Understanding: EQUIVALENT EXPOSURE

 

Understanding Equivalent Exposure 

Maintaining exposure value while using different settings. 

  

Shutter Speed x f-stop = Exposure Value

EQUIVALENT EXPOSURE

If we have a normal light meter setting of 500 at f11 for a subject and we want to open the shutter up for a LONGER time to allow for blur and thus we set the shutter to 250 we now must adjust the aperture to KEEP THE LIGHT THE SAME as it falls on the film. The shutter was set to let in MORE light so the aperture must be set to let in LESS light to keep the amount equal. So, we move the aperture to f16 and now we have an EQUIVALENT exposure. If we move the shutter to 125 the aperture would move to f22 which is about as small as most cameras will go. All of this works because each setting of the aperture AND the shutter are each half or double the one next to it.

If you’re changing from 1/125 to 1/60 that means the shutter speed was doubled. Allowing more light to enter the lens, meaning the amount of light must be cut in half for the two exposures to equal. 

Now you must change the aperture from f/8 to f11

Meaning the Equivalent Exposure Value is: 

1/125 x f/8 = 1/60 x f11

Remember: When you double the shutter speed you must change the aperture (closing or opening by one stop) by half, cutting the amount of light entering the camera in half and vice versa.

When you change the shutter speed, you must also change the aperture to maintain proper exposure and prevent your photography from being over or underexposed.