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Checking Infinity Focus

 
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MomoInu



Joined: 15 Aug 2006
Posts: 68
Location: Santa Fe, NM

PostPosted: Mon Sep 04, 2006 1:59 am    Post subject: Checking Infinity Focus Reply with quote

Most of you, after doing the converson will want to check to ensure that you have infinity focus with your converted lens. Some may not bother and assume that conversion went according to plan. After trying several times, it seems that there are several issues to look out for. First and foremost, does your Canon camera accurately focus on infinity? Second, can you accurately manula focus you lens on infinity -- given that these lenses sometimes focus past infinity?

One the first issue, I've found that the autofocus on the Canon 5D is unreliable -- sometimes it is right, other times it is wrong. Mine will probably go back to Canon for a check and adjustment in a couple weeks. On the second issue, I think that with the stock focusing screen it extremely difficult to know if you have infinity focus or not. I have even swapped in a Brightcreen focusing screen with a split image in the center and large microprism collar and even with this focus screen it has proven to be a challenge. I don't remember having this much trouble with my Contaxes, but then again I never really thought about it much either.

In the 2 examples that follow, infinity focus (to the extent I actually acheived it) was better reached manually focusing the lens (I did an earlier test where the AF actually focused better). The AF version of the same shot was less sharp. With these 2 examples I also want to raise a third issue. What "should" proper infinity focus "look like" in a Canon 5D file? I am new to Canon DSLR's, and have no Canon lenses, so I have no basis for comparison, no basis for saying wether these images are properly focused and sharp consistent with what "sharp" normally looks like. After seeing these 2 examples, please give me you opinions based on your experience with such files.

These 2 examples were taken using a Canon 5D and the Zeiss N VS 24-85 lens. The lens was set at 85mm and wide open (f4.5). Outside of what was applied in the Standard raw conversion process, no sharpening was applied (and yes, they do look sharper with some sharpening applied, but not appropriate to add here for comparision). For each example, the whole frame wis presented followed by a 100% crop from the center. What do you think -- are they as sharp as you would expect, or think they should be?

Example 1:

100% crop:

Example 2:

100% crop:
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conurus
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Location: Vancouver, Canada

PostPosted: Mon Sep 04, 2006 6:42 am    Post subject: Reply with quote

Base on our email communication I think your OEM mount thickness is 1.60mm, your conurus mount thickness is 6.12mm, and you tried shimming either a single 0.5mm or a stack of shims totalling 0.29mm. You originally have a total shim thickness of 0.79mm in your lens. Are these correct?

Would you mind letting me know your shim thickness for example 1 and example 2 respectively?

Also, maybe there is nothing wrong with your camera. It got a small AF square, but the actual AF zone is larger than what the viewfinder suggests. And if there are multiple objects in the (large) AF zone, the camera may choose the closest object (I don't know the exact algorithm). Even a shadow on the ground is very visible to the AF sensor, although the human brain is trained to ignore that.
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MomoInu



Joined: 15 Aug 2006
Posts: 68
Location: Santa Fe, NM

PostPosted: Mon Sep 04, 2006 1:38 pm    Post subject: Reply with quote

Yes, the figures you have are correct. The shim thickness used in both examples is .500mm. When I tried just using the shims totalling 29mm, focus went well past infinity.

I agree that there may indeed be nothing wrong with my camera. It may be a bit inconsistent, but that just may be the nature of the deisgn.

In the examples above, I limited the camera's AF points to just use the one central AF point (so it wouldn't accidently pick up a nearer object and focus on that instead).
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conurus
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PostPosted: Mon Sep 04, 2006 3:58 pm    Post subject: Reply with quote

I would suggest you try 0.29mm shim instead for these tests. The VS24-85 is not what I would call a true zoom. It is a bit varifocal. Therefore, try not to worry about it focusing well past infinity at some focal lengths. I took my NX and my 24-85 to an Alaskan cruise last year and when I point the lens at 24mm to something at infinity, the AF locks at somewhere between the 2m mark and the infinity mark.

Instead, try to copy Zeiss' backfocus as closely as you can. Your original thickness is 1.60mm + 0.79mm = 2.39mm. We add 4.00mm to this because that is the difference between Contax N and Canon mounts. So we should aim for a new thickness of 6.39mm.

If you shim 0.5mm, you now have 6.12mm + 0.50mm = 6.62mm, a 0.23mm difference from the original. It is like having a 0.23mm extension tube all the time. It was not the original optical design and lens performance may be negatively impacted.

If you shim 0.29mm instead, and since your conurus mount thickness is 6.12mm, you will end up with 6.12 + 0.29 = 6.31mm. The 0.02mm difference is small enough that it doesn't matter.

I hope it makes sense. In fact if you try a new test with 0.29mm shim, it would settle once and for all if my installation instructions concerning mount thickness measurements are accurate and that would help the entire community tremendously. (In my installation instructions I am effectively claiming that you do not need to fine tune anything but all you need to do is just to copy the Zeiss shim thickness the best you can.)
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MomoInu



Joined: 15 Aug 2006
Posts: 68
Location: Santa Fe, NM

PostPosted: Mon Sep 04, 2006 6:50 pm    Post subject: Reply with quote

OK. Are you suggesting that the examples shown are not sharp, and that they should be sharper?

When I did the shim calculation, per the instructions, it indicated that I should remove .29mm of shims. From the instructions, "Calculate the difference in thickness, d = x + 4mm - y. Remove the combination of copper spacers whose sum of thicknesses is the closest to d." What you've just described is the opposite of that -- basically use shims totalling d, rather than remove.
From my measurements (your mount measured 6.10mm),
d = (1.6 + .79) + 4.0 - 6.10 = .29
So if I remove .29mm from the shims, I am left with .50mm to use in the lens mount. This would be as close to Zeiss' backfocus as I can get. Right?
Here is an example of a test done with removing .50mm of shims using only .29mm of shims as you suggest. The first image is the 100% crop of the image focused on infinity (85mm wide open), and the second is focused far past infinity.

100% crop, .29mm shim example, focused at infinity (or as close as I could get):

100% crop, .29mm shim example, focused well past infinity:
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conurus
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PostPosted: Mon Sep 04, 2006 9:52 pm    Post subject: Reply with quote

MomoInu wrote:
When I did the shim calculation, per the instructions, it indicated that I should remove .29mm of shims. From the instructions, "Calculate the difference in thickness, d = x + 4mm - y. Remove the combination of copper spacers whose sum of thicknesses is the closest to d." What you've just described is the opposite of that -- basically use shims totalling d, rather than remove.
From my measurements (your mount measured 6.10mm),
d = (1.6 + .79) + 4.0 - 6.10 = .29
So if I remove .29mm from the shims, I am left with .50mm to use in the lens mount. This would be as close to Zeiss' backfocus as I can get. Right?


It was a huge mistake on my part. We use shims closest to the difference d. Both my installation instructions and my example were wrong. In particular, the example came from my 17-35. I just took it apart to check and found that I used 0.62mm of shims, not removing 0.62mm of shims. I was doing one thing but saying another. Sad

My apologies for making that huge mistake. I didn't know what I was smoking when I wrote that, but I fixed it immediately. Thank you for pointing out that error.
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MomoInu



Joined: 15 Aug 2006
Posts: 68
Location: Santa Fe, NM

PostPosted: Tue Sep 05, 2006 12:56 am    Post subject: Reply with quote

So according to what should be right, I "should" use .29mm of shims in my converted mount. However that amount, as shown in the example, clearly goes well past infinity and easy to conclude that more shims are needed. My measurement of the orginal Contax mount could be off, but it probably isn't off by that much. I guess I'll know better when I can get my hands on a precise, standard measure 2mm block.

WIth .50mm of shims installed in the mount, infinity focus looks as sharp as it does for .29mm (which definately reaches infinity), without going noticably past infinity. In your estimation, should infinity focus on the 24-85 at 85 and wide open look sharper than shown in my examples? It seems OK, but I have nothing to compare it against.

I would try reducing the shim amount to say .40mm and see how that works, but my combination of shims does not allow that. The only shim thickness choices I have in the range are .50 or .29. Perhaps when I convert the next lens, I might have a few more shims to select from.
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conurus
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PostPosted: Tue Sep 05, 2006 1:27 am    Post subject: Reply with quote

I just tried the same thing with one of my 24-85's. Wide open at 85mm at infinity, the lens is just not very sharp, in fact I perceive it to be a little worse than the 2 examples you posted.

When the AF locks, it is just a hair closer than infinity as read out from the distance scale. If I turn the focusing ring all the way past infinity until it stops, it is quite blurry. Sorry my DPP crashes when I use the cropping tool so I am not able to show you the blurredness, but my copy focuses significantly past infinity as well.
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