Understanding Reciprocity:

• Exposure reciprocity is about the relationship between Shutter speed, Aperture, and ISO
• Exposure reciprocity says that different combinations of Shutter speed, Aperture, and ISO can give the same exposure.

Reviewing "stops" and exposure

Let’s look at how shutter speed, aperture, and ISO affect ambient light exposure. If you remember before, strobe exposure is not affected by shutter speed.

As we mentioned before, shutter speeds, apertures, and ISO’s are usually changed in units called “stops”. An increase of 1 “stop” cuts the exposure in half. For example, changing the shutter speed from 1/100^th to 1/200^th, the aperture from F2.8 to F4, or the ISO from ISO 800 to ISO 400 all cut the exposure by ½. That means there is half as much light affecting the exposure. If you were to make all 3 changes, you reduce the exposure by 1/8^th, or “3 stops lower”.

One stop differences in exposure

Let's look at 1-stop differences again. The following all have a 1 stop difference:

• Aperture (decreasing exposure): F2,F2.8, F4, F5.6, F8, F11, F16, F22, F32
• Shutter speed (decreasing exposure): 1/15th, 1/30th, 1/60th, 1/125th, 1/250th, 1/500th, 1/1000th, 1/2000th
• ISO (increasing exposure) ISO 100, 200, 400, 800, 1600

Most serious photographers have these stops committed to memory.

Most of the time, people shoot at ISO 100, and change the aperture or shutter speed. If you "stop down", which means decrease the aperture by 1 stop, but increase the shutter speed by 1 stop, the exposure remains the same.

The following 5 settings all have the same exposure (assuming the ISO doesn't change)

• F2.8, 1/1000th
• F4, 1/500th
• F5.6, 1/250th
• F8, 1/125th
• F11, 1/60th
• F16, 1/30th

If you increase the shutter speed by 1 stop, but decrease the aperture or ISO by 1 stop, the exposure will remain the same. For example, the following 4 shots will have the same exposure:

• F5.6, 1/100^th, ISO 100
• F8, 1/100^th, ISO 200
• F8, 1/400^th, ISO 800
• F4, 1/200^th, ISO 100