Skip to main content
Photography

Return to Answer

Added analysis of low ISO w/ adjsted exposure vs. high ISO w/ specific exposure.
Source Link
jrista
jrista
  • 70.8k
  • 15
  • 164
  • 314

EDIT:

Further investigations to cover Matt's assertions about ISO have lead to another sample image. The image below is split into four bands, two representing exposure following Matt's methodology, and two representing exposure where ISO is kept as low as possible. The assertion was that when trying to expose a scene, you set aperture and shutter then use the highest ISO possible without blowing out highlights, and correct exposure via post processing by reducing EV, to produce an image with the lowest noise. This contradicts the normal assertion that one should use the lowest ISO and adjust aperture and/or shutter to achieve a correct exposure to maintain the lowest noise possible.

Noise Test

The first and third bands in the image above were shot by choosing a specific shutter speed and aperture, then upping the ISO as far as possible without clipping highlights. The second and fourth bands in the image above were shot by choosing ISO 100 and a specific aperture, and adjusting shutter speed to achieve a correct exposure (no ETTR.) Both images were corrected in post production by using Lightroom's "Auto Tone" feature, which caused a reduction in the high ISO image's exposure by a little bit, and pretty much left the ISO 100 image the same.

There is a difference of three stops ISO between these two shots, and the increased noise level of the ISO 800 image is very clear in the shades. In the midtones, there is some observed increase in noise in the ISO 800 image over the ISO 100 image. In all tonal levels, fine detail has been lost to one degree or another in the ISO 800 image vs. the ISO 100 image. This can be observed by examining the remote control in its cradle, the fronts of the palm, and in any shadow tones. In the mid tones and highlights, noise levels are not specifically high enough to cause any significant degradation in print. The noise level in midtones and shadows is, however, likely to intrude onto fine details, such as one might pick up with macro photography or any photos that push the limits of resolution of a lens or sensor.

While negative exposure compensation in post processing has indeed reduced the noise level of the high ISO image to more acceptable levels, there is no question that using the lowest ISO possible will result in lower noise than even post-process negative exposure compensation of a high ISO image.

The real question is, is it always appropriate to use the lowest ISO possible? If you can expose the scene you are trying to capture without any undesirable side effects, such as blurring, underexposure, etc. then choose the lowest real ISO you can (expanded ISO settings usually achieve a lower ISO setting through digital, rather than analog, means...so using ISO 50 Expansion for example, should be avoided.) If you can not expose the scene you are trying to capture without any undesirable side effects, which might be the case when photographing sports or wildlife, photographing concerts, or doing much of any kind of indoor photography involving any action, then increasing the ISO to the minimum acceptable level that will allow you to expose your scene properly (i.e. eliminate motion blur, expose at the right level, etc.) should be chosen.

If you have the headroom, overexposing by choosing the maximum ISO you can without clipping highlights, and applying some negative exposure compensation in post processing, can help mitigate the effects of noise of very high ISO, and will be a better choice than choosing an ISO that is too low, then applying some positive exposure compensation in post processing (which will just heighten the effects of noise.)

EDIT:

Further investigations to cover Matt's assertions about ISO have lead to another sample image. The image below is split into four bands, two representing exposure following Matt's methodology, and two representing exposure where ISO is kept as low as possible. The assertion was that when trying to expose a scene, you set aperture and shutter then use the highest ISO possible without blowing out highlights, and correct exposure via post processing by reducing EV, to produce an image with the lowest noise. This contradicts the normal assertion that one should use the lowest ISO and adjust aperture and/or shutter to achieve a correct exposure to maintain the lowest noise possible.

Noise Test

The first and third bands in the image above were shot by choosing a specific shutter speed and aperture, then upping the ISO as far as possible without clipping highlights. The second and fourth bands in the image above were shot by choosing ISO 100 and a specific aperture, and adjusting shutter speed to achieve a correct exposure (no ETTR.) Both images were corrected in post production by using Lightroom's "Auto Tone" feature, which caused a reduction in the high ISO image's exposure by a little bit, and pretty much left the ISO 100 image the same.

There is a difference of three stops ISO between these two shots, and the increased noise level of the ISO 800 image is very clear in the shades. In the midtones, there is some observed increase in noise in the ISO 800 image over the ISO 100 image. In all tonal levels, fine detail has been lost to one degree or another in the ISO 800 image vs. the ISO 100 image. This can be observed by examining the remote control in its cradle, the fronts of the palm, and in any shadow tones. In the mid tones and highlights, noise levels are not specifically high enough to cause any significant degradation in print. The noise level in midtones and shadows is, however, likely to intrude onto fine details, such as one might pick up with macro photography or any photos that push the limits of resolution of a lens or sensor.

While negative exposure compensation in post processing has indeed reduced the noise level of the high ISO image to more acceptable levels, there is no question that using the lowest ISO possible will result in lower noise than even post-process negative exposure compensation of a high ISO image.

The real question is, is it always appropriate to use the lowest ISO possible? If you can expose the scene you are trying to capture without any undesirable side effects, such as blurring, underexposure, etc. then choose the lowest real ISO you can (expanded ISO settings usually achieve a lower ISO setting through digital, rather than analog, means...so using ISO 50 Expansion for example, should be avoided.) If you can not expose the scene you are trying to capture without any undesirable side effects, which might be the case when photographing sports or wildlife, photographing concerts, or doing much of any kind of indoor photography involving any action, then increasing the ISO to the minimum acceptable level that will allow you to expose your scene properly (i.e. eliminate motion blur, expose at the right level, etc.) should be chosen.

If you have the headroom, overexposing by choosing the maximum ISO you can without clipping highlights, and applying some negative exposure compensation in post processing, can help mitigate the effects of noise of very high ISO, and will be a better choice than choosing an ISO that is too low, then applying some positive exposure compensation in post processing (which will just heighten the effects of noise.)

Source Link
jrista
jrista
  • 70.8k
  • 15
  • 164
  • 314

Regarding the statement:

Is lower ISO always better?

There seem to be a variety of opinions on this topic, and while they may seem mutually exclusive, I am not certain that is the case. There is no cut and dry "Yes, X ISO setting is always better." I think which is better is very dependent on context...on what it is you are trying to shoot, and what kind of light is available.

The two points of view expressed so fare include:

  • Use the highest ISO possible to maximize saturation (and therefor minimize noise) without clipping highlights.
  • Use the lowest ISO possible to minimize noise while using the correct shutter and aperture to achieve a proper exposure.

The general consensus is that opting for the lowest ISO possible to achieve a proper exposure is the best approach. That statement is heavily laden with hidden meaning, however, as the lowest ISO possible may not necessarily be ISO 100. You may be forced to use a higher shutter speed or a narrower aperture to achieve the necessary creative effects you desire, forcing you to use a higher ISO to maintain proper exposure. You may also face issues with available light and reach the limits of your lens (i.e. maximum aperture) and be forced to use a higher ISO to achieve a correct exposure. I think that is the point Matt Grum has been trying to make.

Barring any specific creative needs (i.e. action-freezing shutter speed or large DOF via small aperture), the lowest ISO setting with the necessary shutter and aperture to produce a "correct" exposure (i.e. an exposure that is neither over or under exposed, or if you follow ETTR, one that has not blown out any highlights) is, still, the best practice. This can be easily demonstrated, as the sample shots below show. This set of examples is shot with several things in mind:

  1. There is no requirement for a short shutter speed, so it could be as long as necessary to get a correct exposure.
  2. There is no requirement for depth of field, so the widest aperture will do.
  3. The available light is fixed, and can not be changed.
  4. A tripod and cable release will be used to eliminate any camera shake.

alt text

The clearest shot is, as you can see, the ISO 100 shot. At maximum aperture, ISO 100 required a 0.6 second exposure time. This is pretty long, but since there was no motion, a lengthy exposure is not an issue. The ISO 1600 shot is still properly exposed, and used a shutter speed exactly four stops faster than the ISO 100 shot. Despite being properly exposed, there is obviously additional noise. The final shot was another ISO 100 shot at the same settings as the ISO 1600 shot, with the exposure value adjusted by +4EV in Lightroom. The noise level in this shot is obviously FAR higher than the other two since it has been digitally enhanced.

Given these examples, we can come to a few logical conclusions:

  • Using the minimum ISO for scenes with no motion when shooting with a tripod will produce the lowest noise possible.
    • Common for landscapes and still life scenes.
    • Uncommon for sports, wildlife, bird, or any other action photography.
    • Depends for things like portrait shots, however since you can usually control the amount of available light for portraits, a low ISO should usually be possible.
  • Using the minimum ISO for a scene that allows you to expose correctly will minimize noise, but may not be the lowest possible noise.
    • If you need to freeze action with a high shutter speed, or increase your DoF with a narrow aperture, you may need to increase ISO to maintain exposure.
  • Shooting with the lowest ISO regardless of the situation is a bad idea.
    • This will often result in under exposure (possibly severe) when available light is not optimal.
    • This will result in considerably more noise when exposure is corrected in post processing.

I think a good way to learn what ISO settings are best for various scenes and lighting is to set your ISO to Auto, use manual mode, take a few shots, and review the results. The camera's automatic exposure metering will always try to create a "proper" exposure, and when you have a scene with a broad range of tones, it should choose the right setting most of the time. You can also try manually setting a higher or lower ISO than the one the camera chooses automatically, and re-take a shot to see the results. Outside of maybe landscape and still life photography, you probably won't find a single "correct" or "best" ISO setting. However, I do believe the general rule of thumb will always be:

Use the lowest ISO you can while maintaining a proper exposure for the type of shot you are making.

For still life, that will probably always be the minimum native ISO (not using any kind of ISO expansion). For landscape, that will probably be lower ISO's, such as 100 or 200. For action photography, including sports, wildlife, birds, kids, etc., the lowest ISO may change for every shot, and may vary from ISO 200 through ISO 3200 or beyond, and it will be very dependent upon available light. Lots of light will allow you to use lower ISO, less light will dictate higher ISO. Regardless of what ISO you use for action shots, another good rule of thumb is:

Its always better to actually get a shot than miss one because you don't like the camera settings necessary to capture it.

Even if you have to use ISO 3200 to get decent indoor sports shots with your f/1.4 lens, at least you'll get the shots. Those ISO 3200 shots will have lower noise than the ISO 1600 shots that you underexpose then correct via post processing, as shown by the (rather extreme) example above. De-noising algorithms are also pretty advanced these days, and can greatly reduce the noise level of a high ISO shot to more acceptable levels. That again makes using a higher ISO that will ensure a correct exposure a better option than a lower ISO that will likely underexpose and require post-process correction.