Deliberate Under-Exposing of Wildlife Images

[bjanes]

Geoff (arbitrage) beat me to it. ISO is just a brightness control. With a camera like the D850 that's largely ISO invariant, you can shoot underexposed at a lower ISO and bring it back in post, however, it's exactly the same as if you had shot at a higher ISO (the "correct" ISO) in the field. As mentioned, deliberate underexposure is good to manage highlights, but usually not necessary.

In short, if you were to shoot underexposed by -2 at ISO 800 and that gave you 1/1000th of a second, once you brought it back to the computer and increased brightness by two twos, the noise would look identical to a properly exposed shot at ISO 3200 at 1/1000th. ISO is just a volume knob for brightness that you can turn up or down. Doesn't matter if you do it in the camera or on the computer - the results are going to be the same if you're within a few stops of the correct exposure (on a modern Nikon / Sony).

Personally, I just shoot the correct exposure in the field unless I have a compelling reason (saving highlights) not to.

As Steve points out, ISO adjusts brightness and does not affect photometric exposure if the f/stop and shutter speed do not change, assuming that illumination and scene reflectance are constant. The units of exposure are lux seconds. If you are using manual exposure, you can set the shutter speed and f/stop according to depth of field, freezing motion, etc according to what is needed. You can then set the ISO so that significant highlights are on the right side or the histogram. This is what Jim Kasson refers to metered exposure in the article that Ed Erkes quotes.

As an example, you require 1/250 s to freeze motion and f/8 for depth of field. You would set the camera to these values and starting with base ISO increase the ISO to place the highlights to the right of the histogram. Let's assume that ISO is 3200; this is the metered exposure. To gain two stops of highlight headroom you could set the ISO to 800 and take the shot at 1/250s and F/8. Photometric exposure has not changed. If the sensor is essentially ISO-invariant (most Sony sensors, including the Nikon D8XX series) you would brighten the image in post gaining two stops with no change in image quality.

To see how ISO invariant your camera is go to Bill Claff's Shadow Improvement site. The Nikon D5 and Canon 5D M4 are not ISO invariant and should not be used in this way.

Bill

 

[Craig McGowan]

As Steve points out, ISO adjusts brightness and does not affect photometric exposure if the f/stop and shutter speed do not change, assuming that illumination and scene reflectance are constant. The units of exposure are lux seconds. If you are using manual exposure, you can set the shutter speed and f/stop according to depth of field, freezing motion, etc according to what is needed. You can then set the ISO so that significant highlights are on the right side or the histogram. This is what Jim Kasson refers to metered exposure in the article that Ed Erkes quotes.

As an example, you require 1/250 s to freeze motion and f/8 for depth of field. You would set the camera to these values and starting with base ISO increase the ISO to place the highlights to the right of the histogram. Let's assume that ISO is 3200; this is the metered exposure. To gain two stops of highlight headroom you could set the ISO to 800 and take the shot at 1/250s and F/8. Photometric exposure has not changed. If the sensor is essentially ISO-invariant (most Sony sensors, including the Nikon D8XX series) you would brighten the image in post gaining two stops with no change in image quality.

To see how ISO invariant your camera is go to Bill Claff's Shadow Improvement site. The Nikon D5 and Canon 5D M4 are not ISO invariant and should not be used in this way.

Bill

Many thanks Bill for this information including the 'Photons to Photons' link. All extremely worthwhile advice to factor into my future low light shooting.

Thanks again,

Craig