Accurately determining F-stop with a home made pinhole

[M Carter]

I've seen this question asked before, thought I'd post my technique. Mine's a converted Isolette, with a DIY short bellows so I could keep the shutter and cable release, and still be able to close the camera up. It doesn't look elegant when opened, but it's a nice, pocketable camera with "real" film advance and a shutter. I wanted a thicker hole for softer edges, used a needle, sandpaper, soft drink can, scanned it to judge the size and roundness, etc.

Anyway, I had estimated F135 to F180. I setup a test still life with a large gray card, white styrofoam blocks, dark printed fabrics, spot metered the whole thing to find out the dynamic range, and worked it to where there were specific areas relating to exposure zones (IE, shadows with just texture, shadow detail, various highlights, etc). I used a 1K softbox and calculated exposure time for a range of 5 stops of F90 and higher, and reciprocity based on an APUG member's test data vs. Ilford's questionable chart.

I shot those 5 brackets on HP5+, and in the darkroom rewound the film and then loaded the roll into an RB67 with a 65mm lens, and advanced to frame 7. Shot it at the proper exposure calculated for a non-pinhole (I think it was F16 at 1/4 second).

I processed and dried the film with a hair dryer, and made 5x7 prints with print exposure time based on a max black test of a blank frame with a 2.5 filter. Made prints of the RB shot and all 5 brackets and dried them.

I had a large gray card in the center where I took my primary exposure from. The RB print had a much larger gray card than the wide pinhole - I punched a hole in that area and held it over the various exposures. And determined the F90 exposure matched the RB exposure quite closely. Other exposures seemed quite close to 1-stop density differences (no densitometer here, just went by a 1/2 stop guide card I keep handy on various papers).

Also got a great look at how reciprocity affects tonal range - the highlights in the RB shot were perfect, holding faint detail in styrofoam blocks, more in rough white fabric and so on. But all the pinhole frames were fairly blown out; the F90 image looks like it was close to a stop overdeveloped. Good to know for shooting where reciprocity comes into play.

Anyway, my method seems pretty sound, but let me know if I slipped up somewhere!

 

[ciniframe]

Sounds like a lot of work.
For myself I use a clear plastic ruler marked in millimeters, as fine a lines as I can buy. Then I use a 25mm SLR lens backwards as a magnifier and look at a blank sky, place the edge of the pinhole on a mark and estimate the diameter in fractions of a millimeter.

Lets say I measure .4mm and I'm going to use the pinhole at 113mm from the film. Ok, divide 113 by .4 and the answer is, f282 somewhere between the full stops of f256 and f360, closer to f256

So, working with ISO 6 photo paper on a bright sunny day I have an exposure of 43 seconds for f256, so add a little fudge factor for the slightly slower lens opening gives me 60 seconds.

This was a real experience with a new pinhole camera, the paper negative was almost spot on, just a wee bit overexposed.

 

[TheToadMen]

This is how I measure the pinhole size:

I take a piece of blank film and make a mark of exactly 1 cm on it. Then I put it in my enlarger and turn it on. Then I measure the projected line, let's say 22.5 cm.

Then I put the pinhole in the same enlarger and measure the diameter of the projected circle. Let's say 0.9 cm.

Now I calculate the size of the pinhole.
The line is 1 : 22.5 and the pinhole = x : 0.9. Then the pinhole size is x/1 = 0.9/22.5.
Then x = 0.04 cm = 0.4 mm.

If you now measure the focal length (= distance from the pinhole in the camera to the film plane, also in mm), then you can also calculate the F-number like mentioned above:

F-number = focal length / pinhole size

 

[M Carter]

Sounds like a lot of work.

Maybe an hour or so, from shooting to printing. Enjoyable though, I like doing this stuff.

My flatbed does 1600 DPI (and I still own some Schaedler flexible precision rulers) so I was able to get a decent measurement of the pinhole. But I wanted to do something to test exposure accurately for the exact size and exact distance to the film plane, this seemed like the most no-nonsense way. If I could match the gray density of a know exposure, seemed like I'd get a solid answer. (And my film-test still life stuff is pretty easy and there's always some kind of lighting setup here).

I do enjoy testing things like this, all the way to final output. I also used a blank frame to check for light leaks as long as I had film loaded (my first go-round had a bad leak), and to make sure I had the image circle optimized (just a hint of circle in one corner, maybe a couple mm) I wanted to use the full film frame but not "waste" much image circle, looks like that's right on. I always feel better if I can do a "real world" test vs. math, calculations & guessing.

Also found a recip chart that seems much more accurate than Ilford's "all our films are the same" data. Using Ilford's chart, I got some heavy overexposure, it's too aggressive.

 

[TheToadMen]

Maybe an hour or so, from shooting to printing. Enjoyable though, I like doing this stuff.

Not THAT I can relate to!

 

[NedL]

I like your empirical approach: find the time that matches what you want.
My approach is similar, I don't care what the F-number is, I just need to have a starting point to calculate exposures.
Since I use paper negatives almost exclusively, it's pretty easy to empirically find the "base sunny 16" exposure time.

So, when I make a new camera, I take it outside on a nice mid-afternoon "sunny 16 day", and make a paper negative at my best guess.
Come in, develop, examine the negative, then repeat until I've got a good negative. It never takes more than 3 or 4 tries to get to a good exposure. For each of my pinhole cameras, I just have to remember what time goes along with it. Some of my cameras are "20 second cameras", some are 40s, or 45s or 60s or 90s. My instant film camera is 7s, but that one needs reciprocity correction when figuring exposures....

I jot the times in a little book, in case I don't use a camera for a while and forget... but for the most part it is easy to remember 1 number for each camera.

Over time, using the cameras, the "base number" gets refined. For example, one of my favorite coffee can cameras started at 16s, but over several years it's been refined to ~20s. Those refinements are easy once you start making longer exposures, and seeing the difference a minute makes on a 5 or 8 minute exposure. I don't remember a very specific number for each camera, just "this camera is 20s, but leans toward a little faster..."

 

[M Carter]

I'll probably just stick with one for now! But shooting film, I like at least trying for really optimized negs via testing, and I keep a binder full of notes and my test prints for different film and dev combos. I've done very little long exposure stuff, so reciprocity was interesting - glad I found some reliable data here vs. Ilford's chart. Really interesting to find how reciprocity affects the highs, but development can control that. I'm always glad to test this stuff and not come home with some epic shot that could have been better.

Actually, I did test the camera quickly before really dialing it in (and realized I had a bad light leak), but for lith printing, I really like how the process marries the pinhole look.

1st post-assembly test - the light leak actually lined up in a pretty cool way:

 

[TheToadMen]

That's a great image and print. Pinhole and lith do make a good match, don't they?

 

[NedL]

cool!

 

[Chan Tran]

The OP suggested a way to measure the TStop while others suggest ways to measure the FStop. Both are interesting.

 

[M Carter]

That's a great image and print. Pinhole and lith do make a good match, don't they?

Indeed. I confess to a slight case of "pinhole dissing" in the past - "they're not sharp, they're toys", etc. But there's a young guy on the Facebook lith group who does astounding work, really has a great eye and his work stands together as a whole. Both processes are very "transformative" which interests me much more than straight-up photography and printing. And both sort of "hand you things" for free - even though my pinhole has a viewfinder, it's for an 80mm lens, so framing is somewhat of a guess, and for low angles I just point and hope. And then with lith, that first test print usually points me to a different interpretation of the neg than what I had visualized. It's more the norm that I go "whoooo, look at that!" when lith jacks with local contrast as so on; but I've done enough lith that I can recognize what's going on and push things farther along. It's like the muse can smell that developer heating up and she says (it's a she, and she's freakin' hot) "here's what I think, dude".

I won't be shooting nothing-but-pinhole by any means, but glad to have it in my back pocket - literally in my back pocket, the Isolette is a damn handy camera for pinhole!

 

[TheToadMen]

I won't be shooting nothing-but-pinhole by any means, but glad to have it in my back pocket - literally in my back pocket, the Isolette is a damn handy camera for pinhole!

Then this thread might interest you (about World Pinhole day 2017)
http://www.apug.org/forum/index.php...2017”-april-30th-show-us-your-results.145155/

 

[M Carter]

Thanks - I've been aware of the pinhole day but this is the first year I've had one. I really like those sorts of "the world got a little smaller today" things, like print exchanges and so on. So I'll try to keep it in mind - come on, dramatic clouds!

I just ordered some old series IV filters and mounts so I should have some filter options by then, too.

 

[ciniframe]

About the only medium I've shot in pinhole is photo paper, for ease of handling and fairly quick feedback with minimal equipment. The sweet spot for paper, at least for me, is 5x7. Of course the camera will certainly not fit in any pocket! I've resigned myself to a very large bag. But since I have to bring a tripod anyway....well it's just part and parcel to the process. The problem with my setup, 5x7 paper, held in a curve with a radius of 113mm, is reloading in the field. Haven't quite got that working and will probably have to invest in one of those popup tent like changing bags. I also shoot 4x5 and of course that's easy, just bring however many holders you are willing to carry.

 

[M Carter]

Wonder if you could cut 16x20 paper into long strips and make some sort of roller transport? Three shots before reloading!

I looked at various DIY medium format designs, but I wanted a reliable film transport vs. something made from wooden dowels.

 

[Helinophoto]

The easiest;
Shoot the pinhole beside a ruler with a dslr or phone.
Enlarge the shot on the monitor until 1cm on the ruler is 10cm long on screen.(use the same ruler to measure in the monitor).

Measure the hole, divide by 10.

Done

 

[M Carter]

The easiest;
Shoot the pinhole beside a ruler with a dslr or phone.
Enlarge the shot on the monitor until 1cm on the ruler is 10cm long on screen.(use the same ruler to measure in the monitor).

Measure the hole, divide by 10.

Done

My target hole size was well under a millimeter; even when scanned at 1600 DPI next to a Schaedler Precision Ruler (half-millimeter markings - god I want a new one of those, mine is beat to hell), I couldn't get an exact read, thus actually running film through the thing. Then again, I thought an hour of anal retentiveness would pay off for me! Enjoying seeing how many ways to do this there are though.

 

[Helinophoto]

That's weird, I measured a 0.3 hole like this with no issues and a rather lousy ruler.

You can use a similar method by using an enlarger, if you have a transparent ruler, raise the enlarger with the ruler instead of film, until 1 cm is 10 cm on the board.
Replace the ruler with the metal piece with the hole, measure the hole on the board, divide by 10.

You should be able to measure 2-3 mm on the board if your hole is around 0.2-0.3mm ..?

 

[Jim Jones]

For years I used an optical comparator from Edmund Scientific much like this one: http://www.peakoptics.com/index.php..._id=39&zenid=62a168885f1b49bf3cddb91bdf995177. It enabled me to accurately size and quality check even very small pinholes.

 

[MattKing]

A friend of mine uses micro drill bits and a miniature drill press to drill something like a 0.2mm hole, and then uses that hole as a standard, combined with the technique described above using an enlarger, to calculate the size of a pinhole.
The tiny drill bits are quite reasonable in cost: https://hobbyking.com/en_us/mini-drill-bit-set-14pc.html?___store=en_us