Using Light Meter inside Camera Obscura

[holmburgers]

Haven't read all the replies, but indeed, if you're inside the camera, set the aperture to f/1.0, which is equivalent to telling the meter there is no aperture. With a known ISO, the shutter speed will tell you the right time for exposure.

An oft misunderstood concept. In theory, the same technique (using f/1) would work under an enlarger too.

[MattKing]

Is a Sekonik L-358 sensitive enough to appropriately measure the light intensity in a camera obscura?

I would use my Ilford EM10 enlarging meter.

But that only works because I have experience with it in the darkroom.

And it might not be sensitive enough either.

[BrianShaw]

Ideally, I'd still like to try and somehow meter the exposure from inside, but again, as I am taking the reading from the film plane, the light will not pass through any aperture, ....

Well, this confuses me. Having been in a Camera Obscura once or twice... the light comes into the camera through a hole (aperture). Whether that needs to be set on the meter or not I have inadequate eperience. I thought so, and still think so... but holmburgers point about measuring this light just as one would measure exposure on an enlarger table makes perfect sense. I'd belive him, or look up a book that talks about measuring enlarger exposure, or do some test strips. If the meter is sensitive enough you can do what you want to do with no problem. If not sensitive enough, follow the direction given in Chan Trans earlier post.

[holmburgers]

Think about when you're metering with a view camera. You're metering the light of the scene with no "attenuation" whatsoever. Your aperture on your lens however, will of course choke out some of the light as it makes its way to the film. So we tell the f/stop to our meter to account for this.

But if we're already inside the camera, there's nothing that's going to attenuate the light further at this point. By using f/1.0 in the expsoure calculation we are effectively eliminating this altogether, putting 1 in the denominator.

This meter goes down to EV -2, which I think should be plenty to measure daylight pouring through an f/9 aperture.

[BrianShaw]

Ummm. Where's that "embarassed" emoticon? Yes, of course!

[holmburgers]

[Chan Tran]

Well, this confuses me. Having been in a Camera Obscura once or twice... the light comes into the camera through a hole (aperture). Whether that needs to be set on the meter or not I have inadequate eperience. I thought so, and still think so... but holmburgers point about measuring this light just as one would measure exposure on an enlarger table makes perfect sense. I'd belive him, or look up a book that talks about measuring enlarger exposure, or do some test strips. If the meter is sensitive enough you can do what you want to do with no problem. If not sensitive enough, follow the direction given in Chan Trans earlier post.

I actually think there isn't enough light for a good reading. So in my first post I did tell the OP to make some measurement inside the camera and see if he would get any reading. If the light is strong enough above EV-2 for ISO 100 which is only about 6 lux. At ISO 6 you only need about 1.7 lux for a 1 second exposure. If we were to expect exposure to be longer than 1 seconds than the meter can't measure that low a light level. Enlarging meter I think is in order or may a color analyzer in the white channel.

[Joe VanCleave]

If the light inside is too dim for the meter to respond, then meter the external scene with the meter, and use as the aperture setting on the meter the lens's aperture diameter divided into the distance from lens to photo paper. Thus, you would be treating the camera obscura as a large view camera and would meter as you would with any other camera.

Depending on how you mounted the lens to a window or other external opening, you could build a flap-like door adjacent to the lens, which you open in order to meter the external scene's illumination.

-Joe

PS: I would add that measuring directly the aperture of your lens, and dividing that figure into the focal length of the camera obscura, already takes into account the so-called bellows factor, so that no additional computation would be required beyond metering the external scene's illumination.

PPS: If employing the metering flap adjacent to the lens, you would meter the external scene through the flap with the meter set to its reflective metering mode - again, just like you would meter a scene when using a conventional camera.

[Discoman]

I tried making a room into a camera obscura before. Instead of a regular light meter, I pointed a digital (yes, I said it) at the wall the image would be on, and simply started making exposures at a fully open aperture for increasing amounts of tine until I got an image. Then I simply transferred those times over for the known values of the paper I was using and made some exposures that way.

[PanaDP]

Haven't read all the replies, but indeed, if you're inside the camera, set the aperture to f/1.0, which is equivalent to telling the meter there is no aperture. With a known ISO, the shutter speed will tell you the right time for exposure.

An oft misunderstood concept. In theory, the same technique (using f/1) would work under an enlarger too.

I don't think f/1.0 is equivalent to having no aperture. If that were the case, apertures larger than f/1.0 would be impossible, yet there are lenses faster than f1 and they certainly do not magnify light.

Personally, I'd do it with a meter that can read in lux and do a couple sheets of test strips. That's the simplest way you can calibrate your knowledge without doing anything that might confuse the issue.