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02-28-2011, 07:05 AM   #1
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ismaelg's Avatar

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How to test and compare 2 similar lenses?

Hi,

I've been tinkering with photo gear for decades. Some tests are more subjective than others. But I'd like to go a bit further.
I've always been very happy with my copy of the DA18-55 AL kit lens in my K100DS. Some of my best pictures come from this lens.
I just got myself a K20D with a DA 18-55 AL II. I'd like to trade or sell one of the lenses towards a WR copy so I can fully exploit the weather sealed capabilities of the K20.
So I'd like to test both lenses side by side. In theory, the AL II should be superior but variations in both copies can make them close. Not surprisingly, initial not very scientific tests have them very silimar.
What kind of test can I do to determine if in fact my AL II copy is superior enough?

The "winner" will be used in the K100DS so I can get a WR version for the K20.

Thanks,

02-28-2011, 07:48 AM   #2
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I would look at several ssimple points some a little subjective some not.

First of all, since you are talking about using this with the K100DS, then I will assume that the number 1 pririty will be focus accuracy, since yo cannot adjust the AF with the K100.

Try both lenses in varying lighting, using a tripod so framing does not change, and shoot the same image exactly noting the focus point and look for the best focus. Don;'t use a chart but a real image.

Also while doing this test consider focusing speed. DO this by switching AF off and manually focusing to min focus then turn AF on and let the camera focus. do the same for focusing from infinity. See if one lens gets there faster and / or more accurately than the other.


When doing the tests out doors, do one or two scenics, at similar apertures and FL for both lenses and then pixel peep looking for distortion, CA edge sharpness etc, assuming focusing tests are equal for both, these should decide
02-28-2011, 07:59 AM   #3
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Thanks Lowell,

When checking on focus accuracy, does it makes any difference to try different focal lenghts? or is that irrelevant as long as it is kept constant?

Thanks,
02-28-2011, 12:22 PM   #4
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QuoteOriginally posted by ismaelg Quote
Thanks Lowell,

When checking on focus accuracy, does it makes any difference to try different focal lenghts? or is that irrelevant as long as it is kept constant?

Thanks,
I would try at least both ends, maybe one middle focal length.

As I said above, you should do this with real scenes, not targets, and evaluate based upon what you have focused on.


Your focus target should be relitively flat, to give the lens and camera the best chance of finding correct focus. Focusing on a sloping object will introduce errors since you are never quite sure what the sensor sees. your target surface should be bigger than the focus sensor location dot on the screen. For scenics, use a building wall or other flat surface in the middle fo the frame, for close up stuff, pick a typical subject but again try to have the focus sensor location dot over a flat surface.

I have never tried doing side by side tests like this with an AF lens, I have done it with MF several times to compare different lenses I have,

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