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Thread: Lens question

  1. #1
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    Lens question

    I have a query in regards to Canon lenses (and it may be a stupid one). I know with the L series lenses (which I'm tempted to buy today) that if you take 24-105 for example on a crop frame camera it will end up being 36-whatever. However on lenses Canon has "designed" for crop sensor cameras does it take this into account? For example is the 18-55 really 18-55 or does it end up being 24-whatever even though Canon say they're designed for crop sensors?

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    Senior Member ham's Avatar
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    24-105 mm is a 24-105 mm lens no matter what camera it's on.

    If it's on a cropped camera, it's like cropping the image by a 1.6 factor, so you have a similar angle of view as you would with a 1.6x longer lens.

    If you have a cropped camera, and an 18-55 mm you have the effective angle of view of a 29-88 mm on a full frame camera.

    If you put a 24-105 mm lens on it (L, EF or EF-S makes no difference here) you have the effective angle of view of a 38-166 mm lens on a full frame camera.

    In reality, unless you have, or are used to full frame cameras, then forget those numbers, and simply work with on the number it has on the lens when choosing a lens.

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    Awesome, thanks. I was just trying to go through reviews and deciding on 17-40 or 24-105, maybe even 70-200 and they're all like "if they're on crop sensors..." then go on about changing the entire review and now the lens isn't as wide and blah blah, gets confusing.

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    Senior Member clemmb's Avatar
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    Squidy,
    Yes, the crop factor still applies. Since 35mm cameras have been around since forever it seems, what is normal is using this as the baseline. A 35mm negative is approximately 24mmx36mm in size. The cross sectional measurement is thus 43.27mm. This would make a 43.27mm lens the normal lens, however, no one make one so a 50mm is considered normal. A full frame DSLR sensor has the same basic measurement so a 50mm lens is normal. The crop sensor is smaller,approximately 15mmx22mm. Normal for this sensor is ~ 26.63mm. Comparing the two, full frame/crop, 43.27/26.63=1.62. This is where the 1.6 crop factor comes from. On the full frame 50mm gives a 1 to 1 magnification. A 100mm lens on the full frame gives a 2 times magnification. A 50mm lens on a crop sensor gives a 1.6 time magnification.
    The EF-S lenses that say they are made for digital cameras are optimized for the smaller sensor on the crop camera. If you were to mount an EF-S lens on a full frame camera you would see the image was a circle with soft edges we call vignetting. This is because the image projected by the lens is smaller than the full frame sensor. Because of this the EF-S lenses can be made smaller and at lower cost.
    Hope this helps.
    Mark

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    hee hee hee hee, everyone likes to recommend lenses

    There are a couple of ways to test the short end of the focal length to see if is wide enough for your typical subject matter - my favorite is to simply dial the zoom to 24 and then looking through view finder zoom to the widest setting, take a look a the edges, zoom back to 24 and step back until you get the same framing.... unless you hit a wall first . You can do the reverse on the long end to compare the 18-55 vs. 17-40 and the wide side of the 24-105. (I noticed you didn't mention the 17-55 2.8 which is a terrific lens for the 650)

    If you can live with 24 being the widest - then the 24-105 is a pretty darn good lens and will be useful in the FF world if you ever move to that environment and priced fairly attractively. The new 24-70 f4 should be a terrific lens - but won't be available for awhile (anyone know the guess as to when it will ship?)

    Quote Originally Posted by Squidy View Post
    Awesome, thanks. I was just trying to go through reviews and deciding on 17-40 or 24-105, maybe even 70-200 and they're all like "if they're on crop sensors..." then go on about changing the entire review and now the lens isn't as wide and blah blah, gets confusing.
    If you see me with a wrench, call 911

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    Quote Originally Posted by clemmb View Post
    The EF-S lenses that say they are made for digital cameras are optimized for the smaller sensor on the crop camera. If you were to mount an EF-S lens on a full frame camera you would see the image was a circle with soft edges we call vignetting. This is because the image projected by the lens is smaller than the full frame sensor. Because of this the EF-S lenses can be made smaller and at lower cost.
    Hope this helps.
    To be exact, the image is not smaller, it is the same size as an image of a Full Frame lens with the same focal length. But the corners are very dark and not very sharp, so the usable part of the image is smaller - which is fine, because the smaller sensor in APS-C cameras doesn't capture those outer parts of the image anyway.
    So 50mm is 50mm, no matter if EF or EF-S. So why do camera/lens manufacturer build lenses for crop cameras? Because it allows them to built smaller, lighter, and possibly less expensive cameras and lenses.

    I always found it easier to wrap my brain around these things by thinking about the angle of view, or how much of the scene will fit in the picture:

    Mount a 50mm on a full frame, shoot from your kitchen window into the backyard, and a certain part of the backyard will be captured in the picture.

    Mount a 50mm lens (EF or EF-S doesn't matter) on an APS-C camera, take the same shot, and the camera will capture a smaller part of the backyard - even if you used the very same EF lens as with the full frame. The reason is the smaller sensor, which captures only the center part of the image that is projected on the sensor plane.

    Now if you want to capture exactly the same picture as with the full frame camera, you would have to use a lens that has 50mm/1.6 = 31.25mm focal length.

    Back to your example: 24mm on full frame is considered wide. On a crop (APS-C) camera, 24mm gives you an angle of view that is the same as 24mm*1.6 = 38.4mm focal length on a full frame. That is not considered really wide. You would need a 24mm/1.6 = 15mm lens for the same wide angle of view. Again, if that is an EF or EF-S lens doesn't matter.

    Regarding image quality: Using the 17-40L on a crop camera doesn't have any advantage (image quality wise) over the EF-S 17-55mm or EF-S 15-85mm. The 17-40L is heavier and has its price tag because it needs more glass to project a decent image on a larger image circle (well, and build quality and sealing). The quality of the center part of the image (the part that is captured by the APS-C sensor) is not better than the one projected by the two EF-S lenses. But L is still nice I hear

    Edit: Re-reading this thread, it occurs to me that I did not really contribute much that others haven't said already Oh well, got carried away.
    Last edited by ahab1372; 11-15-2012 at 05:08 AM.
    Arnt

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    Quote Originally Posted by ahab1372 View Post
    Edit: Re-reading this thread, it occurs to me that I did not really contribute much that others haven't said already Oh well, got carried away.
    Welcome to the club of carriedawayness - or at least the support group that knows they have a problem.... Hello, my name is Mike and I get carried away....
    If you see me with a wrench, call 911

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    Senior Member clemmb's Avatar
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    Quote Originally Posted by Busted Knuckles View Post
    Welcome to the club of carriedawayness - or at least the support group that knows they have a problem.... Hello, my name is Mike and I get carried away....
    Agreed
    Gotta love this forum
    Mark

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    Sure do
    Arnt

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    Wait, so the EF-S lenses have the same image quality as the L series, but the L series is just better built/weather sealing etc?

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