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03-05-2010, 11:41 AM   #1
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"This lens is soft" and other myths

This article confirms a long discussion I had yesterday with the Sigma tech guy in Ronkonkoma who custom tuned a lens to my camera body with stunning results. I looked around the web and it was about the best explanation I've read about why it pays to have non-oem lenses custom calibrated to your specific camera, not just "brought up to spec" or exchanged:

Here's the story, from Lensrentals.com:

"Three to 4 times a week we have the following conversation:

“The lens you sent me frontfocuses, its not good.”
“OK, we’ll overnight you a replacement.”

Then the first lens comes back and its perfectly fine when we check it out. But the customer is very happy with the replacement lens, it worked great even thought the first one didn’t. So what has happened? Its rather simple, actually, and like most examples of Anti-logic it stems from a wrong assumption: the customer knows his/her camera is ‘fine’ because it works with fine with their other lenses – none of them front focus or back focus.

The key to the puzzle is the definition of ‘fine’. Most people assume that ‘fine’ means ‘perfectly calibrated’. In reality cameras are like any other manufactured item, calibration is within a given tolerance range. We don’t have privvy to what the actual tolerance range Canon, Nikon, or the other manufacturers (except Zeiss and Leica) consider acceptable, so lets arbitrarily say the manufacturer will consider a camera or lens to be ‘in specifications’ if its + or – 3 ‘focus units’ from perfect. We can assume they reached this number because anything within + or – 3 focus units will be within the depth of field of a wide aperture (probably f2.8) lens.

Lets consider that I have a camera body that is -2 focus units from perfect, and a lens that is +2 focus units from perfect. Both are considered ‘fine’ according to the manufacturers definition, although they certainly aren’t perfect. However, the combination of a +2 lens on my -2 camera will be absolutely perfect, I’ll love the lens on my camera . After my experience with this one lens on one camera, I will write Sonnets on the various online forums about how great it is, and will tell anyone who doesn’t like it that they must be a bad photographer. I will have become the most dreaded online lifeform, a FLAO (Fanboy with Loss of All Objectivity).

But what if the lens was -2 focus units from the theoretical perfect and I put it on my -2 focus units from perfect camera? Well it depends. If the lens is say an f4 maximum aperture, probably not much: the depth of field from an f4 aperture lens may well mask a bit of front focusing or back focusing. You might notice the lens frontocuses 3 feet in front of the subject at 20 feet if you pixel peep, but since the depth of field is 10 feet the subject is still in focus and the lens seems fine. I will probably describe the lens as very good, but not descend to complete FLAOdom.

But if its an f1.4 lens with a very shallow depth of field, the front focusing will be noticeable: the subject will be out of focus and soft. If I know how to do a front/backfocus test I may have figured out the problem, but here’s the kicker: if I sent the lens in to the manufacturer to fix the problem they would check the lens out, say it was fine (because it is fine, its within specifications) and send it back. Ony if I send the camera and lens together to be calibrated would the fact that the two together are out of focus be apparent, and then the manufacturer would be able to fix the calibration.

Ah, but there’s no free lunch. If the camera calibration was adjusted as part of the fix, I might find that another lens in my kit that used to be great, now backfocuses a bit. In the past, many full time pros who were aware of these issues, would send their entire collection of cameras and lenses to the manufacturer to be calibrated together. This was one of the original reasons Canon and Nikon formed their Professional Services groups. Most of the rest of us just made do, or sent copy after copy of a given lens back until we got one that was sharp ON OUR CAMERA."

ready the whole thing at
https://www.lensrentals.com/news/2008.12.22/this-lens-is-soft-and-other-myths

The real horror story is at the very end of the article.


Last edited by LouD; 03-05-2010 at 11:44 AM. Reason: bold face
03-05-2010, 11:56 AM   #2
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Yep. Which is why I'm glad Pentax put microcalibration into the K20D and K-7, and Yvon Bourque at PENTAX DSLRshas some nice, freely downloadable and printable focus calibration charts. Takes some time, but is well worth it for your AF lenses...
03-05-2010, 12:16 PM   #3
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And the reason I am using the debug menu firmware hack on my K10D.
I am getting a much better hit rate since I started using it.
03-05-2010, 12:39 PM   #4
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Or try swapping one lens between three or four "identical" in-spec bodies.

One of my (many) excuses for having accumulated four K200D bodies - happy combinations abound. Just don't make the mistake of tryin' to build a "preferences" matrix for dozens of old lenses and six bodies -- there be madness in that there briar patch!

H2

03-05-2010, 12:49 PM   #5
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Does Pentax in-camera lens correction work with non-oem lenses? Also, does Pentax, or any 3rd party company, offer tweaking of Pentax lenses to bodies like the KX that don't have in-camera correction? I have a FA50 that I'm sure is in factory spec, but probable could be improved.
03-05-2010, 01:47 PM   #6
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Yes you can adjust Sigma lenses with the AF adjustment, but sometimes certain lenses share the same "ID" with other Sigma lenses so the camera recognizes them as the same lens....so that can really throw things off. So the exif data for a Sigma might not match the actual lens. I have a bunch of new Sigma lenses, but have never had this issue, but have seen others who have had it. Also, Sigma can adjsut lenses for a k-x. They do not adjsut the camera...they tweak the lens to match the camera. Something with the spacing of the mount. They use shims.
03-05-2010, 01:51 PM   #7
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yeah, that's what they did in my case. They left the camera alone and adjusted the lens.

The rep said they recommend people send in their cameras with their lenses for calibration, but less than half do so.
03-05-2010, 01:56 PM   #8
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Honestly, I just think people don't like sending their camera off to any company other than Pentax or whoever made the camera. But, it is not a bad idea. But you gotta figure there is normally a cost involved. And I don't like the idea of shipping my expensive lenses unless I really have to.

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