A&P Please forgive the noob "zoom" question about lenses

Discussion in 'Lifestyle' started by YourMomOnRyeBun, Jan 12, 2010.

  1. YourMomOnRyeBun

    YourMomOnRyeBun New Member

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    I'm learning...please be patient.

    Let's say you have these two lenses to work with and for sake of conversation say they are both of the same "quality".

    18-70mm
    70-300mm

    You find that the object you want to photograph looks best at 70mm, so essentially you could use either lens. The object would be moving in less than ideal light.

    Which lens would be better to use? Why?

    I'm trying to understand how the lens works. I know the amount of light that reaches the sensor is key, especially when you are limited with the light you have to work with.

    When a lens is all the way "zoomed in" or "zoomed out" will this change the light that enters the lens if they are both at 70mm?
     
  2. Cesium

    Cesium OT Supporter

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    Too many other variables. Can you tell us the actual lenses in question?

    Focus speed and available aperture will be big factors for fast action.
     
  3. YourMomOnRyeBun

    YourMomOnRyeBun New Member

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    I'm trying to understand how the lenses work, not actually chose between two lenses to buy. So lets assume the available apperature settings are the same.

    Same brand
    Same quality glass
    Same focus speed
     
  4. White Stormy

    White Stormy Take that, subspace!

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    most cheaper lenses have a variable aperture. the maximum aperture would be different at 18mm than it would be at 70mm. a smaller aperture (higher f number) means less light is getting in, so it would likely be slower than if it had a larger aperture (lower f number). the variable aperture is one reason why he was asking for specifics.

    if image quality was EXACTLY the same, and they both had the same constant aperture, and neither had any distortion or anything, you should get the same photo at 70mm regardless of which lens you're using.

    The whole reason you might get one of them over the other strictly for use at 70mm, though, is that image quality and performance between lenses is NOT exactly the same. Some are sharper and perform better at their minimum focal length, some at their max, some don't perform well at min or max, etc.
     
  5. YourMomOnRyeBun

    YourMomOnRyeBun New Member

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    So in general you CANNOT say that a lenses function better at max or min focal length? It depends on that particular model?

    I guess that answers my question, sort of. In general, you can't make the generalization. lol
     
  6. tetsuo

    tetsuo And shepherds we shall be...

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    another thing that you would want to consider is which way you would possibly need to go. if you 70 is the max that would work for the situation, use the 18-70, if 70 is the minimum that you might need, use the 70-300
     
  7. BeachBoy

    BeachBoy New Member

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    I still don't see an answer to the question

    18-70 f2.8
    70-200 f2.8

    both have the same aperture.

    at 70mm, at the same aperture (anywhere from 2.8), would there be a difference in the pictures.

    would a DOF be different shooting 18-70 at 70mm f2.8 than 70-200 at 70mm f2.8? Would something else be different (I guess sharpness), would the boken be different (I guess yes), etc?
     
  8. Cesium

    Cesium OT Supporter

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    All other variables the same, you'll get the same image. In the real world, however, you'll likely see barrel distortion at the wide end of a zoom lens and pincushion distortion on the long end. Focus speed will vary as well. Also keep in mind that focal length changes slightly with subject distance. A 70 mm lens is usually only true 70 mm at infinity focus. And as already stated, each lens has it's sweet spots for performance. Usually a zoom is sharpest in the middle of its range.
     
  9. YourMomOnRyeBun

    YourMomOnRyeBun New Member

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    These are the things I need to know...thanks for all the info. Like I said, I am trying to learn...
     
  10. tenplanescrashing

    tenplanescrashing Active Member

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    70mm, or any other is not the same from lens to lens. 70mm on a 18-70 will look *slightly* different than 70mm on a 70mm, and it is because of the glass elements in the lens.

    However, the differences can and may be minimal so it isn't something to beat yourself up over. I like a little bit of overlap in my lenses (12-24, 17-50, etc.) and I can definitely tell a difference in the 17-24mm range between my two wider lenses.

    In reference to the aperture setting, if the 18-70 is f/3.5-4.5, the max aperture is 4.5 at 70mm. If the 70-300 is f/4-5.6, the max aperture at 70mm is f/4.
     

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