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Old Feb 22, 2006, 11:43 AM   #21
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Jim,

At F/2.8, manual focus is usually sharper than auto focus. However, there are some shots where autofocus is very bad and sometimes even better than manual. The manual focus adjustment on the Tamronis too gross in my opinion,making subtle adjustment a bit of challenge. Of course thevery bad focused shotscould be due to some error on my part. Inanyevent,does superior results with MFsuggest that the camera or lens is more likely at fault?

Also, do you think I'm better off with the Minolta or the Tamronfor my 28-75 mm?

Thanks again,

Scott
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Old Feb 22, 2006, 11:56 AM   #22
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Magnum wrote:
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How does focus error differ from softness? What is backfocus and frontfocus?
Focus error can be the cause of softness.

If the camera focuses in front of your focus point, it's got a front focus problem. If it focuses behind your focus point, then it's got a back focus problem.

Because depth of field is very shallow at wide open apertures, focus errors can be far more noticeable.

To get an idea of how depth of field works, see this handy online calculator. The larger the aperture (smaller f/stop numbers), and the closer your focus distance, and the longer your focal length, the shallower your depth of field (less of the scene in focus as elements in the scene are further away from your focus point).

http://www.dofmaster.com/dofjs.html

But, keep in mind that if you use a wider focal length lens, you'll need to be closer to your subject for it to occupy the same percentage of the frame. If you use a longer focal lengh lens, you'll need to be further away from your subject for it to occupy the same percentage of the frame.

If you've got a backfocus or front focus issue, you shoudl be able to tell by testing with a good lens and test targets. If you are focusing on something and what is behind your focus point is sharper, you've got back focus. If you are focusing on something and what's in front of your focus point is sharper, you've got front focus.

That's what a test like my books stacked at close distances can show. If the text on the book closer to the camera compared to the one I'm focusing on is sharper, I'd have a front focus issue. If the text on the book behind the one I'm focusing on is sharper, then I'd have a back focus issue.

Things like AF sensor and mirror alignment can cause these types of issues, and a focus problem is going to be more obvious shooting at larger apertures and closer ranges.

But, again, any lens is going to be softer wide open (largest apertures) compared to stopped down some. So, just becauase a lens may look a bit soft at wide open apertures, doesn't mean you have a focus calibration problem. You'd need to test it to find out.

From looking at your map, it looked like it's probably a bad lens or camera to my eyes. But, do some controlled conditions test with a sharp prime to see for sure if it's a backfocus or frontfocus problem.

KM uses a 50mm f/1.7 for AF alignment purposes, since a good prime is going to be much sharper than a zoom lens, has larger available apertures making AF problems easier to discern, and doesn't suffer from barrel or pin cushion distortion like you'd have with a zoom lens. A "normal" focal length prime (like a 50mm), will also have a more even focus plain across the frame compared to a zoom lens.

If the camera is not backfocusing or front focusing with a good prime, then it could be a lens issue causing that much softness. If so, I'd see if the dealer will swap it out, or you can send it back to Tamron for them to check it out.

Or, if it's a camera problem, see if the dealer will swap it out (or you can send it to KM for alignment).

If you look at the test I posted that I did, the text in the center is sharper than the text on the books immediately in front of or behind it. That's what I was looking for. At larger apertures at that range and focal length, depth of field is *that* shallow.

That's one reason you need to use a tripod when conducting those kinds of tests. If you lean any at all after locking focus at wide open apertures at the ranges you need to test from, you'll have an out of focus target.

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Old Feb 22, 2006, 1:25 PM   #23
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Jim,

Thanks for all the info.

Does"soft" mean an image that is lower in contrast, less sharp, or both?

Scott
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Old Feb 22, 2006, 1:55 PM   #24
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Contrast and sharpness are impacted at wider apertures with most lenses, and both will normally improve with smaller apertures.

With zoom lenses, focal length will impact performance, too (they may not perform as well at one extreme or another from wide to long).

But, the term soft normally refers to the sharpness part.

To get a better idea of how aperture impacts lens perfomance, take a look at some of the MTF charts at http://www.photodo.com/nav/prodindex.html

You'll see some links to how to interpret the charts if you click on one and scroll down to the bottom of a chart, along with articles discussing things like sharpness (is it blurry or not), and contrast (difference between bright and dark) when comparing lenses.

But, you're not measuring things like color, flare resistance, distortion, AF speed, and other things that can impact how well a lens works for you.

MTF ratings and charts also assume that you're using a lens on a 35mm camera (for lenses designed for both). Left of the graph is the center of the lens, and you're going further away from center as you move to the right on the graphs.

But, if you're using a model with an APS-C size sensor that's smaller than 35mm film, you don't care about the part of the lens that's furthest away from center. So, a lens that might have soft edges on a 35mm SLR, may be fine on a DSLR with a smaller sensor.

You really need to take each lens on a case by case basis, looking at both tests and getting user opinions on how well a lens performs on a given camera type (and you can find sites with user opinion sureveys on lenses like http://www.photozone.de ).

Scroll down to the bottom of this page and select Minolta AF from the User Performance Surveys section to see user ratings for lenses available for the 5D (including third party lenses). Just press the start query button on the page that comes up after you select Minolta AF.

http://www.photozone.de/8Reviews/index.html

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Old Feb 22, 2006, 2:25 PM   #25
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P.S.

A 4 page review of your Tamron:

http://www.photo.net/equipment/tamro..._Di/index.html

Some comparison photos shot at wide open apertures (f/2.8 ) and various focal lengths (wider or longer zoom setting) comparing the Sigma 24-70mm f/2.8 EX DG to the Tamron 28-75mm f/2.8 XR Di. Note that your Tamron is a smaller and lighter lens compared to the Sigma.

http://www.pbase.com/fstopjojo/24702875

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Old Feb 23, 2006, 2:28 AM   #26
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Jim,

My initial test indicates that I've got a frontfocus problem!

At F/2.8, I let the AF lock on to a book 6 feet away. Buta book 2.5 inches nearer was much sharper. Checked at both 50 and 75 mm.

So I'm going to ask for a different camera body and repeat this test that in the store before I leave. If the store only has a demo to swap, should I avoid that, because of possible dirty sensor, rough handling, etc.? It would be nice to have the camera this weekend for a big event.

Thanks for the test procedure.

Scott
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Old Feb 23, 2006, 10:28 AM   #27
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Make sure your camera is parallel to the test targets, and I'd try not to use anything with a reflective surface, making sure you've got adequate contrast in the targets for testing. You'll have better luck gauging any potential AF sensor alignment problem in light that is not too low.

A prime is recommended for this type of testing, too (a zoom lens has a lot more distortion, depending on focal length). KM (at least in the U.S.) uses a 50mm f/1.7 for aligning AF sensors.

Make sure to use a tripod (if you lean after locking focus, you'll skew the results), and make sure you're selecting a sensor in the center of your target and you're getting a focus lock (steady versus blinking green light).

If a 50mm prime behaves the same way, then it's most likely a camera problem. If it works fine and the zoom doesn't then it's most likely a lens issue.

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Old Feb 24, 2006, 1:42 AM   #28
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Jim,

I easily duplicated the FF problem at the camera dealer.The test you suggested made thepoor focusobvious at a glance. Another Tamron lens was tried, (same model) with the same problem. Given the good review in that link you provided (thanks!), it looks like a bad batch from Tamron. The equivalent KM lenswas so much of an improvement there wasn't any question about the problem. The dealergave me theKM lensat no additional charge (Price diff was $50 retail).

I also compared camera bodies. In the quick test, where I was mainly looking for differences in focus, the two cameras seemed identical.

The upside to all this is that I learned a lot about some elements of photography and operation of the 5D in a very short-time. The virtues of adversity, eh?I'm also hoping that I don't find other problems with the camera, but instead findthe KM5D satisfactory and maybe even rewarding.

Jim, I'm indebted to you for numerous replies to my questions and the amazing anticipation of what I else I needed to know.

Thank you,

Scott
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Old Feb 24, 2006, 2:34 AM   #29
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I feel bad about even suggesting the Tamron branded lens (even though the optical design is the same for both the KM and Tamron branded lenses).

The lens coatings and electronics may be a bit different (rumour has it anyway), but that Tamron has a good reputation (and a 6 year warranty). I know of one KM DSLR owner that has both of them.

Problems do occur from time to time though, with any brand or model. I'm glad everything worked out.

P.S.

More often than not, I'll go with an internet vendor (but one with a good reputation) when buying products.

But, this story is a good example of why buying from a reputable local dealer can sometimes be worth a bit of extra money, too. A good dealer (like you apparently have) can help you work through any issues you run into without the need to ship items back and forth if problems occur.

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Old Feb 24, 2006, 3:22 PM   #30
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JimC wrote:
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I feel bad about even suggesting the Tamron branded lens (even though the optical design is the same for both the KM and Tamron branded lenses).
Jim,

Actually, yourecommended the Minolta not the Tamron. I ended up with the latter because of haste on my part. After that you noted thatmy choice of theTamron was reasonsible givengood user experience, reviews, and design. All true.So, no tarnish to your rep!

Scott

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