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View Full Version : My first boxCam How can i find exposition time?



MauroBs
12-20-2010, 12:32 PM
Hi, i follow this forum a lot of time and i find very beautiful work.
I'm doing my first camera. It's in black polyplat and it seem like that work. It's 2 box that compenetrate for focus and i mount a meniscus lens that i take from an old photolens.
For viewfinder i use a shit of translucent paper coat with olive oil for more trasparency.
I'd like to shot in 4x5 plane film, but i have a problem:
How can i find the F value of lens? and how can find exposition time?

I ask to you because i can't know who better you can answer.
Thanks a lot

Mauro

I put here some pics of my cam.

JoeVanCleave
12-21-2010, 01:43 AM
To find the f-stop of the lens: Focus the lens image upon your ground-glass, and measure the distance between the lens and the screen, in millimeters. Then divide this number by the diameter of your lens, also in millimeters. This number is your working f-stop for the focal distance you've set the camera to.

When focusing upon far-distant landscape objects, the f-stop will be a smaller number than when focusing upon a close object. So-called "infinity focus", where you focus upon a far distant object, is the shortest focal length your camera will be required to focus to, for any given lens; if you focus with this lens upon closer objects, the nested box or bellows camera will have to be extended out. How close you can focus is thus limited by how far out the two halves of the box camera can extend, or how far out a bellows camera can extend.

If you choose to use so-called "Waterhouse stops" on your lens (to stop it down to a smaller aperture), just measure the diameter of each aperture stop and divide this into the focal length, to arrive at the working f-stop number. You can place the aperture stops on either the front or the back of the lens.

About exposure times: you have to know the "ISO speed" of your film or paper. Then you take a light meter reading of the scene to be photographed, with the ISO speed set on the light meter. The meter will then give you a series of f-stop values and associated shutter speeds. Since you've already figured out your f-stop value previously, reference this f-stop value on your meter and it will indicate the required exposure time needed.

If you're using fast photographic film and wide-open apertures, you'll find in bright sunny daylight that you'll need shutter speeds of fractions of a second, two quick to accurately time by hand with a lens cap shutter. Conversely, if you use slow photographic materials like ortho lithographic film, or paper negatives, then you an shoot dimly lit indoor scenes with shutter speeds of several seconds. Whether you want to do the one or the other depends on whether your camera lens has a working mechanical shutter that's able to make exposure times of fractions of a second. Another alternative, if you don't have a mechanical shutter on your lens, is to stop the lens down to a very small aperture, which cuts down the exposure enough to get sunny exposure times of a second or longer, sufficiently long to accurately time by hand using a lens cap shutter.

Good luck with your project, hope to see some results soon.

~Joe

MauroBs
12-21-2010, 03:39 PM
Thanx for the istruction, now i think that i try to find my fstop.
I think that i use paper for experiment.
Usually i take 100 ISO for paper speed, it's right for you?
Or paper speed is less?
Thanks a lot for your suggestion

Mauro

I link some camera img because in the preview post i forgot it. Attached files http://f295.f295.org/uploads/img_4780_4658.jpg (http://f295.f295.org/uploads/img_4780_4658.jpg) http://f295.f295.org/uploads/img_4783_8621.jpg (http://f295.f295.org/uploads/img_4783_8621.jpg) http://f295.f295.org/uploads/img_4785_8801.jpg (http://f295.f295.org/uploads/img_4785_8801.jpg)

MauroBs
12-21-2010, 04:07 PM
Ok thanx, i found that my aperture is f7, at max extension of, and i understand that if i squeeze more i recalc it right?
Now i'm doing some diaphragm for extend DOF.
Thanx for all.

JoeVanCleave
12-21-2010, 05:09 PM
Mauro, I like your camera design.

Regarding the ISO of B/W photo paper, it doesn't really have a true "ISO" in the same way that film does, but from experiments many people have done, it appears to vary from 2 to 25. Keep in mind that graded paper has a sensitivity to UV and blue light only, and multicontrast paper also has a bit of sensitivity into the green. So, if you are shooting in bright daylight, the paper's sensitivity should pretty well match the scene's spectral range, whereas if you're shooting under incandescent or tungsten lighting the required exposure will be much longer than what your meter would recommend (your light meter is reading from UV all the way to red, but the paper doesn't have much sensitivity to red, although indoor lights put out plenty of red).

I use Freestyle Photo's "Arista" house brand of graded paper for negatives, grade 2 in glossy resin coated finish. I rate its ISO at 12, providing I'm using fresh liquid paper developer mixed 1:15 and at 68f in temperature.

You should do some ISO speed tests of your paper and developer combination, to arrive at an optimal value. If you under-rate the paper's ISO, you'll get exposures too dense (the negative will be too dark), while if you over-rate the ISO then you'll have under-exposed images (the negative will be too white). I'd recommend setting up your darkroom in the daytime, set up a scene outdoors, camera on tripod, then do a series of exposures using the camera's f-stop and an appropriate exposure time for various ISO settings on your light meter, then develop them and see which one gives you the best exposure.

When you evaluate your negatives, if you have clear, sunny blue skies in the image then ignore their density on the negative; they'll always appear blown-out to almost black in the negative, since there's so much UV and blue in the sky. You want instead to evaluate your exposure based on the main landscape or subject portion of your scene.

~Joe

MauroBs
12-29-2010, 08:30 AM
Thanks a lot for your advice, is was very usefull, if i have time i'try to shot.
Thanks a lot.