Curious...

Discussion in 'Beyond the Basics' started by BoblyBill, Apr 2, 2008.

  1. BoblyBill

    BoblyBill TPF Noob!

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    I have four cameras now (Canon Elan IIe, Canon D30, Pentax K100D, and a Canon 40D), and have found the lens crop somewhat confusing. For that longest time I thought that I have it all figured it out, but now I'm not so sure. For excample, I have the EF-S 18-55mm for my Canon 40D that doesn't compare at all to my Sigma 17-70mm at 18mm vs 17mm. I would understand if they were just a little different (see as the one has one smaller mm to go), but if I put a .45x wide angle conversion on my 18mm it is as if it would be 18mm on the 17-70mm. With that being said (and probably in the most confusing way possible), does Sigma make lens that do the 1.6x conversion on the lens so that if I shoot with that lens on my Pentax it is actually as if I was shooting 17mm on a 35mm camera?
     
  2. doenoe

    doenoe TPF Noob!

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    ok, here is my answer.......but im not sure if im right:
    The EF-S lens is made so it doesnt have a crop factor on the crop camera's. So the 18mm isnt cropped and the 17mm is......so you get a difference.
    Or something along those lines.....just wait for the peeps who do know what they are talking about ;)
     
  3. BoblyBill

    BoblyBill TPF Noob!

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    I'm pretty sure you are right.
    Then what do you think it means that they say the lens was made for APS-C sensors?
     
  4. Digital Matt

    Digital Matt alter ego: Analog Matt

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    17mm is 17mm is 17mm, no matter what lens or mount type. The EF-S lens mount is smaller, set further back, taking advantage of the smaller imaging circle needed for the smaller sensor. The lenses are cheaper and lighter. 17mm is still 17mm however.
     
  5. Village Idiot

    Village Idiot No longer a newbie, moving up!

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    No

    If you want to shoot equivalent to 17mm focal length with a 1.5x camera, you need a 25.5mm focal length and a 1.6x needs 27.2
     
  6. Big Mike

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

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    I think V.I. has it. The 'crop factor' on the Pentax is 1.5, and the Canon is 1.6, so there is a subtle difference between the two.

    As for the lenses, I'm with Matt. 17mm is 17mm. Lenses do not have 'crop factors' and the numbers aren't compensating for anything. 17mm is 17mm.

    Some lenses (like Canon's EF-S) are just smaller and have a smaller image circle for the smaller sensors...but that doesn't change their focal length in any way.
     
  7. BoblyBill

    BoblyBill TPF Noob!

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    This totally confuses me. As I would agree a 17 is a 17, though with different cameras you get different angles of view. As in a 35mm, a 17mm would produce a much wider shot than with a DSLR. My problem comes from the fact that it seems to me, that the Sigma lens on my Pentax was made for digital cameras (if it was put on a 35mm it would be a 11.33mm) where as my 18mm on my Canon is a 18mm for a 35mm and produces a 28.8mm shot with my 40D. Does make that sense?
     
  8. BoblyBill

    BoblyBill TPF Noob!

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    You are right. Crop factor is not a good word to use for a lens. And with that, would 3mm (17*1.5=25.5 vs. 18*1.6=28.8) make that big of a difference? The difference that I'm seeing is bigger than that... It could very well be that I'm just seeing things :).
     
  9. Big Mike

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

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    The wider you are, the more of a difference will be. For example, the difference between 10mm and 12mm is 20% (this is one reason I wanted the Canon 10-22mm rather than the off brand 12-24mm lens).

    Also, remember that part of what you see, has to do with the viewfinder magnification. It's usually only the high end cameras (pro bodies) that give you a 100% view in the viewfinder. Lesser cameras are usually closer to 95%. This may not be the same for each of your different cameras.

    Also, I've heard that some lenses aren't exactly accurate with their focal length. It might say 17mm...but it might actually be 16 or 18mm. I don't know if that's true...but if it is, it might be a factor when comparing different lenses & cameras.
     
  10. Socrates

    Socrates TPF Noob!

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    Try this:
    http://support.nikontech.com/cgi-bi...TEmcF9zZWFyY2hfdGV4dD1keA**&p_li=&p_topview=1

    Note especially the wording "apparent focal length."
     
  11. BoblyBill

    BoblyBill TPF Noob!

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    It seems then... that 3mm would be that much of a difference... I'll post pictures tonight to show this. Thanks for all of your input.
     
  12. Village Idiot

    Village Idiot No longer a newbie, moving up!

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    All current DSLR lens focal lengths are based on a 35mm format. A 17mm lens designed for a crop sensor is a 17mm lens with the crap factor applied to it. A 17mm lens designed for a crop sensor will not give you a wider focal length than 17mm on a 35mm camera. If you could fit a 17mm lens on a medium format camera with a 43mm sensor, then it would. The problem is that if it's designed for a crop sensor digital camera, it's designed so the glass is use to maximum area of the sensor which also cuts down on weight and cost as stated above.

    If you try and fit a lens designed for a crop sensor camera on a 35mm camera and if actually works, you'll get a picture with black bordering it in certain areas. This is the lens tranmitting an image circle that's not large enough to cover the entire sensor.

    That's why Nikon's D3 can only shoot at 4mp with the DX lenses on it. They do not effectively cover more than 4mp worth of are on the sensor and there for, would not produce a complete picture like you would normally see with a matched lens and body.

    Trying this on a Canon with a 35mm sensor will break parts at the EF S lenses designed for their 1.6x crop sensor bodies sits further into the body where as the EF mount lenses don't sit as deeply into the bodies.
     

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