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01-06-2011, 12:59 PM   #1
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Focusing issues

I bought a Pentax-A SMC 50mm f1.7 as recommended on the forum

I'm using on a K10D.

I'm getting occasional brilliant photos, and hundreds of just slightly out of focus photos, especially in low light.

Any ideas? I've got Shake Reduction on focal length of 50mm, and I'm focusing it so it looks perfect in the viewfinder.

Bless,
Chris

01-06-2011, 01:22 PM   #2
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I think the problem is that the stock K10D focussing screen (and every stock screen for the Pentax DSLRs that I'm aware of) does not accurately represent the true DOF for large aperture (wider than around f/4) properly. So, even when it looks sharp through the viewfinder, once you see the image the actual DOF was smaller than you saw through the finder and what looked sharp before might be in an out-of-focus area.

My understanding is that this is a tradeoff made to make the viewfinder brighter, which isn't entirely crazy... unfortunately these APS-c viewfinders just aren't as nice as with full-frame cameras.

I think possible solutions are:
-practice with the current screen until you can figure out when you have the "most in focus" point where you want it (this could be hard)
-replace the focussing screen
-use the focus confirmation green hexagon (if you do this you could suffer from front/backfocus issues just like with autofocus)

Another possibility entirely is that your focussing screen shims are incorrect and you need to recalibrate so that the viewfinder image is correct (if your focussing screen is not at the exact same distance from the subject as the sensor then what you see through the viewfinder will not be the same as what the sensor sees).

Last edited by sewebster; 01-06-2011 at 01:31 PM.
01-06-2011, 01:41 PM   #3
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You could also get a viewfinder magnifier... it should help you see better, but I don't think it really solves the underlying problem.
01-06-2011, 01:53 PM   #4
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Ok. I have tried blurring it in and out of focus both ways, and it doesn't seem to improve it. If that was the case, wouldn't it look blurry the times it has taken good photos? Because it doesn't...

Have other people had the same problem?

BTW, you can check out some of the photos here on my Flickr feed.


Last edited by bigonroad; 01-08-2011 at 02:47 AM.
01-06-2011, 01:58 PM   #5
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QuoteOriginally posted by bigonroad Quote
Ok. I have tried blurring it in and out of focus both ways, and it doesn't seem to improve it. If that was the case, wouldn't it look blurry the times it has taken good photos? Because it doesn't...
Based on my idea that the problem is the focussing screen, then no, it wouldn't necessarily look blurry when it took good photos.

QuoteQuote:
BTW, you can check out some of the photos here on my Flickr feed.
Well, the photo you linked to here had a shutter speed of 1/8s. That is quite slow. Basically I would not complain about blurry photos unless you have a shutter speed of over 1/100s or so. Otherwise it could just be handholding technique or subject movement. Even at 1/100 you can have problems depending on the focal length or subject...
01-06-2011, 03:41 PM   #6
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Agree with Sewebster. You can't expect sharp shots with a 50mm at less than 50th sec all the time, even with SR. Try 100th sec. or more.
If you're still getting blurry shots, it's probably your focusing technique. I use MF almost all the time with all my lenses, and I'm good at it, but I do find the FA 50 f-1.7 a little tricky at times. If you are going past, then back past, then slowly to focus, as you indicated, you're probably not concentrating on your focal point. Don't look at the entire scene, or even the entire main subject. Ignore it and concentrate on one small thing (such as an eye) to concentrate the focal point. The more you narrow your vision to one particular focal point the more consistently sharp the shot will be.
01-07-2011, 08:39 AM   #7
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Focusing manual takes practice. Lots of it. I have a similar lens, M 50mm 1.7 and also a A series 50mm 1.4. I use both on my K-x pretty often, my other 50mm lenses get used now and then but those are the main two I use. I get plenty very nice pictures with both, but lots more that are slightly out of focus. Ditto for every manual lens I have, it's just not easy to get good sharp focus every time.

I find that I get better results at smaller apertures, which means better depth of field, but if I shoot at anything near wide open I can expect plenty out of focus shots. I guess the main thing is getting accustomed to manual focus. I'm not sure how to describe it, but it's a matter of seeing the image well enough. I usually try to find some detail, when it's clear I can be pretty sure I have good focus.

Three words...practice, practice, practice.
01-07-2011, 10:35 AM   #8
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I believe the key to MF is more in how you concentrate your vison than in how well you actually see. I'm 62, and my eyes aren't as good as they once were, but my keeper rate is actually better. I've leanred to concentrate totally on one small spot that I know is the best place to center the hyperfocal distance. Once I have determined the composition, I ignore everything execept that tiny focal point while focusing. My common practice is to go just past focus quickly on both sides, then slowly slide into optimum focus. The thing to concentrate on most is where your focal point sort of pops with increased clarity and contrast. (It actually looks similar to a posted image where the person has boosted the contrast too much.) The tinest movement to either side of this pop is still "in focus" but not at optimal focus. This pop is at the center of the hyperfocal distance, which of course varies with the apature.
The hyperfocal distance narrows with wider apatures (smaller f-numbers), so the wider the apature the more critical optimum focus becomes and the less latitude you have for DOF to compensate for the slight front-focus/back/focus characteristics almost all lenses contain. So, the wider the apature, the more MF gives you an advantage to hit the pop point. Thankfully, the wider the apature the more obvious the pop point. I have much more trouble finding the pop point at f-8 to f-11 than at f-2.8.
As Pete says: practice, practice, practice. But I would add, practice most on the way you concentrate your vision.

01-08-2011, 02:46 AM   #9
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Indeed, I think the slow shutter speeds have been an issue. I'm always attracted to low light stuff, which is why this lens is just a godsend so far, compared to my F3.5 kit lens.

I spose, I'm pretty happy with those ones, and in normal light, I've not been having any problems...

Thanks guys!
01-08-2011, 10:51 AM   #10
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QuoteQuote:
As Pete says: practice, practice, practice. But I would add, practice most on the way you concentrate your vision.
Ron - Very good, I couldn't think of a way to put it, but you're describing the same thing I try to see when focusing. I just hit 55, had to start wearing glasses about 5 or 6 years ago, it definitely has an impact but I still get plenty good shots. My keeper average may be improving too, I'm not sure. Some days I can get loads of keepers, like the day I shot Pelicans all day and kept 140 shots, other days like yesterday I'm lucky to get 2. In fact I don't think I got but one yesterday and it was mediocre at best...
01-08-2011, 11:19 AM   #11
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Pete: I've found as I get older, my vision seems good sometimes, not so good others. It's best when I exercise. I have hypoglycemia. If you have it, or diabites, yours will be better if you exercise too. It's better when I spend most of my time looking straight ahead or up, worse when I spend time looking down (has to do with the way these movements change the physical curvature of your eyes).
One advantage I know I have, just as with shooting a gun, is I use my dominant eye to look through the viewfinder.
One thing I find kind of strange, is I'm so farsighted, I can hardly see details of my hand in front of my face, but I can focus very accurately in macro mode.
The first thing for anyone interested in using, or improving, their MF abilities is to make sure the diopter is adjust to your vision.
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