Luminous Landscape Forum
Site & Board Matters => About This Site => Topic started by: Jeff Kott on December 12, 2008, 08:42:12 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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But that would be obvious, wouldn't it? The target would be sharp and the angled ruler would be off.
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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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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.
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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
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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I've always had success with Bart van der Wolf's moire technique
http://www.openphotographyforums.com/forum...mp;postcount=68 (http://www.openphotographyforums.com/forums/showpost.php?p=56292&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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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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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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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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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 can´t believe that that´s a step too far.
Rob C
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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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I've always had success with Bart van der Wolf's moire technique
http://www.openphotographyforums.com/forum...mp;postcount=68 (http://www.openphotographyforums.com/forums/showpost.php?p=56292&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 (http://digitaljournalist.org/issue0812/tech-tips.html) 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
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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 (http://www.geocities.jp/pen_pen_pentax/focus/) (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 (http://www.dchome.net/viewthread.php?tid=284785) and a no-brand one here (http://web.archive.org/web/20071011164059/http://canon-dslr.com/Canon_Jan05/Canon_SLR_Focus_Test.htm))
For the DoF repartition problem, it's easy to compute with DoFmaster (http://www.dofmaster.com/dofjs.html) - 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.
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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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I've always had success with Bart van der Wolf's moire technique
http://www.openphotographyforums.com/forum...mp;postcount=68 (http://www.openphotographyforums.com/forums/showpost.php?p=56292&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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(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 (ààà) free (period) fr) or post here and I'll render unto Caesar the things which are Caesar’s.
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. . . 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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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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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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http://diglloyd.com/diglloyd/free/BrandNewBlur/index.html (http://diglloyd.com/diglloyd/free/BrandNewBlur/index.html)
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Joe,
I checked out the link you posted. Frankly, I'm not sure what this has to do with LensAlign?
Bud
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Joe,
I checked out the link you posted. Frankly, I'm not sure what this has to do with LensAlign?
I think the lesson to be learned is that it's insufficient to test merely one part of the image frame before making adjustments.
I'd be surprised if the LensAlign documentation didn't mention this, though.
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I finished calibrating all of my lens for my Canon 1Ds MkIII with my new LensAlign. I used 10x the focal length as the distance between the sensor plane and the focus chart on the LensAlign. Lenses were all tested at their maximum aperture.
The micro adjustments needed were as follows:
16-35 f2.8 L +6
24-105 f4 IS L +7
100 f2.8 Macro 0 (also the sharpest lens of the bunch)
100-400 f4.5/5.6 L -3
70-200 f2.8 IS L +3
24-70 f2.8 L - the front focusing was so bad that it still was not aligned at the maximum adjustment of +20.
I spoke to Canon tech support afterwards and the tech agreed with me that it must be a lens problem as the others were all adjusted within a relatively close range. This lens is 6 years old and I'm sending it to Canon for service.
The 16-35 f2.8 is the original version and seemed to be the softest lens of the bunch. I might consider a Canon prime wide angle. Does anyone have firsthand experience with how the newer Canon 16-35 II lens compares to the original 16-35?
Cheers and Happy New Year!
Bud James
North Wales, PA
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The 16-35 f2.8 is the original version and seemed to be the softest lens of the bunch. I might consider a Canon prime wide angle. Does anyone have firsthand experience with how the newer Canon 16-35 II lens compares to the original 16-35?
I do. The second version is better in all regards to the first, but it's not perfect. Corner sharpness was the biggest issue for me because I have typically used these lenses at larger apertures (typically f4) and higher ISO ratings. There are unofficial lens tests here (http://www.16-9.net/lens_tests/) which compares both lenses to other brands in separate tests. You can also rent the lens from Lens Rentals (http://www.lensrentals.com/rent/canon-16-35mm-f2.8-l-ii/for-canon) to get a first hand opinion.
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Bud, that is interesting information, how would you rate sharpness improvement from a 3 or 6 unit correction?
what is one unit of micro-adjustment? are the units the same for all canon cameras? e.g is it scaled for the 50D?
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I made a LensAlign Lego this evening.
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Bud, that is interesting information, how would you rate sharpness improvement from a 3 or 6 unit correction?
what is one unit of micro-adjustment? are the units the same for all canon cameras? e.g is it scaled for the 50D?
The difference is very slight between a 1 to 3 unit micro adjustment. It seems that the telephoto lens are affected more by the finer gradations. This probably has to do with the shallower depth of field when lensaligning telephotos wide open as compared to shorter lenses.
I don't know what the unit of measure is for a micro adjustment. My previous postings was results achieved with my 1DsMkIII. I also have a 40D, but it does not have the micro adjustment feature.
Bud James
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I don't know what the unit of measure is for a micro adjustment.
I don't think there is a unit of measurement. It is just an arbitrary scale.
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I made a LensAlign Lego this evening.
That brings a smile to my face! Thanks for sharing that.
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I made a LensAlign Lego this evening.
If you let my 11 year old son incorporate this into his Lego Star Wars battle cruiser, you might be able to side step the patent infringement. LOL!!!
Bud
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If you let my 11 year old son incorporate this into his Lego Star Wars battle cruiser, you might be able to side step the patent infringement. LOL!!!
Bud
Some where deep down in the darkest part of my cellar I should have a Lego Mindstorms set - the one where you can write code and control motors and sensors of all kinds. Maybe I should call in sick and build a Completely Automatic Calibration Monster Thing X5000?
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Some where deep down in the darkest part of my cellar I should have a Lego Mindstorms set - the one where you can write code and control motors and sensors of all kinds. Maybe I should call in sick and build a Completely Automatic Calibration Monster Thing X5000?
Perhaps you can build a device that mounts, tests and set the micro adjustments for each of our lens unattended incorporating your "LegoAlign". That way I can spend more time watching my Luminous Landscape Video Journal collection.
Speaking of LLVJ, aren't we due for a new chapter? I think edition 17 was release last March.
Cheers.
Bud James