Calculating exposure with no lens

I know this is kind of a strange question, but I hope someone can help anyway!

I'm trying to figure out how to properly meter a film exposure with no lens on the camera - in other words, controlling it just with exposure time/shutter speed. I assume that if I were to calculate an exposure time for a hypothetical f/1.0 lens (hypothetical because I know they exist, but I definitely do not have one!), it would still be too long of an exposure, correct? How many stops does an f/1.0 lens absorb (in other words, how many stops should I decrease my exposure time to convert from an f/1.0 exposure to a "no lens at all" exposure)?

Well, the size of the aperture and the distance to the film still define an f-number, with or without a lens. If the diameter of the aperture is the same as its distance from the film then it will be F/1.
You can approximately ignore light absorbed by a lens -- it's not very much. Without the focusing of a lens, you'd get about medium gray all over your film, and no image.

Welcome to photrio!

The f/1.0 is not a good assumption! That merely identifies the marginal ray angle to the image plane, or the concentration of light per unit area. Outside of a pinhole arrangement with a defined marginal ray, it is not appropriate to think in terms of f/# for a lensless system!

OP, what you’re asking for is a radiometric calculation determined by the geometry of the imaging system including the light source as well as the irradiance at the lensless hole. This will give you the amount of energy falling on the film.

From there, you need to convert the irradiance determined above to an exposure value and know where that value is on the film’s characteristic curve.

It is not an easy problem to wrap your head around, but omce you have then you can solve it.

Look up radiometry and start reading. . In particular you would be interested in the equations/geometry for a “simple radiometer” setup, which is what you are describing.

-Jason

A sensitometer exposes film without a lens. Since the ISO is equal to 0.8 over the exposure (in Lux-sec) needed to obtain a 0.1logD density on the film, you can get a handle on how much light in Lux-sec is needed to just get some density on the film.

ic-racer is correct. If you then need to convert the results of a sensitometry test to the exposure on some lensless arrangement that you are trying to build, then radiometric modeling of your setup is required to tell you things like how far away and how bright a light source needs to be.

A faster approach is to simply do a series of exposure tests when you set up your lensless camera.

like a pinhole camera???
a known "aperture" to start, and a known measure of the light sensitive material (asa/iso), then you back into the "speed" of exposure...
... or i don't really know, but that's how i'd start!


welcome!