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Thread: Lenses

  1. #1
    Site Rules Breach - Permanent Ban mandab99's Avatar
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    Lenses

    Hi
    I am super new to photography and I am trying to get my head around lenses. I am trying to understand the difference to a frame when using two different lenses but using the same focal length.

    EG - 18-55mm & 55-250mm. Same object shot at the same distance with the same focal length of 55mm.

    Would the only difference be dept of field?

    Thanks!!!!
    Manda

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    Sunrise Chaser
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    Quote Originally Posted by mandab99 View Post
    Hi
    I am super new to photography and I am trying to get my head around lenses. I am trying to understand the difference to a frame when using two different lenses but using the same focal length.

    EG - 18-55mm & 55-250mm. Same object shot at the same distance with the same focal length of 55mm.


    Manda ,
    Would the only difference be dept of field?

    Thanks!!!!
    Manda
    Manda, 55mm is 55mm, Should be no effect on Depth of Field, If it were 18mm against 250mm , Then there would be a difference
    Canon : 30D, and sometimes the 5D mkIII , Sigma 10-20, 50mm 1.8, Canon 24-105 f4 L , On loan Sigma 120-400 DG and Canon 17 - 40 f4 L , Cokin Filters




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    Member adrian078's Avatar
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    In theory if the lenses were perfectly manufactured with no optical faults and were exactly the same and you had the same aperature, the image would look the same.

    However, in practice quality and lens construction varies, even 2 lenses exactly the same model would vary(although this would be minimal).

    The other thing to keep in mind is that lenses don't perform their best at extremes e.g. for the 18-55mm lens, it might suffer from more barrel distorition or have more chromatic aberrations at 18mm. Generally you will lose some sharpness at the extremes.
    Last edited by adrian078; 18-03-2011 at 12:05pm. Reason: typo

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    Sunrise Chaser
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    Quote Originally Posted by adrian078 View Post
    In theory if the lenses were perfectly manufactured with no optical faults and were exactly the same and you had the same aperature, the image would look the same.

    However, in practice quality and lens construction varies, even 2 lenses exactly the same model would vary(although this would be minimal).

    The other thing to keep in mind is that lenses don't perform their best at extremes e.g. for the 18-55mm lens, it might suffer from more barrel distorition or have more chromatic aberrations at 18mm. Generally you will lose some sharpness at the extremes.
    I'm guessing Manda does'nt even know about barrel distortion , Let alone Chromatic aberations , This is a new to photography thread and I kept my answer to the question as simple as I could , But you are right , Still 55mm is 55mm

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    Mmmmm I think I can complicate things sometimes when it is quite simple. Thanks for your replies much appreciated and thank you Adrian for going deeper into it, it helps me to get my head arount it.

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    The Only Easy Day Was Yesterday
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    Hi Manda,
    this will give you a direct comparison of image quality of the two lenses.
    http://www.the-digital-picture.com/R...mp=0&APIComp=2

    Cheers
    John

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    Member adrian078's Avatar
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    Quote Originally Posted by Wobbles View Post
    Hi Manda,
    this will give you a direct comparison of image quality of the two lenses.
    http://www.the-digital-picture.com/R...mp=0&APIComp=2

    Cheers
    John
    Excellent website John! Haven't seen this one before.

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    Member Gas's Avatar
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    Very good site indeed! Thanks for the heads up.
    EOS 550D

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    I think Manda might be onto something that has not been fully addressed yet.
    Correct me if I'm wrong, but isn't DOF is affected by the aperture of a lens? In other words, the lower the numeric f-stop, the shallower the DOF.
    Assuming Manda is talking about EF-S lenses, using 18-55 at 55mm the maximum aperture is 5.6. If swapping to a 55-250mm, then at 55mm this lens has a maximum aperture of 4.5.

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    Great link, thanks so much John!
    Toad, I'm thinking your right.....unless anyone else has some insight to this?

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    Member nexus's Avatar
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    Quote Originally Posted by mandab99 View Post
    Great link, thanks so much John!
    Toad, I'm thinking your right.....unless anyone else has some insight to this?
    Yes toad is right if the aperture (f stop) is different, but if the f stop is the same all else being equal the image should look very similar.
    Canon EOS 50D w/ Phottix BP-40 Grip | 17-40 f/4L | 24-70 f/2.8L | 70-200mm f/2.8L IS | 430EX II | YN560 | Cactus V5 | Benro C-257EX + B2 Ballhead
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