decreasing the ISO by 3 stops, and this gives 3 stops of highlight protection. The image on the LCD from the JPEG preview is somewhat dark, but you can still see what you are doing. You could decrease ISO by 2 stops to get a better LCD preview with a brighter image and still get 2 stops of highlight protection. Most raw converters such as ACR and LR do not apply a linear exposure multiplier and brightening exposure by more than 3 stops is not advisable.

A third post tells how to adapt this strategy with auto-exposure.

Cheers,

Bill
 
bjanes said:
Graham,

That is more or less what I said in my previous post in this thread. You are correct that ISO is not a part of exposure (measured in lux seconds), presuming that the illumination does not change and this is important. When we make use of ISO-invariance to preserve the highlights, we set the shutter speed to what is necessary to freeze motion and the aperture to get the depth of field we need. In manual mode, would note what ISO is needed for an ETTR exposure with these settings and then decrease that ISO by 3 stops, keeping the shutter speed and aperture the same. We then brighten the image in post.

In this post, Jim Kasson explains the theory of this strategy using the Nikon D850.

In a second post, he explains how to do this in manual exposure mode. Shadow improvement is not that significant below ISO 400 with the D850, so personally I don't bother to use this strategy below ISO 400 which is where the D850 becomes ISO-invariant. Jim recommends then decreasing the ISO by 3 stops, and this gives 3 stops of highlight protection. The image on the LCD from the JPEG preview is somewhat dark, but you can still see what you are doing. You could decrease ISO by 2 stops to get a better LCD preview with a brighter image and still get 2 stops of highlight protection. Most raw converters such as ACR and LR do not apply a linear exposure multiplier and brightening exposure by more than 3 stops is not advisable.

A third post tells how to adapt this strategy with auto-exposure.

Cheers,

Bill
Click to expand...
Gather ye photons while ye may.

My approach seems to generally agree with yours. I would be simply stay in base iso and use blinkies to find the exposure where the brightest important highlight is just barely not blinking, then raise that by 2/3 stop. I know the 2/3 because of testing in rawdigger for my particular camera that puts that spot right at the edge of being blown. There is no point to using ISO beyond base to pushing exposure further to the right. If it was impossible to get the shot at base ISO then on my camera which is not invariant it is better to use ISO to get the shutter speed or aperture I needed.
 
You must log in or register to reply here.
Back
Top