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05-29-2012, 01:24 PM   #1
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the Bad-copy paranoia, Tammy 28-75 tests

Hello everyone!

I recently bought a Tamron 28-75, after having seen all the good reviews on this forum and all. I'm a beginner and actually have no idea what kind of sharpness I should expect from such a lens. PLUS, there is always the "bad copy" problems, where some people complain about their copy not being sharp enough or having a bad AF etc... so I'm getting paranoid as well. I'd say I'm glad to have this lens, but still the question remains, do I have a good "copy" of it ?

The AF seems fine enough, sometimes backfocuses a bit, but it may be just me not yet being used to the K7 AF (so far I have been shooting mostly with the 10-17 and 50 f1.7 M).

So I went for a walk and took a few pictures, a lot of them with the aperture open wide, which I think gives a nice bokeh. Here is the Flickr album, all the exif are copied from Lightroom. All taken with a K7, with 3-star Jpg straight from the camera :

Tamron 28-75 test - a set on Flickr

My question would be : how does it compare with your copy of the lens ? What kind of test could be more revealing ?


PS: Sorry moderator, this is not really a "troubleshooting problem", maybe should be in the beginner's corner?

05-29-2012, 02:29 PM   #2
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Nice set, I personally don't have this lens but I think photos like these aren't a good "test".

I believe K-7 has the AF adjustment feature which allows you to fix a back/front-focusing lens so you can get the sharpest AF. Here's a link to the review of how to do it ---> Fixing Front and Back Focus - Introduction - PentaxForums.com
I've adjusted my DA* 55, DA* 50-135, and Sigma 105 Macro in this manner (using a yardstick standing straight up against the wall with camera on tripod) and now they are almost always spot-on with AF.
05-29-2012, 02:55 PM   #3
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QuoteOriginally posted by Julie Quote
I've adjusted my DA* 55, DA* 50-135, and Sigma 105 Macro in this manner (using a yardstick standing straight up against the wall with camera on tripod) and now they are almost always spot-on with AF.
Interesting method.

Do you just take a number of photos changing the adjustment until it looks clearest or most in focus?

I've always read about photographing a chart on a 45 degree angle.

Your method sounds very simple to achieve good focus.
05-29-2012, 03:01 PM   #4
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QuoteOriginally posted by mtansley Quote
Interesting method.

Do you just take a number of photos changing the adjustment until it looks clearest or most in focus?

I've always read about photographing a chart on a 45 degree angle.

Your method sounds very simple to achieve good focus.
Yes, that's exactly what I did. I chose a number that I wanted to focus on and kept changing the AF adjustment by 1 until I got it focused perfectly.

05-29-2012, 03:57 PM   #5
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QuoteOriginally posted by Julie Quote
I believe K-7 has the AF adjustment feature which allows you to fix a back/front-focusing lens so you can get the sharpest AF. Here's a link to the review of how to do it ---> Fixing Front and Back Focus - Introduction - PentaxForums.com
Hi Julie- the sticky article on this forum is good, but the recommended tool is rather expensive. Here is a way to adjust AF for the cost of a couple sheets of paper, and it is JUST as accurate.
05-29-2012, 04:44 PM   #6
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Side note - the set looks good and sharp.
05-29-2012, 07:17 PM   #7
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For starters, there's nothing apparently wrong with your copy from what I see in those sample shots.
As others mentioned, it is hard to get empirical results with those type of shots because you have focus-able subject matter in-frame at varying distances from the lens. Flatter subjects that fill the frame are far better for judging optics. Nonetheless, I see sharp, in-focus elements across the frames. I doubt you have any alignment issues, or overall soft focus.

a) If you focused on a brick wall, with the camera "perfectly" parallel to it, and you ended up with a photo that was in-focus on one side but not the other, then you might have an alignment problem.
b) If in the same test, you manually focused so that it was clear in the viewfinder but came out a little fuzzy then you may have a general focus issue.
(Note that both of these could be camera problems, especially if you similar results with other lenses)

As far as AF adjustment goes, you've already gotten some very good advice in the other replies.
05-30-2012, 12:26 AM   #8
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hello, thanks everyone for your answers!

I think I will try the few tips get the lens in focus, but I tried some "near-macro" shots yesterday and it always got what I wanted in focus. If there is a front focus or back focus issue, it is a very slight one anyway, and even if the AF does hunt a bit sometimes, it does less so than the 10-17 when the subject is a bit too close to it. I never had big focusing problems with, say, the manual lens, even though it sometimes is front-focused or back-focused, but mostly because sometimes the catch-in focus triggers when the focus is in the center, and when it is not what you meant to focus.

06-12-2012, 04:18 AM   #9
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I haven't yet had the time to check the focus properly, but since it seems that the AF is a recurring problem on pentax dslr, here are a few examples of how good or bad it can be. I made a few other test shots in a medieval fests. When people are not moving, the focus is generally ok, like in this picture :
IMGP0845 | Flickr - Photo Sharing!
and this one
IMGP0630 | Flickr - Photo Sharing!

When targets that are a bit far off are moving, it's a bit more random though. While this one is perfect :
IMGP0764 | Flickr - Photo Sharing!

This one, while targetting exactly the same thing, is focused on infinity :
IMGP0764 | Flickr - Photo Sharing!

It's really hard to explain that, it happened on other lenses, but generally when it was too dark for the focus to aim correctly.
However, it might as well be me not handling the camera so well. On some occasions, when the targets are moving, the focus follows quite nicely, as on these two pictures (they are not beautiful pictures though):
IMGP0781 | Flickr - Photo Sharing!
IMGP0782 | Flickr - Photo Sharing!
06-12-2012, 04:53 AM   #10
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I was never happy with the Tammy on my K7. Images from my K10 and K5 are sharp, as are flash images with the K7 , but natural light images with the k7 always disappointed me. I just keep the lens away from my k7. Don't know what exactly the issue is,
06-16-2012, 06:12 PM   #11
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QuoteOriginally posted by volthord Quote
So I went for a walk and took a few pictures, a lot of them with the aperture open wide, which I think gives a nice bokeh. Here is the Flickr album, all the exif are copied from Lightroom. All taken with a K7, with 3-star Jpg straight from the camera :

Tamron 28-75 test - a set on Flickr
No idea whether your lens is faulty or not, but your eye is working perfectly!

Lovely "test" shots... reminds me of my days living in the Vercors (but that's gotta be the Chartreuse!).
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