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Thread: Question about focal length and wide angle

  1. #1

    Question about focal length and wide angle

    I have a fundumentals question that will guide a Populist-like foamcore build I'm working on. I want to build for "wide angle" on 35mm film and am questioning my design logic.

    My Altoid-tin pinhole camera is super shallow (19mm). With 35mm film, images are wide angle and the frames span about 60mm -- the width of the tin. I don't know how to calculate the actual angle of view (i.e., no sight-lines on the camera), but I know it's pretty wide. Is there a way to compute the effecitve 35mm focal length? i.e., it's not as simple as comparing to a 19mm lens on an SLR, is it?

    Now I love my Populist, but I want to get wider. More like the Altoids tin. So I figure I'll decrease the film/pinhole distance. I measure the Populist distance as 25mm. But hey, that should already be pretty durn wide angle, shouldn't it? Shots look kindof "normal". Anyway, I figured I'd indent the front of the camera to get the pinhole closer to the film plane. Should I consder widening the masking to get more image? How will that help with wide-angle? I haven't measured, but the framing is masked to approximate a standard 35mm frame regardless of the projected image circle. I can widen the mask like a Panoramic Populist...

    Do I have this right, or am I missing something critical?

    Thanks in advance.

  2. #2

    Question about focal length and wide angle

    There are no rules, actually, pinhole cam. design is absolute free will and imagination...

    Best is to make various drawings of top view and figure what you want ? taking angle ? film length ? etc. everything is possible

    Paul

  3. #3

    Question about focal length and wide angle

    You can do either some basic trig or direct measurement to find the angle of view for a pinhole camera. For example, I have a camera that uses a 6x6 section of 120 film and has a focal length of about 25mm. Using a trig calculator like this: http://www.carbidedepot.com/formulas-trigright.asp

    I put in a=25 and b=30 and find that half my angle is 50.2 degrees, so my horizontal field of view is 100.4 degrees. Using the table in this article, that's equivalent to a 35mm lens with a 15mm focal length http://en.wikipedia.org/wiki/Angle_of_view

    Note that my vertical field of view is also 100.4 degrees, which is off that chart for 35mm field of view. Similarly, my diagonal field of view is 159 degrees, which is also off of that chart. I did the math once and I think the diagonal field of view of my camera is equivalent to 10.5 degree focal length on 35mm. Of course, comparing 120 film to 35mm isn't apples to apples because the film is so much bigger.

  4. #4

    Question about focal length and wide angle

    Ah. This is exactly what I needed. I read that Wikipedia article before I posted to re-ground myself. I'm assuming that b=30mm is half the frame width (i.e., your image frame is 60mm wide)? It looks like 120 is usually a framing of 56mm.

    I'll use those calculations to play around with possibilities... if I decrease the horizontal framing, I see how my Angle of View plunges. So I can vary pinhole distance and the width of my frame to hone in on the wide-angle effect I want.

    Played around with the Populist: pinhole-to-film distance = ~25mm. Framewidth = ~40mm. So AOV calculates to 77-degrees which is maybe a 23mm focal length 35mm equivalent. That don't seem right... but maybe it is...

    polka: I know. I get that. I love the flexibility and creativity in pinholeland. But I don't want to go build something and then discover that "hey, this thing isn't doing what I expected!". I may as well do some planning before I start cutting and gluing.

    Thanks guys...

  5. #5

    Question about focal length and wide angle

    Yes, my film frame is 60mmx60mm, which is why I used 30mm for that side of the triangle. Here's a basic drawing for my camera.

    Attached files

  6. #6

    Question about focal length and wide angle

    Isn't 30mm too small?

    If you draw a circle with radius 30mm on a 60x60mm square, you "miss" the four corners. The diameter of the circle covering the whole square would be square root of (60^2 + 60^2) which is about 85. So then you should use 42 in the formula.

    -peter

    ps: of course, as Polka already mentioned, there are no rules

  7. #7

    Question about focal length and wide angle

    Quote Originally Posted by 2033
    Isn't 30mm too small?

    If you draw a circle with radius 30mm on a 60x60mm square, you "miss" the four corners. The diameter of the circle covering the whole square would be square root of (60^2 + 60^2) which is about 85. So then you should use 42 in the formula.
    Yes, as I mentioned in my first post, using 30mm provides the horizontal field of view and the diagonal field of view is greater. For the purpose of judging 35mm focal length equivalents, it doesn't matter as long as you compare to the proper item in the Wikipedia chart.

    Personally, I don't visualize framing in terms of diagonal field of view, so I usually don't use it. However, if a single number is given for field of view, it is usually the diagonal one.

  8. #8
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    Question about focal length and wide angle

    Quote Originally Posted by pmc
    Played around with the Populist: pinhole-to-film distance = ~25mm. Framewidth = ~40mm. So AOV calculates to 77-degrees which is maybe a 23mm focal length 35mm equivalent. That don't seem right... but maybe it is...
    If you want to use the frame numbers on the backing paper for film advance, then I suggest a width that is a multiple of 4.5, 6 , or 9 cm. That way you can put a red window on the back of your camera and you will not have to guess or count turns.

  9. #9

    Question about focal length and wide angle

    I found this (paraphrased from here):
    To get a "normal" looking image, measure your film/paper diagonally, then build a camera with a pinhole that is that same distance away from the film or paper. Thus, to produce a "normal" 4x5 in. pinhole image, build a camera that is 6 in. deep (the diagonal measurement of the 4x5 in. film).
    35mm film frames = 36x24 so the diagonal is 43.27mm. I'd want that depth for a normal image. That compares to ~50mm lens equivalent on the table here. For what it's worth, it turns out my Populist isn't much deeper than my altoids tin. So I'm going to try the same depth but use a wider image mask (more like a Panoramic Populist).

    I also noted this:
    Image diameter is approximately 3-1⁄2 times the size of your camera’s focal length. Thus, a 2 in. deep camera would give you approximately 7 in. coverage (7 in. diameter circular image).

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