Converting meter readings for pinhole

sirch

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The winter evenings are long in Westmorland and there is very little on the idiots lantern these days so I have been messing around with pinholes and flash guns but I am struggling to get the exposure right. I don't accurately know the size of the pin hole which doesn't help but online pin hole calcualtors give an f stop of around f200. If I meter at say f8 I recon there are about 10 and a half stops between f8 and f200 so am I right in thinking that I need to fire the flash 10 times?

Well I'm obviously not because it didn't work but it's hard to know if that is because I am miles off with the the pinhole f stop or that 10 flashes is not equivalent to 10 stops of light.

I've tried various things, and manged to get well-under and well over but nothing close, any ideas?
 
The winter evenings are long in Westmorland and there is very little on the idiots lantern these days so I have been messing around with pinholes and flash guns but I am struggling to get the exposure right. I don't accurately know the size of the pin hole which doesn't help but online pin hole calcualtors give an f stop of around f200. If I meter at say f8 I recon there are about 10 and a half stops between f8 and f200 so am I right in thinking that I need to fire the flash 10 times?

Well I'm obviously not because it didn't work but it's hard to know if that is because I am miles off with the the pinhole f stop or that 10 flashes is not equivalent to 10 stops of light.

I've tried various things, and manged to get well-under and well over but nothing close, any ideas?
10 flashes would be just over 3 stops of additional light... each stop requires 2x the previous amount. I.e. when combining lights it's 1,2,4,8, etc
 
10 flashes is not the same as 10 stops. For each stop, you double or halve the amount of light. For a series of stops, the doubling or halving is sequential. If it's one flash for f8, it's two for f11 and four for f16, etc.

Call it f256 for easier number crunching. f256 is 10 stops away from f8. Doubling the number of flashes sequentially for ten steps results in 1024 flashes. You can crunch the number with a calculator - do 2 to the power of number of stops you want to adjust for. 2^10 = 1024.
 
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Thanks, I knew my thinking was off ... but ... a reading of f8 with one flash exposed with 10 flashes gives something way under but that can just about be seen with the naked eye and can be scanned to show an image. An f22 reading with 16 flashes gives something way over but again that can be just about seen and can be scanned. So the answer is somewhere between those two, 1024 would be miles over exposed :thinking:

I guess I need to do more thinking about what is going on.
 
Ilford make a couple of pinhole cameras as well as film, so why not drop them an email about this Chris? Reciprocity could be an issue as well, so I think you might get an answer that we could all benefit from.
 
Thanks, I knew my thinking was off ... but ... a reading of f8 with one flash exposed with 10 flashes gives something way under but that can just about be seen with the naked eye and can be scanned to show an image. An f22 reading with 16 flashes gives something way over but again that can be just about seen and can be scanned. So the answer is somewhere between those two, 1024 would be miles over exposed :thinking:

I guess I need to do more thinking about what is going on.

10 flashes at f8 power is 3 and a bit stops extra exposure. If that had resulted in a correct exposure, your pinhole would be a bit smaller than f22 (about f25).
16 flashes at f22 power is 4 stops extra exposure. If that had resulted in a correct exposure, your pinhole would be f90.
(16 flashes at f22 power is equivalent to 128 flashes at f8 power.)

So, your pinhole's effective aperture must be somewhere between f25 and f90.

Where did the pinhole come from, and what's its distance from the film? And what is the film?

If the amount of over and under exposure looks roughly the same, try 4 flashes at f22 power.
 
I made the pinhole with a 0.4mm drill bit but how accurate the drill bit and resulting hole are I don'y know. The pin hole is about 75mm from the film (may be more like 73mm) and I am using Ilford Delta 400, 120. I used an online calculator to get a f value of IIRC about f180.

BTW, I have used a scanner in the past to measure the size of a pin hole but this is drilled in a tin can so I thought scanning it would be too difficult but I might have a go at that to try to get a more accurate size.
 
Would a constant light source be better?, you can get a torch called a skyray king off flebay pretty cheap and they are mental, the one I got (about the size of a coke can) is brighter than main beam on my car.
I've used it with a brolly a few times at wedding for night shots.
 
I made the pinhole with a 0.4mm drill bit but how accurate the drill bit and resulting hole are I don'y know. The pin hole is about 75mm from the film (may be more like 73mm) and I am using Ilford Delta 400, 120. I used an online calculator to get a f value of IIRC about f180.

BTW, I have used a scanner in the past to measure the size of a pin hole but this is drilled in a tin can so I thought scanning it would be too difficult but I might have a go at that to try to get a more accurate size.

Chris, one stop over or under on that hole size would take you from 0.28mm to 0.56mm; 2 stops (which should still give you a reasonable image) would allow a range of hole size from 0.2mm to 0.8mm. I reckon it's pretty likely to be within that range, so calculating on the basis of 0.4mm should still leave you within 2 stops. 75/0.4 gives you an aperture of f/188, or a 2-stop range of f/94 to f/376!

BTW, you didn't say what the film was (that I noticed).
 
Would a constant light source be better?, you can get a torch called a skyray king off flebay pretty cheap and they are mental, the one I got (about the size of a coke can) is brighter than main beam on my car.
I've used it with a brolly a few times at wedding for night shots.
Possibly but any exposure over a few seconds starts to introduce reciprocity failure which is another variable and I feel it would be better to keep it as simple as possibile, at least initially.

Chris, one stop over or under on that hole size would take you from 0.28mm to 0.56mm; 2 stops (which should still give you a reasonable image) would allow a range of hole size from 0.2mm to 0.8mm. I reckon it's pretty likely to be within that range, so calculating on the basis of 0.4mm should still leave you within 2 stops. 75/0.4 gives you an aperture of f/188, or a 2-stop range of f/94 to f/376!

BTW, you didn't say what the film was (that I noticed).
Ilford Delta 400, 120 (second line of post #7 :) )

I started out hoping that latitude would save the day but given that I have tried about 4 frames with different total expsoures and nothing has worked reasonably well I am wondering if anything else is going wrong with the process. I am cutting 120 film into individual frames and then putting it in a home made pinhole camera, I obviously cut the film in the dark and the camera is as light tight as I can make it but I am now wondering if there is a light leak or something. As I said above I can retrieve a very low-contrast image in the scanner but would like to do better.

Typing the above gave me an idea though, I haven't painted the inside of the camera black, may be that would help.
 
Well I managed to measure my hole (stop making up jokes :) ) and it's a long way off round, about 0.3mm one way and around 0.4mm the other

So I blacked out the inside and it made a difference, still way over, metered at f22 for one flash, so did 16 flashes and got this.

Can-Selfie-2.jpg

Obviously not the best photo and much wider angle than I was expecting but I'm still pondering why it is so far off from the expected f-stop.
 
An oval of 0.4 x 0.3 is likely about 1/3rd of a stop less than a circle of 0.4 (0.28 is one stop less than 0.4). If we assume 0.35 effective diameter, that would give f214 - about 6.5 stops smaller than f22, which means about 90-100 flashes at a power set for f22.

If you're getting over-exposure with 16 flashes, I'd be wondering if the flash is putting out the right amount of light, or whether something else is amiss with the lighting/metering. Could check through all of the flash and metering procedure. That said, I'd just set up something with a constant light source, meter for a given aperture, adjust for the pinhole, and then do the calculation for reciprocity failure. Reciprocity shouldn't be a big deal - you're only looking for an exposure time of 90-100 times the f22 time. If your indoor setup for f22 is, say, 1/4 sec, then your pinhole time is 25 secs + reciprocity compensation.
 
So I know nothing about flashguns, I never use them. However, would I be correct in thinking that you are metering when the flash is firing, thereby getting a total exposure time that is the ambient light exposure plus the effect of the flash? If so, what is the exposure time for say F/8?
 
Well I know nothing about pin hole cameras but if the camera is open for too long, in between flashes, it's possible to get a secondary image from ambient light which would make the shot unsharp.
 
So I know nothing about flashguns, I never use them. However, would I be correct in thinking that you are metering when the flash is firing, thereby getting a total exposure time that is the ambient light exposure plus the effect of the flash? If so, what is the exposure time for say F/8?
I have a flash meter so it only takes a reading when the flash fires so you get the sum of flash plus ambient for 1/5000 (or whatever) of a second.

Well I know nothing about pin hole cameras but if the camera is open for too long, in between flashes, it's possible to get a secondary image from ambient light which would make the shot unsharp.
I have been shooting these in a very dimly lit room so virtually no ambient.
 
I just about managed to get a daylight shot, it's been persisting down all day and was 1/30 at f16 for ISO-400 at mid day so I just stuck the pinhole camera out of the back door. The process was the same as the other shots but I used a new roll of Delta 400 which gave a calculated exposure around 20 seconds and I got a well exposed negative.

DaylightTestDelta400.jpg

The question now is: are some of the issues I have been seeing due to the use of flash or had I inadvertently fogged the previous roll of film, or both. If I get chance I'll try another flash shot tonight
 
The question now is: are some of the issues I have been seeing due to the use of flash or had I inadvertently fogged the previous roll of film, or both. If I get chance I'll try another flash shot tonight

Only one way to find out …..I'd put a wager on that you had fogged the film as it seems too strange that all the experiments that you did turned out to be a failures.

Fingers crossed that this time will come up trumps!
 
20 seconds is comfortably in the ballpark. If the effective aperture is the f214 I estimated earlier, then the metered time would be 5.65 secs, and 11.5 secs with the reciprocity calculation. 20 secs after calculation crunches back to a smidge over 8 seconds metered, so a shade more than half a stop over-exposed. That indicates that the pinhole size and distance are about right in the sense that the calculations for a correct exposure match the real world result. Fogging of the first film is a possibility. It's feasible that the 10 flashes for f8 still resulted in under-exposure, while the 16 for f22 (for a pinhole of f90) resulted in over-exposure. (It all depends on how much fogging there was.) Worth having another go with the flash, although it's a lot of flashes: for f214, 90 flashes at power for f22.
 
In an effort to just change one variable at a time, I stuck to 10 flashes at f22 but with new film and the result was a few stops under but not a mile away. Anyway I've got one of wrinkles ironed out :)
Delta-f22-10flashes.jpg
 
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