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Lens Question

Discussion in 'General Technical Discussion' started by gbenic, Jul 19, 2005.

  1. I am sorry about the silly question, but I just gotta know! If I have a 70-200/2.8 and a 70-300/4-5.6 and set the aperture to 5.6 on each lens, will they perform the same in moderate light? I guess what I am asking is if you stop a lens down, do you give up the light-gathering capablities?

  2. jfrancis


    May 8, 2005
    Orlando, FL
    They wil have the same light gathering capability at the moment of exposure, but the 70-200 will provide a brighter viewfinder image, and of course will have superior sharpness and contrast. The 70-200 will also focus faster.
  3. ckdamascus


    May 14, 2005
    New Jersey
    A side effect of faster, modern lens on modern bodies is "automatic diaphragms".

    A lens will be 'wide open' until the point of exposure, therefore you gain the benefits of faster autofocus (since the sensor gains more light), and a brighter viewfinder.
  4. Chris101


    Feb 2, 2005
    Hey Greg, they probably will not perform the same. The 70-300 will be at or near it's maximum aperture, while the 70-200 will be closed down two stops. Therefore the 70-200 will be sharper and have higher contrast.

    If you stop them down to f/11 they will probably perform very similarly. The 70-200 will always have vr, a brighter viewing image and quicker focusing 'cause of the afs. The 70-300 will always be lighter, smaller and have the added range above 200mm.
  5. Thanks for the replies. I was just using those lenses as an example. So owning and f2.8 lens is more than just the light gathering. It affects AF, sharpness, and brightness. I have so much to learn. Thanks again.
  6. Chris101


    Feb 2, 2005
    Just a bit of explanation Greg: The AF and viewing brightness ARE due to it's light gathering ability. The lens focuses wide open, so the faster the lens, the easier and faster it focuses.

    Also when you are looking through the lens to compose, you get the advantage of the faster lens's light gathering power as well - the image is brighter because you view through it wide open.

    Sharpness is NOT directly related to the aperture. However most lenses are sharper when stopped down two f-stops from thier maximum aperture. (This is a 'rule of thumb' not a 'hard and fast' law of optics.)

    Therefore, when you stop a faster lens down to match the aperture of a slower lens, the faster lens will have the exact same light gathering power as the slower lens (for exposure), but it will be stopped down so that it is in it's 'sharper zone'.

    There are, of course exceptions. Compare a Phoenix 70-210 f/4.5 to a Medical Nikkor 200mm f/5.6. Stop down the Phoenix and it still will not match the sharpness of the Nikkor. So lens design, materials and manufacture also play a (big) role in sharpness.

    Once you stop a lens down to small apertures though, say f/11 or smaller, the hole the light goes through is so small, that all lenses act about the same. Any smaller than f/11, and the blades of the aperture interact with the light and the image gets worse. That's what they call diffraction. Here is a picture of diffraction through a small, round, pinhole so you can see how diffraction can ruin a picture:

  7. Thanks Chris. The explanation confirms some of my suspicions and teaches me more.
  8. tjgreen

    tjgreen Guest

    Greg, I'm also still learning - keeps you on your toes, doesn't it?

    Just another thing to think about, that helped me decide to spend for the faster f/2.8. I want to use a tele-converter, but on the 70-300, that puts my max aperture way out. On the 2.8, if I lose a stop or two, I'm still in a very workable range. I definitely will use tele-converters, since I can't afford to buy the really long, expensive lenses.
  9. PJohnP


    Feb 5, 2005
    Greg :

    There's another whole side to "fast glass" that's not as obvious. Most lenses, making a generalisation, are not at their best wide open, and usually start to achieve their best results stopped down a stop or two (and sometimes more). If you look at Bjørn Rørslett's evaluations, for example ( http://www.naturfotograf.com/lens_surv.html ), he makes comments on better apertures for shooting effectively.

    What does this mean with respect to your question ? Well, if you buy an f/1.4 lens like the Nikon 85mm, you could be shooting with sharpness at f/2 or f/2.8 where a slower lens wouldn't even be capable, which allows you more flexibility in low light (e.g., stay at lower ISO, shoot with faster shutter speed).

    OTOH, if you do have more opportunity to shoot in bright light or not fast action imagery, you can buy "less capable" lenses (note quotation marks), stop them down, and get wonderful results. Why do that ? Well, some of these "less capable" lenses cost less (again, Bjørn discusses a number of these lenses on his site). Lower costs could mean more lenses for you !

    So this is definitely a double-edged discussion, balancing utmost capability against lower costs. The highest capablity with high costs may not (or may) be necessary for your needs. A little thinking about this could lead you into some rather interesting combinations of lenses for your kit.

    John P.
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