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Thread: About Focal Lengths, magnification and Field of View

  1. #1

    Default About Focal Lengths, magnification and Field of View

    Anybody have good resources to read up on how all these affect digital SLRs?

    I thought I got it already. But the more articles I read around. The more confused I am.

    First I thought the 1.5x crop factor in DLSR on your yesteryear lenses gives you a 1.5x magnification as well. E.g. 100mm becomes a 150mm lens. But after reading an article by Thom Hogan (could be somebody else out there.) I was told there is no magnification at all. You just get the Field of View of 150mm from a 100mm lens. E.g. If you shoot a pic in Film scanned it at 6mp. Then shoot another pic in digital 6 mp. Your digital is bascially a crop of the film version.

    Then again today. I was reading Focus mazagine while attending to some business. The article regarding the AF-S 300mm VR lens states clearly on a digital body, the focal length will be increased to 1.5x and the Field of View is reduced (not in the exact same words.)

    Am I missing a dimension here?

  2. #2

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    First, you'll need to understand what is "crop factor". It has to do with the size of the standard 35mm frame relative to the digital camera sensor. Pls see:
    http://www.dpreview.com/learn/?/Glos...r_Sizes_01.htm

    Focal length is focal length and its determined by the optics in the lens. This WILL NOT change simply because the sensor size has changed. The effect of the change in sensor size merely gives it an effect that is EQUIVALENT to a change in focal length.

    This article explains it very well:
    http://luminous-landscape.com/tutori...dslr-mag.shtml
    Last edited by jumbocrab; 24th July 2004 at 10:30 PM.

  3. #3

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    Thats right, the focal length does NOT change with DSLR's, only the FoV. Magnification is still the same on a 300mm lens regardless of whether it is mounted on a 35mm camera or a DSLR, only the field of view is changed, i.e. how much of the image can be seen.

    Nikon themselves perpetuate this myth. From its FOCUS magazine, an extract of a sentence on its AF-S VR ikkor 200mm f/2G lens: "When used with Nikon D-SLR cameras, the focal length of the AF-S VR Nikkor 200mm f/2G IF ED increases by a factor of 1.5. Similarly, picture angle also depends on whether the AF-S VR Nikkor 200mm f/2G IF ED is used with a 35mm film camera or a Nikon D-SLR.."

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    just imagine that the smaller sensor size, is MUCH smaller than the actual 35mm film you're using, then you'll get the idea

    the lens still remain as they are, but the Circle of Confusion may be affected due to the smaller sensor

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    You unconfused first part
    But then added another confusion in the latter part.

    I know you know the answer but poster is still as confused.


    Quote Originally Posted by sehsuan
    just imagine that the smaller sensor size, is MUCH smaller than the actual 35mm film you're using, then you'll get the idea

    the lens still remain as they are, but the Circle of Confusion may be affected due to the smaller sensor

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    let me try again.

    first part:
    the sensor (CCD/CMOS) in the camera is much smaller than the 35mm film it was meant to expose, hence there is a lot that gets chopped off the image. the resulting image, seems to be shot with a "longer" lens, but the lens still remains unchanged, because it's still the same glass etc... your perspective still remains as per the physical lens focal length, e.g. 70 to 200mm etc, but the Angle of View (or Field of View) gets affected by the smaller sensor.

    second part:
    actually, don't trust magazines 101%. writers still can make mistakes, you know? besides, how else to explain this concept to everyone, is an uphill task, because not everyone's a technically inclined individual... essentially, the 300mm lens remains a 300mm lens, but the Angle of View is changed to that of a 450mm lens, but the perspective remains the same that of a 300mm lens. hope you're not further confused!

  7. #7

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    Quote Originally Posted by sehsuan
    let me try again.

    first part:
    the sensor (CCD/CMOS) in the camera is much smaller than the 35mm film it was meant to expose, hence there is a lot that gets chopped off the image. the resulting image, seems to be shot with a "longer" lens, but the lens still remains unchanged, because it's still the same glass etc... your perspective still remains as per the physical lens focal length, e.g. 70 to 200mm etc, but the Angle of View (or Field of View) gets affected by the smaller sensor.

    second part:
    actually, don't trust magazines 101%. writers still can make mistakes, you know? besides, how else to explain this concept to everyone, is an uphill task, because not everyone's a technically inclined individual... essentially, the 300mm lens remains a 300mm lens, but the Angle of View is changed to that of a 450mm lens, but the perspective remains the same that of a 300mm lens. hope you're not further confused!
    How about the DOF, are they affected as well?

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    Quote Originally Posted by Brandon
    How about the DOF, are they affected as well?
    optically speaking, the DOF is not affected - but since the Angle of View is so much reduced, you'd have to use shorter focal length to frame up a picture the same way, so you'd end up with apparently less more DOF. the lens does not change, but it affects the way you do your framing, so hence it seems to make the lens have a deeper DOF than it would on a full frame camera. hope this helps!

  9. #9

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    Thanks guys. It's clear as distilled water now.

    Maybe I should write to Nikon for causing me so much confusion? It looks like Nikon has been releasing press releases with the wrong claims. How can a magazine from a major lens maker contain such a big error?

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    well, it is an easier way to describe, isn't it?

    i doubt the article was written by nikon though.

  11. #11

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    Quote Originally Posted by sehsuan
    optically speaking, the DOF is not affected - but since the Angle of View is so much reduced, you'd have to use shorter focal length to frame up a picture the same way, so you'd end up with apparently less more DOF. the lens does not change, but it affects the way you do your framing, so hence it seems to make the lens have a deeper DOF than it would on a full frame camera. hope this helps!
    Thanks a lot

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    OMG! why did i write, "less more DOF"?

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