I always photograph in the manual mode when photographing products and shooting in the studio because it allows me to choose both the shutter speed and aperture. Until few months ago,  I occasionally didn’t remember which command dial was for aperture and which was for shutter speed. It wasn't such a big deal, more of a frustration and humiliation when it was happening in front of customers or assistants. I had an ‘AHA’ moment a few months ago when I figured out how to remember which command dial changes aperture and which command dial changes shutter speed. Keep in mind that I’ve been shooting with Nikon for years so this tip might be different for other cameras. However, most cameras (not Nikon DSLRs that I’ve been using so far) let you assign different functions to the command dials so you might want to change the settings in your camera to use my tip. 

Remember that we are talking about adjusting aperture and shutter speed with command dials. Take a look at your camera now. The aperture blades are located in the lens and when you make changes to aperture with the command dial, the aperture blades change & move in your lens. The shutter curtains, which move with a varying speed depending on what shutter speed you set, are located in your camera’s body. When you adjust the shutter speed with the command dial, the changes therefore take place in the camera. In Nikon DSLRs, the front command dial (sub-command dial) changes aperture and the back command dial (main command dial) changes shutter speed. Take a look again at your camera. The lens with the aperture blades is in front part of the camera so the front command dial (sub-command dial) will change the aperture. Accordingly, the shutter blades (in you camera) are behind the lens (or in the back) so the back command dial (main command dial) changes the shutter speed. I know it sounds very obvious and straightforward but it took me a few hmm … years to notice it, which makes things much easier to remember which command dial does what. I hope this little tip will also help you with remembering which command dial changes aperture and which command dial changes shutter speed. By the way, if you shoot in Aperture Priority or Shutter Priority, the same rule applies, meaning the back/main command dial changes shutter speed and the front/sub-command dial changes aperture.