# Maximum focusing distance when using Canon FD to EF adapter?

I have an old Canon A1 camera, and a 50mm 1.4 ssc FD lens for it. I also have a Canon 30D, but sadly no 50mm lens for it. I was planning on getting a (lensless) FD-EF adapter to use the 50mm on my 30D. I know that I won't be able to focus to infinity, but does anyone know what the maximum focusing distance actually is?

Or, is there a way to work that out if we assume the adapter to be an extension tube of a certain length?

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Canon EF has a flange distance of 44mm. FD has 42mm, plus the thickness of the adapter... this may be enough info to work something out but I don't have the math for it :) – Staale S Feb 22 '11 at 14:48
According to the thin lens formula, 1/y + 1/x = 1/50 where y = lens-to-image distance (mm) and x = lens-to-sensor distance. Using this as an approximation and taking a 14 mm adapter pushes x to a minimum of 42 + 14 = 56 mm. Solving for y gives 50*56/(56-50) = 467 mm = about 1.5 feet. (That's why these are primarily macro photography solutions.) Equivalently, if you want to focus to y mm (from the lens), you need x = 50y/(y-50). E.g., for y = 5 m = 5000 mm, x = 50.5 mm implies the adapter must be no more than 50.5 - 42 = 8.5 mm thick. – whuber Feb 22 '11 at 18:31
Wish we had more examples of how bad the quality is with various adapters with lenses at various f stops with various lenses. – user7150 Nov 3 '11 at 3:20

You'll have to be within a foot or two. I used a glassless adapter with my 30D and an FD 135/2.8 and had to be within roughly 6 feet to focus on the subject. I have an FD 35-70, too; if you don't get an accurate answer by tonight I'll try to measure the exact distance for you with it at 50mm.

I also have a cheap (I think it was \$1 from KEH) off-brand adapter with glass in it. I found that I had to stop down my 135/2.8 lens to f/8 in order to get good pictures. I ended up selling the 135 (which was a shame, it was a fantastic lens, compact, lightweight, and well-built) and buying a Nikon non-AI 135/2.8 (also well-built but about 50% larger and heavier than the Canon). It wasn't a very expensive tradeoff, luckily: the Canon sold for about \$10 and I bought the Nikon for around \$40; but a 50/1.2 lens is much more expensive, so if it's still around a 4x markup, you might almost be better off getting a micro-4/3 camera instead!

Oh, and I also, sadly, found that my \$250 all-plastic EF-S 55-250/4-5.6 is sharper than the Nikon at all overlapping apertures and focal lengths (i.e. wide open the cheap modern consumer 5x zoom is sharper than a 40-year-old near-professional fast prime). Just something to consider if you are going to get into manual focus lenses: they are much more fun but, unfortunately, not as sharp as a lens with modern construction and coating.

Edit after performing the test:

I used my FD 35-70/3.5-4.5 at 50mm, wide open (I guess that means f/4) with my glassless FD-EOS adapter (a generic one from eBay, \$15 shipped) on my 30D. I set up a ruler with the 0"(inches) point at 6" from the plimsole line (heh) then took pictures of a pack of cards (1) just beyond what I would consider "sharp" focus, (2) in the middle, as near as I could tell, of the sharp focus range, and (3) just inside of what I would consider sharp focus (i.e. pictures (1) and (3) were somewhat blurry, while (2) should be sharp). Picture (1) was at the 10.5" mark of the ruler, (2) was at 9.75", and (3) was at 9". I wasn't sure if that meant, with the narrower DOF of your 50/1.2, your maximum focus distance will be just under 16.5" or at 15.75", so I stopped down to about f/8 and did my best with the reduced light to determine the maximum focus distance, and the threshold seemed to be about 10.5" again. So I think you won't be able to focus beyond about 16 inches.

I'm not positive I set everything up at exactly right angles there, so you might be able to capture another half-inch or so, and maybe even another quarter-inch on top of that if your lens focuses to beyond infinity.

Here is a 100% crop of roughly the center of the 15.75"-from-the-sensor middle-of-the-focus-range capture. Note that the books in the background are 14.5" past the end of the ruler (32.5" away from the sensor). What I would normally call beautiful, buttery-smooth bokeh is, in this context, an incredibly-frustrating why-can't-I-focus-on-that problem.

(I'm not entirely convinced the apparent CA on the black circle of the Ace is actually CA: upon very close examination of the box, that circle is actually a dark purple, and I can just barely see a tiny little leakage of reddish ink on the edges of that circle; that 100% crop is about 2x actual size so it's much more visible in the picture. Of course, it could just be a characteristic of the lens, which is a consumer zoom and from the film era).

All of which reminds me: your 50/1.2 would probably make a great macro lens! I've been using the 35-70 with a cheap extension tube set (heads up: while off-brand air may be the same as Canon air, off-brand threads are not as smooth as Canon threads) as my macro lens, and at f/3.5 under anything other than bright sunlight (and even then, as the lens itself usually shades the subject) it's pretty difficult to figure out where exactly the focus point is. Obviously you still have to stop way down to take the picture, but having eight times as much light would probably help you a lot when trying to center the focus on the middle of your subject!

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Excellent, thanks drewbenn! Wow, 16"... I thought I might have got a little more reach than that, but it seems to tie in with Matt Grum's predictions. I guess i'll just have to wait until the day I can add a proper EF 50mm to my collection :) – ltn100 Feb 23 '11 at 15:38

I think the max focus distance winds up being a couple of feet so it's pretty useless with a glassless adaptor. I don't know what the quality of the cheap focus correcting adaptors is like but I'd guess it probably isn't worth it, given the cost of a good adaptor (the original Canon ones sell for about a grand).

FD telephotos can be made to focus to infinity with a glassless adaptor but they have much more space behind the rear element than a 50mm lens.

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http://www.amazon.com/Canon-Mount-Adapter-Infinity-Focus/dp/B001D8X72G says that it's product will "Focus to Infinity", so I think you'll be fine, if you buy the right product. Of course, the way that they manage to do this is with a slight optical distortion.

Alternatively, you might just consider buying a 50mm 1.8, they aren't that expensive, and the quality is actually pretty good. It's only a little more than the FD-EF adapter is.

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Thanks for the answer, but that product contains a correction lens to enable infinity focus. As I said in my question I am after a lensless adapter because I dont want to suffer quality loss. – ltn100 Feb 23 '11 at 15:31