Pinhole technical question -> Sharpness

[CDJ990]

Hey guys,
Sorry if this is in the wrong section of if this is a double post (I'm new here and I think my last post failed as I auto logged out while writing).

Given that my exposure time has to be kept constant and I want to limit vignetting; am I better off increasing the aperture of my pinhole in order to increase my film size or will the trade off end up resulting in a softer overall image? I'm much more concerned about sharpness than I am about angle of view. :confused:

I'm very confused as of now so any help would be great.
Thanks Heaps,
Chris

[Tim Meisburger]

Hi Chris. I don't want to put you off this forum, but for pinhole questions you may do better at www.f295.org. Are you aware of that website? Otherwise, you will surely find your answer here as well.

[DannL]

Hello Chris,

As Tim has already noted there a a lot of sites dedicated to pinhole photography on the web. Here's another that you may want to your favorites . . . Mr. Pinhole (http://www.mrpinhole.com/calcpinh.php) and his handy pinhole calculator.

Before I can delve further into your request for info, I would need to understand the restriction you have for . . . "my exposure time has to be kept constant".

Dann

[CDJ990]

Hi Guys,

Thanks for the feedback. Sorry about the confusion, I hadn't heard of F295 but it looks like the right place, all I know is the technical ability of people of this forum is immense. I'll post on F295 so if mods want to delete this go to town.

However, DannL, what I mean by my exposure times have to be kept constant is because I'm doing a 6 week exposure. I know it's extreme (why I like it I guess) but I can go longer or shorter for logistical reasons, basically thats just how long I can have the camera in the location for. This means that I can only really change exposure with the focal length and the aperture. In other words I can't just decrease the aperture size and increase the focal length and film/paper size because then I'd need a longer exposure time, and more time I don't have.

p.s. it seems I can't register at F295 its been disabled :(

[Nathan Potter]

Well, roughly speaking there is an optimum aperture for pinhole resolution determined somewhere in pre history (actually by Petzval I think) that goes as D = (2 X f X wavelength)^1/2; that is D = the square root of (2 X f X wavelength). where:

D = diameter of the pinhole
f = the focal length (pinhole to film distance)
wavelength = about 0.5 µm

Following this relationship will provide you about the optimum resolution.

The effective f/no is simply the pinhole to film distance/pinhole diameter.

You have a further constraint in that you wish to minimize vignetting. Since there is no lens I think you want to say minimize light fall off at the edge of the film. This means you need to use a longer focal length depending on the size of the film you wish to use.

I'll, for example, assume you will use 4 X 5 inch film and further assume that to minimize light fall off you want to use a FL of 300 mm. (roughly a 2X tele).

Now for optimum resolution we have in mm. units D = (2 X 300 X 0.0005)^1/2 which equals about a 0.55 mm. (550 µm) diameter pinhole.

The effective f/no. of the 0.55 mm. pinhole is f/pinhole diameter which equals 300/.55 = f/545

Use f/545 to calculate your exposure for six weeks.

Now that is a long exposure so I'm guessing you will need to use a set of neutral density filters if you intend to use an open pinhole for the full time.

Play with my numbers for a different set of conditions - and good luck.

Nate Potter, Austin TX., Chatham MA.

[CDJ990]

Well, roughly speaking there is an optimum aperture for pinhole resolution determined somewhere in pre history (actually by Petzval I think) that goes as D = (2 X f X wavelength)^1/2; that is D = the square root of (2 X f X wavelength). where:

D = diameter of the pinhole
f = the focal length (pinhole to film distance)
wavelength = about 0.5 µm

Following this relationship will provide you about the optimum resolution.

The effective f/no is simply the pinhole to film distance/pinhole diameter.

You have a further constraint in that you wish to minimize vignetting. Since there is no lens I think you want to say minimize light fall off at the edge of the film. This means you need to use a longer focal length depending on the size of the film you wish to use.

I'll, for example, assume you will use 4 X 5 inch film and further assume that to minimize light fall off you want to use a FL of 300 mm. (roughly a 2X tele).

Now for optimum resolution we have in mm. units D = (2 X 300 X 0.0005)^1/2 which equals about a 0.55 mm. (550 µm) diameter pinhole.

The effective f/no. of the 0.55 mm. pinhole is f/pinhole diameter which equals 300/.55 = f/545

Use f/545 to calculate your exposure for six weeks.

Now that is a long exposure so I'm guessing you will need to use a set of neutral density filters if you intend to use an open pinhole for the full time.

Play with my numbers for a different set of conditions - and good luck.

Nate Potter, Austin TX., Chatham MA.


Thanks so much for your help Nate, it has cleared up a lot of issues for me so thanks a lot.

[Jim Jones]

A pinhole camera used in sunlight for six weeks will require either extreme filtering to reduce the light or a film that is very insensitive to light, such as Diazo film or cyanotype. Six week pinhole exposures may be practical by artificial lighting, but calculating the effect of reciprocity failure on the exposure might be difficult. Reducing the pinhole size below the optimum diameter to extend exposure time will quickly result in increasingly unsharp images. One way for a modest increase in exposure time with a optimally sized pinhole is to increase the focal length and image size.
With more information on the long exposure project, perhaps we can be more helpful.

[Jac@stafford.net]

A pinhole camera used in sunlight for six weeks will require either extreme filtering to reduce the light or a film that is very insensitive to light,[...]

Pinhole Resource (http://www.pinholeresource.com/) once published a pinhole image done by an Italian photographer of they sky showing the sun's path of about 12 months. A single exposure. It worked, although the sun's image was reversed due to profound reciprocity effects.

[ShannonG]

Solar cams are a fun project,I have 2 going right now.I put them up on the 4th of July and will harvest the images next 4th of July.The reciprocity failure is huge but the result can be interesting.Lots of sample images and tutorials,as well as pinhole calculators. on the web If your using home made cameras,its a good idea to set up a few different ones with different pinhole sizes and focal lengths,and a couple different films take notes and see what you get.Pinhole art for me anyway is a lot of trial and error.Experiment and have fun. Here is a shot of some cameras i have made,all hold 4X5 film these are regular cameras. solar cams have a much smaller hole.106493