You may have already read my article about using manual mode with your external flash, which allows you to shoot in “manual” mode but get automatic exposure via the flash. That article can be found here. That particular mode of operation is for when you have a fixed ISO setting, which is the normal case while using "manual" mode.
What about manual mode without flash? It turns out that you can still get ‘manual mode’ to provide automatic exposure. You make this happen by selecting “Auto ISO sensitivity ON” in the “Shooting” menu (on Nikons, of course).
You can see this mode at work in Manual by adjusting the shutter speed or aperture and observing the ISO indicator value changing to keep up with your selections. When you attempt to set your exposure to go beyond your maximum ISO limit you have set, you’ll see the exposure indicator show "under exposure" (and by how much) since it won’t go beyond your specified maximum ISO limit.
Why on earth would you want to automate Manual Mode? If you’re using a long lens, it’s handy to be able to set a specific aperture and also force a (specific) high shutter speed. If you hate to give up automatic exposure just because you want to specify both an aperture and shutter at the same time, this is the secret sauce that lets you have your cake and eat it, too. I guess I must be getting hungry; that’s too many food references to ever put into a paragraph, let alone a single sentence.
If you’re worried about getting quality results (and you should be) then you may not want to use this technique unless you have a camera with a sensor that can handle pretty high ISO values without getting excessive noise. I have had far more shots ruined by motion blur than I have lost due to image noise. If you have a long lens (e.g. 500mm or beyond) then there’s basically no such thing as too fast of a shutter speed to capture fast-moving subjects.
To calibrate your “Auto ISO”, do some testing up front to determine what’s a reasonable upper-limit ISO. Two key points to consider here are color noise and loss of dynamic range. I always assume that I’m going to be doing some post-processing of my (RAW) shots anyway, so a small amount of color noise in the un-processed shot is completely acceptable. The upper limit of color noise should be where you can no longer clean it up in your image editor without excessive loss of resolution. You should consider noise reduction that only operates on the colors and not the ‘luminance’ channel; you will retain resolution but the image will have a slightly sandy-grained look.
A camera like the D610 is still coasting with values like ISO 4000, but a camera like my D7000 is already a little short of breath at ISO 1600. Dynamic range goes out the window with high ISO; don’t use a technique like the one presented here for something like landscape shots, unless you’re forcing a particularly slow shutter speed.
Some camera models offer “Minimum shutter speed = Auto”, where the camera will select the “1/focal length” in the “ISO sensitivity settings” menu (with additional adjustment options to get values like 1/(focal length * 2) ). I find that this type of shutter speed control is fine for static subjects, but is often unsuitable for fast subjects like flying birds or action sports. I’d rather crank up the shutter speed to cryogenic levels and leave it at subject-freezing speeds.
This technique is, of course, not appropriate in all situations. As always, pick the right tool for the right job.