When determining exposure, is ISO typically taken into consideration secondary to aperture and shutter speed or is it equally as important?

Question

In other words, do professional photographers typically pick the aperture and shutter speed they want and then after those are picked adjust the ISO until their exposure meter shows the shot as being well exposed? Or is ISO equally used along with aperture and shutter speed in the initial exposure setup of the shot?

For me, ISO seems like something used after the fact to compensate to make the exposure meter be where I want. Is that what I should be doing?

One more comment: I often start a shot out thinking, this shot should have a high aperture, or this shot should have a slow shutter speed. Does anyone start a shot thinking, this shot should be ISO 800?

Answer 1

It depends on your priorities. You have to start somewhere on the exposure triangle and prioritize accordingly depending on the shot you want. It depends on the creative impact of each parameter.

Examples from my photography:

• For landscapes, aperture is the priority to get the depth-of-field I need in the shot. Then I set the ISO as secondary to maximize image quality, maximize dynamic-range and minimize noise. I then let the shutter-speed be whatever is needed to expose the image as desired.
• For architecture, aperture is still the priority. I often set a long shutter-speed secondary to blur out people and then let the ISO be where it needs to be.
• For sports, I first set the ISO to the highest acceptable value for the intended print size. Then I set the aperture to something bright but maybe not the widest to account for focusing at the wrong distance. I let the shutter-speed set itself in that case.
• For creative work, aperture is 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.

ADDENDUM:

These are examples from my photography. None is an absolute rule either and I do select shutter-speed first on some rare occasions. The point is that 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 that, a carefully chosen ND filter can shift things to get something I deem a more desirable result.

Answer 2

Aperture and shutter speed are important creative controls which alter the look and the mood of an image (in addition to ). ISO is not a creative control. It required from a technical standpoint in order to amplify the analogue signal prior to readout/digitization in order to reduce read/quantization noise. It is also required to obtain the correct output brightness level for the image preview (or the image itself if you are shooting JPEG).

As such I will always set the aperture / shutter speed first, ISO gets set to whatever is necessary for the shot. If the necessary value is too high, then that is an indication that there is not enough light. In this case you must chose to compromise on shutter speed, aperture or both, and again set the ISO to whatever is necessary to get the shot. I have no problem at all with using auto-ISO provided I'm still in control of the aperture / shutter. For a given shutter speed / aperture combination reducing the ISO will actually make the noise worse. Think of auto-ISO as auto-noise-reduction. It's totally different to full auto.

Whilst you could argue that noise can affect the mood of an image, digital noise is clumpy / bandy / generally ugly looking so if I want a grainy image I'll do it in post with a film grain emulator.

Answer 3

I find it interesting how different photographers use ISO based on their camera choice and experience. Specifically: film vs digital. In fact, while cmason and I are saying the same thing, we both approached this differently.

When shooting with film, you are basically locked to the ISO of the film. Sure, you could rewind a given roll, use a leader retriever, and swap rolls whenever you want, but most of the time and for most people that's simply not practical. You choose your film before a given shoot based on what you expect circumstances to be. There are many characteristics of each film that would make people choose one over the other, but for the sake of discussion lets say only ISO is considered. If I'm going to be outside at a family picnic I will likely choose a different film than if I'm in a dark house, or if I'm setting out on a hike intending to use the tripod for everything. I would always carry a small variety of film with me so that I could switch from 400 to 800 if I found I needed to (though I'd likely only switch after exposing the entire roll). Getting some ISO 1600 or 3200 film was for special circumstances and I wouldn't normally have that in my refrigerator ready for use. (Though on the other end of the spectrum I always had ISO 50 Velvia on-hand!)

So, yes, shooting film means I typically picked ISO first. Shutter speed and aperture were chosen later to make the photo. If the film ISO was too high or low, I relied on shutter and aperture to help me get a shot -- perhaps using f22 or even pulling out the tripod for something quick.

With digital it's easy to switch ISO. Often just as easy as setting aperture or shutter speed. I find that I do switch ISO far more frequently than I ever did with film -- and that's a huge advantage, for sure. But still, when I pull the camera out of the bag the first thing I check is ISO and I set it to what I think is a reasonable starting point. Headed outside on a bright day -- no point in ISO 1600, or even 800; 200 is more than plenty. Taking some no-flash indoor photos -- bump it up to ISO 1600 or maybe even 3200. I often work fast (typically chasing a 3-year-old) and find myself working the same old way as with film: setting ISO to something reasonable and then making use of the aperture and shutter speeds to get the shot.

Answer 4

You adjust ISO to allow you to achieve what you desire with aperture and shutter speed. If your lens is wide open, and the shutter speed too slow to allow a quality image, you have no choice but to increase ISO.

It is helpful to think back to film days to understand the relationship. Then, it was not as simple as turning a dial to set ISO (lets drop push processing for now). ISO was controlled by the sensitivity of the film you were using. If conditions were not correct for the sensitivity of your film, you had to change film, if you had other film. If your camera had ISO 64 slide film, and you needed ISO 400, you had to unwind your current film, and put in a new roll.

So, from this old film perspective, ISO is the 'last' thing you adjust, as it was a colossal pain to adjust it back in the day.

Answer 5

I find the end use for an image determines the priority of of the 3 controls used to acquire it.

When I used to shoot images for brochures & promotional material printed on coated paper, acuity was the highest priority and low ISO was the only way to get that. Next came depth of field and I always brought enough light to the scene or setup (typically with external flash) to ensure I got the aperture necessary to get the important areas in sharpest focus. Then shutter speed was limited to what I could sync with the flash.

For the action & sports photos I used to do that were printed on newsprint or uncoated paper, capturing the action was the priority and that meant I was using the fastest shutter speed allowed by the ambient lighting conditions with a high ISO and whatever aperture let me keep my SS where it would freeze the action.

If your end use is posting to Facebook, then obviously the priority is just get a decently exposed image and let the settings fall where they may.