Originally posted by normhead Sorry, I was out, I used the Pentax 100 macro for comparison because it was there. The only Pentax 135 prime tested was the K-135 2.5 and the DA* 60-250 mops the floor with it. I used the FDA 100 because it's a lens that a lot of people use.
But if you have to have a 135 here ya go. The FDA 100 macro does much better against the 60-250 by the way.
I'm not sure what mTF is but lw/ph, is Line Width per Picture Hieght. So if you have an lw/ph of 2350, you can produce a test pattern of 2350 distinct lines on your picture no matter what sensor size or the image height is.
My Tamron 90 macro is another top prime i use. It's very highly rated on every platform it's been tested on. The difference is indestinguishable at first glance but if you really stare for a long time pixel peeping side by side, the consensus is the 90 is a little bit better micro contrast, probably due to less CA.
MTF50 mean you accept to consider that the pattern is still visible if it keep only 50% from original contrast (you have lost 100-50= 50% of contrast).
MTF30 is a common measure and would mean you accept to keep only 30% of original constrast and so you accept a lost of 70% contrast.
Basically the original signal was say black and whites line and the lense resolve light gray vs dark gray lines. You'd need to boost contrast extensively to get back to original contrast. MTF50 tend to be the standard of many review sites and many manufacturers use MTF30 on their spec charts.
We have also to keep in mind this is for a given subject distance (lenses do have quite different sharpness characteristic at different subject distance) and that the number are very specific to a camera sensor. 2 lenses can appear almost identical on an old 10MP sensor but quite different on a 24MP sensor without low pass filter that can go up to 3500-3700 while a old 10MP might not get more than 2000-2100 and a K5 would go up to 2700. Typicall a weaker lense migtht manage to show quite good performance on an old sensor but might struggle with border performance with a sensor with much more resolution.
I know it is a bit sad we don't have better number, but when you show us theses photozone results norm you show us how many dark grey vs light grey lines you can count on a quite old camera sensor. This say nothing on how the lense perform without degrading the signal, how it would perform on a more modern sensor, how it perform for landscape where the subject can be far away or how it perform in contra light.
Counting that we typicall display our picture at quite small size and we can't really see that much detail to begin with, the tendancy to use the resolution metric with MTF50 as the key indicator of lense performance might not be that relevant in the end.
If we take the 60-250 case:
- photozone find the borders are a bit behind and noticably so at f/4 60mm on an K10 but overall that the performance is really, really great.
- e-photozine working on a K5-IIs find that while the lense is very sharp at 60mm f/4, it need f/5.6 by 135mm and f/8 by 250mm to keep this sharpness... And the borders at 135mm and 250mm never catch up the center
- looking at user reviews here, many think the performance is great at f/4 but is really better at f/5.6 so that seems to match.
Still it seems that the bokeh is truely great, as are the colors... And for sure the performance even from the e-photozine review show is certainly good enough for as you are likely to care mostly of the center and you will close down to f/5.6 in many situation when that possible.
Can we say it is stack of prime?
Would it compare favorably to DA*300 or DA*200 ? Difficult to say but maybe we are not that far!
Would it compare favorably to a DA50 f/1.8 (one of the cheapest prime available) or DA70 f/2.4 or worse FA77 f/1.8? I'd say no way it could do that. The wider apperture are really key for portraiture and even if maybe the DA50 is no sharper, what it can offer would look dramatically different than what the 60-250 can do. FA77 is going to have better bokeh too, even at same apperture and DA70 is likely to be sharper, corner to corner.
In the end saying if 60-250 is a stack of prime will mainly depend where you focus is... If it is for landscape closed down or wildlife, that may be very well true. For portraiture and low light that would be quite wrong.