digital pinhole problem? any suggestions?

[Schunn99]

hey all, was not sure were to post this,
but here my problem:

I have digital pinhole problem that I am hoping y'all might be able to answer. I am doing a digital pinhole project in my college photography class. I am currently using a homemade pinhole body cap. I drilled a hole in the body cap and then I made several different size pinholes in aluminum pieces cut from a soda can. The most major one I am using is supposed have an f/sop of around 250. According the measurement I took from scanning the pinhole in and measuring it in PS. I have tried using this cap several ways: straight on the body and using extension tube set that extending out to 68mm. My problem is this: No matter what size the pinhole is or the distance from the sensor, all my images are coming out blurry. the images from the extension set is less blurry but still blurry.
Do you have any suggestions or any clue why this is happening?

Thanks,
Scott Hunnicutt
BFA photography student

 

[Ray Heath]

g'day Scott

not sure what you mean by blurry, pinhole images are not sharp by normal lens photography standards, however by using some simple maths they can be designed to be as sharp as possible

the maths should take into account the distance from the pinhole to the film plane/sensor (somewhat like focal length)

if you are sometimes using extension tubes and sometimes not, then you are changing the distance from pinhole to sensor (focal length)

an equation i've used to good effect;
d = square root of 2.6x0.00055xfl
where d is the pinhole diameter, 0.00055 is the average wave length of light and fl is focal length

this equation then gives you the optimum size pinhole that will produce the sharpest possible image over that fl, you then divide the fl by the diameter of the pinhole to calculate relative aperture

if i roughly measure the distance from the lens mount to the film plane mark on my Canon 400D the fl is about 50mm so;
d=square root of 2.6x0.00055x50 (all in mm)
d=sr of 0.0715
d=0.267mm

calculate relative aperture;
f=50 divided by 0.267
f=187

hope this helps
Ray