How to Improve Resolution in Pinhole Camera

[dickbromberg]

I am relatively new to pinhole photography.
I have some experience developing 35 mm negative film (Kodak Plus X 125). These were images taken on an ordinary SLR camera.

I've put together a few pinhole cameras by replacing the lens with a lenscap with a pinhole. I made the pinholes using .001 brass shim stock and drilled the pinholes. I have been able to keep the tolerances to about plus or minus .002 mm. I also chemically blackened the brass after the pinholes were drilled.

There are a few pinhole calculators on the internet and I used a pinhole diameter recommended for the camera's focal length.

I would like to build a pinhole camera from scratch and want to understand how the focal length effects the resulting image. I understand the usual distortions result from the infinite depth of field, and these are fine.
What I want to do is maximize for resolution.
Thanks

 

[Dave Wooten]

use a "normal" focal length for the format...i.e. 150 mm for 4 x 5, and caluclate the best dia pin for the format. See Renners book or go to f295 ..the pinhole website.

 

[nze]

Perfect hole help to get better result ( laser drill is a good solution)

 

[Removed Account]

One thing is that the pinhole technically doesn't "focus" the image, it simply reverses the light that is transmitted to it (although I am not sure of the technical name for the effect). A pinhole camera's "focal length" would be more accurately described by the angle of view. A pinhole camera's "focal length" is just the angle of view equivalent to a lens of that focal length on the same film format. Sorry, I couldn't think of a way to describe that more clearly. Beyond having as close to a perfect pinhole as you can obtain (ie. drilled as smoothly as possible on as thin a stock as possible) the only other method I can think of to improve resolution would be to try different formulas for optimum pinhole size or conduct experiments on the shape of the pinhole. If you are in to physics, and your brain doesn't ache from the effort, you can work on coming up with your own formulas. One thing I might suggest would be to use a specific wavelength of light in your calculations and when taking your pinhole shots try to use filters to keep the light within close tolerances to the pinhole. That way you will avoid the problem of different wavelengths of light from the same object landing at different points on the film.

These are just my ideas and I am no expert on the subject, so if I've said something incorrect I'm sure someone will come along and point it out

- Justing

 

[JBrunner]

Once you have achieved the "perfect" pinhole, (laser or micro drilled SS) the resolution can still be no greater than the size of the pinhole. The best way to increase the apparent resolution is to use a larger format.

 

[Pinholemaster]

My 'sharpest' pinhole work is always on a short focal length 8x10 camera.

Here's a good resource to check out: http://www.pinholeresource.com/

Or just buy a well made camera: http://www.pinholecamera.com/

 

[dickbromberg]

Maximum Resolution Follow Up.

use a "normal" focal length for the format...i.e. 150 mm for 4 x 5, and caluclate the best dia pin for the format. See Renners book or go to f295 ..the pinhole website.

Thank you Dave.
I will read Renners book and try F295 the pinhole website.
If you know what the "normal" focal length is for 35 mm film is, I would appreciate it if you would let me know what it is.

Dick B

 

[DarkroomExperimente]

normal is usually considered the length of the diagonal of the negative

so for the usual 24mmx36mm format for 35mm film a "normal" lens would be 43mm or so

...although in practice 50mm is what seems most commonly accepted as a normal lens for 35mm

 

[rwyoung]

"normal" focal length is generally considered to be the same as the diagonal measurement of the film used.

There is some wiggle room to this but for 35mm film, between 45mm and 55mm is "normal".

 

[Ole]

Once you have achieved the "perfect" pinhole, (laser or micro drilled SS) the resolution can still be no greater than the size of the pinhole. The best way to increase the apparent resolution is to use a larger format.

I thought so too, but I was proved wrong. Thanks to diffraction, an optimal pinhole gives a resolution slightly better than the size of the hole!

 

[PHOTOTONE]

If you want more sharpness in the print, use a bigger negative for your pinhole photo. A negative the same size as the print (a contact print) will give you the best viewing sharpness from a pinhole photograph.

 

[Mustafa Umut Sarac]

Ole is right , smaller the hole , better the res., darker the image , longer expose times.
If you want max from your camera , use a good tripod.
Some people use lens and pinhole hybrids, they are really good.
May be you can use mura , coded aperture etc.
Research these terms at mit.edu , you will be amazed.
Mura requires computer correction but it worths your experiments.
come to f295 and look diy anamorphic camera experiments , you will like the results far away more than the ordimary pinhole.
i m there as mtumut , pm me , i will help you.

 

[JBrunner]

I thought so too, but I was proved wrong. Thanks to diffraction, an optimal pinhole gives a resolution slightly better than the size of the hole!

That's really interesting, Ole. Diffraction has always been a term I have associated with degredation of sharpness. Are you privy to principle of the effect you describe?

J

 

[nicolai]

Short answer: the smoother the microscopic surface of the pinhole, the sharper the image. So:

Buy a precision laser-drilled pinhole. You can get them from Lenox Laser among other places.

-- or --

Use the Roger Cline method for making perfect pinholes and zone plates and print them out on graphic arts film.

Use a tripod.

Use larger film.

 

[keithwms]

To improve resolution of a pinhole photograph, optimize the pinhole geometry for a particular wavelength, and restrict the wavelengths captured on your film/paper to that wavelength. Example: use ortho film, which has a narrow sensitivity, and optimize your pinhole for the central wavelength of that sensitivity curve.

And yes, shoot the largest format you can

 

[Rich Ullsmith]

Yes, I was going to say exactly what Keith said: restrict the wavelength. If you use a panchromatic film, a red, green or blue filter will narrow it down. Those different wavelengths correspond to little discs of light hitting the film; eliminate one and you'll get a better edge. There is extensive discussion of this on the pinhole forum, under sharpness and wavelength.

 

[Ole]

That's really interesting, Ole. Diffraction has always been a term I have associated with degredation of sharpness. Are you privy to principle of the effect you describe?

J

Dead Link Removed (about 2/3 down the page) gives a good summary of the different ways of calculating the optimal pinhole size, and what different formulas are used.

The basic idea is that monochromatic light through a pinhole will not spread evenly, but form a set of concentric rings on the film due to diffraction - at some radii, the light from opposite sides of the hole will be in opposite phase, and you get a dark band there. So by adjusting the hole diameter so that the first ring of extinction overlaps the edge of the (non-diffractional) light spot, the size of the CoC can be reduced to a little smaller than the hole. The different formulas for optimum diameter give slightly different diameters, but the result is the same: Getting a diffuse ring to overlap a diffuse circle to a smaller or greater extent. That's why they all work.

 

[Eric Rose]

Enlarge the hole to fit a Copal 0 and put a lens on the camera LOL. I just couldn't resist. Sorry.

 

[Murray@uptowngallery]

Diffraction is a concern with lenses; when aberrations are designed/manufactured out of a given lens, physics still limit what can be accomplished. The term diffraction-limited and how to avoid diffraction effects then become important to 'lensed' photographers, especially in smaller formats.

At a fundamental level, without diffraction, one could not create images with pinholes. There is no glass, there is no refraction (air is on either side of the pinhole, so there is no interface of two different indices of refraction), and there is no reflection (shiny areas inside pinhole are not how the image is created; rather, they are just artifacts that may create flare or other asymmetries). Rayleigh's paper is available on many website's if you can read 19th century English. He may have said in different language something similar about neither reflection nor refraction.

To contrast with the practical photographer's abject fear of diffraction, most if not all highly respected and referenced optical textbooks begin with a discussion of diffraction, as a fundamental or first principle. The cited examples analyzed are pinholes and slits.

I am pretty convinced that the geometry for optimal pinhole images correspond with optimal geometry for diffraction. There are others far more qualified than me to dispute this, but if you have any occasion to study and compare the geometry of diffraction and the geometry of poctorial pinhole optimization, the suggestion is strong that they are one and the same.

I hear 'diffraction effects' blamed for less than optimal pinhole images, usually attributed to a 'too small' pinhole. I haven't compared 'too small' to 'too large', but suspect both are moving away from 'optimal'. Even Rayleigh acknowledged that sharper results might be obtained with smaller holes, but exposure times became too long.

Inconsideration of the previous paragraph's observation, I'm confused by diffraction's role in less than optimal pinhole imaging. I might argue that diffraction isn't 'enough' or not ideal enough.

Matt Young's paper is pretty readable and either explains limits of pinhole resolution or references other papers.

For a practical discussion of relative resolution, contrast & MTF, see Carlsson's paper 'Revenge of the Simple-Minded Engineer'. It analytically justifies the use of smaller pinhole constants than Rayleigh's 1.9. 1.562 is shown to correspond to maximum MTF at low spatial frequencies (larger details).

Sparrow Limit may or may be a practical lower limit for pinhole geometry.(found this in microscopy; not sure if relevant to us).

The 'focal effect' is not very pronounced, but comes from the geometric conditions under which diffraction occurs.

 

[Sandeha Lynch]

I'd agree that 4"x5" Ortho sheet film can be pretty sharp ...

 

[Murray@uptowngallery]

On the Diffraction Limit for Lenless Imaging by NIST's Klaus Mielenz confused me even more- the math for pinhole irradiance is all from diffraction theory...but there is still a diffraction limit for lensless imaging...I don't NEED to know this to take pictures...I just find it intriguing.

 

[Murray@uptowngallery]

To get back to practicality (you DID say you were new), using a smaller pinhole constant (1.5-1.6) than Rayleigh did (1.9) will in the opinion of many help. If you use paper negatives, graded paper only responds significantly to blue (425 nm or so) and UV. VC paper responds to UV and blue thru green, roughly 550 nm at the high end.

A blue filter will likely improve paper negative sharpness, and remember to chose a bluish wavelength for calculations (say 425-475 nm, not terribly critical).

If you are shooting film, b/w, a contrast filter, kept dust free, of yellow, orange or red flavor, depending how much of that 'look' you can live with with help. This is a trick for cheaper lenses with poor chromatic aberration performance...by excluding wavelengths multi-wavelength focal issues are reduced (talking glass here).

For color film, all I can think to do is use a smaller pinhole constant. I use 1.56 based on Carlsson's paper. Jim Jones (APUG & f295 & elsewhere) prefers 1.5.

I will still pursue my own quest to understand the gibberish I posted earlier.

Murray

 

[Joe VanCleave]

It's nice to hear from fellow F295'ers here on APUG. I've personally used many different sized formats of pinhole cameras, and consistently find that larger negatives yield superior image quality - with the cavaet that by 'superior' I am suggesting image detail and, to a lesser extent, tonal range are better with larger negatives, along with no evident emulsion granularity for contact prints.

Of course, there are some genres of images that purposefully work better enlarged from small gauge pinhole negatives, where low resolution and obvious emulsion granularity are part of the aesthetic desired; in this respect one has to make a personal judgement about what quality of image works best for your purposes.

Back to sharp pinhole images: I shoot now almost exclusively on grade 2 RC paper negatives. Several reasons: 1) Large format panchromatic sheet film is expensive; 2) Graphic arts films, though cheaper than panchro, can't easily be controlled contrast in high-key daylight exposures; 3) Ortho paper is sensitive to shorter wavelengths, permitting a smaller sized pinhole to be used; 4) Much less problems with dust and other processing artifacts with paper negatives; 5) Paper negs can still be contact printed to yield high-quality prints.

The main cavaet to using paper negatives is the long exposure time, as compared to panchromatic sheet film. My experience with APHS graphic arts film as an in-camera negative material is that it is no faster than graded paper, and doesn't come in various contrast grades.

I have also used multigrade paper negatives, but the blue/UV of daylight activates the high contrast portion of the emulsion, making it look very much like APHS. This is why I use grade 2 RC paper for negatives. Freestyle's Arista brand is very good, and inexpensive.

One other key to sharp pinhole images: if you are shooting lots of 'close up' compositions, where most of the subject matter is much closer than 'infinity', you can get by with a smaller-than-Rayleigh pinhole diameter; most formulae for optimal pinhole size are assuming the subject matter is at infinity; when the subject is much closer, image softening from geometric optics outweighs any effects of diffraction from having too small of a pinhole.

Regarding the theory of optics and pinhole, I'm not an optician or mathematician, only a darkroom dabbler. But I believe that, without diffraction, pinhole imaging would still be possible, because the pinhole acts as a 'phase selector', isolating wavefronts of only one phase from all the light of various angles reflecting from the subject.

A final thought is about large negatives yielding sharper prints: if you enlarge a small-gauge pinhole negative to the same size as a contact print from a large format pinhole negative, the differences are very obvious; added to that is the fact that the optimal pinhole formulae favor longer projection lengths over shorter ones; so if we compare a small gauge camera with large format camera of the same angle of view, the larger format will have the longer focal length, which permits a larger F-number to be used, resulting in a sharper image.

~Joe

 

[Murray@uptowngallery]

Thanks Joe...good hands-on answer.

Joe is the man for paper neg experience.

 

[miryclay]

Smaller hole, rounder hole, bigger format helps improve apparent sharpness and ortho film should though I haven't tried it.

I also use stand development in PMK developer to increase edge effect and apparent sharpness.

Hope that helps.

Mike