Don Clary
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 p.1 #1 · Tuning Contax-Zeiss adapters to Canon lenses | |
JameelH and I have been tuning 4 Fotodiox adapters to 4 Zeiss lenses 15mm f3.5, 21mm f2.8, 35mm f2.8PC, and 50mm f1.4. . The 15mm was tested on a 1Dm2; the other 3 were tested on a 5D. None of the 4 lenses would focus at infinity with the Fotodiox adapter; all stopped at some distance before infinity.
The Fotodiox is a black finished, well-machined adapter, with a silver ring in the center, where the Zeiss lens seats. It is larger in diameter than the common red & white boxed, F-S labeled eBay Chinese adapter that I prefer. The Fotodiox has a knurled outer edge that measured a consistent .064 thickness on all 4 examples. The silver seating ring measured a consistent .0585 thickness on all 4 examples.
I precision cut the adapters by grinding them on my gemstone faceting machine. I use a water-cooled 8 disc (called a lap), coated with diamonds. I cut first with 260 grit diamonds and finish with 600 grit diamonds.
Jameel tested the lens and adapter first, before making any adjustments, by photographing a suitable infinity subject. The fine detail of the subject needs to be placed in the center of the viewfinder. I prefer a telephone or high power line because it is a constant diameter subject receding in size as it approaches infinity, and is a high contrast black against a blue sky.
Jameel shot with the lens wide open, with some focus bracketing (at the infinity stop, slightly before the stop, and more before the stop). Then he shot at the infinity stop, with the lens stopped to an optimum aperture, usually f8. I take each picture into Photoshop at 100% pixels, and crop the center. Then I bring in all crops into Photoshop at 100% at the same time and compare the sharpness side by side.
I have extensive experience testing lenses, and rely on a little known fact: Stopping down a lens can increase the contrast, and stopping down greatly sharpens the edge of the image. But at the center of the lens, there is almost no difference in resolution between wide open and stopped down, even when viewing 100% crops.
If any of the wide open, bracketed images is sharper than f8 at the infinity stop, then that sharp bracket is were the lens is correctly focused. This means the lens is focusing past infinity. Then you remember this setting on the lens when you want to focus at infinity, and dont focus using the viewfinder. 100% pixels in Photoshop is more accurate than anything you can see on a focusing screen, when using extreme wide angle lenses.
But lets assume that f8 at the stop is sharper than any of the bracketed exposures. That means the adapter is too thick, and that you are relying on DOF (depth of field) to get some approximation of sharpness.
I thin the adapter by a small amount, .001 or .002, remount the lens and test again. I grind it in small increments, trying not to grind past infinity. When the wide-open image is extremely close in sharpness to the f8 (at the infinity stop) image, then the adapter thickness is correct. I am grinding on the Zeiss side of the adapter, so there are no problems with the adapter being loose in the Canon bayonet. All of the adapter dimensions on the Canon side of the adapter have not changed.
The final correct thicknesses are:
Zeiss 21mm .0555
Zeiss 15mm .055
Zeiss 35mmPC .055
Zeiss 50mm .054
I ground the 50mm to .055 and it was not yet at infinity, and the next grinding increment, .0015 less (to .0535) took it very slightly past infinity. I was amazed at how precise this adapter thickness needs to be. A human hair is approximately .001 in diameter. This thickness adjustment would be extremely difficult to do by milling, but is easy to do by grinding. But keep in mind this is 100% pixel peeping with a wide-open lens.
The thicknesses quoted above apply only to the combination of the Fotodiox adapter and Zeiss lens. I have tuned 3 F-S Chinese adapters for 3 Zeiss lenses, and that correct thickness was .060. I use the version that has the screw stop to prevent excessive rotation. I have been thinking why the F-S adapter thickness is different than the Fotodiox. My conclusion is that the Canon bayonet flange of the adapter is at a different distance (.005) relative to the rear seating edge of the adapter. I prefer the F-S adapter to the Fotodiox, since it is cheaper and quicker to modify. The Zeiss side of the F-S adapter is perfectly flat. On the Fotodiox, I must grind the knurled outer rim from .064 down to .0585 before I can begin to thin the silver seating ring itself.
Below is the 50mm wide open at the original .0585 thickness, and the best image at slightly before infinity at .0535. Compare this to the f8 image at .055. It clearly shows that the wide-open image is sharper at the correct thickness, than the f8 image is with the adapter .001 thicker than correct, and that DOF does not completely mask this focus error. In other words, the lens wide open at correct adapter thickness is sharper than the f8 image when the adapter is one human hair thickness different from correct thickness! This shows that if you must stop down the lens excessively, trying to get sharp infinity focus, the adapter is not the correct thickness, and you are not getting the sharp image you are paying for in the Zeiss lens.
50140585 = 50mm f1.4 original thickness .0585
50140535 = 50mm f1.4 final thickness .0535
508055 = 50mm f8 , 001 too thick at .055
15350585 = 15mm f3.5 original thickness .0585
1535055 = 15mm f3.5 final thickness .055
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