Pinhole Exposure Time Based on Light Meter- Help

imaguitargod · Feb 26, 2014

Hello folks! First time poster here so I apologize if I did not place this in the correct section. I have a question about exposure times for pinhole cameras using a hand held exposure meter. Ive been banging my head against this problem for a few months now and I must be missing a piece of information as I still cant figure it out.

I basically have two pinhole cameras (one with an fstop of 119, and one with an fstop of 138) and will be using 400 speed film (occasionally 200 speed). I wish to create an exposure chart (or even be able to make the calculations on the fly) for them both based off of the readings I will be getting with my Capital CDS F4 Exposure Meter (I can include the PDF of the instructions for said meter if anyone wants to check it out). For the life of me I cant figure it out.

Derrel · Feb 26, 2014

Photographic Tables, F/stop, Shutter Speed, ISO and EV

f/16,22,32,45,64,90,128 are the full stops, so f/119 is I'd estimate 1/3 stop "faster" than f/128. Make sure to check into reciprocity factor for the film is use; check the film manufacturer's data sheet. f/138 is probably about 1/3 stop slower than f/128. If you can get a shutter speed reading at f/16, then keep doubling the time as you move higher in f/number...about six full time values LONGER than the time needed for f/16 along the line ought to be close--then, compute the needed reciprocity factor value.

imaguitargod · Feb 26, 2014

Derrel said:
Photographic Tables, F/stop, Shutter Speed, ISO and EV

f/16,22,32,45,64,90,128 are the full stops, so f/119 is I'd estimate 1/3 stop "faster" than f/128. Make sure to check into reciprocity factor for the film is use; check the film manufacturer's data sheet. f/138 is probably about 1/3 stop slower than f/128. If you can get a shutter speed reading at f/16, then keep doubling the time as you move higher in f/number...about six full time values LONGER than the time needed for f/16 along the line ought to be close--then, compute the needed reciprocity factor value.

Thanks for the quick reply! Ok....maybe I should have said this originally.....I'm a super beginner when it comes to the technical aspect and langue of photography. Also, the exposure meter I'm using is super simple ( pdf of the instruction manual here to get an idea of what I'm using: http://www.cameramanuals.org/flashes_meters/capital_f4_exposure_meter.pdf )

Gavjenks · Feb 26, 2014

What film are you using. Reciprocity failure (takes non-linearly longer amounts of time to get the same exposure as exposure time increases) is going to be the main issue here. It will have probably more of an influence than anything else you might consider.
And reciprocity failure depends on film type. We can help you look it up if you give us a brand and type of film.

Your exposure conversion table will be for one specific type of film, therefore. And you'll need a different table for different film.

imaguitargod · Feb 26, 2014

Gavjenks said:
What film are you using. Reciprocity failure (takes non-linearly longer amounts of time to get the same exposure as exposure time increases) is going to be the main issue here. It will have probably more of an influence than anything else you might consider.
And reciprocity failure depends on film type. We can help you look it up if you give us a brand and type of film.

Your exposure conversion table will be for one specific type of film, therefore. And you'll need a different table for different film.

Oh yes! RF, I am familiar with that one at least. I am using Kodak 400 and 200 speed film (35mm, C41).

Gavjenks · Feb 26, 2014

It's more specific than that. There's T-Max, Tri-X, etc. Let's assume you're using T-Max, though for example:
http://static.photo.net/attachments/bboard/001/001v15-7112484.JPG

Anyway, before you get to that:
1) Set your ISO on the meter like it says to
2) Point to subject and take reading. Let's say it reads "7" (after you do the high/low thing in step #3 in the instructions). It's kinda hard to tell you more about how to do that since the images aren't in color and I'm not sure which part moves. But your reading is 7 EVs
3) Rotate the ring so that "7" in the little notched area lines up with the arrow in the middle.
4) Look at f/32 on the dial, and see what shutter speed it is lined up with. In this example (7 EV) it looks like it would be 15 seconds?
5) Take that number and make it 16 times slower (f/128 is 4 stops slower than f/32). In this example, that would be 128 seconds.
6) Plug your new number into the chart I linked to above. Metered time is ~120 seconds, so for T-Max film, the reciprocity adjusted time is roughly 1000 seconds, or 20 minutes.
7) Shoot your image at 20 minutes exposure.

Don't worry about +/- 1/3 EV for your two lenses, you don't have precise enough tools for that to matter.

In darker scenes, f/32 might be off the scale. If so, just use, say, f/2 instead. And instead of 16x larger number than from the meter, use a 4,096x slower time. (f/2 is 12 stops faster than f/128, 2^12 = 4,096).

Gavjenks · Feb 26, 2014

For T-Max 400:

[TABLE="width: 249"]
[TR]
[TD]Meter's reading[/TD]
[TD][un-RF corrected][/TD]
[TD]Your shutter[/TD]
[/TR]
[TR]
[TD]at f/32[/TD]
[TD][/TD]
[TD]seconds[/TD]
[/TR]
[TR]
[TD]1/60[/TD]
[TD] 4/15[/TD]
[TD] 1/4[/TD]
[/TR]
[TR]
[TD]1/30[/TD]
[TD] 8/15[/TD]
[TD] 1/2[/TD]
[/TR]
[TR]
[TD]1/16[/TD]
[TD]1[/TD]
[TD]1[/TD]
[/TR]
[TR]
[TD]1/8[/TD]
[TD]2[/TD]
[TD]3[/TD]
[/TR]
[TR]
[TD]1/4[/TD]
[TD]4[/TD]
[TD]7[/TD]
[/TR]
[TR]
[TD]1/2[/TD]
[TD]8[/TD]
[TD]18[/TD]
[/TR]
[TR]
[TD]1[/TD]
[TD]16[/TD]
[TD]54[/TD]
[/TR]
[TR]
[TD]2[/TD]
[TD]32[/TD]
[TD]140[/TD]
[/TR]
[TR]
[TD]4[/TD]
[TD]64[/TD]
[TD]300[/TD]
[/TR]
[TR]
[TD]8[/TD]
[TD]128[/TD]
[TD]800[/TD]
[/TR]
[TR]
[TD]16[/TD]
[TD]256[/TD]
[TD]2400?[/TD]
[/TR]
[TR]
[TD][/TD]
[TD][/TD]
[TD][/TD]
[/TR]
[TR]
[TD]at f/2[/TD]
[TD][/TD]
[TD]seconds[/TD]
[/TR]
[TR]
[TD]1/500[/TD]
[TD]8[/TD]
[TD]18[/TD]
[/TR]
[TR]
[TD]1/250[/TD]
[TD]16[/TD]
[TD]54[/TD]
[/TR]
[TR]
[TD]1/125[/TD]
[TD]33[/TD]
[TD]140[/TD]
[/TR]
[TR]
[TD]1/60[/TD]
[TD]68[/TD]
[TD]300[/TD]
[/TR]
[TR]
[TD]1/30[/TD]
[TD]136[/TD]
[TD]800[/TD]
[/TR]
[TR]
[TD]1/16[/TD]
[TD]256[/TD]
[TD]2400?[/TD]
[/TR]
[/TABLE]

imaguitargod · Feb 27, 2014

Thanks!!! I'm going to go and try this out. I'll post if I have a question about it!!!

limr · Feb 27, 2014

He said he was using C41, though. Those values are for black and white film. Is the RF of the Kodak color films similar to TMax?

Gavjenks · Feb 27, 2014

I dunno, limr. It changes dramatically even for the same type of film at different speeds. So almost certainly not. You really need to know like... the exact brand and type and speed you're using if you want spot on exposure.
Either that, or spend huge amounts of time doing test shots and plotting the curve yourself.

I was just choosing a random film as an example. In case that wasn't clear, guitargod, you will probably get some wacky exposures if you literally follow the chart. You have to go find your own correct RF chart and recalculate the rightmost column.

imaguitargod · Mar 2, 2014

Looking at the film canister I have, it just says Kodak 400 (it's color) and in the bottom left corner it says DXN (I think this is their Ultra-Man series)....I'm looking on Kodak's website and their RF chart only goes up to 100 seconds. Going further into their technical documents online, it states " For critical applications with longer exposure times, make tests under your conditions." HA! Thanks, Kodak.... I may just have to use their 100 second baseline and attempt to extrapolate a curve then experiment....

Gavjenks · Mar 2, 2014

imaguitargod said:
Looking at the film canister I have, it just says Kodak 400 (it's color) and in the bottom left corner it says DXN (I think this is their Ultra-Man series)....I'm looking on Kodak's website and their RF chart only goes up to 100 seconds.

Break out some graph paper, plot a curve, and try to follow the same sort of shape and acceleration up to guess at higher numbers if needed.
And/or then just go do it, and see what you get and adjust accordingly.

Any new homemade equipment tends to involve some test shots of your living room and mucking around, even if you think you have all the info you need at first, before you nail it down completely.

imaguitargod · Mar 2, 2014

Gavjenks said:
imaguitargod said:

Looking at the film canister I have, it just says Kodak 400 (it's color) and in the bottom left corner it says DXN (I think this is their Ultra-Man series)....I'm looking on Kodak's website and their RF chart only goes up to 100 seconds.

Click to expand...

Break out some graph paper, plot a curve, and try to follow the same sort of shape and acceleration up to guess at higher numbers if needed.
And/or then just go do it, and see what you get and adjust accordingly.

Any new homemade equipment tends to involve some test shots of your living room and mucking around, even if you think you have all the info you need at first, before you nail it down completely.

Yep! That's what I'm doing right now, starting to play around with it. It's going to go a lot smoother now that I know how to use the light meter and such. Thanks again, Gavjenks!