New Lens Tests

Discussion in 'Photography Beginners' Forum' started by Innocence, Oct 3, 2006.

  1. Innocence

    Innocence TPF Noob!

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    I've been doing a whole lot of reading and it seems that when people buy lenses, some get "bad" stock? Or...something, and they return it and get a "better" copy..haha I don't understand this.

    How do we super newbie amateurs tell if we have a "dud"? Are there a set of tests we're to do when we purchase lenses? Like, I'm sure I couldn't tell, unless each and every one of my shots had some terrific vignetting or CA haha.

    What are these "duds" anyway? Is it just the review person making stuff up or do manufacturers regularly output bad ones?

    Thanks!! I'm learning lots from you all!! :hug::
     
  2. Digital Matt

    Digital Matt alter ego: Analog Matt

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    Some lenses make it through quality control, even though they do not perform optically as well as they should. Some tests you can do are, take a photo of some small text, like a book. Shoot wide open, and at each aperture. View the photo at 100% and inspect the sharpness. It all depends on the quality of the lens you bought in the first place. If you are buying top quality Canon or Nikon glass, and it is visibly not sharp wide open, especially when compared with sample images on the net, then you probably have a bad copy.

    If you bought a cheap lens, it's probably not going to be very sharp wide open anyway. If it doesn't get reasonably sharp by f/8 or f/11, then something is wrong also.
     
  3. Big Mike

    Big Mike I am Big, I am Mike Staff Member Supporting Member

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    As far as I can tell...

    Lenses are very precise...even the slightest misalignment can cause some of the common problems like mis-focus, excessive chromatic aberration etc.

    Lenses are manufactured in a factory, by the thousands. There is a quality control process...but they obviously don't catch every thing. The level of quality control is probably different for each lens, company or person doing the job. Some lens designs are probably more susceptible to problems than others. There are lots of factors.

    There are tests that you can do yourself. If you find that you get a bad copy...you can almost always return it to your place of purchase or return it to the manufacturer. They should either fix the problem or replace the lens itself.

    This is one reason why I like the idea of buying from a local store that I can walk into. If there is a problem, I can actually talk to someone face to face.
     
  4. Christie Photo

    Christie Photo No longer a newbie, moving up!

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    Say, Matt.... YEARS back I heard at a Mamiya presentation (I think it was Dean Collins) that each type of lens performs at it's optimum at a certain aperture. Say... their 80mm for the RB was supposed to perform best at f8, sharper than at f16. Do you know anything about this?

    Pete
     
  5. Digital Matt

    Digital Matt alter ego: Analog Matt

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    From what I've read, common problems like barrel/pin cushion distortion, CA, light falloff, and poor sharpness start to decrease towards f/8-f/11 on most lenses, and then reappaer as you get closer to f/22. It's usually 3-4 stops down from wide open. I'm not sure of the exact scientific reason.

    I see a huge increase in sharpness with my Tamron 17-35, when shot at 17mm and f/8, vs 17mm and f/2.8. At f/8, it's sharp from corner to corner, where at 2.8, it is soft at the edges. The Canon 50mm f/1.8 howerver, is pretty sharp wide open, and gets noticeably sharper by f/5.6 already, because the maximum aperture is larger.
     
  6. Big Mike

    Big Mike I am Big, I am Mike Staff Member Supporting Member

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    I've always read, and found myself, that lenses really get better once you stop down at least a stop or two from wide open. My 50mm is certainly better at F2.8 than it is at F1.8.

    The last course I took, the instructor said that F8 is the sweet spot for most lenses. Sounds believable enough for me. When using my kit lens (18-55) I often keep it at F8 and get pretty good results.
     
  7. Philip Weir

    Philip Weir TPF Noob!

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    There is a general optical principle, and that is "a lens is the best at 2-3 stops down from wide open" Doesn't matter what the widest aperture is.

    www,philipweirphotography.com
     
  8. Johnboy2978

    Johnboy2978 No longer a newbie, moving up!

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    So in doing a test like shooting a book, at what distance should you place it in looking at a 70-200mm lens for example?
     
  9. Digital Matt

    Digital Matt alter ego: Analog Matt

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    Distance is unimportant, as long as the text will be readable at 100%. Check all focal lengths of a zoom lens, as there will likely be differences.
     
  10. fmw

    fmw No longer a newbie, moving up!

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    Innocense, the front lens element is always convex. As you get close to a subject, the focus from the center of the image to the corners can be different.

    Tape some newsprint on the wall and set your camera up on a tripod making sure the front element of the lens is parallel to the subject. As was mentioned above, shoot the newspaper at different apertures. You can see differences in sharpness between the center and the corners by reading the text and judging the sharpness of the characters. You will notice that, wide open, the lens will probably show some softness in the corners which will improve as you stop down.

    I've made the same tests with the bricks on the side of my house.

    Some lenses are designed to be used wide open. I had a 500mm F2.8 Nikkor at one time that was designed in this manner. It was sharpest wide open and got softer in the corners as you stopped down. It was designed to allow the user to shoot distant subjects at high shutter speeds for sports, wildlife etc. This is the exception, though. Most lenses show their best sharpness across the image a couple of stops closed down from wide open.
     
  11. Innocence

    Innocence TPF Noob!

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    Alright! Thanks everyone! Advice noted.

    The retailer will understand if after a day of purchase, I run in and tell them my lens is bad? :)
     
  12. Big Mike

    Big Mike I am Big, I am Mike Staff Member Supporting Member

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    The better retailers will have a return policy, yes.
     

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