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Flange to Film distance

 
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jjlphoto



Joined: 17 Jan 2007
Posts: 102

PostPosted: Wed Jan 17, 2007 6:13 pm    Post subject: Flange to Film distance Reply with quote

With all the issues surrounding Contax CY lenses and EOS DSLR bodies regarding loss of infinity focus with a 1.5mm adapter, I wonder if anyone has considered that the true EOS DSLR flange to film distance is closer to 44.1mm? Many folks who cannot use the commercially available 1.5mm thick adapters are happy to find a 1.4mm thick adapter works fine. (including myself)

That being said, is it worth considering that the Conurus conversion be made .1mm less than prersently done to allow for this Canon discrepency?

Thanks, John
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conurus
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Joined: 15 Aug 2006
Posts: 795
Location: Vancouver, Canada

PostPosted: Wed Jan 17, 2007 7:56 pm    Post subject: Reply with quote

Hi John,

Welcome to the board! This is going to be very long, and therefore divided into sections.

*** 1. We discourage flange distance tuning

conurus discourages the practice of tuning a N-mount lens by altering its flange distance. The existing flange distance of the OEM lens is assumed to be correct, and at conurus, we replicate that with very tight tolerance, made possible by hand-matching and accurate measurements.

We have never received any problem reports with lenses tuned to their theoretical flange distance. We follow the "if it ain't broke, don't fix it" principle here.

*** 2. All Contax N series lenses focus to infinity on Canon EOS bodies

As autofocus lenses, all lenses in the N-series focus a little beyond infinity. Even if you do not trust us doing the proper job and as a result the flange distance is slightly longer than it should, they will still successfully focus to infinity.

Even the widest focal length, 17mm, has been repeatedly shown to focus to infinity correctly by multiple samples of the Contax N 17-35/2.8 that we converted to Canon EF mount. Furthermore, lens tests have shown that there is *no* chromatic aberration problem at 17mm. Both of these are accomplished by sticking with the theoretical flange distance. No tuning is required.

*** 3. Problem with some lenses does not imply problem with all lenses

We are aware of the fact that the manual focus Distagon 15mm and maybe even the Distagon 21/2.8 will not focus to infinity unless you use a thinner-than-theoretical Y/C mount to EOS adapter. But we have never experienced any infinity-focus or CA problems with Contax N mount lenses.

conurus believes we should apply a technique on a lens-by-lens basis. Remember "if it ain't broke, don't fix it". Zeiss knows best about their floating element lens design, and they specifically tune each lens' flange distance before it goes out the factory. conurus' job is to replicate their tuning to the highest possible degree of accuracy, not trying to improve upon it, which seems impossible to us.

*** 4. You have control over your flange distance, however.

We want to give all customers peace of mind that they *do* have control over this important parameter. You will be able to fine tune the film-to-flange distance of any lens by adding or removing copper shims inside the lens. No grinding is required! JIS screwdrivers and a micrometer are the tools needed, and the procedure is way easier than grinding down an adapter. Please review section 2 of this web page:

http://en.conurus.com/install.html

If you wish to reduce the film-to-flange distance by 0.1mm, you may be able to do so by taking out a 0.1mm shim. The standard sizes for those shims are: 0.03mm, 0.05mm, 0.10mm, 0.30mm and 0.50mm, although each lens varies as to what is inside. Additional shims may be ordered from the parts department of Contax.

We even sell the JIS screwdrivers on our web site. You *do* have control over this. We want you to have peace of mind.

*** 5. Flange distance of EOS is in fact 44.00mm

We performed measurements and concluded that the film-to-flange distance of a EOS film body is in fact 44.00mm. (There are 3 different film-to-flange distances - the one I am citing is the focal plane of the lens.) We have no idea why numerous problems occurred in the manual focus Y/C to EOS adapter world and you have to fine tune the flange distance, because we have no experience with any Y/C mount equipment. There has been a conjecture that DSLRs have a different flange distance from film SLRs. This is not possible - such a change would affect Canon's own EF lenses as well. There must be an unknown, yet-to-be identified reason why no Canon user has experienced any difficulty going from a film Canon to a digital Canon.

*** 6. You don't need to be an engineer to be a photographer

When you get your converted lens back from conurus, it works out of the box. Plug, and play!
a, you do not need to worry whether adapter thickness is 1.50mm or 1.40mm.
b, you do not need to apply epoxy glue to affix some focus confirmation chip on your mount adapter, and hope you get the alignment correct and the glue does not come off.
c, you do not need to install an AF pin to make a 1-class camera believe a lens is attached. All N-mount lenses are AF, so of course when we machined the mounts, they always engage the "lens present" detection switch of 1-class cameras.
d, you do not need to worry about what focusing screen to use (unless of course if manual focus is a part of the style of your photography).
e, you do not need to remember to set the aperture to wide open to focus and stop down to meter. Aperture is electronically controlled by the camera body.

_________________
I am a bird and I uncovered the E, EF and N-mount communication protocols!


Last edited by conurus on Thu Jan 18, 2007 12:44 am; edited 2 times in total
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jjlphoto



Joined: 17 Jan 2007
Posts: 102

PostPosted: Wed Jan 17, 2007 8:21 pm    Post subject: Reply with quote

Thanks for the lengthy explaination. I have a theory as well. Since your mount actually replaces the Contax mount, you method is very precise and exacting, hence it works perfectly.

Since twist on adapters for CY lenses are indeed that, an additional piece of metal twisted on, there may be some issues with gapping and compound tolerances adding up, more surfaces, etc, etc. A user merely adding that 1.5mm adapter probably winds up with a lens that sits too far away from the film plane than it was designed to sit. So by using that thinner 1.4mm adapter gives the user that added room so when it all adds up, a CY lens with a 1.4mm adapter probably is exactly where it should be compared to a CY lens that had its mount actually removed and replaced with a Canon EF mount. (Theoretically speaking of course)
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Andi



Joined: 17 Jan 2007
Posts: 27
Location: Europe

PostPosted: Thu Jan 18, 2007 12:45 pm    Post subject: Reply with quote

As Conurus conversions do work with Flange to Film distance like it is published by Canon, it should be considered that the published Flange to Film distance of the Contax/Y mount is not correct?

I do not want to shift the discussion away from the conurus conversion, but maybe this can be discussed sometimes later on FM
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jjlphoto



Joined: 17 Jan 2007
Posts: 102

PostPosted: Thu Jan 18, 2007 2:19 pm    Post subject: Reply with quote

Andi wrote:
As Conurus conversions do work with Flange to Film distance like it is published by Canon, it should be considered that the published Flange to Film distance of the Contax/Y mount is not correct?


Personally, I do not feel that could be possible. It would not be the Zeiss way to have any such inaccuricies. For me, it seems more plausable that the act of stacking an adapter on top of an existing mount is why the CY lens has problems of ending up being too far from the film plane, where-as Conurus is replacing the entire mount, so there is no opportunity for compound tolerances and gapping of stacked pieces interfering with the total distance.

Here is another way I looked at it:
-Which would be closer to being exactly 2mm thick?
a- One piece of metal machined to be exactly 2mm.
b- Two pieces of 1mm thick metal stacked together.
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