# Quick Question about pinhole size

Discussion in 'Pinhole Photography' started by EASmithV, Sep 9, 2009.

1. ### EASmithVMember

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My friend found a big piece of construction ducting laying around, and just for the hell of it, we're going to build a pinhole camera with a focal length of about 152cm. I looked at a pinhole size calculator, and is it true that my pinhole will be 17mm wide? I'm going to have it cover an 8x10 sheet of paper.

2. ### reellis67Subscriber

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When I used the tool that I use to determine pinhole size, a focal length of 1520mm (152cm) requires a pinhole size of 1.64mm, which would be about f/925...

- Randy

3. ### Tom MillerMember

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A quick calculation based on the formula in Eric Renner's book tells me that the pinhole should be 1.46mm wide.

Many people use this calculator to determine pinhole size: http://www.pinhole.cz/en/pinholedesigner/ It will likely give a slightly different result.

4. ### DWThomasSubscriber

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1.7 mm sounds reasonable, confirmed via Pinhole Designer. Maybe a power of ten fell by the wayside with mm vs cm? f/894! You better include a lawn chair and a cooler of beverage with your photo gear!

DaveT

5. ### reellis67Subscriber

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Dave,

That's what I was thinking too! Don't wait too long into the day to start those exposures...

- Randy

6. ### RalphLambrechtSubscriber

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1.45 mm if you're after sharpness and 1.75 mm if you're after resolution. Anything in-between will work fine as well.

7. ### keithwmsMember

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Ralph, care to explain your reasoning?

8. ### RalphLambrechtSubscriber

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Reason for what, Keith?

9. ### keithwmsMember

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Your distinction between sharpness and resolution.... and why you'd recommend different aperture sizes to get one or the other. The way you wrote that, it implied to me that 1.45 would give better sharpness but inferior resolution, whereas 1.75 would do the opposite. I don't get that... unless you have some info about what spatial freqs are better transmitted at either extreme or such?

10. ### RalphLambrechtSubscriber

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Keith

Indeed, that is exactly what I meant.

Since you mentioned spacial frequency, I assume that you're familiar with MTFs. An MTF comparison between a pinhole size, optimized for the Airy disk, compared to one optimized for the Rayleigh criterion reveals the difference. If someone is after the optimum pinhole size, one must decide what to optimize it for. Our eyes have a clear preference for contrast over resolution, and therefore, the Airy disk is commonly preferred. However, the Raleigh criterion results in higher resolution at the cost of contrast and perceived sharpness.

Diffraction optics dictates that the pinhole is as large as possible to minimize light spreading, and geometric optics dictates that an ideal pinhole is as small as possible to optimize image clarity. Considering the Airy disc and the Rayleigh criterion leads to two theorems for an ideal pinhole diameter and suggests that there may be more than one right answer.

1. The smallest pinhole possible is based on the Airy disc to optimize image sharpness.

d = sqrt(2.44 * wave length * focal length)

2. The largest pinhole necessary satisfies the Rayleigh criterion to optimize image resolution.

d = sqrt(3.66 * wave length * focal length)

Again, the quest for the optimal pinhole diameter is generally fueled by the desire to create the sharpest pinhole image possible. Contrast and resolution are both aspects of sharpness, but human perception typically considers high-contrast images to be sharper than high-resolution images. Consequently, unless resolution is more important than perceived sharpness, my proposal for the optimal pinhole diameter is based on George Airys diffraction-limited disc.

I hope this explains it a bit better.

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i love this place!

12. ### keithwmsMember

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Ah okay, thanks Ralph, I figured it was an MTF argument. Interesting. I was guessing that it was all a wash when one takes into account wavelength spread, but I will think about it some more. Interesting!

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