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02-05-2009, 02:10 PM   #46
Just1MoreDave's Avatar

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I suggest comparing it for sharpness with the kit lens. Set up a tripod in some place with reasonable light, a target with some detail, some element that will help determine where the plane of focus is, maybe even some color. I used a dining room table once, in a bright room with a detailed table covering. You may need a few shots to get the exposure right and make sure the kit lens has the same field of view. Instead of squinting through the viewfinder, you can download the shots to a computer to check your focus.

Once the setup is right, just one shot with each lens at f8 should tell you all you need to know. I would also take a series of shots with the A28/2.8 in Av from f2.8 to f22 at each full stop. Look for exposure differences that may indicate the lens is not stopping down as much as the camera thinks it is.

The test should show if the prime outperforms the kit and by how much. If the kit lens wins, send the prime lens back and feel great that you saved money.

02-05-2009, 06:19 PM   #47
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QuoteOriginally posted by Just1MoreDave Quote
I suggest comparing it for sharpness with the kit lens.
I agree.

I have the M28/2.8 - different optical formula, so not directly comparable. But some say the A is better and others the M, which I take as confirmation that it's a tossup. Anyhow, when I compare my 28 against the 18-55 (both versions), the 28 wins *easily* at any aperture larger than around f/8. By f/8, the 18-55II does well enough that it's hard to tell a difference. The 18-55I is still noticeably softer in the corners.
02-05-2009, 07:19 PM   #48
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I was just reading another post (in the Rumors section) and someone had posted a link to some Ken Rockwell reviews (I know, love him or hate him ) and Ken had just posted a review of the SMC Pentax-A f2.8 28mm lens.

Here's his comments about sharpness:

Used properly, it's sharp!

The sample I had was much softer on the right side wide-open, but fine on the left. This went away stopped down.

I sw these results both with an 8x Peak loupe and with 3600 DPI scans, both on Fuji Velvia 50.

f/2.8: Great center, but the corners are soft.

f/4: Better than f/2.8.

f/5.6: Sharp all over.

f/8: Optimum; very sharp all over.

(diffraction limits sharpness on all lenses at f/11 and smaller) "

So, maybe try using the lens at smaller f-stops unless you really need f2.8.

And, just for grins, I took a couple of pics with an SMC Pentax-A f2.0 50mm lens (only "A" lens I could quickly get my hands on). If I had shake reduction on and inputted "50", then 50mm showed up as the "mm" setting. If I turned the camera on with SR off, then "----" showed up as the mm setting. It correctly reported the f-stop and shutter speed (as far as I could tell).

Also, I wear glasses, but frequently don't wear them when taking pics - so if I forget to change the diopter setting I have a heck of a time manually focussing. Good luck.
02-05-2009, 10:14 PM   #49
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Did happen to see Ken's review, but frankly didn't think too much of the specific data, on the presumption that if a given example isn't consistent right to left, something's out of whack. I definitely wasn't shooting all wide-open, and I don't think anything past f11. As for the kit lens, well, half the reason for the prime is to do better than the kit lens is. I've been presuming f8 is where the best sharpness should be, though.

I'm waiting on a magnifying eyepiece, which might arrive tomorrow: my eyestrain was bad all day today, to the point nothing really looked right, camera or not, but it's clear that manual focus with a lens this wide in a crop sensor finder and this stock screen aren't going to cut it a lot of the time. (Pity, there, I like the field of view I get. ) Anyway, I'm letting things and my eyes rest for the most part until then, then I think I'll do some tests with a tripod and all, and get scientific about checking focus and AF around and all. (Something I'd also been hoping to have two sharp primes to check against each other. And the screen alignment. And the AF.... And any front/back focus, and how to get that info all stored. so many variables, no real known quantities.

Kind of a project.


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