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07-29-2012, 08:40 PM   #1
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FA-31 f/1.8 Limited

Had my first serious outing with with FA-31 on my K-5. The optics are superb (and I am currently using a Marumi SHG Protection filter...to protect my investment...and this lens is surely and investment), I mean astounding. I'm not a pro, indeed, I'm still a newbie compared to many on this forum. The K-5 is my first really serious camera (it's awesome) and I'm still riding the it's learning curve. Back to the FA-31....the thing is built like a tank...a bit on heavy side for street shooting, but the focal length and angle of view are near the traditional "standard lens." Whatever this lens renders, it renders beautifully, provided of course that my manipulation of the K-5's setting haven't reduced it's IQ to that of a mere mortal lens from one of the "big 2" manufacturers. So, yes I think the lens is awesome, but I need to advice on getting the most out of it. Sometimes I'm able to get great selective focus and bokeh and sometimes not, even at the same f/stop. Why? Am I expecting this lens to perform in ways that a "standard" lens is not meant to? Is it an AF issue or could the lens be suffering from front or back focus? Should I be setting the AF to use only the center focus point? Sorry for rambling, but if anyone can school me on this lens....I'd appreciate it (also trying to sort the same things out with the FA-77 LTD and FA-43 LTD). Thanks!!

07-29-2012, 08:48 PM   #2
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As well as aperture, depth of field also depends on distance from the subject. So getting closer to your subject with any lens will produce a more out of focus background.
07-29-2012, 08:56 PM   #3
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You are rightt, the FA31mm is a awesome lens. For me, the key is the higher keeper rate with the FA31mm than with my other lenses.

I use center focus point, and as pointed by Calsan, you need to come close to the subject if your want some bokeh. But the lens is also superb for landscape and outdoor.

I canot comment on the bacl/front focus. There are several threads in the K-7 forum on that.

Enjoy this great lens...
07-29-2012, 09:22 PM   #4
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QuoteOriginally posted by CMG Quote
Sometimes I'm able to get great selective focus and bokeh and sometimes not, even at the same f/stop. Why? Am I expecting this lens to perform in ways that a "standard" lens is not meant to? Is it an AF issue or could the lens be suffering from front or back focus? Should I be setting the AF to use only the center focus point?
If I understand your question you are not getting consistent focusing? Are you using the lens wide open? (f/1.8) If you are not used to very fast lenses you must keep in mind that the DOF can be very thin. So, is nothing in focus? or is something in focus just not what you wanted?

If something is in focus, just not what you wanted then using center point only will allow you to better select what the camera is going to focus on. If you have it on multi then you will get situations where the camera selects something entirely different than you wanted to focus on. It has no way to read your mind.

If nothing seems to be in focus it could be that the thin depth of field is causing you problems, for example if the available DOF is 1" and the camera locked onto the subject's nose, you might not notice that the tip of the nose is in focus but nothing else is. It could also be a front focus / back focus adjustment needed but test everything else before you start making AF Fine adjustments, that is a good way to just chase in circles if you do not understand and test properly.

07-29-2012, 10:15 PM   #5
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I usually keep at f/2.8 with this lens so I don't get too much unintentional blur. Also the sharpness goes up a notch there too. You should also still get that beautiful bokeh at f/2.8 on backgrounds too.
07-29-2012, 11:18 PM   #6
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These limited lenses are my first really fast lenses. I need to get a feel for the very shallow depth of field that you get wide open. I suppose I sometimes forget that while selective focus is one great aspect of these lenses, using the superb optics at middle and higher f/stops is also part of the package. I think it's easy to get caught up in the "bokeh" obsession, especially as a relative newbie, and forget that lenses of this quality do many things very well.
07-29-2012, 11:30 PM   #7
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QuoteOriginally posted by jatrax Quote
If I understand your question you are not getting consistent focusing? Are you using the lens wide open? (f/1.8) If you are not used to very fast lenses you must keep in mind that the DOF can be very thin. So, is nothing in focus? or is something in focus just not what you wanted?

If something is in focus, just not what you wanted then using center point only will allow you to better select what the camera is going to focus on. If you have it on multi then you will get situations where the camera selects something entirely different than you wanted to focus on. It has no way to read your mind.

If nothing seems to be in focus it could be that the thin depth of field is causing you problems, for example if the available DOF is 1" and the camera locked onto the subject's nose, you might not notice that the tip of the nose is in focus but nothing else is. It could also be a front focus / back focus adjustment needed but test everything else before you start making AF Fine adjustments, that is a good way to just chase in circles if you do not understand and test properly.
I'm getting things in focus in and around the chosen focus point, though I'm not quite sure how large an area in the viewfinder is covered by a focus point, it's just that sometimes when shooting wide open I expect a more dramatic selective focus...sometimes a get it, sometimes I don't.
07-30-2012, 08:00 PM   #8
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QuoteOriginally posted by CMG Quote
though I'm not quite sure how large an area in the viewfinder is covered by a focus point,
It is easy to fall into the trap of thinking of the focus points as points. In reality they are areas, and anything within the area can end up being the target the camera locks onto. While it would be nice if the AF point was small enough you could be sure that you could focus on an eye for example, in fact the focus area of a single 'point' might cover the eye, eye brows and even the nose. So what you think and what the camera thinks it should be focusing on can be quite different. In can be very frustrating until you understand this. The red indicator is not the focus 'point' but only indicates which focus 'area' is being used.

Here are two threads that might have some value to you:
https://www.pentaxforums.com/forums/pentax-k-5/192356-k-5-autofocus-thumbs-up...rustrated.html

https://www.pentaxforums.com/forums/pentax-k-5/177572-all-fuss-about-auto-focus-point-size.html

07-30-2012, 11:23 PM   #9
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QuoteOriginally posted by jatrax Quote
It is easy to fall into the trap of thinking of the focus points as points. In reality they are areas, and anything within the area can end up being the target the camera locks onto. While it would be nice if the AF point was small enough you could be sure that you could focus on an eye for example, in fact the focus area of a single 'point' might cover the eye, eye brows and even the nose. So what you think and what the camera thinks it should be focusing on can be quite different. In can be very frustrating until you understand this. The red indicator is not the focus 'point' but only indicates which focus 'area' is being used.

Here are two threads that might have some value to you:
https://www.pentaxforums.com/forums/pentax-k-5/192356-k-5-autofocus-thumbs-up...rustrated.html

https://www.pentaxforums.com/forums/pentax-k-5/177572-all-fuss-about-auto-focus-point-size.html
Thanks! I'm going to check out those threads.
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