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Author Topic: LensAlign question  (Read 11835 times)
Jeff Kott
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« on: December 12, 2008, 07:42:12 PM »
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I just read Michael's article on LensAlign and this looks like an interesting and worthwhile product.

The only question I have regarding the device is that since the target for focus is to the left of the engraved metal ruler, it seems to me that a lens with field curvature might show front or back focus even if the autofocus at the center of the lens is dead on.

I hope I'm missing something - please let me know if I'm wrong.
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DarkPenguin
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« Reply #1 on: December 12, 2008, 08:33:10 PM »
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But that would be obvious, wouldn't it?  The target would be sharp and the angled ruler would be off.
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Jeff Kott
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« Reply #2 on: December 12, 2008, 09:07:16 PM »
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Quote from: DarkPenguin
But that would be obvious, wouldn't it?  The target would be sharp and the angled ruler would be off.


So you're suggesting that for lenses with field curvature you make the autofocus adjustment by ignoring the angled ruler. If that's the case, what's the point of using the LensAlign for those lenses?
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DarkPenguin
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« Reply #3 on: December 12, 2008, 10:33:55 PM »
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Quote from: Jeff Kott
So you're suggesting that for lenses with field curvature you make the autofocus adjustment by ignoring the angled ruler. If that's the case, what's the point of using the LensAlign for those lenses?

No, but you should be able to figure out how far out it is by not invoking the AF.  Once you know where 0 is on the periphery you can adjust accordingly.

Just a guess.
« Last Edit: December 12, 2008, 10:34:45 PM by DarkPenguin » Logged
ErikKaffehr
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« Reply #4 on: December 13, 2008, 01:25:36 AM »
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Hi,

Yes you are right, except that any lens having that much curvature of field as described by you should be spared for it's deserved duty as a paperweight. I don't think that the LensAlign device should be used format filling but probably at the center of the image.

Seems to me to be a smart device.

Best regards
Erik


Quote from: Jeff Kott
I just read Michael's article on LensAlign and this looks like an interesting and worthwhile product.

The only question I have regarding the device is that since the target for focus is to the left of the engraved metal ruler, it seems to me that a lens with field curvature might show front or back focus even if the autofocus at the center of the lens is dead on.

I hope I'm missing something - please let me know if I'm wrong.
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ejmartin
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« Reply #5 on: December 13, 2008, 09:16:20 AM »
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I've always had success with Bart van der Wolf's moire technique

http://www.openphotographyforums.com/forum...mp;postcount=68

which is a clever way to use the aliasing (moire pattern) generated by high frequency structure in the target when it is captured on the sensor.  Quick, accurate, and best of all free.
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emil
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« Reply #6 on: December 13, 2008, 09:56:09 AM »
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It would be nice to have more information on the "Lite" version, such as how it is aligned. Besides the fact that it doesn't have the rear targets for alignment, I believe the only other difference is the lack of tripod mount. But how is it aligned?
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barryfitzgerald
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« Reply #7 on: December 13, 2008, 10:30:57 AM »
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There is a point that nobody has mentioned so far.
And that is. You will get variations in the distribution of the DOF with various focal lengths.

Shorter focal lengths have a larger DOF area behind the point of focus, longer tele lengths have a more even distribution.

For example, a 28mm lens will have by it's nature a noticeably larger area "behind" the point of focus, than in the front. Ditto for 50mm too. If you go about calibrating your AF for even distribution from the focal point, you will run into trouble.. thoughts??

;-)
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Jeff Kott
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« Reply #8 on: December 13, 2008, 01:32:09 PM »
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Quote from: barryfitzgerald
There is a point that nobody has mentioned so far.
And that is. You will get variations in the distribution of the DOF with various focal lengths.

Shorter focal lengths have a larger DOF area behind the point of focus, longer tele lengths have a more even distribution.

For example, a 28mm lens will have by it's nature a noticeably larger area "behind" the point of focus, than in the front. Ditto for 50mm too. If you go about calibrating your AF for even distribution from the focal point, you will run into trouble.. thoughts??

;-)

Barry, that's a good point, one of several issues that I think need to be taken into consideration.

There's your point on different focal lengths, field curvature, focus shift as you stop down and somehow I think the distance between the lens and the target may make a difference in the extremes (5 feet versus infinity).

I'm not sure how to take account of all of these factors, but I certainly don't think that fine tuning autofocus is a trivial endeavor.
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Rob C
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« Reply #9 on: December 13, 2008, 01:45:07 PM »
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I have a sneaking suspicion that the next generation of cameras might well have an improved version of their current adjustment system, a system whereby you simply pop in the lens, the body reads it and runs a rapid self-test and then sets its own correction.

I cant believe that thats a step too far.

Rob C
« Last Edit: December 14, 2008, 05:32:25 AM by Rob C » Logged

Jeff Kott
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« Reply #10 on: December 13, 2008, 01:45:41 PM »
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I just reread my post. I think I could have more clearly said that taking those four factors into account, it seems like it would be easy to do more harm than good in fine tuning the AF.
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bjanes
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« Reply #11 on: December 14, 2008, 09:55:09 AM »
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Quote from: ejmartin
I've always had success with Bart van der Wolf's moire technique

http://www.openphotographyforums.com/forum...mp;postcount=68

which is a clever way to use the aliasing (moire pattern) generated by high frequency structure in the target when it is captured on the sensor.  Quick, accurate, and best of all free.

Emil,

Thanks for the link to this ingenious method. One could combine that method with some of the ideas posted by Chuck Westfall and come up an excellent and low cost method. The LensAlign sounds like a good method, but IMHO is quite overpriced for a few pieces of plastic and a stainless steel ruler. One could buy a stainless steel ruler and a small mirror at Office Depot and devise a home brew setup. I would like to see some posts on how this could be done.

Bill
« Last Edit: December 14, 2008, 09:56:17 AM by bjanes » Logged
NikoJorj
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« Reply #12 on: December 14, 2008, 11:45:53 AM »
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Quote from: bjanes
One could buy a stainless steel ruler and a small mirror at Office Depot and devise a home brew setup. I would like to see some posts on how this could be done.
There are already lighter focus test charts on the web, in this kind (I got one really neat at home, from a single sheet of paper, with little tabs for easy assembly, and moreover it does not only works with pentax cameras   but just can't retrieve its URL). (Edit : I found how-to instructions with a nikon version    here and a no-brand one   here)

For the DoF repartition problem, it's easy to compute with DoFmaster - and anyway as long as the 0 target is sharp, result is OK.
The DoF extension can hide any moderate stop-down focus shift - but anyway it is to be considered as it affects also real-world focusing.
Field curvature can be set aside by using only the center part of the image, at the expense of resolution...
And the main variable is maybe subject distance, which I've heard can significantly affect focus accuracy (ie some lenses/cameras combo performing very well with a camera/test chart distance of 2m and not so well with the camera/snow leopard distance of 35m) : best tested with the real-world value... that may involve big test targets (hence the need to DIY).

Edit : Re-thinking to it, there is another variable to estimate : for a given camera, the reproductibility of results (ie AF accuracy)... Not to mention the need for this accuracy of course.
« Last Edit: December 14, 2008, 01:58:30 PM by NikoJorj » Logged

Nicolas from Grenoble
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« Reply #13 on: December 14, 2008, 04:10:25 PM »
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Quote from: NikoJorj
Edit : Re-thinking to it, there is another variable to estimate : for a given camera, the reproductibility of results (ie AF accuracy)... Not to mention the need for this accuracy of course.
Yes, and I think it's fascinating that this hasn't been mentioned as a serious point to consider.

For instance, see the 1D MkIII focusing problems, which could occur even with static subjects.

This is why it's important to not just test once, but repeatedly. I shoot an odd number of images - typically five - for similar tests.
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Jan
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« Reply #14 on: December 14, 2008, 06:07:54 PM »
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Quote from: ejmartin
I've always had success with Bart van der Wolf's moire technique

http://www.openphotographyforums.com/forum...mp;postcount=68

which is a clever way to use the aliasing (moire pattern) generated by high frequency structure in the target when it is captured on the sensor.  Quick, accurate, and best of all free.

I've been using a similar technique for a number of years. If the target is suitable and has sufficiently close spacing of lines, as in a test chart or a banknote, the color aliasing is so obvious at the point of focus, you don't even need Live View to manually focus very accurately. Using the viewfinder, what one is seeing is presumably the aliasing that the sensor's AA filter is supposed to remove, but doesn't fully remove. The fact that such aliasing is not fully removed by the AA filter allows one to see such aliasing in Live View mode, but with much reduced intensity.
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NikoJorj
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« Reply #15 on: December 15, 2008, 04:38:52 AM »
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Quote from: NikoJorj
(I got one really neat at home, from a single sheet of paper, with little tabs for easy assembly, and moreover it does not only works with pentax cameras   but just can't retrieve its URL)
I found the image in a back drawer of my PC... It is really small for short working distances only, probably better blown-up to a 13x19" or 17x22" sheet for normal use.

If you're the creator of this neat little thing or know who it belongs (or can read it if it's written on it), please send me a message (nikojorj () free (period) fr) or post here and I'll render unto Caesar the things which are Caesars.
« Last Edit: December 15, 2008, 04:43:05 AM by NikoJorj » Logged

Nicolas from Grenoble
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« Reply #16 on: December 15, 2008, 08:40:35 AM »
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Quote from: Jeff Kott
. . . somehow I think the distance between the lens and the target may make a difference in the extremes (5 feet versus infinity).
I agree. There must be some minimum distance between target & camera. For example, the Sony EX-1 video camera instructs users to use a distance of 3 meters when adjusting the backfocus. I used this distance when adjusting my 1Ds3 bodies and it works well on telephotos down to 85mm. Focal lengths shorter than that make it almost impossible to evaluate where the critical point of focus is, and the camera-to-target distance must be reduced to less than 3 meters. It seems logical that this short camera-to-target distance may not work for distances of, say, 10 meters to infinity.
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« Reply #17 on: December 15, 2008, 02:55:00 PM »
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Quote from: Chris_Brown
I agree. There must be some minimum distance between target & camera. For example, the Sony EX-1 video camera instructs users to use a distance of 3 meters when adjusting the backfocus. I used this distance when adjusting my 1Ds3 bodies and it works well on telephotos down to 85mm. Focal lengths shorter than that make it almost impossible to evaluate where the critical point of focus is, and the camera-to-target distance must be reduced to less than 3 meters. It seems logical that this short camera-to-target distance may not work for distances of, say, 10 meters to infinity.


I heard from Canon that the best distance is half the focal length x10 - so a 35mm would be tested at 1.75 meters.

The chart I use now is A2 and still a bit small for that distance..

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budjames
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« Reply #18 on: December 25, 2008, 09:26:37 AM »
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I just received my LensAlign Pro yesterday. The first lens that I calibrated was my Canon 24-105 f4 IS mounted on my 1Ds MkIII. I had the body and lens calibrated by Canon 2 weeks ago and I was not happy with the results. Images were still soft.

I used LensAlign to check the focus at all of the indexed focal lengths on the lens barrel: 24, 35, 50, 70 and 105. The lens front-focused about the same amount at all focal lenths at the max aperture of f4. I ended up with a +6 micro adjustment.

The results are a definite improvement.

Then I calibrated my Canon 16-35 f2.8 L. A +5 provided the best focus wide open at each of the indexed focal lengths on the lens barrel. Oh my gosh, this lens is not really sharp even after calibration because the LensAlign ruler is at the right edge of the image where the lens exhibits distortion. I think that I might be in the market for a Canon wide prime after this experience.

The remaining Canon lens that I now have to calibrate are: 24-70 f2.8 L, 70-200 f2.8 IS L, 100-400 f4.5/5.6 IS L, and a 100 f2.8 Macro.

Although my Canon 40D does not have micro adjustment capability, I'm going to check my Canon 10-22 f3.5/4.5 S lens and 24-105 f4 IS L on the body to see how it fares. I think that I might have a 5D MkII in my not to distant future to replace my 1 yr old 40D.

Merry Christmas everyone!

Bud James
North Wales, PA
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Bud James
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JoeAdair
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« Reply #19 on: December 26, 2008, 11:55:54 AM »
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http://diglloyd.com/diglloyd/free/BrandNewBlur/index.html
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