Home › Forums › Photography Q&A › Calculating distance to fill the frame
- This topic has 5 replies, 3 voices, and was last updated 3 years, 5 months ago by James Staddon.
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October 29, 2020 at 8:30 am #54834Frazer FamilyParticipant
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
October 29, 2020 at 10:56 am #54835David FrazerParticipantIn 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) = 25mmThis 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
- This reply was modified 3 years, 5 months ago by David Frazer.
October 29, 2020 at 11:20 am #54838David FrazerParticipantOctober 29, 2020 at 7:57 pm #54901Frazer FamilyParticipantThat’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.
October 31, 2020 at 1:44 pm #55016David FrazerParticipantIt’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.
October 31, 2020 at 2:34 pm #55025James StaddonKeymasterThat 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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