I shoot a lot of macro images with flash and custom light mods. Most often I set my camera to F/18 and 1/200 of a second with 1/4 flash power.

Every time the light changes I need to adjust my ISO. I've tried AUTO ISO but since I'm focusing very close, the camera doesn't get the exposure metering right with my flash and light mod.

I wonder why the camera can't apply AUTO ISO after the image was taken such that the image get the right amount of brightness?

  • \$\begingroup\$ Are there reasons for F/18 and 1/200 s? \$\endgroup\$ Commented Aug 26, 2021 at 19:31
  • \$\begingroup\$ Yes. I use F/18 for having a greater DOF and I leave the shutter speed at 1/200 because it is easier to adjust the ISO every time the light changes since the SS is limited by 1/250 (sync speed) \$\endgroup\$
    – Arji
    Commented Aug 26, 2021 at 19:51
  • \$\begingroup\$ This excellent answer is definitely related if not quite the same question \$\endgroup\$
    – Philip Kendall
    Commented Aug 26, 2021 at 20:19
  • 2
    \$\begingroup\$ If your flash is the main light source and is set to 1/4 power, why is the light changing? \$\endgroup\$ Commented Aug 26, 2021 at 22:03
  • \$\begingroup\$ @MikeSowsun "custom light mods". P/4 tends to be a lot for most external flashes when the actual subject is centimeters away so most of the light will be wasted. Comparatively small amounts of wasted light getting to the target after all will make quite a difference, as will some leaves or other material interposing itself between flash and subject. Also the subject and background themselves may be light or dark while you still want to make use of most of your available dynamic range. \$\endgroup\$
    – user98068
    Commented Aug 28, 2021 at 10:29

4 Answers 4


Because ISO adjustment changes the gain applied to the analog sensor signal before digitization.

Whether that results in better performance vs. a brightness adjustment after digitization is not necessarily given, (see steven-kersting's answer about ISO invariance), but in many cameras it is different.

If you have an ISO invariant camera, you can indeed forget about setting ISO if you're going to post process anyway.

Addendum: I should add that the setting "Auto ISO" at least in some cameras can set the ISO value to "non-natural", i.e. non full stop (doubling/halving steps) values, eg. 125. These values may not be available for manually selection and are actually digital expansions from the nearest real value. (I'm talking from experience with my entry level Canon EOS here, YMMV)

  • \$\begingroup\$ This is not true for almost all digital cameras today. ISO is not done by adjusting the gain on the sensor. Most have a single gain, some more recent sensors are dual gain. If you dig into camera reviews on dpreview and other sites hey tell you whether the sensor is single or dual gain. \$\endgroup\$
    – the_limey
    Commented Aug 30, 2021 at 6:26
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    \$\begingroup\$ @the_limey It's still analog gain done prior to digitization. \$\endgroup\$
    – Michael C
    Commented Aug 30, 2021 at 16:42
  • 1
    \$\begingroup\$ Probably a LOT of cameras in actual use today predate the "iso invariant" trend .... that predates going iso-variant (dual gain) ... \$\endgroup\$ Commented Aug 31, 2021 at 12:01

In fact, ISO (auto or otherwise) is applied to the sensor readout after the image is taken; but the camera has no way to review the image to determine the exposure is correct (and what is correct anyway?).

These days there are a lot of cameras that are very nearly ISO invariant. And with one of those cameras it is possible to leave the ISO set to minimum for all images; and then brighten them in post, with negligible impact to the results compared to the camera increasing the brightness with auto ISO. Effectively, you can change the ISO in post with essentially the same results.

This chart shows the benefit of using a higher ISO as light levels decrease for a Nikon D750, and Olympus EM5, and a Panasonic GX-9 (there are many others with similar performance)... around half a stop of improvement on the chart is about the minimal limit detectible visually.

enter image description here

  • \$\begingroup\$ Comments are not for extended discussion; this conversation has been moved to chat. \$\endgroup\$
    – Philip Kendall
    Commented Sep 2, 2021 at 18:46

@ths answer is spot on about Auto ISO but also about ISO. This gain is applied on the sensor-readout before being digitized. Since the Analog-to-Digital conversion has limited bit-depth, the process is not reversible.

Auto ISO has another effect in that the selected ISO on most cameras except in Manual mode will affect other parameters. So if you allow the camera to use a higher-ISO automatically, it could also change aperture, shutter-speed and flash output. None of these can be changed after the fact and so it would be impossible to apply the Auto ISO behavior.


You can. ISO on a digital camera is the same as a brightness adjustment post capture, because the ISO of the sensor is fixed -- it's a function of the design and manufacture of the sensor itself. So ISO shifts are done by brightening the image after it's taken (see below for a caveat to this).

You can leave the ISO at the natural ISO of the camera and brighten images yourself in post. In general, I see that cameras do a better job with Auto ISO in-camera vs. dialing up exposure in Lightroom, as the camera handles noise better. But your mileage may vary.

Some recent cameras/sensors have "dual gain" capabilities to offer different base ISOs. E.g. my Fuji XT-4 base sensor ISO is 160. It also has a "high gain" sensor mode that kicks in at ISO 800. All other ISO values are emulated in-camera by brightening the image post capture.

  • 1
    \$\begingroup\$ "the ISO of the sensor is fixed -- it's a function of the design and manufacture of the sensor itself" <- sorry, but this is wrong, at least in practical terms. The ADCs on the sensor can be set to have different gains, and this does give different results from a purely digital brightness adjustment. \$\endgroup\$
    – Philip Kendall
    Commented Aug 27, 2021 at 13:47
  • \$\begingroup\$ You are wrong @PhilipKendall. Go read the reviews for recent cameras on dpreview. They will tell you if the sensor is single or dual gain. The adcs are not programmable to be different gains at different isos. That isn’t how modern sensors work. \$\endgroup\$
    – the_limey
    Commented Aug 30, 2021 at 6:32
  • \$\begingroup\$ From dpreview review of the Nikon Z fc “ The sensor is also highly ISO invariant, which you can see demonstrated in our test scene, where we've taken photos at different ISOs and brightened the Raw files. In layman's terms, this means that you can shoot at low ISOs (to preserve highlights) and brighten the image in Photoshop or similar and get the same result as if you boosted the ISO in-camera.” \$\endgroup\$
    – the_limey
    Commented Aug 30, 2021 at 6:41
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    \$\begingroup\$ "ISO Invariant" doesnt mean the ISO doesn't change, it means the noise performance holds up well as ISO is increased. \$\endgroup\$
    – Eric S
    Commented Aug 31, 2021 at 14:57

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