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10-06-2016, 12:10 PM   #1
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Lens comparison testing - best practices

Just got a sweet 18-135 from the marketplace and want to test it against my Tamron 18-250 just for fun. I don't want/need to do exhaustive testing (100's of images) but I'm interested in general how they compare. Lots of stuff on the internets about how to test lenses but prefer Pentaxian advice here.

I figure quick and dirty test: using tripod, take pictures of some static stuff (bookcase, picket fence, some guy's house) at 18 55 135 etc using both (to their limits), at various apertures and then take a peek at the RAW images. What else is useful to point at to test this sort of stuff?

10-06-2016, 12:33 PM - 3 Likes   #2
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Accuracy test - at roughly 45degrees photo of a yardstick or target you can use for calibration, just to see if it's front or back focusing. Use the Live View and the optical focus, see if they agree.
The fence etc. should tell you if there's any decentering, but the 18-135 seems pretty good.

You'll like the focus motor, it will spoil you.
10-06-2016, 12:34 PM - 2 Likes   #3
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Sunny day, tripod, ISO 100, 2 sec timer (or remote trigger), Av mode, try different zoom settings and re-focus after changing zoom (even if your lens is parfocal).
You can test for "worst case" scenarios, where you use wide open aperture (lowest possible f-number at given zoom; this will change on variable aperture lenses) and take a photo of something at minimum focus distance and something near infinity, at the widest and longest zoom setting and some focal length in between. This will let you find where the lens performs "the worst", as zoom lenses can be optimized at the widest, longest, or somewhere in the middle. Lenses can also be optimized at near focus or at infinity. This test will let you learn how the lens is performing in general. You also most likely notice any possible defects that the lens might have with these tests.

Another test would be to focus on something that is reasonably far away. I think they say 50x the focal length. That means 50mm * 50 = 2500mm = 2,5m. Then use f8. This would give you something close to the "best performance." Most lenses should be at least decent at this setting. This test doesn't tell the full story like the above test, but it can show you if something is wrong with the lens, like decentering issues or lens element separation.

Just make sure you focus right! You don't want misfocusing problems to give you the wrong idea. Use AF.S, single point, with a big, contrasty subject. Or you can use manual focus, with live view, and in live view enable Focus peaking as well as the Digital Zoom when you focus. AF itself can be wrong with some lenses (called back or front focus), but AF testing is another issue. You can do it alongside these tests. You see, sometimes the factory tolerances of the lens and the camera are at opposite ends, so the camera thinks it locks focus, but it is always consistently off. If you suspect this, you can use a focusing chart and then the function AF Fine Adjust to correct this. There are numerous threads about this, but it seems this is not a common problem since the K-50. Still, just letting you know, so you can differentiate between image quality and AF problems - often people confuse the two and conclude "camera no good!"

And congrats on the lenses! The 18-135mm is quite popular on these forums
10-06-2016, 12:36 PM - 2 Likes   #4
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Start with the reviews here and see how each behaves at it's ends, especially at max reach (135 and 250mm) as usually lenses that have so wide focal lenght cover (like Tamron) usually suffer at the longest focal length. It might end up Tamron being a bit better at 135mm... I had a Sigma 18-300mm and it was fine from 18 up to some 120-150mm but beyond 180mm it was pretty much useless. Sold it, bought a used Tamron 70-300mm Macro for half of it's nominal price (it's cheap even when new) and to my big surprise it's worth more than what I paid for it. From my personal experience, you can always expect to get more from lenses that don't reach from one to the other extreme. You can do direct comparisons but I think you'll be comparing apples and oranges. I tried to compare three lenses, two 28mm F2.8 primes and one 30mm F1.4 lens. It's only 2mm difference in focal length, but they seem to be noticeably different at F2.8, two of them being wide open and one stopped down... So, numbers don't mean much... Instead, I decided not to compare them but to find out what are their weak and strong points and use them accordingly. I ended up using Sigma 28mm F2.8 MiniWide for landscapes only, Sakar 28mm F2.8 as an optional macro lens when Pentax-M 100mm F4 macro and Tamron 90mm F2.8 macro aren't the best choice and the 30mm F1.4 EX DC Sigma for portraits and theatre... However, you can still do a comparison but only to see which can be more suitable for this or that...

10-06-2016, 12:48 PM - 2 Likes   #5
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Sounds like you have it pretty well thought out to me.

Congrats on the new 18-135mm too, I received a used one last week from Ebay in mint condition and it's going to be my 'walk around' lens for sure and I love the bokeh it produces.

You might want to check the front/back focusing though as I discovered mine needed a +5 adjustment on both my K-30 and the K-50 I gave to my daughter. It's always worth checking whenever you get a new lens as manufacturing tolerances mean it could be slightly off (but still within acceptable ranges).

If you haven't got any focusing charts you can download a free set from Yvon Bourque's site here: #ebooks4pentax . They are under 'Free Samples' in the left column.

For best results print them out at high quality on a laser printer, I used our Brother inkjet printer and I wish I had taken the PDF file to a local office supply place for printing as mine are a bit 'fuzzy' which doesn't help when trying to do a critical focus test!
10-07-2016, 09:48 AM - 2 Likes   #6
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Focus can make such a huge difference, I repeat the test shots three times with the same setup, refocusing each time.

Cloudless daylight is common here so I wait for that. Shadows are useful but you don't want to take so long that the shots have very different shadows.

I take an initial shot at f8 in Av with one lens and adjust the settings if needed for a good histogram. Then I can lock exposure and take shots at several different apertures by just turning the aperture dial.

I use photoshop to take 100% crop tiles of the center and one corner. Then I line up the tiles of each lens and aperture side by side so differences are more obvious. It's also easier to post the tiles here at full size.
10-07-2016, 04:01 PM - 1 Like   #7
Des
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Interested to see the results, having the 18-135 and having previously used the Tamron for 7 years.

My guess is that between say 24mm and 70mm at f8 there won't be very much between them, although I expect the 18-135 to be a little better. From my experience, I'd say the Tamron is very good between 35 and 50, stopped down (but any reasonable lens should be). The 18-135 will be better wide open across the common range, the gap in centre sharpness will narrow as they are stopped down. Neither is great in the corners, although each improves on stopping down. More distortion at the wide end with the Tamron. Tamron is much more prone to CA/PF. Bokeh is better on the 18-135.

Maybe you will prove me wrong. Lots of good advice about testing methods here.
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