Canon FD lenses and infinity focus

Introduction

The classic lens formula is: 
1/S + 1/I = 1/F
"S" is the lens to subject distance.
"I" is the lens to image plane distance.
"F" is the lens focal length. All distances must be measured in the same units (millimeters are recommended).

To focus at infinity, the term "1/S" goes to zero and "I" is equal to "F". In fact this is what the marked focal length of a lens means -- at infinity focus the distance from the lens to the image plane is the focal length.

The question arises where we measure from on a multi-element lens that may have a large thickness. The proper answer is that we measure from front and rear nodal points which are neither marked nor trivial to determine the location of. This issue is not being fully addressed here, but it is important to be aware of.

Canon FD lenses on EF mount cameras

The "registration distance" of a lens is the distance from a surface on the rear mounting flange of the lens (which mates against the flat surface on the front of the camera body) to the image plane. For FD cameras this is 42mm, and for EF cameras (modern EOS bodies) this is 44mm. This means that even if we were able to mount an FD lens so that the two registration surfaces mated, we would have an extra 2mm of extension, which would make infinity focus impossible. The situation is even worse because an adapter adds some thickness. On top of this the FD mount ring itself prevents the two surfaces from being mated and forces some additional thickness unless removed.

I decided to make some measurements using a 50mm and a 200mm FD lens. I added an adapter (without glass) to each lens, set each lens to infinity focus and then measured how close it actually measured.

The 50mm lens focused at 240mm. Plugging this distance into the lens formula gives 

1.0 / I = 1.0 / 50.0 - 1.0 / 240.0
Solving for I gave me 63.7 mm (so we have 13.7 mm of extension)

The 200mm lens focused at 3310mm. Performing the same calculation for this lens gave me an image plane distance of 212.9mm (an extension of 12.9 mm).

An average of these two values is 13.3 mm and we can subtract 2mm for the difference in registration distances for the two cameras. This tells us that the adapter itself adds 11.3 mm of extension.

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Tom's Digital Photography Info / tom@mmto.org