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11-04-2009, 08:19 AM   #1
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would a manual lens experience front/back focus problems?

I try to focus on the center of a subject using a M 50/f 1.7, the camera focus indicator would kick in, however, there's a little margin where that focus indicator indicates that the subject is in focus. Sometimes I would get a sharp picture but sometimes the focus's is a little off to the front or to the back. Is this just my inability to look properly through the viewfinder? I hope so..

11-04-2009, 08:43 AM   #2
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A manual lens does not experience front or back focus by virtue of the fact that it is MF. The indicator is a guide but you will determine exactly how you want the lens to focus. If you continue to experience problems focusing, trying mounting on a tripod and using live view with a magnification, or purchase an after market split prism viewfinder.
11-04-2009, 08:49 AM   #3
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I believe the original poster was asking whether the AF confirmation indicator can give an incorrect reading (i.e., FF or BF) for a manual focus lens. The answer to this is yes. It's odd but it's true. If the camera is unable to identify the correct focus plane, it will give an incorrect reading no matter if the lens is AF or MF. I've seen this with an AF lens that would provide incorrect readings when the lens was set to MF. I could accurately focus the lens manually, but I had to ignore the AF confirmation indicator. Very odd but very *solvable* by simply trusting your eyes and not the camera. I've only seen it on this one lens as all of my other true MF lenses worked fine.
11-04-2009, 08:53 AM   #4
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If you experience this with a MF lens then you yourself is to blame.
Go to the eye doctor or something.
AF lenses experience BF or FF due to the fact that you are letting the camera focus for you.
It determines focus by virtue of what it was programmed to do..at times it is a little off because that is what the camera sees.
Most of the lenses have a little BF or FF issues but are sometimes deemed within the acceptable limits that not unless you pixel peep, you will see it.
To make lenses spot on ('coz focus programs can only do so much) is that newer cameras have incorporated lens focus fine tuning.
This started with the K20D in which you can fine tune upto 20 lenses and the camera would have a memory of these lenses as you individually mount them.

11-04-2009, 08:56 AM   #5
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QuoteOriginally posted by Rich_A Quote
I believe the original poster was asking whether the AF confirmation indicator can give an incorrect reading (i.e., FF or BF) for a manual focus lens. The answer to this is yes. It's odd but it's true. If the camera is unable to identify the correct focus plane, it will give an incorrect reading no matter if the lens is AF or MF. I've seen this with an AF lens that would provide incorrect readings when the lens was set to MF. I could accurately focus the lens manually, but I had to ignore the AF confirmation indicator. Very odd but very *solvable* by simply trusting your eyes and not the camera. I've only seen it on this one lens as all of my other true MF lenses worked fine.
Oooppsss..sorry, this is the more appropriate reply.
I think I have to go to the eye doctor for not reading it well enough!
11-04-2009, 09:11 AM   #6
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First, make sure your diopter is adjusted properly in the viewfinder. The AF indicator (the green hex, not the red square) could be off a little or it may be that the AF system is picking up something else in the background or foreground, very common in a scene where there are objects or whatever close together but at different depths. Using your lens wide open at f/1.7, you will have to be very precise in your focus. A split image viewfinder is very helpful.
11-04-2009, 10:08 AM   #7
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Focus indicator (green hex) is based off of the body's AF system and can have FF/BF. What looks in focus is based on the focus screen's alignment, and can be off separately, but should both be good to go (the green hex being lenient on what is in focus).

Get a focus chart, read the instructions and focus on the line with white surrounding it. Also try some trap-AF to see how much play the green hex has and get a feel for it.

And the diopter of course, point your camera at a plain white wall, and make the focus brackets as sharp as you can. I have good vision but my dipter setting is all the way to the right.
11-04-2009, 11:06 AM   #8
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QuoteOriginally posted by blu3ness Quote
there's a little margin where that focus indicator indicates that the subject is in focus.
I think this is the main source of why the AF indicator isn't foolproof. One reason this happens is that the focus sensor isn't just a single point - it takes up more or less the entire center bracket etched into the viewfinder. So the indicator may light if *anything* within that range is in focus. But DOF is so shallow with a 50/1.7 that you can easily have one part of that region be in focus and another part not.

Unfortunately, simply switching to trusting your eyes isn't foolproof either, sicne the viewfinder shows you too much in focus. You can test this by shooting a sheet of printed text on an angle - look to see how many lines of text appear in focus in the viewfinder compared to in the picture itself. Chances are you'll find around twice as many lines in focus in the viewfinder as in the picture.

Despite these two issues, it is possible with practice to learn to focus reasonably well with a manual lens. The main trick is to learn to "see" the entire in-focus zone, not just look at one specific point. If you can see the entire in-focus zone in the viewfinder, you can better anticipate what part of it will turn out not be in focus in the picture and adjust accordingly. You will also have a better sense of when the AF indicator is potentially telling you something you *don't* care about is what is in focus.

11-04-2009, 11:14 AM   #9
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Original Poster
Thank you marc, that explains a lot. I was actually trying to focus on some lines of texts at 45 degree angle. The center focusing bracket through the viewfinder targets 2 paragraphs of text, I tried to focus on the upper paragraph, but the focus indicator makes no distinction between whether the top or the bottom paragraph is in focus. I have also tried a focusing chart, it seems that the focus indicator will stay lit with a tolerance of at least 3 mm on the chart. (within 3 mm from the center line the focus indicator stays lit).

Can anyone else confirm this? I just want to make sure there is no fault with my camera or the lens.
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