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  1. #1
    Markster's Avatar
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    SLR: Is what you see really what you get?

    I have a second-hand 28-70mm zoom. I'm wondering if it's really not as sharp as I thought. I can't quite tell if I'm just not focusing too crisply or what.

    My canon SLR focuses and meters at "wide open" so maybe it's something that kicks in when the iris closes or maybe I'm just imagining it.

    Granted I've only had the lens for a few rolls of film now, but either I'm flubbing the focus (a real possibility) or it's coming out a tad blurred.

    Is what you see what you get with regards to sharpness? Seems pretty sharp, crisp, and in-focus in my viewfinder. When I get the print back it lacks a bit. Just a tad soft.

    What's the best way to test sharpness? I suppose I could compare similar zoom levels with my prime lenses? I could compare 28mm, 50mm, and the 80mm on my telephoto with the 70mm on this one, but that takes a lot of time and effort.

    I'm not sorry I got the lens. It's quite a time-saver. If it's a little soft I will live with it. I paid a lower price for it than I would have if it were new. It's more than paid for itself. It would be good to know, though, if I'm going to take a nice photo such as a landscape or a portrait that this might be the second choice to reach for in my bag.
    -Markster

    Canon AE-1P 35mm | 50mm/f1.8 FDn | 28mm/2.8 FD | 70-200mm/f4-5 FD | 35-70mm/F2.8-3.5 Sigma FD

  2. #2
    holmburgers's Avatar
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    28mm at f/3.5 is going to have lots of DOF, so focusing errors might be harder to spot in the viewfinder. I'd recommend zooming in to 70mm on the subject, focusing, and then backing off.
    If you are the big tree, we are the small axe

  3. #3
    Markster's Avatar
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    Sounds quite do-able. I'll try that for a while.
    -Markster

    Canon AE-1P 35mm | 50mm/f1.8 FDn | 28mm/2.8 FD | 70-200mm/f4-5 FD | 35-70mm/F2.8-3.5 Sigma FD

  4. #4
    Paul VanAudenhove's Avatar
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    Have you examined the negatives with a loupe? Or are you judging based on prints? Sorry if that sounds obvious, but the negative is the first place to start examining for mistakes.

  5. #5

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    Quote Originally Posted by holmburgers View Post
    28mm at f/3.5 is going to have lots of DOF, so focusing errors might be harder to spot in the viewfinder. I'd recommend zooming in to 70mm on the subject, focusing, and then backing off.
    Its actually a bad idea, most still zooms do not hold focus when zooming they have a lot of breathing, there are some noted exceptions (11-16 Tokina, 70-200 Nikon) but most do not hold their focus throughout the zoom range.

  6. #6
    holmburgers's Avatar
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    Hmmm, that I didn't know. I do know that Canon recommends this practice in the manual for their 35-70mm f4.
    If you are the big tree, we are the small axe

  7. #7

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    Suprisingly the older slower zooms are much better in that regard, but most current fast zooms fall very much on the bad side of the field, ask anyone trying to shoot video with stills lenses. Its a nightmare!

  8. #8
    lns
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    A lens might be less sharp in the corners just because it's a cheaper design; most people can live with that for casual shots. If your lens is not sharp generally, though the focus is good, you have a bad lens that you probably should return.

    To test, I'd put the camera on a tripod, focus carefully and take some boring pictures of something like a newspaper page tacked to the wall, at a variety of focal lengths and apertures. Take notes to remember which is which. Look at the film (easier if you use b&w or slide film) and prints. If you want to use color negative film, I'd also take the same shots, at the same focal length and apertures, with a normal lens or two, so you can compare the prints. One roll should be enough to see if you have a bad lens

    There are some more advanced instructions on the internet, of course, but this should work as a cheap and dirty test.

    -Laura

  9. #9
    nick mulder's Avatar
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    Breathing I thought was a change in FOV as you rack focus - I didn't know it applied in the reverse sense... I would have said a 'back focus issue'.

    Quite willing to be corrected here

    ...and as an unhelpful aside, one of the peculiarities of an SLR is that you never actually see what went to film - the film sees it, aside for some top and tail getting here and there of the mirror, you see almost everything but what went to film...
    Cleared the bowel problem, working on the consonants...

  10. #10

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    What you see is what you get, unless the mirror or focussing screen is out of adjustement. If the problem only occurs with this one lens, it's the lense's fault, otherwise you should get your camera checked.

    These kit lenses are not the sharpest anyway... you could try taking a few photos of a measuring tape with the aperture wide open. That way, you'll see if the focus is just off (a different number than your set distance is perfectly sharp) or if the sharpness is generally lacking (no number appears perfectly sharp). Try this at different focal lenghts to be sure.

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