Method to determine pinhole f-stop

[skeletron]

I've been trying to figure out an easy and accurate way to measure a pinhole f-stop value so that I can start metering my shots instead of making a somewhat blind guess. What I'm currently thinking:

-Take a body cap to my DSLR and cut a hole in the center, line the edge with high density foam/felt/or something else that can block light.

-Press the pinhole to be tested up to the DSLR+cap+foam and determine what shutter time gives a good exposure.

-Take the shutter time and ISO speed and look up the corresponding f-stop value on an EV table.

Would this work or is there something I'm not taking into consideration?

Thanks.

 

[Micky D]

You can down load this free "pinhole designer" program HERE.

It's very simple and it gives you everything you need to know.
You can dial-in the film you are using and it will print out an exposure chart taking reciprocity failure into account.

 

[thetooth]

check the zero image website they have a calculator for this . plus it's a good website i have 2 of there camera's .

 

[Carlos Cruz]

scan the pinhole and measure in PS ;-]

 

[skeletron]

Thanks for the link Micky D.

And I didn't know Zeroimage had a calculator. I'll look into that, too.

One concern I have, though, is making home-made pinholes. Depending on how thorough I am or how many beers are in me (I have to get my pinhole material from somewhere), the pinhole diameter will vary. Is this variance, however slight it may be, have a large effect on the final exposure? I don't really have any way of accurately measuring the diameter of the pinholes I make.



EDIT:

scan the pinhole and measure in PS ;-]

Good idea. 🙂

 

[Micky D]

Is this variance, however slight it may be, have a large effect on the final exposure?

Remember that there is an optimum focal length for the size of the hole too!

 

[russianRF]

If you are really concerned about it, your best bet is to buy laser-cut pinholes. One good place is here

 

[Al Kaplan]

Don't you know an automobile mechanic or such who has a micrometer?

Digital sensors have a linear response to light intensity so your exposure calculations should be accurate. On the other hand film suffers from something called reciprocity failure at very long and short exposure times. At the short end some films will appear a bit underexposed even at 1/1000 second. Going in the other direction, increasing exposure from 5 seconds to 10 seconds to 20 seconds still might not result in what you'd consider another stop's worth of negative density.

Back in the day when film ruled this made it a lot easier to arrive at a useable exposure.

You can make more than just a tiny pin prick with the tip of the point and still get images as sharp as any pin hole image will be. When you go to a smaller hole that you start getting diffraction effects which soften the image. The light rays that contact the hole's edge as they pass get bent slightly, softening the image. The bigger the hole the smaller the percentage of light rays get bent (diffracted). Push your pin or needle in some aluminum foil until the hole is the size of the shaft diameter. Lay the foil on a hard surface and flatten any jagged edges with your finger nail. You might have to go back and forth a few times before you have a nice clean round hole.

You can blacken the aluminum foil and the hole itself by holding it close to a candle flame. Soot is good and black!

 

[uhligfd]

Well, how about exposing for the sensor ISO setting at varying speeds through the sensor (it is digital, so this involves no waste, just a little time) and learn what exposure duration gives the best image (keep camera on tripod and seeing one image). [For this purpose let's say: 10 sec optimal exposure through pinhole]

Then keep the iSO and view as is and put on a lens of about equal focal length (angle of view); measure exposure. [Let's say this one is 1/125 sec, f/8]

How many stops is it from 1/125 sec to 10 sec? 125 - 60 - 30 - 15 - 8 - 4 - 2 - 1 sec - 2 sec - 4 sec - 8 sec, or about 10 stops. So your pinhole is about ten stops from f/8 - 11 - 16 - 22 - 32 - 45 - 64 - 90 - 128 - 180 - 250. Ok?

However, depending on focal length (= distance of pinhole from sensor in this case), f/240 is a bit small. Look up the optimal pinhole f stops and compute it all backwards to find the one that is optimal.

10 stops different from a known f stop on the lens is ideal: 10 stops equal 2^10 - = 1024 = 1000 fold increase in exposure. How simple. If the equivalent lens stop exposure time is 1/125, then expose through the pinhole at 1000 * 1//125 sec or 8 sec and you are all set. (Luckily, digital has no Schwarzschild effect at long exposure times ...)

 

[rbiemer]

A few things for you to consider;
>Beer/soda can aluminum is maybe not the best choice--it is very hard to drill. I find using disposable pie pans to work better. Or the trays that some baked goods come in ( Here in the US ).
>I don't use sewing needles to make my pinholes. It was too hard to find out their diameter (at the time I started this, I didn't have a micrometer). Instead I use micro drill bits. They typically come in a set of twenty different sizes--numbered from 60 or 61 to 80. The smallest is .0135". About #10 a set and they last quite a while for the kind of use they get drilling a few pinholes.
>as mentioned up thread, the Czech Pinhole Designer program is very useful. And the price is right! Lots of other good stuff at that site also.
>Eric Renner's book is also a good source of info and inspiration.

Rob

 

[Sjixxxy]

scan the pinhole and measure in PS ;-]

I second the notion. I did this for a pinhole camera I made in college during Photo III. My exposures ended up being so accurate that my resulting prints ended up with such good tonality that the instructor was amazed that I was doing my work with pinhole.

 

[JoeV]

I've had good luck making pinholes in 2 mil sheet brass shim stock. I use a loupe to visual their quality, and with careful practice you can get them quite smooth and round.

I've seen images from another website showing laser drilled pinholes that appear to have a rough edge from the way the laser blasts away the metal. Although they're supposed to more accurate in size than homemade, my experience is that smoothness is just as important as getting the correct size for your focal length.

I've also heard from others (mainly on F295) that the EMS electron microscope pinholes are very accurately made and don't have the rough edges of the laser drilled.

I've had good luck measuring pinhole diameter using a scanner and blowing up the scan in PS and measuring pixel width and correlating that to the scan resolution. But I've also used a loupe and placed the pinhole adjacent to a millimeter scale, and by backlighting the pinhole can estimate the size to a quarter of a millimeter, which seems to be accurate enough.

Finally, although there are advantages to getting the optimal sized pinhole, often the resulting image quality of pinhole doesn't have to rely strictly on sharpness. Larger formats of film or paper will intrinsically get you more sharpness, just because the optimal pinhole size for longer focal lengths renders a higher f-number.

~Joe

~Joe

 

[Baldadash]

for mac users:
http://www.concepthouse.com/products/PinholeCalc/
also,
I use Snapple caps & jar lids for apertures
http://www.flickr.com/photos/art209/2485741326/
http://www.flickr.com/photos/art209/3057207248/
-b