Originally posted by pschlute I am reading every post on here as carefully as I am able. I am not a lens engineer, just a photographer.
What I did not understand was the term "compressing". How does a lens compress light ?
Surely if a lens has say a baffle that restricts the spread of the light so only aps-c area is covered, all that is happening is the "spread" of light is affected. I do not understand how its intensity or brightness can be affected at all. My window example a few posts up demonstrates that restricting the passage of light does not change it's intensity/brightness.
It is like an inverted teleconverter. But instead of enlarging the projected image like a TC do, focal reducer compress the projected imaged to a smaller area.
It takes the captured image from a 43 mm image circle and compress it down to a 28mm image circle.
As then the same amount of light cover half as large area the intestity of light doubles.
It is kind of like using a magnifying glass out in the sun to set stuff on fire.
Quote: If this were the case, why do I get the same reading with my FF camera as my aps-c camera ?
Because the intestity of the light is not changed. But if you FI use a teleconverter that change the projected image you will get different readings with or without the teleconverter.