Here's another way to look at it.

You need to understand magnification ratios somewhat, and how big your subject is. "Life size" magnification means that the subject will be the same size on your film (56mm x 56mm for the Hasselblad), as it is in real life. So, If you're photographing a coin that is 2 inches in diameter (about 50mm), it will also measure 2 inches (or 50mm) on your film. "Half lifesize" would mean this same 2" coin would measure 1" on your film, 1/4 life size the coin will be 1/2".

Now, in order to reach Life size, or in magnification ratio terms, 1:1, you need extension equal to the focal length of your lens. For your 80mm lens you would need 80mm worth of extension in order to produce a life size image on film. This will also result in a 2-stop reduction in light, meaning you will need to increase your exposure by two stops.

40mm worth of extension will provide you with half life size, or 1:2 magnification ratio. This will result in a 1-stop reduction in light.

20mm worth of extension will provide you with 1/4 life size, requiring a 1/2 stop of exposure compensation.

Thus, the 55mm extension tube will provide you with a little better than half size reproduction on your 80mm lens (slightly greater than 1:2 ratio), requiring a 1-stop compensation in exposure.

Keep in mind here, that all of the above is measured with your lens focused to infinity. Higher levels of magnification are possible if you use the built-in helicoid along with the extension tube.

Michael H. Cothran