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05-31-2015, 09:28 PM - 7 Likes   #1
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Figuring out Da 15 focusing - How I did it

I first acquired a Da 15 limited a little over 4 years ago, and have owned one (a couple different copies) for most of the time in between. I've shot thousands of images with it. While it has a laundry list of great characteristics: size/weight, flare resistance, colors, contrast, center sharpness, etc, I have always been frustrated with it's edge/corner performance. I shoot landscapes, so edge-to-edge, front-to-back sharpness is important, and I require large DOF. I've spent more hours contemplating other K-mount wide angle options than I care to admit because, despite my best efforts, I often have ended up with images from the Da 15 that I really like, but on closer inspection have areas on the borders/corners that are disappointingly blurred. I tried the Da 12-24 for a bit and liked it, but sold it as I ultimately didn't think the size/weight sacrifice was worth it for me. While I've known, academically, that field curvature of the Da 15 was likely causing some of my frustrations, I hadn't gone through any sort of testing process to see if I can confirm this. Earlier this evening, I spent about a half an hour putting the lens through its paces to try and clarify the field curvature issue a bit. I thought I'd post my process here, in case anyone out there is having the same frustrations I have had. Warning: this is by no means scientific, but it's at least somewhat systematic, and I feel like I understand the lens much better now than I did before. Note this was done with a K3. Here goes...

1) Find a suitable subject. I went to a local park and found a wide open area where I had grass in the foreground, a walking path/trees/lake in the midground, and a line of foothills/mountains in the background. I oriented myself so that I was shooting in a direction perpendicular to the mountain range in the background (so the mountains on the far left and far right sides of the frame were approximately the same distance from my camera). I positioned my camera low to the ground, on a tripod, so that the elements in the frame ranged from about 2 feet from the front element to infinity.

2) Place camera on tripod, set exposure mode to manual (M), set drive mode to 2-second delay.

3) Determine smallest aperture that gives a suitably sharp result for your purposes (before diffraction begins to degrade image sharpness to an unacceptable degree). This will obviously be subjective. For me, I began at f/5.6 and (after confirming that the hard stop at infinity was accurate on my lens' focus scale) manually focused to infinity and shot an image. I then shot identical images at f/8, f/9.5, f/11, f/13, f/14, and f/16. I reviewed the images, zooming in to 100% in the rear LCD and looking at the portion of the background mountain range (i.e. infinity) at the center of the frame (about 1/3 of the way from the top of the frame in my composition). Once I could see a noticeable drop-off in sharpness in this region between one image and the next, I considered that my limit. For my lens and purposes, I was able to go to f/11 without much effect from diffraction. I thus determined that this was the smallest aperture I would be willing to use when taking a landscape image.

4) Determine how close you can focus before the borders at infinity begin to degrade due to the lens' field curvature. I began by manually focusing to infinity hard stop using f/11 aperture. I then took several images, each subsequent one focused slightly closer, noting with each where the focus was set on the focus scale. I found that I barely had to move focus off the infinity hard stop before the edges at infinity began to degrade noticeably, whereas the center at infinity did not degrade for much longer - this is the field curvature of the lens coming into play. Once I determined how close I could focus before the infinity edges began to degrade, I set the focus scale to that point, took an image, and examined the foreground at 100%. I found that the image was sharp from about 6-8 feet to infinity. Everything closer to the camera was noticeably blurred. I then experimented with progressively closer focal points, to see if I could take another single image with all the remaining foreground elements in focus. By the time I focused close enough to get the immediate foreground elements in focus, the edges at 6-8 feet had begun to degrade slightly, even though the center at 6-8 feet looked great (again, this is the field curvature rearing its head).

I ultimately discovered that, for my purposes, when taking a landscape image on a tripod, I can shoot 2 images (f/11, first image with L side of infinity symbol aligned with f/4 mark on distance scale, second image with L side of infinity symbol aligned with f/16 mark on distance scale), and after focus-stacking in PP, emerge with all elements from about 2 feet to infinity, across the frame, sharp. For compositions with elements closer to the camera than 2 feet, more images will be required. With a bit more playing around, I also determined what the best single-image compromise for my purposes is: f/14 (not much change in sharpness due to diffraction...I found this became much more pronounced at f/16 and beyond), L side of infinity symbol aligned with f/8 mark on distance scale (this will do a reasonable job getting everything from 2 feet to infinity in focus, though the infinity borders and the immediate foreground will be a bit soft).


So what did I learn?

1) For landscape shooting with high DOF requirements and critical sharpness needed across the frame, the Da 15 will not likely be able to achieve this with a single image. You'll either need to focus stack, or sacrifice sharpness in some parts of the image. For me, there are enough other pros to the Da 15 (primarily size/weight, which blows any other wide K-mount lens out of the water) that I'll be keeping it and becoming adept at focus stacking when I really need critical sharpness across the frame.

2) Most of the border/corner sharpness issues that I've observed with the Da 15 relate to field curvature, not to the border/corner performance of the lens per se. Put another way, it is capable of producing border and (all but extreme) corner resolution that is pretty good.

3) From here on out, when I shoot with the Da 15 (in most circumstances), I'll be using MF, and will be leveraging the settings I determined above. No AF, no focusing in Live View, just MF using the distance scale on the lens. I now know exactly how my copy performs and how to get what I want from it.

4) It is important to test your lenses systematically to learn how to get the best out of them. I should have done this with the Da 15 years ago.

Hope this is helpful for someone...

Brandon

05-31-2015, 09:57 PM   #2
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Very nice, Brandon. Thanks for sharing your experience. I too, wondered if MF a little back or front can bring up the sharpness in corners, but never tried as you did. Good job!
06-01-2015, 01:45 AM   #3
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QuoteOriginally posted by bpv_UW Quote
I first acquired a Da 15 limited a little over 4 years ago, and have owned one (a couple different copies) for most of the time in between. I've shot thousands of images with it. While it has a laundry list of great characteristics: size/weight, flare resistance, colors, contrast, center sharpness, etc, I have always been frustrated with it's edge/corner performance.
/snip ...

Not my style of shooting. I never carry a tripod, never plan my shots. So boring. Ever since I got a DA16-45 I've just about given up on my DA15. It is grossly over-rated. What is the point of having a f/4 lens if it is only useable stopped right down and AF is so unreliable ?
06-01-2015, 01:59 AM   #4
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thanks mate a good read

06-01-2015, 02:20 AM   #5
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The Sigma 8-16 is a lot sharper both in the center and in the corners then the DA 15. And to top it off it also has some more focal length to play with. Yes, the lens is bigger, but if you're going to carry a tripod anyway what's that little bit more of a lens going to matter?
06-01-2015, 03:23 AM   #6
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Worth sharing. Thanks.
I recently bought a 8-16, and took the opportunity to send my DA15 and DA10-17 off for adjustment and/or repair.
When I get them back I plan to do some testing to decide how I will be going about my ultra-wide stuff in the future. Something similar to your approach will now be part of that process
06-01-2015, 03:25 AM   #7
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QuoteOriginally posted by Clavius Quote
The Sigma 8-16 is a lot sharper both in the center and in the corners then the DA 15. And to top it off it also has some more focal length to play with. Yes, the lens is bigger, but if you're going to carry a tripod anyway what's that little bit more of a lens going to matter?
If he found the 12-24 too big/heavy, I have a strong suspicion the Sigma would also fail here
06-01-2015, 05:25 AM   #8
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I mostly use the DA15 for a foreground object in front of a more distant background, in which case soft edges are irrelevant. Great lens. Nevertheless, I'm really interested in seeing what the DA and D FA wide zooms on the Pentax lens roadmap look like.

06-01-2015, 06:09 AM   #9
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The biggest thing to me is to focus on areas that are more on the edges rather than on the center. I guess because of field curvature if I focus on the center portion of the image, the borders will often be really sharp. At the same time, if I focus on the borders and am stopped down a little, the center usually will be fine (I normally shoot at f5.6 or f8).

As to whether or not a tripod is necessary, it probably isn't, although when you are shooting at small apertures, it is helpful to be able to use isos that are low still.
06-01-2015, 12:53 PM   #10
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Thank you for the info, now I have to do some work with my DA15. Can anyone imagine doing all this testing back in the film days?
06-01-2015, 01:03 PM   #11
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QuoteOriginally posted by kh1234567890 Quote
/snip ...

Not my style of shooting. I never carry a tripod, never plan my shots. So boring. Ever since I got a DA16-45 I've just about given up on my DA15. It is grossly over-rated. What is the point of having a f/4 lens if it is only useable stopped right down and AF is so unreliable ?
Good point.
06-01-2015, 05:53 PM   #12
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QuoteOriginally posted by Wingincamera Quote
Thank you for the info, now I have to do some work with my DA15. Can anyone imagine doing all this testing back in the film days?
Back in the film days, 90% of our shots were 4x6 prints and we never saw most of these issues unless we made a big print or were viewing slides on a big screen. For most of us, the biggest print we ever had made was an 8x10 and they still looked pretty good. I have scanned some of my old film shots that I considered my better ones and they don't look as good on a 27 in. monitor as they do in the album.
06-01-2015, 07:34 PM   #13
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QuoteOriginally posted by grahame Quote
Very nice, Brandon. Thanks for sharing your experience. Good job!
QuoteOriginally posted by beachgardener Quote
thanks mate a good read
QuoteOriginally posted by Wingincamera Quote
Thank you for the info
You're welcome, glad it was helpful.


QuoteOriginally posted by Clavius Quote
The Sigma 8-16 is a lot sharper both in the center and in the corners then the DA 15. And to top it off it also has some more focal length to play with. Yes, the lens is bigger, but if you're going to carry a tripod anyway what's that little bit more of a lens going to matter?
Have you compared the 15 and 8-16 directly? I'd be interesting in seeing it, if you did. I have thought a lot about the Sigma, but size/weight is an issue. When I hike/backpack (not much now due to young child (soon to be children) at home - but more again once kids get a bit older) is when I do most of my landscape shooting. I carry my body and lenses in a modest-sized chest pack, and the Sigma (or the Samyang 14, or the Da 12-24 to a lesser degree) eat up most of that space). With my current line-up, I'm able to fit the Da 15, Fa 31, and Fa 77 with ease, and it keeps the weight down (which is important). I also like to be able to use a CPL (or occasionally an ND filter for long exposures), and neither the Sigma or the Samyang allow that.


QuoteOriginally posted by Sandy Hancock Quote
Worth sharing. Thanks.
I recently bought a 8-16, and took the opportunity to send my DA15 and DA10-17 off for adjustment and/or repair.
When I get them back I plan to do some testing to decide how I will be going about my ultra-wide stuff in the future. Something similar to your approach will now be part of that process
I'd be interested in hearing your thoughts and seeing any comparison images after you've done this

-Brandon
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