Light meter for pinholes

Ces1um said:

So how does the double sided emulsion affect your photos? Just curious. Is it more difficult to print/enlarge, or to scan? Or are you worried more about scratching the emulsion? I'm just wondering because when these films are used for their intended purpose and developed as designed the emulsion itself is quite difficult to damage. I've been thinking about using it cut down in a pinhole camera and then processing it at work. one 8x12 sheet should get me four 4x5 photos (coincidentally filling two toyo film holders just perfectly).

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Damage was my primary concern about the double sided emulsion. Some using the film for photographic use say it's very soft. That could be partially a consequence of different chemistry and pH I suppose. The emulsion is reasonably tough after it's dried, but my understanding is development in those trays with ridges or other patterns in the bottom could cause damage. If one tries to cut it prior to development; e.g., make four 4x5s out an 8x10 handling can be a little touchy. I've done it with one of those office guillotine cutters using a sheet of clean paper under the film, but that's not a regular thing.

There are those who say the double layer reduces sharpness -- maybe if one is shooting with a lens that could be seen. I'm not bothered by it for pinhole work. There are people who strip the emulsion off one side, but wow, no, not this guy!

Since I mostly shoot the X-ray film in 8x10, I just contact print it. My ancient Epson 3200 can scan transparencies up to 4x9 inches which works for 4x5 if desired, but that's my scan limit. (Yeah, s'pose I could torment myself with two scans and stitching, but ...) I have "scanned" by using "the other technology" to photograph the negatives on a light box with some success. But for rare occasions of actually exhibiting the work, I do contact prints. I created a masking arrangement that can plant an 8x10 on a chunk of 11x14 paper so one can see the entire negative, rebates and all (something I do not generally do otherwise). There's a bit about that exercise in one of my PBase galleries. (Backing up a level in that hierarchy will get you to pretty much everything lens-less I've done "in modern times.")

tezzasmall said:

Wow! Those exposures on the card from 2 seconds upwards can't be right surely? I've used FP4 in a 35mm TTL Pentax with a lasered pinhole and the exposures were always in the ball park of a few seconds...?...

Terry S

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The card came from the camera manufacturer. It is specifiuc to f/207 - an aperture much smaller than you would be likely to use with a 35mm film pinhole.
I don't use FP4, so I print my own tables using the "Pinhole Designer" program, and then shrink them down using a photocopier. I didn't have on hand an easily accessible jpeg of what I use, so decided to post the jpeg I had of the card from the manufacturer. The times on the card are similar to my table for Plus-X (which I still have a few rolls of).
There is a definite reciprocity related "edge of cliff" with all the times for all the films I've looked at. The T-Max films and Ektachrome films I've used just have that edge farther into the long times.
If you take a photo in bright sun, the times are short, and there is little reciprocity failure. If you take something at dusk - be patient.
This evening portrait required a three minute exposure, and would have benefited with more (our friends are great sports):


Shots that I took nearby just half an hour or so earlier involved exposures of less than 15 seconds.

MattKing said:

This evening portrait required a three minute exposure, and would have benefited with more (our friends are great sports):

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That came out quite well -- you are fortunate to have patient friends!

I use the spot meter to find the EV im working to, then an app to find shutter speed then another app to allow for reciprocity for the particular film im using also allows for filters.
A bit convoluted but works.

Here's a quick-and-dirty trick taught to me by a friend who occasionally teaches a build-your-own-pinhole-camera class. Take your effective f-stop and change it to the closest value that's ten times a "standard" stop that appears on your meter. Then find the exposure at that standard stop and multiply by 100. You'll need to add time for reciprocity on top of that.

Example: Suppose your pinhole is f/207, so pretend it's f/220. Your meter shows 1/15 second at f/22, so multiply by 100 to get 6 seconds (not exact, but close enough for pinhole). Then figure on the reciprocity.

I use the highest f-stop on my meter, then correct for the actual aperture of the pinhole camera with the following formula. Assuming the highest f-stop on my meter is 128:

Exposure Time = (Camera's f-Stop / 128)^2 x (metered exposure at F/128).

Since the camera's F/stop is a constant (assuming its pinholes are fixed), and I'm always metering using the highest aperture value on my meter, then the first part of the formula can be reduced to a single number, which I fix to the camera with a label. Each of my pinhole cameras has such a number label affixed.

Example: Camera is F/300, the highest aperture value on my meter is 128:

(300/128)^2 = 5.49

So I would label the number 5.49 on the camera. I take a meter reading, reference the exposure time (in seconds) opposite f/128 and multiply that value by 5.49 - I keep a cheap calculator on hand for this purpose. YMMV - you might use the light meter app and calculator on your phone.

Of course, this doesn't take into account reciprocity failure, since I often shoot paper negatives, which have little issues with reciprocity in daylight exposure.

~Joe

MattKing said:

It isn't a calculator or wheel. It is just a little card, and is specific to one film and one pinhole.
I take a reading for f/22 - my meter gives me a time.
I read that time on the table, it gives me another time to actually use.
Here is the card, as supplied by the pinhole "manufacturer (it is for FP4):
View attachment 188146

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I think these numbers are actually off for a few reasons, the primary one being that a light meter displaying "f/22" actually uses an internal value of f/22.63. The same issue arises for all EVEN numbered f/stops. Those use an oddly rounded value, for historical reasons. For calculating exposure compensation, the precise value should be used. You would take the precise f/stop of your pinhole at the desired focal length, divide by the precise value indicated by the meter, and square the result. The correction factor is then multiplied by the indicated exposure time to arrive at the correct pinhole exposure.

A further issue is that the times indicated on a light meter are not the times used internally, in some cases. Times on a light meter internally follow powers of 2: 1, 2, 4, 8, 16, 32, 64, etc. A light meter will display 15 but use 16 internally, 30 but use 32 internally, and 60 but use 64 internally. The correct internal time numbers should be used or the exposure will be off.

All that considered, your table would look like this: The correction factor is the square of (207 / 22.63) = 83.67. Multiply 83.67 by the indicated times to get the correct corrected time, fixing the cases where the meter is using a different time internally: 1/1000 (really 1/1024), 1/500 (really 1/512), 1/250 (really 1/256), 1/125 (really 1/128), 1/60 (really 1/64), 1/30 (really 1/32), 1/15 (really 1/16), and similarly for 15, 30, 60, and 120 seconds. The nominal numbers are OK for direct use from the meter, but when multiplied by a large factor for extrapolating pinhole exposures, the inaccuracies multiply as well.

Regards,

Don

Don:
If you convert the variance into fractions of stops, you will see why the margin of variation due to these numerical round-offs is not statistically or practically important in the context of most pinhole film exposures.
Especially when you use a black baseball cap as your shutter timing mechanism!

I did a few practical calculations and found the timing error to be around 5%, which is not significant. The error does multiply, but as the overall exposure time does as well, the error remains about the same if the uncorrected values are used. The rounding of certain exposure times and f/stops produces NO error when transferring light meter reading to a shutter/iris as the variance is accounted for or perhaps within the mechanical/marking tolerance of a mechanical shutter. Electronic cameras “know” the real values, as do light meters.

Trying to keep the corrected light meter time when using an exposure time correction factor table is tedious and not worth the effort. I guess if I was going through the effort of making a table or calculator of timing correction factors, I would use the corrected f/stop values as there is little effort involved in doing so and is not confusing to the end user as the f/stop used is not obvious in the resulting correction factor or corrected times.

Practically, no real difference, I agree.

Best regards,

Don