Calculating distance to fill the frame

Home Forums Photography Q&A Calculating distance to fill the frame

Tagged: ,

Viewing 6 posts - 1 through 6 (of 6 total)
  • Author
  • #54834
    Frazer Family

    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?


    David Frazer

    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.

    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.


    David Frazer

    So by extension…

    Frazer Family

    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.

    David Frazer

    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.

    James Staddon

    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). 🙂

Viewing 6 posts - 1 through 6 (of 6 total)

You must be logged in to reply to this topic.

Would you like to participate?

Create an Account!

Pin It on Pinterest