scanning pinholes?

[PHOTOG]

I tried to search it but came up empty.

Can anyone refferance me to how to calculate f-stop by scanning the pinhole?

[taco]

I tried to search it but came up empty.
Can anyone refferance me to how to calculate f-stop by scanning the pinhole?
I'm not sure if I understand your question right, but if you want to scan your pinhole with a scanner to see how large it is and then to calculate the f-stop; here one possibility:
Scan the pinhole in 600 dpi and count the pixels, then turn the hole ± 90° to the left or the right and scan it again. If the amount of pixels is the same: congratulations, your hole is perfectly round. If there is only a small difference, try it nevertheless.
Then calculate:
600 dpm = 236 dpcm (centimeter)
1 cm = 236 pixels
1 mm = 23,6 pixels
0,1 mm = 2,36 pixels
With this, you can calculate the size of your pinhole, e.g. 8 pixels = 0,339 mm
Based on that, just use the pinhole designer
http://www.pinhole.cz/en/pinholedesigner/
to determine the f-stop according to your focal length

[MartinS]

...then turn the hole ± 90° to the left or the right and scan it again. If the amount of pixels is the same: congratulations, your hole is per...fectly round.


Just to be exact, this means you have at least two cross-center dimensions equal, wich does not necessarily mean you have round pinhole at all :-)
Besides that, good instructions, nevertheless :-)

[taco]

Just to be exact, this means you have at least two cross-center dimensions equal, wich does not necessarily mean you have round pinhole at all :-)
Besides that, good instructions, nevertheless :-)
Must be good, David Balihar from the pinhole designer which I give as reference is a Czech ;)

[durian]

I scan the pinhole together with a ruler. Then I load the scan in the Gimp. With a line I can see how many pixels is one millimeter on the ruler. I then measure the pinhole the same way, converting pixels to millimeters. F-stop is simply focal length divided by the pinhole diameter.

Simple but it seems to work.

-peter

[renon]

First I scanned my pinholes with the scanner I use for scanning the negatives. In the meantime I bought a cheap USB microscope (100x enlargement). With the USB microscope I take a picture of the pinhole together with a part of a high precision metal ruler. Rest of the procedure as described by durian.

[Zou]

I scan the pinhole together with a ruler. Then I load the scan in the Gimp. With a line I can see how many pixels is one millimeter on the ruler. I then measure the pinhole the same way, converting pixels to millimeters. F-stop is simply focal length divided by the pinhole diameter.

Simple but it seems to work.

-peter


I did that as well. Simple and effective.

[gneissgirl]

Yet another variation on the theme:
Scan the pinhole at 1000 dpi.
In Photoshop or similar editing software, zoom in to maximum magnification so you can see individual pixels, and with the pencil tool (or whatever tool allows you to color one pixel at a time), count the pixels across the diameter of the hole. Selecting the pixels with the tool and changing the color of each pixel as you count just makes it easier. No need to scan a ruler along with it.
Each pixel is 0.001 inch, so the diameter in pixels = the diameter in thousandths of an inch. Convert to mm from there, if you wish.

[pinholeblender]

I use the ruler tool in photoshop. Its buried under the eye dropper. I scan at the highest resolution my scanner will do. Next I make the pinhole area file the screen. Use the ruler to measure across the pinhole. Read the diameter in the info window. The units this window uses is controlled by the "unit and ruler" preferences under Photoshops general preferences.

[gneissgirl]

Thanks, Chris! That sounds much easier! I knew about the ruler tool in Illustrator, but I didn't think to look for it in Photoshop. Very direct solution! Let photoshop do the work.

[durian]

Just for fun and illustration, a scan of my first pinhole. Size 19/48th mm, slightly less than 0.4 mm. I cut out the scanned ruler and aligned it with the pinhole.

-peter
Attached files http://f295.f295.org/uploads/img175a_4210.jpg (http://f295.f295.org/uploads/img175a_4210.jpg)