Calculating distance to fill the frame

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  • #54834
    Frazer Family
    Participant

    Let’s say I have a chalk board that is 3 feet wide by 2 feet tall. How far away do I have to stand with my crop-frame camera and a 70 mm lens in order to fill my frame?
    What about if I have a 200 mm lens? Or a 35 mm lens?

    Or I could flip the problem upside down and ask: what if I change the chalk board to be 6 feet wide by 4 feet tall.

    I assume there must be a formula to calculate this, but I haven’t managed to find it. Does anyone have the answer?

    Evelyn

    #54835
    David Frazer
    Participant

    In the field I find a trick like this is much quicker, though not perfectly acurate: hold out your hand at arm’s length.
    – 1 finger width = 800mm
    – fist width = 120mm
    – index to pinkie streched out = 80mm
    – span (thumb to pinkie stretched out) = 50mm
    – two spans (same as above but both hands, pinkie to thum) = 25mm

    This website makes it really easy to get the exact number, assuming you are using only rectilinear lenses. (so no fisheyes) It also calculates depth of field. Note: the site contains ads and I have not looked at the other pages on the site. https://www.pointsinfocus.com/tools/depth-of-field-and-equivalent-lens-calculator/

    For those interested in the mathematical formula, it’s simple trigonometry. You do have to use a different formula depending on whether or not your lens extends when changing focal distance. https://www.pointsinfocus.com/tools/depth-of-field-and-equivalent-lens-calculator/#angle-and-field-of-view

    formula

    Attachments:
    #54838
    David Frazer
    Participant

    So by extension…
    formula2

    Attachments:
    #54901
    Frazer Family
    Participant

    That’s great @dfrazer. I love how you said “simple trigonometry”. I have forgotten most of the trigonometry I ever learned, but I would have fun studying out the formulas if I had the time. I think that realistically I would use your field trick. I assume that it is for crop frame?

    Another thing: I noticed in the link that you sent for the depth-of-field-and-equivalent-lens-calculator, that “crop-frame” is not an option under the sensor size. Which of those options would most closely resemble my Nikon D3100?

    Thank you for your help.

    #55016
    David Frazer
    Participant

    It’s probably best to check the trick out for yourself, as different people have different sizes of arms and hands …

    You can select your camera type from the drop-down on that site. So a Nikon crop-frame camera is listed as Nikon DX/APS-C; Canon crop-frame would be Canon APS-C.

    #55025
    James Staddon
    Keymaster

    That is so fascinating, @dfrazer! Thanks for sharing! I’m going to have to find out what the “in the field trick” is for me, with my size hand(s). 🙂

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