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Forum: Pentax SLR Lens Discussion
11-04-2015, 04:05 AM
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Thanks for the feedback :) I agree, there will be variations due to tolerances, quality control, wear and tear etc. Plus, as others have already mentioned, the data used doesn't necessarily demonstrate the lenses' performance in relation to specific subjects or distances - but it's a starting point.
The Popular Photography charts look good.
I think I'll continue to add the current range of Pentax lenses to my sheet (at least where data is available) and see if there is any appetite from others for me to expand it further. If not, at least I'll have the reference I was originally looking for :)
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Forum: Pentax SLR Lens Discussion
11-03-2015, 05:25 PM
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That makes perfect sense. I wouldn't rely on or get comfy with the data I was looking for and am now compiling. And yes, you're right, when you need to take the shot, you take it, whatever. If possible, with some options to choose from later. I agree with all that :)
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Forum: Pentax SLR Lens Discussion
11-03-2015, 04:30 PM
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Well, the DA 35 F2.4 is also sharpest at F4 edge to centre if you believe the tests :)
Maybe I'm alone in being interested as to the most useful aperture range of my existing and future lenses... That's fine. It just seems kind of handy to know where I can grab a bit more light or DOF without sacrificing significant resolution.
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Forum: Pentax SLR Lens Discussion
11-03-2015, 01:58 PM
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So... here's the kind of thing I was thinking of. This is a very, very rough summary of just the Limited primes, the DA35 F2.4 and DA50 1.8 - just to play with the basic layout... Data taken from Photozone reviews with the exception of the DA50 1.8 which was Lenstip. I haven't validated this yet - I may have made an error or two, but as I said, this was mainly to play with the format.
Any constructive input would be appreciated - this layout is fine for me, I think, but if I turn it into something for others to use also (and add a lot more lenses), I'd rather make it broadly appealing :)
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Forum: Pentax SLR Lens Discussion
11-03-2015, 12:02 PM
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Very handy. I have a few M and A lenses that I can add to my table using this data. Thank you! :)
Edit: I just had a quick look at his data, and it's interesting to see how the 35mm lenses seem to peak at f/8...
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Forum: Pentax SLR Lens Discussion
11-03-2015, 11:29 AM
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@osv - thanks, that's very useful, and helpful.
I don't doubt that MTFs based on a single, close distance won't tell the whole story, and that every copy of every lens will differ slightly. However, if I compile my crib sheet as a starting point, I'll at least have narrowed down the most useful aperture ranges for each lens based on admittedly-limited tests. And that's what I'm after - a tool that gets me "in the ball-park" (without simply assuming "a couple of stops down from wide open" and "no slower than f/8"); but not a definitive reference...
Thanks again for the info - much appreciated :)
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Forum: Pentax SLR Lens Discussion
11-03-2015, 09:07 AM
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This is great stuff - thanks for sharing :)
I'm pulling together a table for the lenses I own and I'll post it on the forum when its ready...
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Forum: Pentax SLR Lens Discussion
11-03-2015, 07:32 AM
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Well, thanks for your input and sticking with it this far :) Based on my original post, it seems I'll need to pull the lens performance data together from photozone, lenstip etc.
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Forum: Pentax SLR Lens Discussion
11-03-2015, 06:36 AM
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I've never shot mine wide open, so it's good to get a view on that! :D I'll have to give it a try, just for laughs...
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Forum: Pentax SLR Lens Discussion
11-03-2015, 01:54 AM
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I do get that, and I have no problem using a lens wide open if needed. I was merely wondering if the optimal apertures for each lens had been summarised. No matter, I can pull the info together from various reviews.
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Forum: Pentax SLR Lens Discussion
11-03-2015, 12:56 AM
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My subjective analysis was based on the MTF figures at Photozone for each lens... And yes, F11 shows to be worse than F5.6 on those lenses but still very good on several of them.
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Forum: Pentax SLR Lens Discussion
11-02-2015, 03:49 PM
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So I've now looked at the MTF figures for a bunch of the Limited lenses, and - subjectively - this is what I come up with:
DA15 - very good from F4, best centre at F5.6, best across the frame at F8 & F11
DA21 - good centre from F3.2, best centre F4 / F5.6, best across the frame at F5.6 / F8, still good across frame at F11
DA35 F2.8 - great from F2.8, best across frame F4 - F8, still good at F11
DA40 - great from F2.8, best from F4 - F8, still good at F11
DA70 - great from F2.4 - F2.8, best at F4 & 5.6, great at F8
Although this doesn't include appraisal of CA performance (or vignetting, for example), it makes me question whether it's simply possible to say "one or two stops down to get decent performance"; since some of the lenses perform really well - at least, from an MTF perspective - wide open...
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Forum: Pentax SLR Lens Discussion
11-02-2015, 02:53 PM
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Right, so... as a rule of thumb, then... an f/1.4 lens should start performing really well from, say, f/2.8 onwards, while an f/4 lens would start kicking in nicely around f/8 - yes? What about the point where diffraction starts to become an issue?
If I can get to the point where, instead of a lens by lens list, I can reliably work off a formula, I'd be genuinely happy to do so.
Very interesting! :)
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Forum: Pentax SLR Lens Discussion
11-02-2015, 02:13 PM
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Agreed, and for the lenses where I can't remember the optimal range of apertures, this is where I tend to end up... but some are very adequately sharp, with little to no CA, at f/2.8 or f/3.5, and that gives a decent amount of extra light to play with if you're aware of the fact...
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Forum: Pentax SLR Lens Discussion
11-02-2015, 01:38 PM
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Hi Michael... good idea, but I'm not sure that would give the whole story. The kind of information I'm talking about would, for example, tell me that my 50mm f/1.7 lens is soft wide open with significant lateral CA, performs best *between* f/4.0 and f/11, and after that diffraction kicks in.
For the lenses I use often, I know this stuff anyway... But, if I think about my DA15 and DA70 - neither of which are getting much use right now - I honestly couldn't tell you the optimal range of apertures. If I was out shooting and decided to use either of those, I'd have to make a best guess. It would be nice to have a crib sheet with me.
Also, for those who don't yet have a particular lens, it might be useful to know where it shines.
As I say, if such a compilation doesn't exist, I'm happy to pull the information together for myself (and others, if interested)...
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Forum: Pentax SLR Lens Discussion
11-02-2015, 11:57 AM
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Has anyone compiled a list of Pentax lenses and their optimal aperture ranges for sharpness / lack of CA / lack of diffraction? Seems to me this would be useful resource. I'd guess most of the information can be gathered from the in-depth and user lens reviews here on the site. If it hasn't already been done, I may pull one together... Otherwise, if one exists, I'd be grateful if someone could point me towards it :) Thanks...
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