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  1. #1

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    Sharpness and wavelength

    I have been making phole images for a few years, and have several home-made boxes. I projected all the pinholes on the wall, got an exact measurement of size, did the math to figure out the optimum "focal length," and inspected the pinholes under a microscope for any defects.

    I'm happy with the images, yet they don't appear as sharp as some others I see.

    Talking physics with the neighbor, he said that I would get a sharper image if I filtered out one of the primary colors. He said since I have all these different wavelengths squeezing through the pinhole and overlapping, if some were eliminated the result would be a sharper image.

    Well, I do that already. I shoot mostly landscape and cloud movement, so I have R25 filters on everything.

    But. . .since I'm filtering the shorter wavelength part of the spectrum (blue), and allowing the longer wavelengths (red) to pass, is this the cause of a less-than-sharp image? I guess that is my question: when using a pinhole aperture, will shorter wavelengths produce sharper images than longer ones?
    If so, I need to start culling 58mm polarizers from the junk sales.

    Love to hear some ideas on this.

  2. #2
    Ole
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    The pinhole should be optimised for the wavelength you want to use; there is no physical law that says that one is inherently sharper than the other.

    Well - when you get right down to the microscopic scale, shorter wavelengths will produce a sharper image than longer ones - or at least allow greater magnification. That's why they make electron microscopes. But that's totally irrelevant to pinholes.
    -- Ole Tjugen, Luddite Elitist
    Norway

  3. #3

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    Perhaps by filtering the shorter wavelengths, I need to recalibrate the optimum distance of pinhole to film plane? This would be much easier for me, since the film holders are adjustable. Remanufacturing and measuring a new set of pinholes would not be fun for me.

    I think the formulas I used assumed a wavelength in the middle of the visible spectrum. Maybe I need to recalculate, shift that number to the longer end.

  4. #4

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    Pinhole Designer has a box to change the wavelength of the light, it default to a middle of visible spectrum. I tried a few different filters, and didn't see much difference when I was playing around. Though if you want the sharpest, you might want to calculate for blue light, and use a strong blue filter. Also put a UV blocking filter on it. Many people suggest a user constant in Pinhole Designer of 1.5 to 1.56

    And sharpness can sometimes come down to the size of the film you use. If you use 35mm, it must be enlarged by a lot to have an 8x10 print, but if you use 8x10 film, there is no enlargement. Looking at images on the web you are normally looking at a size reduction too, especially if it is coming from a large format like 8x10, this will increase apparent sharpness.

  5. #5

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    You know, I didn't think of a UV filter but that would make sense with unfocused light. And I have more than a few laying around.

    Can't use a blue filter, because it goes against my sky/cloud separation. I have a couple 58mm polarizers, I'll compare that to the R25 next time I get out. BTW, it's 4X5 sheets, and I'm not saying it's not sharp. Just not like some I have seen.

    Someday I will learn how to scan and post.

  6. #6

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    It seems to be a long journey towards "perfection". I've only just started, but the information above came mostly from Jim Jones and a few other people at www.f295.org

    In my limited testing I did try a few different things including putting filters over the light source, and filters over the "lens". And they made little difference. With the dark red filter over the "lens" you should be getting minimal UV to the film, and a lot of film already rejects UV light, but I still wonder if it impacts things in a negative way. However the IR light might be messing with the sharpness, so you might try an IR blocking filter.

    I never did try a polariser over the pinhole, that might be real interesting. I'll have to try that and see what happens. It's possible that polarized light could increase sharpness a little, and would be worth a try.

  7. #7

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    Try making a photo without any filtering; the filter itself can make things diffuse. I tried filters to improve contrast; but the images lost sharpness and I've never done it again.

    Also, to satisfy my curiousity: are you using paper negs or film?

  8. #8
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    I had same question...film or paper, but with a red filter on paper I imagine he'd be complaining about no image rather than unsharp ones.

    With a 25 filter, try about 600-620 nm or so in Pinhole designer. I second the 1.5-1.56 numbers for pinhole constant. These lower numbers are supposed to be better for MTF, contrast and perceived sharpness on larger details (see Carlsson's '...Revenge of the Simple Minded Engineer' paper on Google).

    My experience has been that small details in subject matter like landscapes enhance my feeling of lack of sharpness because the details in the smaller formats I shoot are probably too small for pinhole's relatively low resolution.

    I tend toward wide angles so very close and large objects work better for me.

    For more fun at closer than landscape/infinity distance, look up the Paul Prober/PinPlus site and look at his so-called Prober-Wellman paper for pinhole size at macro-range distances...the hole required gets significantly smaller.

    If you don't want to wade thru the math (most don't), lean toward a smaller aperture for closeups...ballpark 1.25 constant.

    Only problem with optimizing a camera for a specific distance other than infinity is it becomes uncooperative at infinity when you need it. I made one for 1" and it was lousy at 4 feet. It became a tool for only one job.

    Stop by and visit.join f295.org too!
    Murray

  9. #9
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    Using Pinhole Designer, there is a noticable difference in recommended pinhole diameter between wavelengths near the blue and the red end of the visible spectrum. Long ago I compared resolution tests between blue sensitive film and Kodak IR film with a 25 filter. There was a modest difference in optimum pinhole size, well within the tolerences that many people accept. The gain in sharpness by using the UV end of the spectrum may be more than ofset by the lack of panchromatic film response.

    Rich, your neighbor would certainly be right in regard to simple lenses with no color correction. In theory he is right in regard to pinholes. but the gain is very slight in practical pinhole photography.

    Quality filters shouldn't degrade sharpness except for any change they make in color.

    As Greg says, larger formats can record more subject detail. The blur in a pinhole image is proportional to the pinhole diameter. That diameter increases in proportion to the square root of the focal length. For equal angular coverage, the film size is proportional to the focal length. Thus, a 4x5 pinhole camera can record subject detail about twice af fine as can a 35mm pinhole camera.

  10. #10
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    I limit the spectrum of light in my pinhole shooting by using ortho media, such as paper negatives or graphic arts film; these are mainly sensitive to UV and blue. I find this more convenient than starting with faster panchro film and having to use a filter.

    I believe larger formats, suitable for contact printing, are key to sharp images, along with a smaller sized pinhole optimized for blue/UV-sensitive ortho media. Of course the downside of the restricted tonal range of ortho media may offset any aesthetic advantages to a sharper image; only you can decide that for yourself.

    If you decide to shoot mainly closeup images, such as dioramas or still-lifes, then you can get by with a much smaller pinhole size. At such close-in distances, the image-softening effects from geometry are stronger than the image-softening effects from diffraction. Stated another way, most formulae for optimal pinhole size are themselves optimized for objects near or at infinity. For instance, an object that's 1F in front of the pinhole (i.e. the same distance in front as the film plane is behind) the blurr effect at the film plane from geometry is twice the pinhole diameter, which far outweighs any diffraction effects.

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