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10-10-2007, 05:04 AM   #1
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Pentax-M 50mm f/1.4 distortion?

So I bought a Pentax-M 50mm f/1.4 lens. It's in great shape, and I really enjoy using focus trap. But as I was looking thru the camera, the center of the viewfinder has clear distortion -- the image bulges around the center toward the camera. Is this normal?

10-10-2007, 05:40 AM   #2
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Don't know what you mean.
Can you see it on the images, too, or in the viewfinder only?

Perhaps what you see/mean is the very thin DOF?
10-10-2007, 06:14 AM   #3
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I should probably look at some test pictures. But there is a clear barrel distortion in the center of the viewfinder -- about the size of the center circle in the viewfinder. I don't notice any distortion with my other 50mm lens.
10-10-2007, 06:50 AM   #4
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QuoteOriginally posted by konraDarnok Quote
I should probably look at some test pictures. But there is a clear barrel distortion in the center of the viewfinder -- about the size of the center circle in the viewfinder. I don't notice any distortion with my other 50mm lens.
If you do test pictures try to take a brick / block wall, keeping the mortar lines horizontal in the view finder (i.e. parallel to top and bottom edges of the viewfinder), and one corner of the wall alligned with an edge of the view finder.

this will (providing trhe bricklayer was not drunk) give youo a good reference for distortion

10-10-2007, 06:59 AM   #5
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QuoteOriginally posted by Lowell Goudge Quote
If you do test pictures try to take a brick / block wall, keeping the mortar lines horizontal in the view finder (i.e. parallel to top and bottom edges of the viewfinder), and one corner of the wall alligned with an edge of the view finder.

this will (providing trhe bricklayer was not drunk) give youo a good reference for distortion

Sure thing, but is this symptomatic of this lens?
10-10-2007, 07:20 AM   #6
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That is not typical of the lens but, we should make sure what lens you do have. Can you post a photo? I'm only asking on the odd chance it's some obscure version of the lens. Also, post some sample photos. Threads like this are useless without pictures.. lol
10-10-2007, 07:22 AM   #7
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QuoteOriginally posted by konraDarnok Quote
Sure thing, but is this symptomatic of this lens?
Not sure, I never tried.

I have the older SMC Kmount version with 52mm filter but have never tried this test with the lens. I never noticed any distortion but that does not mean it isn't there.
10-10-2007, 07:49 AM   #8
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QuoteOriginally posted by konraDarnok Quote
Sure thing, but is this symptomatic of this lens?
I have the M 50mm f1.4, and I would say this is not typical for this lens. This is the lens I go to if I don't want barrel distortion. Sound's like there might be something wrong with your copy?

10-10-2007, 04:43 PM   #9
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I took an 8.5"x11" sheet of paper with a 1/2" grid and took a shot from 1 foot away (filling the frame) and one shot from about 5 feet away (cropped out the relevant grid area). I don't see any barrel distortion problems. Do you? Clearly, I'm not perfectly square with the wall, but that doesn't look like barrel distortion anyway.

Honest, I see a barrel distortion in the diopter. But I guess that doesn't matter if the shots come out correct.

*shrugs*

Last edited by konraDarnok; 04-10-2008 at 07:53 AM.
10-10-2007, 05:53 PM   #10
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To be honest, i copied them into PSP X2 and streightened the center line with the streightning tool.

If anything, you have a slight perspoective distortion from not being perfectly square to the page when you shot, but othewise there is no detectible barrel distortion.
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