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01-21-2011, 09:07 PM   #1
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Pentax SMCP-FA 645N 80-160mm f/4.5 lens on Pentax 645D Imatest results

I recently purchased the Pentax 645D camera, as well as a new Pentax SMCP-FA 645N 80-160mm f/4.5 lens (ordered it from Japan). I also have a Canon and a Sony system, and have learned through experience to test any lenses that I buy with Imatest, as about 30-40% of the lens copies that I get are not sharp, are decentered,…

I thought I would share some quantitative Pentax 645D sharpness testing results. You can download a spreadsheet that shows the Imatest results, by viewfinder position, for the SMCP-FA 645N 80-160mm f/4.5 lens at 80mm, 120mm, and 150mm, and by f stop. The spreadsheet is at

http://cid-67997732cc8711ff.skydrive.live.com/redir.aspx?page=play&resid=679....Notifications.

I also tried to attach it to this post; it is attached the the same post on Luminous-Landscape, the Medium Format / Film / Digital Backs – and Large Sensor Photography forum, if SkyDrive and my attachment don't work.

I did not test all the lenses at all f-stops, so if you see “NA” in the results, simply choose a different f-stop in the drop down menus. In the spreadsheet, blue signifies a drop-down menu.

Imatest is very sensitive to a number of factors, including lighting, distance to target, RAW conversion, post-processing,… To normalize, I try to keep all of these items the same from lens test to lens test. For my tests, I use Lightroom for raw conversion, Auto-Tone, Auto-White, no sharpening or noise reduction, and a Linear Curve with Black set to zero. I also run through a series of focuses and report the highest LW/PH. I’ll see some decentering this way, but can miss some as well (this is a relatively complex topic). Fundamentally, this approach tells me what a lens can do, and it is the same as what it will do the vast majority of the time.

Key findings:
• I got a pretty good copy of the 80-160mm lens. I run Imatest to determine whether to keep the lens or return it. This one is a keeper, but it’s not quite as good as I hoped it would be
• The best f-stop at all focal lengths, in terms of sharpness, is 9.5, followed by f11, then f8, and then f6.7. I don’t regard the sharpness at f5.6 as usable for my landscape work. The effects of diffraction are pretty large by f16. I did not measure f13.
• Sharpness at 80mm is better than at 120mm, which is better than 150mm. Racking out the lens all the way to 160mm resulted in significant softness, so I think the usable range for my copy is 80mm-150mm, and it is very good from 80mm-120mm.
• One measure of relative sharpness across camera systems is the average LW/PH for the lens-camera system divided by the number of pixels in the picture height. I compared the 80-160mm zoom to my Canon 50mm f1.4 (probably my sharpest lens, and I’ve got quite a few sharp lenses) and to my Canon 100mm f2.8 (non-IS lens). I chose these lenses as they fall within the vicinity of the 80-160mm lens, which is approximately the equivalent of a 65-130mm lens for 35mm systems. The relative sharpness of the Pentax at 80mm and f8 to the Canon 50mm lens at f8 is 91% - the Canon 50mm lens is ~10% sharper. You might say comparing a zoom to a fixed focal length lens is unfair. True. The comparable figure for my Sony Zeiss 24-70mm zoom lens at 50mm and f8, the best zoom that I’ve measured, is 108%. A number > 1 means that the Pentax 645D+80n-160mm lens system at 80mm is relatively sharper than the Sony Zeiss zoom at 50mm + a900! Given this, I believe the Pentax 80-160mm zoom performs quite well at the lower end of its range. This metric is shown in my spreadsheet comparator. The Pentax does relatively better (the ratio increases) at smaller f-stops. The anti-aliasing filter (or lack of it) will also be a factor here.
• The average increase in LW/PH for the Pentax 80-160mm lens vs the Canon lenses was about 130% (it ranged from 110% to 145%). From just the number of pixels, if the lenses were of comparable sharpness, we would expect a 145% increase – maybe more given that the Pentax does not have an anti-aliasing filter and the Canon does. This result makes the Pentax zoom seem even weaker relative to the Canon 50mm lens. However, a 30% increase for (approximately) the same field of view is a 30% increase, and I’ll take it.
• I did not see the edge softness to quite the same degree on the Pentax 80-160mm at 150mm as Lloyd Chambers saw, but I saw some softness that is reflected in the Imatest numbers. I believe the difference is simply copy-to-copy variation.
• The Pentax files sharpen up very well; perhaps better than the Canon files
• For non-Imatest target shots (real world pictures), I was very pleased with my initial results

I hope that you find this interesting and/or useful. If you do your own Imatest testing on Pentax 645 lenses, please contribute and add to this post.


Last edited by JDOAK; 01-23-2011 at 07:16 AM.
01-22-2011, 07:10 PM   #2
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Well thank you for presenting this. Quantitative data is always nice to have. As a long time Pentax user, Iím not too surprised at your results, but the comparison to the Zeiss 24-90 is a bit unexpected. Reminds me of a Mike Johnston column: Once and future kings: itís Pentax and Zeiss, baby, just like the old days Do have plans to test any other lenses?

Thanks,

Tom.
01-22-2011, 08:28 PM   #3
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Very strange -- my post earlier today here simply disappeared -- along with some of my other posts.

I was talking about that Lloyd tests the lens at near infinity, while the Imatest test uses near field targets so that there can be some differences.

CORRECTION 1/22: My mistake. See below.

Last edited by leping; 01-23-2011 at 12:58 PM.
01-22-2011, 11:45 PM   #4
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Hi Leping,
that is strange - you saw them posted at one point? I just checked but there are still recent posts of your hand in the forum.
Mind, I once hand the strange phenomenon of a mis-delivered email. It was a compuserve address, with 8 digits before @. The last two digits had somehow been changed. The person who received my email recognised that I had meant to send it to another address, because that correct address was still visible, but he received it! In other words, digits are fragile.

01-23-2011, 07:13 AM   #5
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Thomas,

I just received a new Pentax SMCP-FA 645N 80-160mm f/4.5 lens. Will post on that after I get the testing done in a few days.

Leping - I saw your reply on LL. What kind of differences have you seen in near field vs far field, especially for zooms, but also in general?

My experience is that Imatest is an extremely good predictor of far field performance. Maybe I've just been lucky, but lenses that have tested poorly in Imatest were noticeably poor in far field (I test both), while lenses that have tested well in Imatest have, without exception, performed well in far field.

One possible reason for that is that I test about 10 different focuses in Imatest. I find the results are very sensitive to focus (I use a $100 laser distance meter, differences of 1/2 inch can result in large changes in Imatest values at 15') , so I try to see what the best the lens can do is regardless of focus. What the lens can do regardless of focus near field appears to be very predictive of how it will perform far field.

Have purchased and tested about 20 lenses to date, have kept 10 lenses across 3 camera systems, so my return rate is about 50%.

Thank you.
01-23-2011, 12:44 PM   #6
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QuoteOriginally posted by Smolk Quote
Hi Leping,
that is strange - you saw them posted at one point? I just checked but there are still recent posts of your hand in the forum.
Mind, I once hand the strange phenomenon of a mis-delivered email. It was a compuserve address, with 8 digits before @. The last two digits had somehow been changed. The person who received my email recognised that I had meant to send it to another address, because that correct address was still visible, but he received it! In other words, digits are fragile.
Hi Tom, Thanks for helping, but it turned out in this case it was totally my bad -- I actually replied to an identical thread at the Luminous Landscape MF digital forum, and of course it is still there.

Really sorry for my mistake!
01-23-2011, 12:56 PM   #7
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QuoteOriginally posted by JDOAK Quote
Leping - I saw your reply on LL. What kind of differences have you seen in near field vs far field, especially for zooms, but also in general?
Hi Jdock - Yes I realized my mistake last night, but the server for this forum was down (Sunday morning maintenance?) and I have to wait till this morning to apologize.

Actually I have no experiences to say any set pattern of difference in performance, near target vs. near infinity. But somehow I have the impressions from readings and communications that, besides obviously with many macro lenses, some tend to perform better at closer focus range.

But, I bet someone could easily find cases to the opposite, so it might be not a simple rule. Nevertheless, there can be often a difference, including differences in field curvature (helping to blur the clouds, which most of us don't care, and helping to make the foreground sharper, which is usually welcomed).

It will be great to learn about your 2nd sample. Eager to see how much variation will show.
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