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10-17-2011, 04:53 AM   #1
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Weird lens behaviour

Hello, i have noticed a weird issue with two of my lenses. I have two 135mm lenses one at 2.8 and another 3.5. My issue is that when wide open the 2.8 is sharper as you move away from the focused point. Im not sure if the lens is just sharper at the corners but the 2.8 should produce better out of focus blur then the 3.5 no? And yes the picture i took was not in the same plane. The items were on an angle moving away which is causing me to wonder. My question really is can a lens at 2.8 be sharper then a 3.5 as distance moves away from the focused point?

10-17-2011, 10:17 AM   #2
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In theory, the depth of field should be smaller at f2.8 than f3.5, so at a certain distance from the focus point, you expect to see more blur at f2.8 than f3.5. But with two different lenses, the theory is "more of a guideline". If focus is on the exact same spot for both lenses and they're both wide open, your results could be caused by factors like these:
  • Contrast on the f2.8 lens is much higher, so apparent sharpness is increased
  • A really curved field on one lens and a really flat field on another, or really high distortion vs. low
  • Really bad corner sharpness vs. really great corner sharpness
  • The f2.8 is really a quarter-stop slower and the f3.5 lens is a quarter-stop faster than marked, or slightly different focal lengths
As the lenses are stopped down, errors in the actual area of the aperture are likely.

Looking at some of my 135mm lens tests, where I tried really hard to eliminate focus errors, I can definitely see the effect of high contrast next to low contrast. The Takumar Bayonet 135/2.5 has a wider field of view than the SMC Pentax 135/2.5, by about 5%. I only have used one 135mm with a curved field. I would not expect to see what you've seen without two or three problems combined. If your f3.5 lens is something awful or has been dropped once too many times and you tested it against the FA 135mm f2.8, that would do it.
10-22-2011, 08:58 AM   #3
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Thanks for the info. The two lenses in question are my Beck 135mm f2.8 and a SMC Takumar f3.5.

Below are the photos. I'm still not entirely sure what is the problem but as you say probably two or more combined. However, the reason I bring this up is that I recently cleaned the aperture blades because it was covered in oil and was sticking. However, I did not touch the focusing ring to get to it. However, since I am experiencing these issues wide open the cleaning of the blades should not be the issue.

Images are Wide Open, 5.6, 8, 16
Beck 2.8
http://dwo.net23.net/forum/gallery/image.php?album_id=5&image_id=98
http://dwo.net23.net/forum/gallery/image.php?album_id=5&image_id=99
http://dwo.net23.net/forum/gallery/image.php?album_id=5&image_id=100
http://dwo.net23.net/forum/gallery/image.php?album_id=5&image_id=101

Images are 16, 8, 5.6, Wide Open
SMC Takumar 3.5
http://dwo.net23.net/forum/gallery/image.php?album_id=5&image_id=102
http://dwo.net23.net/forum/gallery/image.php?album_id=5&image_id=103
http://dwo.net23.net/forum/gallery/image.php?album_id=5&image_id=104
http://dwo.net23.net/forum/gallery/image.php?album_id=5&image_id=105

All the pictures are from the camera and full resolution but only two you really need to see are 98 and 105 for the wide open shots. The others are just in case people are wondering how the Beck holds up to the Takumar. I tried my best to keep the photo in the same area but it seems like the tripod must have moved when I was changing the lenses. However, the difference of dof is very apparent.

Edit: links should be working now. Web space provider needed to review my space for illegal content.

Last edited by epqwerty; 10-23-2011 at 02:23 PM.
10-23-2011, 03:50 PM   #4
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I'm far from an expert but its hard to tell because the lenses seem to be focused at different points. With the beck, the packages of paper sticking out are more in focus but the Canadian sociology book is more out of focus. With the tak, the packages of paper are in better focus but the Canadian sociology is out of focus. Maybe someone more experienced than me can tell something from those, but if not, perhaps it would be better to use a printed target. For this perhaps a printed target with a scale shot at an angle might be more appropriate.

Maybe something like this for and angled target
Focus Testing - photo.net
If nothing else it will tell you much more accurately how your dof compares.

10-24-2011, 03:34 AM   #5
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QuoteOriginally posted by ripit Quote
I'm far from an expert but its hard to tell because the lenses seem to be focused at different points. With the beck, the packages of paper sticking out are more in focus but the Canadian sociology book is more out of focus. With the tak, the packages of paper are in better focus but the Canadian sociology is out of focus. Maybe someone more experienced than me can tell something from those, but if not, perhaps it would be better to use a printed target. For this perhaps a printed target with a scale shot at an angle might be more appropriate.

Maybe something like this for and angled target
Focus Testing - photo.net
If nothing else it will tell you much more accurately how your dof compares.
I will try this out probably by the end of the week and see the results. The chart can be printed on any normal printer? Because ive havent the time to get it done by a professional.
10-24-2011, 04:59 AM   #6
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QuoteOriginally posted by epqwerty Quote
I will try this out probably by the end of the week and see the results. The chart can be printed on any normal printer? Because ive havent the time to get it done by a professional.
Again I'm not an expert, but I use photo paper just to get a smoother print. People often suggest newsprint and such to test out lenses so I would think regular paper would work if its all you have. If it were me I wouldn't bother with a professional print, at least not yet.
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