Exposure modes separate parameters controlled by the photographer from those controlled by the camera. On digital cameras, ISO is treated as a separate parameter which can be either in the photographer’s control (Manual ISO) or in the camera’s (Auto ISO). Once in a mode with more than one parameter to set, the question arises of which exposure-parameter to set first?
The answer depends on your priorities. You have to start somewhere on the exposure-triangle and prioritize accordingly depending on the shot you want. This depends on the creative impact of each parameter.
Here are some examples. Just remember that ultimately it comes to your creative vision and requires a prior understanding of how each parameter affects an image.
- Aperture first for landscapes: This establishes the desired depth-of-field. Then, ISO second to get the maximum image quality, maximum dynamic-range and minimize noise. This leaves shutter-speed to be whatever is needed for a pleasing exposure.
- Aperture first for architecture too:. Sets the depth-of-field as required. Then, blur out people by selecting a long shutter-speed second. The ISO the falls where it needs to be.
- ISO first for sports: Set it to the highest acceptable value for the intended print size. Then, set aperture second to something bright but not necessarily the widest to account for imprecise focus.This leaves the shutter-speed to set itself.
- Shutter-speed first for sports: This is valid and frequently used strategy It has two failings though, one is that you have to guess at which shutter-speed to select. Pick one that is too high and you may get under-exposure. Pick one that is too low and you get a lot of blur. By setting the ISO first instead, the goal is to get the highest shutter-speed possible which results in a correct exposure.
- For creative work, aperture is most often the priority because it has the most impact on the image. If there is any movement, shutter-speed is next and finally ISO. If there is no movement, then ISO gets set before shutter-speed because, in such case, ISO has an impact on the image but shutter-speed does not.
Note that these are not rules, only guidelines. Sometimes shutter-speed becomes a higher priority in one of those cases for example. Each decision is guided by a mix of creativity and constraints. Creative options get chosen first and the last parameter generally falls into place. Even then, a carefully chosen ND filter can shift things to get a more desirable result.