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06-15-2009, 01:48 PM   #16
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QuoteOriginally posted by Douglas_of_Sweden Quote
All zooms are compromises, even good ones. They can impossibly be optimised so that they offer the same quality for all focal lengths. As a almost perfect rule, they are best in the middle of the focal length range, and are suffering most from compromises in the short and long ends. I've shot the DA*50-135 against some of the best prime lenses Pentax made in this focal range: A50/1.7, A*85/1.4, A100/2.8, A*135/1.8. When I say that I shot them against each other I mean identical settings, multiple motives, brick walls on tripod with SR off and remote controle to remove all random differences... Not just one focal length, multiple appertures and iso's etc.

In the middle of the focal length range at f4 or more, it is comparable to the A100/2.8 and close to the legendary A*85/1.4. That a zoom rival excellent primes in this way is rare. In the 50mm end, it is OK, but not as sharp as the A50/1.7, and it has some corner softness. The 135mm end is its worse end: more corner softness, some distorsion. This is not strange: correcting this would have required more glass and made the lens more expensive and heavy and perhaps not as good in some other aspect. It get's noticeably better quite soon when you zoom away from 135mm towards 100mm.

Your consumer sigma zoom on the other hand has 135mm somewhere in the middle of its focal length range. Usually these ~70 to ~300mm lenses are quite bad above 200mm and quite good below 200mm. I don't know this lens first hand, but it is reasonable to assume that you are somewhere near its best range. So you have compared the worst part of an excellent zoom with the best part of another zoom. Not the best way to evaluate the lens.

As for autofocus speed. I don't know your sigma, but what I can say about the DA* lens is that is has a rather long ...searching for the right word in English...focus ring turning (?), and a lot of glass, which means that it takes some time to turn it, especially with the week screwdrive AF engine of the K110 and its old focus algorithms. If the sigma is faster it may be because of a shorter focus ring range and lighter construction(?).
Some times I'd wish my DA*50-135 would focus faster even on the K20D, but on the other hand it is a dream to focus manually, and I hope it will be a bit faster on the K-7.
Hi

Yes I think you have explained very well, Yes I think on my sigma 135mm is the best range of the lens so it would be as good as the worst end of my DA*

I will bring my DA* out and take some more photo and see how it compare with the cheap sigma again.

Thank for your help

Now sounds like my 50-135 is not a bad copy.

Would be nice if someone can upload the bad copy DA* 16-50 up here so I know what bad copy really means

Thank you

06-15-2009, 02:11 PM   #17
D W
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FOCUSING SPEED

Basic physics - when a motor has to do more work it will perform at a slower pace.
Just think of the amount of glass and helicoid that is require to move the focusing on the 50-135f2.8 vs the lighter and smaller Sigma.
06-15-2009, 02:14 PM   #18
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QuoteOriginally posted by D W Quote
FOCUSING SPEED

Basic physics - when a motor has to do more work it will perform at a slower pace.
Just think of the amount of glass and helicoid that is require to move the focusing on the 50-135f2.8 vs the lighter and smaller Sigma.
I am not talking about the AF speed just the accurtecy of the AF. In sigma is always spot on but with DA* have back focusing problem but now I am trying to play with the debug mode if if that work. Thank you
06-15-2009, 07:49 PM   #19
ogl
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I've look at your 100% photos from K110D + DA50-135 and I can say
It's razor sharp. What is your problem?


Last edited by ogl; 06-15-2009 at 08:02 PM.
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