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07-25-2010, 10:29 AM   #1
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Sharpness: DA 17-70 vs. Tamron 28-75

Hi,

Last week, I bought the DA 17-70 f/4. I've been playing with it in and around the house, and think it's a good lens. There's just one "but". I bought it to replace the 16-45 and the Tamron 28-75. According to the sales guy, it should be sharper than the Tammy, claiming it had higher MTF figures. According to me, it is not. I actually compared sharpness by taking photos of some printed paper. With the Tamron, I could make out the individual black dots in a grey section. With the Pentax, I could not. This concerns center sharpness; I have not compared corner performance. I tested the Tamron at f/2.8 and f/4, and the Pentax at f/4 and f/5.6. In all cases, the Tamron appeared quite a bit sharper.

Should the Pentax beat the Tamron in terms of sharpness, or was the sales guy just trying to sell me the lens?

Thanks in advance.

07-25-2010, 10:34 AM   #2
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QuoteOriginally posted by starbase218 Quote
Hi,

Last week, I bought the DA 17-70 f/4. I've been playing with it in and around the house, and think it's a good lens. There's just one "but". I bought it to replace the 16-45 and the Tamron 28-75. According to the sales guy, it should be sharper than the Tammy, claiming it had higher MTF figures. According to me, it is not. I actually compared sharpness by taking photos of some printed paper. With the Tamron, I could make out the individual black dots in a grey section. With the Pentax, I could not. This concerns center sharpness; I have not compared corner performance. I tested the Tamron at f/2.8 and f/4, and the Pentax at f/4 and f/5.6. In all cases, the Tamron appeared quite a bit sharper.

Should the Pentax beat the Tamron in terms of sharpness, or was the sales guy just trying to sell me the lens?

Thanks in advance.
You might take my copy of the DA 17-70 and find that it out performs the Tamron.

I think most of the reviews (Photozone.de) are done with just one sampling of a lens type.

The salesman, using the available data, was trying to sell you the lens.
07-25-2010, 10:38 AM   #3
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So you're saying my copy is bad?
07-25-2010, 10:50 AM   #4
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Having owned both the Tamron 28-75 and DA 17-70, I can say that they are both very nice lenses.

However, I would actually be very surprised if the 17-70 was sharper than the 28-75 at the same aperture. The Tamron 28-75 is a very sharp zoom lens. I don't know what specific data the salesman was referring to as I don't recall reading anything in the past that suggested that the 17-70 being sharper.

That said, there are other nice features the 17-70 has over the 28-75 like the extended focal length range, quiet SDM focusing and quick-shift manual focusing (and in my opinion slightly better overall build quality).

07-25-2010, 10:57 AM   #5
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Have you tried doing any focus adjustments? It is also just possible that you got a below average sample of the lens. Can you try out another at the store?
07-25-2010, 11:06 AM   #6
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QuoteOriginally posted by starbase218 Quote
So you're saying my copy is bad?
No, I'm just saying some copies are better than others.

If you just got the DA 17-70, and can return it. I return it and try another copy.
07-25-2010, 11:15 AM   #7
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Regardless of which lens is, on average, sharper, the salesman WAS just trying to sell you a lens.

That is, after all, his job.

And that is why an educated/informed buyer is a salesman's worst nightmare.
07-25-2010, 11:28 AM   #8
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Instead of thinking that the 17-70 is a bad copy, the Tamron might be a very good copy :-)

I recently had to apply some focus adjustment to my Tamron 28-75. Have you tried to adjust focus for the 17-70?

Why don't you take some photos with the 17-70 and submit 100% crops, perhaps its a good lens, but the Tamron - you got an exceptional copy.

I understand the quandary you are in. You never said which FL you did your testing at. zooms often have a sweet spot, at either end or somewhere in the center. Like on the 50-135, 135 FL is supposed to be the worst end of it. Maybe you are comparing the best end of the 28-75 with the worst end of the 17-70?

best wishes in making your decision,

07-25-2010, 12:21 PM   #9
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The trouble is, I have done this before when I got the 16-45 and compared it to the kit. In one case, the kit was better. In another, the 16-45 was better. Test setup variations were a problem, as was occasional front- or backfocusing. I played with adjustments, tried various focal lengths, etc. In the end, I couldn't get my head around all the differences. In the end I settled on the 16-45 being better because of a series of brick wall test shots showing more detail, and a lack of vignetting.

I know that zooms tend to have a sweet spot in which they perform at their best, but according to at least two sites (photozone and slrgear), the DA 17-70 performs quite uniformly across its range. Nevertheless, I will run some more controlled tests and post the results here.
07-25-2010, 03:20 PM   #10
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Ok, this is going to be a long post, after which you should know what kind of tea I usually drink . I re-tested both lenses at 70mm. I attached the focus target - a tea bag - with some tape so that it would remain upright during the test. Also, to confirm correct focus, I placed a focus test chart directly underneath the tea bag.

First up was the Pentax. I took the following three shots at f/4, 5.6 and 8 respectively:







Narrowing the aperture clearly improves things. Next up was the Tamron at f/2.8, f/4 and f/5.6 respectively:







A clear overall improvement over the Pentax. The grey pattern is already visible at f/2.8, with f/4 and f/5.6 adding additional sharpness.

At this point, I noticed the focus test chart in the Pentax pictures. It was off, indicating backfocus. So I did as follows: starting out from f/8, I retook all the Pentax's shots. But this time, I moved the camera slightly forward, focused, then pulled it back for about 4cm before releasing the shutter. I think the results - from f/8 to f/4 respectively - speak for themselves:







The softness in the f/4 photo probably is due to the narrow DOF and me over-compensating for the focus offset. When that result is disregarded, the DA 17-70 also shows the grey pattern, both at f/5.6 and f/8. So this lens is capable of very sharp results, at least where center sharpness is concerned.
07-26-2010, 03:37 AM   #11
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QuoteOriginally posted by starbase218 Quote
the DA 17-70 performs quite uniformly across its range.
Nope. At the 70mm end it's a bit softer - just like any zoom at their tele end.
Looking at the pics in your next post - those are slightly out of focus, as you already discovered. You need to apply focus correction for this lens.
07-26-2010, 06:36 AM   #12
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Nothing in those photos indicates a "bad" copy of any lens, but just varying levels of excellence. After doing a lot of tests on my lenses, I decided that I am a poor newspaper photographer--meaning a poor photographer of newspapers taped to the wall. There are so many variables involved, especially if you are using autofocus.

I still don't think my tests are something that should be published, but I made a little more sense of them by doing a few of things: 1) I only use manual focus, with my K10d which has focus aids on the screen, 2) I refocus at least 3 times and take a shot at each try, comparing the best, and 3) I use flashes on light stands, rather than available light. Even so, I'm not convinced that the differences I find between excellent lenses are meaningful.

To me, and I'm no expert, the last one looks like a bit of camera shake in the mix as well.
07-26-2010, 12:45 PM   #13
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QuoteOriginally posted by GeneV Quote
Nothing in those photos indicates a "bad" copy of any lens, but just varying levels of excellence. After doing a lot of tests on my lenses, I decided that I am a poor newspaper photographer--meaning a poor photographer of newspapers taped to the wall. There are so many variables involved, especially if you are using autofocus.

I still don't think my tests are something that should be published, but I made a little more sense of them by doing a few of things: 1) I only use manual focus, with my K10d which has focus aids on the screen, 2) I refocus at least 3 times and take a shot at each try, comparing the best, and 3) I use flashes on light stands, rather than available light. Even so, I'm not convinced that the differences I find between excellent lenses are meaningful.

To me, and I'm no expert, the last one looks like a bit of camera shake in the mix as well.
I would be surprised if it's camera shake, considering I held the camera down on a table and used 2-second dely with mirror lockup. But compared to what you did (manual focusing, multiple tries, etc.) my tests are relatively simple.

I just re-checked focus accuracy on another subject. And in this case, focus was spot-on without any adjustments. In any case, I now know that the lens can be very sharp. But now I wonder... is it normal for autofocus to sometimes be inaccurate like this?
07-26-2010, 01:15 PM   #14
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QuoteOriginally posted by starbase218 Quote
I would be surprised if it's camera shake, considering I held the camera down on a table and used 2-second dely with mirror lockup. But compared to what you did (manual focusing, multiple tries, etc.) my tests are relatively simple.

I just re-checked focus accuracy on another subject. And in this case, focus was spot-on without any adjustments. In any case, I now know that the lens can be very sharp. But now I wonder... is it normal for autofocus to sometimes be inaccurate like this?
I could be wrong, but the letters (Kop Thee) in the lower right of that last unsharp photo look like camera shake to me. You can see a faint double to the letter, rather than just indistinct edges as in some of the earlier unfocused images you posted. I would get this occasionally even on a tripod and with a cable release. Sometimes I got something like that because I forgot to turn image stabilization OFF while the camera was on a tripod. Like I said, at the level we are examining lenses, this is hard stuff.

Autofocus is sometimes not locked where we think it is. That's why I test with manual focus.
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