Many current cameras have a “hidden” autoexposure feature, an option I use quite often when working wildlife. If your camera has “auto ISO” buried someplace in the menus, you probable can use this feature.
Three choices control exposure: shutter speed, aperture, and ISO. We generally lock in two of these, and vary the third. In aperture priority you select the f/stop and ISO, and the camera sets the shutter speed. In shutter priority you select the shutter speed and ISO, and the camera sets the f/stop.
With most cameras it’s far easier to change shutter speed and/or aperture than it is to change ISO. After all, with both Nikon and Canon that’s what the command dials by themselves do as default behavior.
Look in your camera’s menus to see if you have “auto ISO.” If so, turn it on for this test. The way auto ISO normally works is that you set a minimum shutter speed and a maximum ISO value. So long as you’re shooting at that shutter speed or higher, the camera will vary the ISO needed for the situation. If the light level drops below this range one of two things happens: (a) in aperture priority the shutter speed continues to slow below your preferred minimum, while the highest ISO value is maintained, or (b) in shutter priority, once the aperture is wide open the camera underexposes, usually with some sort of viewfinder warning indicator, but still using the highest ISO.
But here’s the catch for bird and mammal work: setting a minimum shutter speed for wildlife means you’re probably going to choose a fast speed in order to stop any possible action. What if you intentionally want a slower speed? At the same time, at any given light level you don’t want to work at any higher ISO than necessary; you always want to use the lowest ISO you can get away with, given the situation. That fast minimum shutter speed you selected may force you to shoot at a high ISO even in good light.
OK, back to those three variables that control exposure. Reread that paragraph up above, and you’ll notice that I did not mention the manual exposure mode in which what you set is what you get. In this exposure mode you have to manually set both the shutter speed and the f/stop. Now here’s the kicker: if your camera has auto ISO, most likely you can use auto ISO while the camera is in manual mode, and let the camera vary the ISO needed for whatever shutter speed/aperture combination you select.
However — and this is a major “however” — you must know how to work in the manual mode. I’m always amazed at how many photographers today do not know how to do so. If you’re not sure, read my June 2015 blog post, and your camera’s manual, and practice.
So, switch to manual exposure mode, and turn on auto ISO. Set the highest ISO value to whatever maximum ISO you’re comfortable using with that camera. Set the camera itself to its lowest native ISO. For Nikon cameras, the “minimum shutter speed” choice in auto ISO does not apply when the camera is in manual mode (and I’m pretty sure this is also true for Canons, but not being a Canon user myself…).
What you have basically done is turn the manual exposure more into an autoexposure one. You select the f/stop and shutter speed you want, the camera sets the necessary ISO.
But how can you add or subtract the amount of light (for example, in order to reposition the histogram when shooting RAW) if the camera adjusts the ISO for any given shutter speed/aperture combination? Just as in any autoexposure mode, you use the camera’s autoexposure compensation. This does not affect the shutter speed or aperture you’ve set; after all, in manual mode what you set is what you get. Instead, it changes the ISO the camera selects. Dial in a +1 autoexposure compensation, and the camera raises the ISO by one stop; dial in a -1 compensation, and it drops the ISO by one stop. Since the camera is in manual mode, the shutter speed and aperture displayed in the viewfinder will not change, but there will be some sort of indicator that exposure compensation is in effect. You’ll have to pay attention to what you’re doing, and remember to return the compensation back to “zero” when no compensation is needed. If the light level drops so low that the camera maxes out ISO, your set exposure values will be too low. Just watch the meter display, which in this case will show underexposure.
Should you use this “auto ISO with manual” mode all the time? Heavens, no. As with all modes on your camera, you use what is appropriate to the situation. I personally find auto ISO in the manual mode extremely useful when I’m working wildlife.