Pinhole lens; does exposure change w/ varying focal length?

[dennis lee]

I've just begun experimenting with a pinhole lens on a Century

Graphic 6x9 camera. Will the exposure factor change as I move

the lens board in and out to vary the focal length?

[david_nebenzahl]

Yes, because the effective aperture will change with differnt pinhole-to-film distances.

 

To figure out your aperture, you need to know (roughly) how big the pinhole is. Let's say for the sake of discussion that it's about .010" (a typical size). The aperture is the the distance from pinhole to film ("focal length") divided by the pinhole size. So do the math, and you can figure the aperture. It will probably vary from f/250 to f/600 or more.

 

For those pedants who would say that this is an oversimplification of how to figure aperture, I reply that you're correct but irrelevant. It's a pinhole, for chrissake.

[sjixxxy]

I always figured my pinhole size by running them through my film scanner and reading the diamer in photoshop.

 

As for exposure, once you know its f/stop, figuring exposure is easy. Meter the scene the same you you normally meter it at any f/stop and plug the numbers into this formula. pinhole shutter speed = metered shutter speed * ((pinhole f-stop^2)/(metered f-stop^2)) (I think that is correct) then factor in any reciprocity.

 

Best to probably remember the f/stop at a few given focal lengths and stick with them.

[david_nebenzahl]

<i>Meter the scene the same you you normally meter it at any f/stop and plug the numbers into this formula. pinhole shutter speed = metered shutter speed * ((pinhole f-stop^2)/(metered f-stop^2)) (I think that is correct) then factor in any reciprocity.</i>

<p><b>Do what?</b> You mean to tell us you actually use this formula? I don't believe it.

<p>All you've got to do is have a little "extension" scale handy so you know how to extrapolate from the highest f-stop shown on your light meter. I just wrote down the numbers on a piece of paper:

<p>64 - 90 - 128 - 180 - 256 - 360 ... n

<p>I can mentally figure exposure times: "Lessee, meter says 1/20th at f/45, so that's 1/10th at f/64, 1/5th at f/90, 1/2 [see, I'm mentally rounding here--don't worry, it's fine, it's a <i>pinhole!</i>] at f/128", etc., etc. <b>Much</b> simpler than messing with some ridiculous formula!

[chad_jarvis1]

Not to put too fine a point on it, but actually with a pinhole lens exposure determination should be more exact (than with really big openings), as the potential margin of error increases with smaller apertures. So I would think that the use of some BASIC MATH (what?!?? you actually know how to calculate SQUARES?!?!) is the wise way to go.

[chris_ellinger]

An alternative method of scaling exposure readings is to modify the film speed setting. For example, my zone plate is f/64, but the smallest aperture on my light meter is f/32, requiring a shift of two stops. So adjusting the film speed setting two stops slower, allows me to use the time opposite f/32 on the light meter without any further conversion.

[pvp]

Since the f/number is defined as the focal length divided by the aperture diameter, yes: the exposure will change. I use a 0.5mm pinhole on 4x5, a very convenient size as I need only measure the bellows extension (in millimeters) and double that to get the f/stop.

[j.w.]

For some funny reason, it always ends up being 30sec - 1 min in bright sun for paper negatives, and 5 - 10 sec for film in bright sun. Doesn't matter f/ratio of camera, type of film or paper. I have no idea why this is true, except that's been my experience with pinhole over the last 10 years.

 

Lately I've gone to concentrating more on subject and composition and less on fine-tuning to the Nth degree the proper exposure.

 

Pinhole, as a process, lends itself best to a more intuitive approach, which is kind of counter-intuitive to most LFer's mindset, who generally are not satisfied with "nat's ass" precision; they want to be able to count the polyps, too.

 

Throw some RC paper into the back of your camera, set up paper processing trays, go outside and shoot some scenes, then immediately develop and contemplate the results. Repeat procedure until you're confident of the results in a consistent type of lighting. Then start thinking about the type of scenes that best take advantage of the extreme depth-of-field that pinhole offers.

 

I discovered things worth shooting that I normally wouldn't think of; like insect nests, and closeup details of plants and roots, etc.

 

Pinhole is a new lens for uncovering fresh views of the world. Enjoy.

[stephen hazelton]

Yes.

[woyteg]

Ok, since English is not my native language I got a little bit lost here. My question is: Is the formula "pinhole shutter speed = metered shutter speed * ((pinhole f-stop^2)/(metered f-stop^2)" correct?

 

I just started playing with pinhole photography and while waitong for my "Santa Barbara" I'm trying to learn as much as I can.