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Thread: 7 lines per mm?

  1. #1
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    7 lines per mm?

    Is it possible to make a pinhole camera which will give you the magical figure of 7 lines per mm needed for a sharp print?

    I know that ive been discussing various aspects of this in other threads, anyone not bored with the topic of resolution optimisation (which is a side issue as the many beautiful images posted on this site testifies) please pass on to more fun questions... thank you!

  2. #2
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    7 lines per mm?

    you know what i ment, right

  3. #3
    500+ Posts earlj's Avatar
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    7 lines per mm?

    mesmer:

    Pinhole photography does not lend itself to absolute measurements of resolution (e.g. lines / mm). The smallest granule of detail on the film image is limited by the diameter of the pinhole - there cannot be detail in the image smaller than the hole. Where pinhole image resolution is impressive is in relative resolution, that is the amount of information per sqare inch of film area. Since the optimal pinhole size, as limited by diffraction, increases proportionally less than the area illuminated as the pinhole to image distance increases, the relative resolution goes up as the distance and image size goes up. In order to get 7 lines per mm, you would have to have a pinhole .143 mm or less, and that limits your pinhole to film distance to just over 10 mm, which results in a pretty small image. But a pinhole photo on 8X10 at say, 75 to 150 mm distance, with an optimal pinhole has many times the information in the image than the small image, which at an appropriate viewing distance, make it look sharp to the eye.

  4. #4
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    7 lines per mm?

    Earl,

    thanks for your infomation. basically ive been having a good natured debate with a friend who shoots 8x10 lens photography. i shoot 8x10 with a pinhole and we both contact print. we were interested if one of my contact prints could theoretically approach one of his contact prints for resolution and sharpness such that an objective viewer would not be able to tell the two prints apart. i think it would be possible at 20x24 but im not sure if its possible at 8x10. not that i think that pinhole is in any way inferior; what my friend would give for my DOF!

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    500+ Posts earlj's Avatar
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    7 lines per mm?

    The absolute shaprness of an 8X10 pinhole photograph can never be compared to an 8X10 negative made with a lens. It can be compared to an 8X10 enlargement made from a much smaller negative, though. Pinhole photographs are not sharp in the absolute sense - they are just sharp enough too look great at a normal viewing distance. And as you say, the sharpness of the parts of the pinhole image that are outside of the depth of focus of the lens are more than comparable.

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    500+ Posts Daryl's Avatar
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    7 lines per mm?

    Hmmm.
    I thought lenses were diffraction limited after stopping down.
    Formula was something like 1600/(f/stop).
    So at f/64 the best you could hope for would be 1600/64 or 25 lines.

    This formula would yield 7 lines at f/228 - which is definately pinhole territory.
    At some point does the glass become useless?
    If the glass ever becomes an impediment - then that would mean that there is a region where
    pinhole would outperform glass.

    I was looking at http://nvl.nist.gov/pub/nistpubs/jres/104/5/j45mie.pdf and promptly got a headache.

  7. #7

    7 lines per mm?

    Hmm. Well, oversimplifying (by ignoring diffraction), you'd need a hole of around .07 mm diameter to give 7 lp/mm resolution. That's optimum for about a 3 mm focal length, suggesting there'd be problems with image circle size...

  8. #8

    7 lines per mm?

    Uh, yup. That's the nut I'm trying to crack but it's so @*#@ abstract it's hard to make enough sense out of.

    What I got out of this article is that it's hard to distinguish between the various subtle choices of 'optimal' constants (Rayleigh vs. others) because it's so subjective, but that it IS definitely in one's interest to make an effort to be in the 'ballpark'. But you already knew that before trying to read this article.

    I found another article that said contrast is a factor and some images look sharper with an unexpected diameter than predicted...but he used computer simulations instead of real images...I don't know if that's right.

    I didn't know one could get 7 lp/mm with pinhole. I have been looking for articles that discuss this quantitatively, and thus far I didn't think I had seen anything referenced higher than 2.5-3 lp/mm.

    I wondered if reducing an 8x10 to 4x5 does this because we see some tricky images posted that look TOO sharp to be credible pinhole images sometimes

    I think that diffraction limiting with too-small an aperture and glass is still a different animal than a pinhole because I think (my opinion) the pinhole can only produce a remotely acceptable resolution when it is close to it's optimal circumstances...diameter, spacing , wavelength.

    A lens is still doing some refracting but has diffractive problems at too small an aperture. I don't know if diffraction limiting is one of the evil 7 aberrations. Smaller apertures correct a number of those aberrations independently (mathwise).

    What I'm still working is understanding how image formation shares similarities in both pinhole and glass...both are described (apparently) by the same 2-D Fourier Transform model. It seems that pinhole can only produce a decent image where this model is optimized, yet the 2DFT is relevant (that's all I understand so far) for image formation with large apertures in glass far from the diffraction limit.

    If I get that part, I have my math class paper started.

    I think the choice if 1600 or whatever number one chooses/f-stop has alot of assumptions and qualifiers to go with it. I don't know how far one can run with it, and I have to keep re-reading why format plays a role in determining where diffraction limit is...if it's based on the sliding circle of confusion scale, that may be why it's so slippery....it's based on an assumption of acceptance and maybe viewing distance...I'm not sure

  9. #9
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    7 lines per mm?

    Hi, the math is certainly confusing however there are definately some pinhole photographers that seem to atchive sharp(ish) prints without cheating eg. dianne boss and martha casanave...

    eg. one of dianne boss's recent ones... Attached files

  10. #10
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    7 lines per mm?

    ...and one from martha casanave, beautiful photographs im sure you will all agree... just wish i knew how they do it!

    Attached files

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