PASM and Exposure

PASM refers to the standard exposure modes available on most “serious” cameras. Each letter is a different mode.

Each mode is characterized by which setting(s) you control and which are set by the camera. Settings in this case mean exposure settings, so we’re talking about shutter speed, aperture and ISO.

• (P)rogram mode: Camera controls aperture and shutter
• (A)perture priority: You control aperture, camera controls shutter
• (S)hutter priority: Camera controls aperture, you control shutter
• (M)anual mode: You control aperture and shutter

So, for example, if you have a scene where things are moving really fast, like a sports scene it makes sense that you would want to control shutter speed - make it really fast so things don’t blur.

If you have a landscape and you want the whole thing in focus, then you’ll want a large Depth of Field, which means a very narrow Aperture.

I guess you’d use Program mode if you just want the camera to set everything.

Photography Exposure Triangle