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05-12-2009, 12:43 AM   #1
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BF or something else?

OK,

recently I have been noticing one strange thing with my DA 40 and I don't have an idea what is responsible for this.
Basicaly it looks like the corners are LOT sharper then centre of the frame.
following is at f5.6 but the same thing happens even at f2.8 so I don't think loose iris or something like that can be blamed.
Please have a look and feel free to express your opinion. Camera was sitting on the wall, the test wall was 4m +/- away. I would blame BF but at f5.6 40mm on K10D focused at 4m the DOF is over 2m!!! And because the image isn't sharper only in one part (say upper, lower. left...) tilt or not being parallel to the wall theory is out of window too.
what's going on???

full image:

and 100% crops
centre of the frame:

extreme upper left corner

extreme lower right corner


05-12-2009, 12:50 AM   #2
axl
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PS: the glass is clean and this phenomenom is not consistent! I'm still getting some excellently sharp shots out of this lens.
05-12-2009, 01:41 AM   #3
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Whoa??? I only three more words to say..... WTF?

mike
05-12-2009, 02:27 AM   #4
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Quick you better patent it the world first lens that's sharper at the edges, you could make a fortune.

I have absolutely no idea as to WTF is going on.

You said that it's intermittent as well. This makes it even weirder.

I hope you manage to get it sorted quickly and easily.

05-12-2009, 02:56 AM   #5
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Hi Axl,
the center of this lens should always be better or equal as the borders, says
Pentax SMC-DA 40mm f/2.8 Limited Review / Test Report

But, as suggested there could be a backfocus issue. I'd suggest try an f/4ish f/5.6ish series with that wall once more and then put it through some manual focus brackets. I don't know wether you can perform a closed aperture preview through the viewfinder, maybe that does tell something also. There could be a focus shift when stopping down in the lower f-numbers and nearer distances also.

Best, Georg (the other)
05-12-2009, 08:35 AM   #6
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Hmmmm... maybe the lens is fine and the wall is bowed outwards?

Maybe this is an odd kind of decentering defect?
05-12-2009, 12:52 PM   #7
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Weird. Are you sure the camera focused using the center point? Assuming the center is *ever* sharp, I'd say it has either focused correctly on the side or else aimed for the middle but backfocused for whatever reason.
05-12-2009, 12:59 PM   #8
axl
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Marc if it would be BFed why are the corners sharp?
The field curvature can't be so big!

05-12-2009, 02:19 PM   #9
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Whatever the curvature is, maybe it backfocused by exactly that amount. So if the corners at that distance would be in focus, say, 10cm in front of the center, then maybe it backfocused by 10cm inches. Although if you're right about DOF being 2 meters, then BF of 10cm shouldn't have rendered the center OOF. It would have to BF by 2 meters + 10cm to give that result. I agree, doesn't seem likely.

So we're back to my other questions - are you sure about the camera was using the center focus point, and is the center *ever* sharp? Oh, and you don't by any chance has a blob of grease in the middle of the front element? I know, these are insultingly obvious things to check, but I'm really as stumped as you. If the lens is *never* sharp at the center, then clearly it is simply optically defective. But if it is ever sharp - and your other thread on selling the lens suggest it often is - then something else must be going on.
05-12-2009, 03:34 PM   #10
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QuoteOriginally posted by Marc Sabatella Quote
Whatever the curvature is, maybe it backfocused by exactly that amount. So if the corners at that distance would be in focus, say, 10cm in front of the center, then maybe it backfocused by 10cm inches. Although if you're right about DOF being 2 meters, then BF of 10cm shouldn't have rendered the center OOF. It would have to BF by 2 meters + 10cm to give that result. I agree, doesn't seem likely.

So we're back to my other questions - are you sure about the camera was using the center focus point, and is the center *ever* sharp? Oh, and you don't by any chance has a blob of grease in the middle of the front element? I know, these are insultingly obvious things to check, but I'm really as stumped as you. If the lens is *never* sharp at the center, then clearly it is simply optically defective. But if it is ever sharp - and your other thread on selling the lens suggest it often is - then something else must be going on.
pic is worth 1000 words so here is 100% crop from the same day same camera & lens combo, this time @ f2.8



I'm leaning towards the conclusion that it's some sort of focusing issue, but:
How about aperture blades. Would/could damaged/loose blades cause this "effect"? But if so, wouldn't it be consistent?
05-12-2009, 04:14 PM   #11
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Like I said, I'm stumped. Whatever it is seems odd enough that I don't know if can be diagnosed in just a couple of sample pictures. Maybe dozens, different apertures, using AF on different focus points, also MF, scenes involving material behind and in front of intended focus point to gauge where focus actually is, etc. Aperture blades can probably leave oil deposits on lens elements. I would assume that would be visible in the right light.
05-12-2009, 04:45 PM   #12
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An element is mis-aligned in the lens, it requires collimation
05-12-2009, 11:51 PM   #13
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QuoteOriginally posted by Nesster Quote
An element is mis-aligned in the lens, it requires collimation
what makes you believe so and how sure are you about it?

but it would make sense that the element is not misaligned but loose inside. As I said, I get still many good shots sharp corner to corner, but I get many that look like the one from my OP too.
Is it possible that element would become loose inside the lens?
05-13-2009, 02:54 AM   #14
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Yes that's what I meant - i.e. front to back something doesn't line up.

Are you certain the pictures you get that are sharp are in fact so - you don't just shoot brick walls I hope. There may be times the center is sharp enough but you can't tell about the other bits as there's nothing in the supposed focus plane.

Aperture blades wouldn't do this unless they are clearly perforated etc, in which case you'd be getting a soft portrait kind of effect with a sharp center.

If this was an old mf lens of simple design I'd suspect the rear element was on backwards. Although the effect of this would be greater than what you have.
05-13-2009, 02:31 PM   #15
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I do shoot other things too, I may post some later so you may judge for yourself if it's sharp or not

BR
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