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Old October 4th, 2004, 10:35 AM   #1
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EOS Lenses/DOF and other ponderings

After the ensuing discussions concerning the use of EOS lenses on XL1/XL2, and the related discussion of XL1Solutions EOS lens adapters, I did a little research with some optical physicist friends. OK, I hate to admit to wanting to get technical, but, I thought it was time I better understood the issues rather than blowing off about a subject that I had only passing knowledge. I gleaned several things from my discussions and have some relevant questions, as well as a comment.

First the comment...it is a fact of physics that DOF is influenced only by aperture, focal length and magnification(or distance to the subject). The combination of aperture and focal length can be reduced to f/number. So, for any lens, if the f/number is held the same, and the film plane to subject is the same, the DOF will be the same. There is a caveat, however, that relates to lens quality. Poorly made lenses(aka cheap lenses, in some cases) will suffer in image resolution in a manner that mimics the blur one perceives when out of focus. Thus it can appear that a cheap lens has less DOF than a better lens, at the same f/number and distance, but it's because of lens irregularities that blur the image, not really DOF. So, when comparing lenses, one must account for this. There is also the consideration for pixel size of the array. Pixel size is nowhere near the grain size of film. If the resolution of the image is limited by pixel size, apparent DOF will also be affected. However, for any single sensor, this limitation will apply to all lenses used, regardless of f/number.

Second, and this is my question...
I agree that a lens design is done for one particular focal length. That is to say that the distance from the lens to the focal plane is fixed. Therefore, any lens mount adapter, to work properly, MUST have the same lens to focal plane distance built into its geometry. The exception to this is with the use of a secondary lens, built into the adapter, as in the case of the Canon EOS adapter. (Beleive me, I'm trying to get to my question, here) The XL1Solutions adapter doesn't use any glass, therefore, it must match the lens to focal plane distance of the EOS lenses in order to work properly.

Since my own experience has been that the XL1Solutions adapter works for me, can I conclude that it has the proper geometry? I will note that zoom lenses behave a little differently than fixed focal length primes. I haven't explored the physics of a zoom lens. If the XL1Solutions adapter doesn't have the correct geometry, I would expect to see a blurry image that can't be focused. In fact, I've experienced this with zoom lenses, but everything works well with my EOS primes, and at small magnification factors. The EOS 100-400 zoom, for example, requires that I adjust zoom ratio in order to focus. This would be a good indication that the XL1solutions adapter is not the correct geometry for EOS lenses.

I'd be willing to loan my XL1Solutions adapter to Jeff, or anyone else with the proper equipment and training on this forum, for answers.

This has me quite baffled.
Bill Ravens is offline   Reply With Quote
Old October 5th, 2004, 06:41 AM   #2
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Hi Bill, sorry for the delay in responding, busy day at school. The geometry of the lens, as you refer to it, is the distance from the rear node to the focal plane or chip. This distance is also the focal length of the lens, when focused at infinity. If the distance is increased, just like extension tubes do in still photography, the lens will focus closer to the subject, but will not focus to infinity. If the lens is moved closer, reducing the distance, the lens will not focus as close. My first question would be, does your lens focus at infinity with the XL1 Solutions adapter?

We all know about Depth of Field, but how about Depth of Focus? Depth of Focus is an area or zone behind the lens that has a degree of acceptable focus. The film plane or chip needs to stay within this zone or the images will not appear in focus. In other words a lens can be moved forward and backwards, axially, and Depth of Focus will allow an acceptably sharp image to be focused on the chip.

The only shots I see on his site, with the adapter, are of the 8mm fisheye. Notice that by the time the subject 20 ft away the focus is barely acceptable. I think he is shifting the lens closer to the chip, increasing the focal length (decreasing depth of field) and using the excessive depth of focus to still achieve acceptable image quality. This also enables a shot from only one foot away. So, in effect, there may be a slight decrease in the magnification when compared to the standard Canon EF EOS adapter. But I sincerely doubt claims of only 2x.

Interesting note on the images with the fisheye on the XL1 Solutions site. There is no fisheye (100% barrel distortion) effect in the images. This "phenomenon" is even noted in the text. When I put my Canon 15mm fisheye on my 10D this effect is quite noticeable and requires defishing software to reduce or eliminate the effect. However, when the lens is put on the XL1, the effect is barely noticeable, if at all. The 7.2X crop factor, or magnification reduces the fisheye effect. I suspect his adapter may reduce the crop factor slightly. Something on the order of a 1 or 2X reduction might be possible with this lens.
Jeff Donald
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Jeff Donald is offline   Reply With Quote
Old October 5th, 2004, 10:15 AM   #3
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Bill, lens to imaging plane distance is not a MUST if you want to get good pics. It's true that lenses are optimized w.r.t. all kinds of aberations (chromatic, spheric, distortion...) for use with a specific focal length. Within video applications, nobody will remark any difference if the lens is being used in a different mode (using adapter tubes...).
On the "cheap lens DOF" the effects which are responsable for extra the out of focus blur (lens dispersion and spherochromatism) again, are so small that nobody would recognize the difference in "DOF" properties. Only out of focus color fringing effects are sometimes visible .
Andre De Clercq is offline   Reply

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