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08-23-2010, 04:26 PM   #1
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Trying to understand front focus/back focus thing

I just recently noticed that some my pictures don't seem to be tact sharp with either my FA50 1.4 and my Tamron 28-75 2.8. Is this an issue that normally develops with the camera body over time or is this something that is typically a lens issue. Yesterday, 60% of my photos were slightly OOF. Unfortunately, by the time I was able to make it back to my computer to print a test chart, it was overcast/rainy and approaching sunset 7:30PM. Camera, K20, is about 1 year old

08-23-2010, 04:37 PM   #2
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It depends. If you were shooting at a consistently wide aperture, then your photos might be soft just because of the shallow DOF.

Sit down with the test chart tomorrow, using a tripod, and see if at F1.4, your photos are sharp where they should be.

I've heard that the Tamron is prone to FF, so chances are that there's something wrong with it, but the FA 50mm.

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08-23-2010, 05:47 PM   #3
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Frustrating.. I won't be back home until friday night, so this is going to be a Saturday project. What is the minimum shutter speed that would be acceptable for this test? If it is an issue with the tamron lens, do you recommend trying to correct with the AF Settings in the custom menu (providing it's minimal) or sending it back to Tamron? I purchased it from Adorama when I was up in NYC a couple of months ago.
08-23-2010, 06:43 PM   #4
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If your camera is on a tripod, just let it autoexpose (Av). Try your adjustments (you can always 0 them out), and then exchange the lens if you deem necessary. In the meantime, until Saturday, just try fiddling with the adjustments. You won't break anything by doing so.



08-23-2010, 07:11 PM   #5
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Can i add a question in the same vein, I just purchased an FA77 LTD. And i seem to notice the AF over correct and just change focus slightly from where i think it should be. I seem to be able to tilt downwards or move back ever so slightly for perfect focus to my eye from the VF.

Is this considered front focus or back focus issue?

I'm moving back, but i assume this is considered perhaps a front focus issue? It's not a large concern and love the FA77, but i shall run some more accurate tests to confirm my suspicions. I've just been wondering similar to the OP, Except I have the issues in the VF.
08-23-2010, 07:25 PM   #6
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Don't jump the gun here. Just because you have out of focus pictures doens't mean you have a problem. Most OOF pictures are caused by simply not using a small enough aperture to get everything you wanted within the DOF, or the camera simply not reading your mind about exactly where you wanted the focus to be (even when selecting a focus point, that jus allows you to specify an approximate area, not an exact point). Post some of the pictures in question and describe how you focused, *then* if it seems there might be a problem, a testing with focus test chart might be in order. And if there is a problem, it's as likely to be the camera as the lens.
08-23-2010, 07:55 PM   #7
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QuoteOriginally posted by Marc Sabatella Quote
Don't jump the gun here. Just because you have out of focus pictures doens't mean you have a problem. Most OOF pictures are caused by simply not using a small enough aperture to get everything you wanted within the DOF, or the camera simply not reading your mind about exactly where you wanted the focus to be (even when selecting a focus point, that jus allows you to specify an approximate area, not an exact point). Post some of the pictures in question and describe how you focused, *then* if it seems there might be a problem, a testing with focus test chart might be in order. And if there is a problem, it's as likely to be the camera as the lens.
Thanks, I will use a test chart one day, But i don't have a printer at the moment. Will find some small text or something to try and define the issue further. The issue was noticeably consistent, but i only had the lens on the first time, I always tilted down just a little and it would then be where i wanted.

It would always beep at me the ms after i saw the focus point, not on the focus point. None of my shots are really out of focus even @ 1.8, but this is mainly because i was using MF and even holding my finger and then moving it back slightly to ensure my eye was more accurate than the AF.

It could be those times were i was just too close a minimum focusing distance, or my diopter could need a turn, but i have a feeling with AF it was ever so slightly thinking I'm in front of my center spot. Anyway i will continue to get to know the lens before i adjust any K-7 options.
08-23-2010, 08:04 PM   #8
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Use a newspaper page or something like that. Place a line at some random text line and use that for your focal point. Aim your camera so it is about 45 degrees (or as close as you can get) and take your photo. Make sure your lighting is good. If the focus line of text is out of focus, check for where it IS in focus. If in front of the line, you are front focused. Back of the line, Back focused. Be aware however that this isn't fool proof. I've checked lenses against a chart that Said it was either FF or BF when in fact, anything I took a Normal photo of, was dead on. As noted there are a few different things that can affect your AF results.



08-24-2010, 08:14 AM   #9
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QuoteOriginally posted by Freak Quote
Thanks, I will use a test chart one day, But i don't have a printer at the moment. Will find some small text or something to try and define the issue further.
Still, download a test chart or two to see what they look like, then draw one up by hand. The issue is that you have to understand what makes an unambiguous focus target. A regular sheet of text is *terrible* as an AF test, because there is no way to control which line the camera focuses on. That's because, as I said above, even selecting a focus point only allows to tell the camera *approximately* where to focus. And most focus problems are from people not understanding issues like this, not from any sort of camera or lens issue. A focus test chart is designed so there is only one thing the camera could *possiby* focus on in the vicinity of the selected focus point. Then you know for sure where the camera tried to focus, and the chart will have sort sort of scale - far removed from the focus target so the camera doesn't try to focus on the scale - to allow you to see how well it did.
08-24-2010, 11:11 AM   #10
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In this particular photo, I was focusing on the gas tank (bullet hole sticker).
08-24-2010, 04:37 PM   #11
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Did you select a focus point that was centered on that spot? You have to realize the red dots that appear in the viewfinder are only approximations. Even i the red dot was centered on the gas tank, the steering column and/or the red light would still have been within range of the focus sensor. Subjects like this with lots of different parts at different distances are difficult to nail with AF, but not because of FF or BF - it's because it's impossible to accurately to convey to the camera that you wanted focus on the gas tank and not the nearer objects that were also within range of the selected focus sensor.
08-28-2010, 09:53 AM   #12
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I finally was able to print a chart and get setup near a window with some light. Center point was on the black middle line. Is this close accurate enough with this particular lens: FA50 f1.4
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08-28-2010, 10:03 AM   #13
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This one was with my Tamron 28-75. This one looks like it has a little FF
Should camera be closer?
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Last edited by 41ants; 08-28-2010 at 11:09 AM.
08-28-2010, 10:36 AM   #14
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Looking at both of your charts has me suspicious of your test setup. These are supposed to be photographed at a 45 degree angle so that when optimal focus is achieved, everything in front and in back of the focal point looks the same. It should also look the same left to right. If I look at the 10s on both photos, the one in front (bottom) on the left is more in focus than any of the other 3.

When you take your photos, make sure you are properly lined up on that center line, aiming down, at a 45 degree angle. Or have the chart standing at a 45 degree angle if it's too difficult to aim the camera down (not enough room).

08-29-2010, 10:52 AM   #15
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I agree, the test is set up crooked. But even so, one can see pretty clearly from the FA50 test that things are basically OK with the camera. As I observed, the problem in the motorcycle test was very obviously *not* one of FF or BF - it was simply the camera not reading your mind regarding where exactly you wanted the focus to be. The solution is to be sure to always select a focus point for yourself, and to realize that the focus "point" is not actually a point but actually a largish area of the viewfinder, and the camera might legitimately focus anywhere within that area (you can test for yourself to see just how big an area is covered by each point).
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