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Thread: Crop factor and extension tube

  1. #1
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    Default Crop factor and extension tube

    Please allow me to a ask a newbie question: By adding an extension tube between the lens and the body, in effect it is pushing the lens further away from the sensor plane, will that alter the crop factor?

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    Default Re: Crop factor and extension tube

    Crop factor is the ratio between the CCD/CMOS sensor size and the 35mm film size. That ratio is fixed for a particular camera design and is not affected by the lens.
    As complexity rises, precise statements lose meaning and meaningful statements lose precision.

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    Default Re: Crop factor and extension tube

    Thanks roygoh for explaining.

    LEt me ask the question in a different way:

    1. The smaller sensor of DSLR (less than FF) alone does not explain the crop factor. P&S has even smaller sensor, but crop factor does not apply. However, most would try to convert the focal length of a P&S to 35mm equivalent. NOT to confuse with crop factor. The question why crop factor apply to DSLR and not P&S?

    2. I believe DSLR uses normal 35mm equivalent lens on a smaller than 35mm sensor. If the image plane falls on a smaller sensor, the sensor captures only the center portion (or missing out the corners and edges) of the image, thus appears like an amplification or crop. Hope I explain it right.

    3. Now the question on the image falling on the sensor, if the lens is moved further away from the sensor, will that increase the image plane area, therefore increases the 'crop' factor?

  4. #4

    Default Re: Crop factor and extension tube

    Quote Originally Posted by L.Lee
    Thanks roygoh for explaining.

    LEt me ask the question in a different way:

    1. The smaller sensor of DSLR (less than FF) alone does not explain the crop factor. P&S has even smaller sensor, but crop factor does not apply. However, most would try to convert the focal length of a P&S to 35mm equivalent. NOT to confuse with crop factor. The question why crop factor apply to DSLR and not P&S?

    2. I believe DSLR uses normal 35mm equivalent lens on a smaller than 35mm sensor. If the image plane falls on a smaller sensor, the sensor captures only the center portion (or missing out the corners and edges) of the image, thus appears like an amplification or crop. Hope I explain it right.

    3. Now the question on the image falling on the sensor, if the lens is moved further away from the sensor, will that increase the image plane area, therefore increases the 'crop' factor?
    1. Crop factor applies to (non full frame sensor) DSLR as well as Point and Shoot (which have very much smaller sensors). The difference between actual focal length and its 35mm equivalent focal length gives an indication of the magnitude of the crop factor because the 35mm equivalent focal length is a conversion assuming that the sensor is a full frame. As long as the sensor is not full frame (35mm format), there is a crop factor because this crop factor refers to the sensor size relative to the 35mm format. We seldom read about the crop factor for Point and Shoot camera doesn't mean that there is no crop factor.

    2. I think it's correct.

    3. No. The crop factor relates to the sensor size relative to the full frame 35mm format only. The varying of the image size doesn't affect this relative size. E.g. the crop factor remains at 1.6 for Canon 350D regardless of the focal length used (e.g. whether it's 50mm, 150mm, 300 mm or any focal length). Basically the crop factor is how much the image appears bigger in the frame than the same image on a 35mm full frame format.

    Here's an example :
    Diagonal mm of the frames :
    Full frame : about 43mm
    Canon 350D : about 27mm (i.e. about 1/1.6 of full frame)
    Nikon Coolpix 5700 : about 11mm (i.e. about 1/4 of full frame)

    Assume now that : For a picture of the full moon against a pitch dark sky, on a 35mm full frame, the moon occupies 1/4 of the whole frame (i.e. the full moon image is about 11mm in diameter).

    On the Canon 350D (which has a sensor about 1/1.6 the size of a full frame) , the moon would occupy 1/4 * 1.6 = 40% of the frame. (i.e. 11mm moon on a 27mm diagonal frame = 11/27 = about 40%)

    On a Nikon Coolpix 5700 (which has a sensor about 1/4 the size of a full frame), the moon would occupy 1/4 * 4 = the whole frame. (i.e. 11mm moon on a 11mm diagonal frame = 100%)

    If you zoom out so that the moon becomes half the previous size and now occupies only 1/8 of the full frame (i.e. now moon image is 5.5mm in diameter), the actual size of the moon image will be reduced by the same amount (1/2) on the 350D and 5700 in absolute size as well as relative to their sensor frame so that the moon is now 1/8 x 1.6 = 20% on the 350D frame and 1/8 x 4 = 50% on the 5700 frame.

    In other words,
    5.5mm moon on a 27mm daigonal frame =20%
    5.5mm moon on a 11mm diagonal frame = 50%
    Last edited by Clockunder; 21st January 2006 at 09:15 PM.

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    Default Re: Crop factor and extension tube

    Clockunder, that is a very impressive explanation and I really learnt something on crop factor. thanks for taking the time

    Now I would like to explore further concerning extension tube by asking these questions:

    1. What effect extension tubes has in macro photography?
    2. For the purpose of reduce focusing distance?
    3. The tube pushes the lens further away from the sensor, technically what happen here?

    Thanks,

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