In photography, short for "Indicated Meter Reading." This is the value for f-stop and shutter speed on your camera(for the set ASA/ISO) that will produce 18% middle grey (for B&W images) in the area that has been metered. This is equivalent to Zone V in the Zone system.

Referring to black and white photography now. I'm sure something similar applies in color, but I'm not that experienced with color photography. Often, you will not want to shoot your image right at the IMR, because whatever you have metered will come out as a relatively boring grey. You'll need to fix this in the darkroom later, with a burn or dodge on the affected area. Instead, it's much easier to slightly over- or under-expose your film to get a lighter or darker value in the affected area.

For example, let's say you're shooting a simple portrait of your white friend Susie. You meter her face, and come up with a reading of f-11 @ 1/125 second (it's daylight) for your IMR. If you were to make the image at f-11 @ 1/125 of a second, Susie's face would be 18% middle grey (Zone V) (which is not really what the typical white person's skin looks like) on your negative. Instead, we want to make her face lighter, to make it look more like real Caucasian skin. To do so, we will under-expose the film slightly - say, two stops. We now have an exposure setting of f-22 @ 1/125 second. This now places her face in Zone VII, which is nominal for Caucasian skin. In this way, in our print, Susie will have white skin, as opposed to 18% middle grey skin.

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