Extension Tube and Loss of light

I put my 85L on my MKIII and aimed it at a blank wall with even lighting and let the meter do it's thing, then I added one 12mm extension ring and it read the same reading. I do have my camera set up in 1/2 stops instead of 1/3 so there probably is some difference but I've never had it an issue or even noticed it on either my 5D or MKIII.

12 mm on an 85 is less than 1/3 stop, from f/1.2 to f/1.36. Put it on a 50 or shorter and you'll see more difference. Add more tubes and see the exposure change still more.

I think what I got hung up on is the term "loss of light". Which from what I gather is a mis-leading name.

When I think "loss of light", I'm thinking of a TC which forces the camera to go from f/2.8 to f/4 because it can't let in as much light anymore.

One of these two is named badly because they're not the same thing, it may have the same end result, but it's not the same issue.

No. They're exactly the same thing. In both cases the physical aperture remains the same while the optical distance from the lens' nodal point to the sensor is increased. A 1.4x TC inflicts a 1 stop loss of light because your 200 f/2 with 100 mm aperture diameter is now a 280 mm lens with the same 100 mm aperture for an f-no of 280/100 or 2.8. If you put 80 mm of extension tubes on that same 200 mm lens, you're again seeing 280/100 or f/2.8.

aram535 wrote in post #8582181
I think what I got hung up on is the term "loss of light". Which from what I gather is a mis-leading name.

When I think "loss of light", I'm thinking of a TC which forces the camera to go from f/2.8 to f/4 because it can't let in as much light anymore.

One of these two is named badly because they're not the same thing, it may have the same end result, but it's not the same issue.

You may be hung up too much on the literalness of the phase 'loss of light'. In the case of the TC, the FL is increased with no change in the diameter of the aperture, so the f/stop is effectively smaller so there is less light getting though...which is not 'loss of light'. And in the case of the extension tube, a fixed image circle contains a total number of photons, and when you use the extension tube the image circle is spread out over a larger area and the sensor/film captures only a piece of that circle whose size is related to the gain in image circle size so that it sees less light...which is not 'loss of light' either!

Figuratively speaking, since ability to use as high a shutter speed (for a fixed f/stop) is lost, people state it as 'loss of light' because that's what it appears to have happended. Figurative terms are common in photography...technica​lly a fixed focal length lens and a zoom are BOTH 'prime' lenses -- but the masses don't speak that way! -- while only fixed lens cameras (e.g. movie cameras) might be used with a 'supplementary' lens to increase FL by mounting in the filter threads!

Thank you, those two posts finally drove the point home.