| Pentax 17-70/4 vs Tamron 17-50/2.8 vs Pentax 16-50 vs Sigma 18-50/2.8 and Sigma 17-70
As stated before, first descision is whether you want reach, speed (or both).
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For me speed, although nice is not a big issue, if i want to use a fast lens, I will use a prime which usually beats the fastest zooms easily. At the moment I have smc 55/1.8, smc-f 50/1.7 smc-a 24/2.8 and in screw mount 35/2.0 and 80/2.0 lenses. I'd love to add a faster 24 though.
If you can't take the time to change lenses and cannot carry a second body with the other lens and therefore definately want your zoom-lens to be fast, the descision is clear. For those, I think the Pentax is the better choice, it has a slightly longer range to the wide-angle side (On film I always preferred the angle of view of the 24mm (16mm on APS-C) more pleasant than the more common 28mm (18mm on APS-C, for me a 24mm(16mm) therefore is a must. It also means ith will perform better at 18mm. The Tamron however is also god and much cheaper. For me however the fast zoom which is much larger, brings too much weight and has an immense (Ø77mm) filter-thread, thus raising the price for filters enourmously.
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Image quality is another factor, the longer the zoom-range is usually the lower sharpness and the more difference between the ends and the optimum focal length (usually in the middle). With the Tamron 17-50, Pentax 16-50 and Pentax 17-70 I expect this not to be a big issue.
As vignetting, sharpness and most other lens faults tend to disappear when closing the aperture and most shot are not made wide open, I tend to give lower weight to these as to distortion. When shooting on film, It is impossible to correct this and although this can be done fairly good with digital processing, that only applies if the distortion is barrel-type or pincushion-type. Many lenses, (especially wide-angle-zooms) tend to have a combination (aka wave-type) which is nearly impossible to remove.
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I have seen pictures, mostly from tests, where distortion is plain awful and therefore for me it is a break-point. It is however inherent to zoom-lenses, all zoom-lenses have it, either more or less. That is the price to pay for choosing a zoom-lens.
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For me the zoom range of 17-70 is nearly ideal (starting at 16 would be perfect).
Thus if distortion is not too big, I'd go for the Pentax 17-70 which much better than the (cheaper) Sigma, has a constant aperture and is smaller.
Currently I have no idea whether the distortion of the 17-70 is too large for me or not, I have found few reviews of this lens and can't really compare because I don't have the distortion percentages for my Tokina 28-80/2.8 and smc-A 35-105/3.5 (my favourite zoom for film, digital equal would be 24-70, I use it together with smc-A 24 (hence starting at 16mm would be perfect) to compare with. If distortion is too much, my 1st choice would be the 16-45/4.0
Last edited by KX-Digital; 10-01-2010 at 03:39 AM.
Reason: unfinished
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Originally posted by ftpaddict 
but if it's constent it's okee 
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