[kenyee]

Anyone else's 28-75 look soft at 50 and 75 but not at 28 when wide open?
This is at 28mm (focus on bear's nose):

This is at 50mm:

This is at 75mm:


but at f/4, it sharpens up a bit at 50mm:

and 75mm:


and at f/5.6, it's much sharper at 50mm:

and at 75mm:


It doesn't look like BF/FF so the only thing I can think of is DOF issues...

[Stratario]

https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/35482-my-warra...focussing.html

Check out post #5, item 4.

[kenyee]

"4. If you get the focus tests right at 75mm and F2.8, you will find that at 28mm and F2.8 there will be some backfocusing. This lens can not get both long and short focal distances right. You would have to make some compromises with this lens. My lens slightly front fouses at 75mm and slightly back focuses at 28mm. It is okay in middle focal lengths."

But I don't see BF/FF in these pics, do you? Just seems softer than I expected...

[Stratario]

The ones that look soft to you may be slightly front focused. But you won't be able to see it because there are no object in front of the nose to show it. The depth of field is very narrow at F2.8 and the sharpest area might be just behind the cantelope. Maybe you can do better with a focus chart at 45 degrees with a tripod. This is usually done at 75mm and F2.8 as a starting point.

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Last edited by Stratario; 11-23-2008 at 09:35 PM.

[philmorley]

after a lot of heavy use, I had mine recalibrated. afterwards with very basic and subjective testing once I got it back,

there is almost no difference at 28mm between f2.8 and f8
there is pretty much no difference at f5.6 across all focal lengths.
sharpness at f2.8 declines as focal lenght increases so for me

f2.8 28mm to roughly 50mm is acceptable and by 75mm I prefer to be at f4.0 to maintain sharpness

overall pretty similar to your findings

[kenyee]

Phil:
is yours at 75mm/2.8 this soft? Roughly 6-7' away so DOF should be 2-3"


It's ok at 75mm/4.0 but still what I'd consider to be just ok :-P

[kenyee]

So the new copy looks sharp at 75mm, but backfocuses badly from 28-50.
Here it is at 28 (focus is on bear's nose):

Here it is at 50mm:

Here it is at 75mm:


Hmm...you really can't see much from these small images :-P
In the first image, the 2nd bear is in focus instead.
In the 2nd image, the bear's ears/arms are in focus instead of the nose.
In the 3rd image, the nose is sharp along w/ parts of his black shirt.

Oh well...at least B&H is nice enough to cover shipping for an exchange....time for Tamron roulette again...

[philmorley]

Hi, I will try and take a couple of test shots on weekend (and then work out how to post them

Phil

[kenyee]

Thanks, Phil. If you could, please find a place that'll let you put up the originals (webshots.com does and is free).
FWIW, I took a few portrait photos for testing and the BF is still there...lock on face, shirt is in focus. It's not as noticeable as close range "macro" mode. 75mm is very nice indeed (I can see why people like this) and 50mm is just ok (not as obvious as 28mm).
If the K30D had a BF/FF adjustment for different parts of zoom range of lens, this wouldn't matter at all and Tamron can sell as many bad QC lenses as they want ;-)

[kenyee]

So I got the 3rd lens from B&H and it's been used. Lens and hood are in the same plastic bag (missing 2nd bag)...dessicant is outside bag in box, dust on lens, stains on lens cap and part of lens, sticky feel to lens and box like it was used by a smoker. And it still backfocuses at 28mm, but it's very sharp at 50mm and a bit less sharp but acceptable at 75mm.

The B&H guy apologized because the warehouse shipped it like
that and I guess they don't know why, but you should check any lenses you get as a "exchange" for a defective lens from B&H.

I give up...I'm returning it and getting a refund. This is most sad...

BTW, all tests with the stuffed bears were done on a tripod w/ MLU.