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11-04-2016, 04:17 AM   #16
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QuoteOriginally posted by grahame Quote
I believe you.

I guess aperture size on lenses are not 100% accurate. The levers are just fixed on the ring by screws, and workers adjusted them during assembly. that is why we see a little difference in exposure among lenses. That little difference might count.

Also, optical formulas contribute to the bokeh and the transition from in focus to blurry area. That might contribute to the slight difference in DOF at same aperture setting on the aperture ring.

Just my guess.
Not sure whether it is minor accuracy differences in aperture or more probably your second point, that the optical formula impacts the bokeh, and the out of focus area, so that the apparent separation of foreground and background is more pronounced.

Years ago I did a comparison of a few 135 mm lenses and found that an early preset lens produced more three dimensional images due to the much different optical formula.

As norm suggests you can measure the depth of field and will see it is the same, but it is the bokeh that might be making the FAJ 18-35 seem to have more DOF

11-04-2016, 05:13 AM   #17
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QuoteOriginally posted by kotofei Quote
Yes, DOF depends on focal lenght and aperture. BUT! DOF differs with optical scheme.
No it does not.

QuoteOriginally posted by kotofei Quote
So... retractable 18-50 F4-5.6 is awful. The worst short kit lens in pentax lineup.
What? Did you get that from my review? I didn't write that at all. It's a bit too fragile, but it's optically very good!

QuoteOriginally posted by kotofei Quote
How about sigma 17-70 F2.8-4.5 compared to DA-L WR 18-55 F3.5-5.6 (with quick shift- afaik better kit lens in "18-55" family) ?
The Sigma 17-70 (all three versions) are very very good optically. Superb lenses. They are much more expensive, but better optically (and with a larger aperture and longer range) than the kit lenses.
11-04-2016, 05:20 AM   #18
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QuoteOriginally posted by kotofei Quote
thanks, that was helpful

Not exactly. Yes, DOF depends on focal lenght and aperture. BUT! DOF differs with optical scheme.
DOF only depends on focal length, apperture and sensor size. If these are equal, two different lenses have the same DOF.
What may differ is the character of the OOF areas, so called bokeh.
11-04-2016, 07:02 AM   #19
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Wow that 18-50 is rated awful. Too bad, the 18-55 was so much better than most companies 18-55 it sold a lot of cameras. It's distressing that in this day and age, Pentax would make something rated so poorly. I was thinking of getting one just to save space in my tripping/hiking kit. Scratch that plan.

11-04-2016, 11:19 AM   #20
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QuoteOriginally posted by normhead Quote
Wow that 18-50 is rated awful.
I feel it's rated (by me, for the record) accurately and honestly, and it is not at all awful. Read the text, don't just look at the numbers. A 1004 lens can't very well be expected to perform as well as a 1000$ lens. If the review had included a rating for the 18-55, it would have received better results for handling and build quality, and worse results for AF. It would come out even, or nearly so. And note that the HD version is superb optically.

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