This is a very interesting discussion. Having grown up in the film era, I too have a problem abandoning ISO as one of the anchors of the exposure triangle. I do understand the difference in sensitivity and gain, but I struggle a little with practical application of the "isoless paradigm". Let me try a couple of scenarios, and perhaps it might help to clarify it for me and others.
OK, the point with 'ISOless' is to chose the settings of the aperture and shutter to give the maximum exposure that your pictorial constraints will stand. The constraints are generally:
For the aperture-
- The DOF you want
- sometimes you set the aperture to the 'optimum resolution' setting.
For the shutter speed -
The motion blur you can tolerate/want to use creatively in your picture.
For the overall exposure (scene luminance, aperture and shutter speed)
- not saturating the sensor and thus blowing the highlights.
Subject to those things you always get least noise in the image by using the largest exposure you can stand. You never want to back away from that for the spurious purpose of finding a 'correct exposure'.
Now, if you really want to be cute about it, with
some cameras you can get lower shadow noise (read noise) by setting the highest 'gain' based ISO you can set
for'a given exposure without blowing the highlights. Note carefully here that we are using the ISO purely as a gain control, not as a determinant of target exposure.
Scenario 1 : Ambient light remains stable. Subjects may have different reflectivity. To avoid possible confusion to an internal meter, I check ambient light with a handheld meter, and for a given film sensitivity, I manually set shutter and aperture after making mental correction for film characteristics.
That is 'film method', agreed.
If I carry over my experience from the film world into the digital world, I will subordinate the ISO to a minimum desired shutter speed and my selected aperture, with a tendency to minimize ISO if possible.
Question : How does this change in the isoless world? What is the best iso to select? Why?
The main point is you set ISO
after selecting your exposure parameters, if at all. So in the ISOless world you decide on the aperture you want (DOF or maximum resolution) and the shutter speed (motion blur). Then you meter. Now, if the meter is set to base ISO and shows the picture to be over exposed, you compromise either the DOF or shutter, usually the shutter, since a shorter shutter will generally give a pictorially equivalent result and less motion blur. Of course, if you were particularly trying
for motion blue, you's dial in more DOF, or if you wanted both the motion blur and shallow DOF, you'd fit a ND filter. The other cases depend on whether your camera is or is not ISOless.
- With an ISOless camera, you just shoot, unless the meter shows the exposure to be so low that you feel the need to compromise some of your pictorial constraints, when you choose which of DOF or motion blur you're going to compromise, and dial in more exposure.
- With an ISOful camera, you need to change the ISO to minimise read noise. Essentially, you are trying to set the ISO as high as possible without losing the highlights, essentially similar to 'ETTR'. If the ISO goes over some limit (indicating a lower than acceptable exposure, and thus unacceptable noise) then again you need to compromise your pictorial constraints.
Scenario 2 : Subject stable in terms of reflectivity, but light changes, and I have no ability to measure ahead of time. I set either shutter or aperture and depend on my camera to set the other for a given film sensitivity.
Exactly the same
Question : Dow does this change in the isoless world? What is the best iso to select? Why?
As I said, with an ISOless camera, you leave it on base ISO, with an ISOful camera, you put the ISO as high as you can, keeping the same exposure and avoiding clipping the highlights. Nikon's auto ISO function provides a way of doing exactly this, although a little unintuitively.
- Put it in Auto-ISO, you'd have thought 'M', but no, you set it to A.
- Set the minimum shutter speed to the shutter speed you have selected according to your pictorial constraints.
- Set the maximum ISO to the reflect the noise level at which you want to start compromising your motion blur constraint.
- Set the aperture to your selected aperture.
Now, if the exposure is too large for base ISO, then the camera will start reducing the shutter speed. If it is smaller than optimum for base ISO it will start to raise the ISO, but keep to your selected aperture and shutter speed. If the noise becomes unacceptable, it will start to extend the shutter speed.
The point abot this is you will always have the maximum exposure subject to your pictorial constraints, whereas if you set an ISO and then adjust exposure to be correct, sometimes you'll find yourself selecting an exposure smaller than you could have had.
