Using Light Meter inside Camera Obscura

Leave Me Here · Jul 16, 2012

Dear World,

I'm trying to get my head around the following, any clarification would be greatly appreciated!

- I've turned a room into a Camera Obscura, and I'm trying to use it to expose paper negatives (standard Ilford RC paper).
Is there anyway I can accurately calculate the exposure with a light meter standing inside the Camera Obscura?

I typically rate my paper at ISO 6, then shoot wide open. My lens on the Camera Obscura is f9, however if already standing inside the camera, I'm assuming the aperture on the light meter is irrelevant?

So a working example would be as follows;
- Set light meter to ISO 6, f1.0
- take reading at film plane within Camera Obscura
- expose for indicated time

Am I on the right track??

Cheers,
Matt

ic-racer · Jul 16, 2012

You don't need to know the aperture number when using a film-plane meter. You set the ISO and it tells the shutter time.

Leave Me Here · Jul 16, 2012

Sorry, just to clarify, I'm using a standard incident Light Meter. A Sekonic L-358.

ic-racer · Jul 16, 2012

Leave Me Here said:
Sorry, just to clarify, I'm using a standard incident Light Meter. A Sekonic L-358.

In that case the readings will not allow an accurate prediction. You will have to do some test prints to 'calibrate' your light meter readings.

If you had something like the Horseman film plane meter, you could just hold it near the wall and read the exposure time, though, you would need to read the display from the wall side of the meter.

debanddg · Jul 16, 2012

As said in the post above, what about metering the light falling on the wall where you will finally put your photo paper? Shouldn't it give you an idea about the exposure?

BrianShaw · Jul 16, 2012

http://www.sekonic.com/Products/L-358/Features.aspx#3

Read the sections on Incident Light Metering and Ambient Light Metering.

As ic- said, it might only get you into the ballpark, but seems quite easy to do.

BrianShaw · Jul 16, 2012

p.s.

set meter to ASA 6
Lumisphere retracted
Place meter on "film plane"
Determine light value
Read time for f/9 (or whatever the correct aperture is... you say both f/1 and f/9))
Expose paper for that amount of time... compensating for reciprocity failure if required.

Process, evaluate, and adjust for perfection on subsequent exposures.

ic-racer · Jul 16, 2012

The usual meters out there estimate the film plane light intensity using an equation (too difficult to type here) that takes into account the following:

b = Constant with units of lx⋅cd–1⋅m2
θ = Angle between subject and lens axis
N = Relative aperture (f-number) of lens
F = Lens flare correction factor
f = Focal length of lens in m
V = Lens vignetting factor
Ls = Luminance of subject in cd⋅m–2
T = Lens transmittance factor
u = Subject distance in m

So, yes you could solve the equation for the above variables then predict the exposure.

Much easier to just do some test strips.

Leave Me Here · Jul 16, 2012

Thanks for the replies. A bit more research and I've discovered essentially what I'm dealing with is Bellows Extension Compensation on a large scale. I've jumped from 4x5" where I never needed to worry about it, to UULF 1mx1m upwards, where it is essential.

Now I'm standing inside my camera, trying to work out the exposure with my light meter, when possibly what I should have been doing is working out the focal length or magnification, and apply these figures to the Belows Extension Compensation calculation.

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, and thus that figure should be irrelevant. So when setting/reading the meter, that's what I proposed f1.0, because its the largest the meter will go.

Hope I havent confused anyone! Love to see or hear of some examples of workflow doing shots 1m x 1m (40"x40") or larger.

Cheers,
Matt.

Chan Tran · Jul 16, 2012

Leave Me Here said:
Thanks for the replies. A bit more research and I've discovered essentially what I'm dealing with is Bellows Extension Compensation on a large scale. I've jumped from 4x5" where I never needed to worry about it, to UULF 1mx1m upwards, where it is essential.

Now I'm standing inside my camera, trying to work out the exposure with my light meter, when possibly what I should have been doing is working out the focal length or magnification, and apply these figures to the Belows Extension Compensation calculation.

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, and thus that figure should be irrelevant. So when setting/reading the meter, that's what I proposed f1.0, because its the largest the meter will go.

Hope I havent confused anyone! Love to see or hear of some examples of workflow doing shots 1m x 1m (40"x40") or larger.

Cheers,
Matt.

Use an incident light meter with the flat diffuser. If the light is bright enough to get reliable reading then you're ok. Set the ISO at 25 times the paper speed in your case about ISO 150. Get the reading in EV. If you get EV 0 then use 1 sec exposure. For every EV step higher than 0 halve the exposure time. It should give you exposure of zone V. Measuring this way is really like a spot meter but using an incident meter. Oh you can set the aperture on the meter at f/1.0 then the shutter speed is what you are going to use.

holmburgers · Jul 16, 2012

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 · Jul 16, 2012

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 · Jul 16, 2012

Leave Me Here said:
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 · Jul 16, 2012

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 · Jul 16, 2012

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

holmburgers · Jul 16, 2012

Chan Tran · Jul 16, 2012

BrianShaw said:
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 · Jul 19, 2012

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 · Jul 20, 2012

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 · Sep 16, 2012

holmburgers said:
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.

RalphLambrecht · Sep 16, 2012

couldn't you use a darkroommeter instead?

Chan Tran · Sep 16, 2012

I think darkroom meter makes a lot of sense. I believe this walk in camera has an electrical outlet near by just in case the darkroom meter need AC power.

holmburgers · Sep 17, 2012

PanaDP said:
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 f/1 and they certainly do not magnify light.

If the diameter of the aperture is equal to the focal length (1:1), there is no attenuation of light and nearly 100% will make it through (minus absorption in the glass of course). That's effectively the same as having no aperture. To prove it; take a reading of a scene like normal, suppose the meter reads f/5.6 @ 1/250th. Now, put your lens @ f/5.6 and place the meter at the film plane. At f/1.0 the meter will tell you that you need an exposure of 1/250th.

Now, as for a f/0.95 or larger aperture actually magnifying or amplifying the light, think about this; the image formed by the lens is significantly smaller than the subject that you're photographing. With that in mind, if you can pack the same amount of light into a smaller area, you can indeed amplify the light at the film plane, relative to the subject.

No one seems to believe me on this point, but the reality is that physics proves it. The 2nd point you make about amplifying light is a little harder to wrap my head around, but I believe there's an equation out there somewhere that can show this as well. (I gotta be careful touting the "math", because if you asked me to do it, I'd have trouble...)

Here, the first two equations on the wikipedia exposure value page (http://en.wikipedia.org/wiki/Exposure_value) should be able to prove it if you feel like plugging in some numbers.

PanaDP · Sep 18, 2012

By golly, I tried that and it does indeed work. I stand thoroughly corrected and better educated than before. I can't believe that years of school (much of it in photographic technology) and more years of working hadn't taught me that.

I'm still thinking about the apertures larger than f1.0 but I have a feeling you're also right, that it's the effect of packing the larger area of light into a smaller area. Not exactly amplifying the light but creating the same effect.

johnielvis · Sep 20, 2012

I thought about this and it didn't seem right--so I started on the energy throughput analysis and had a thought "SOMEBODY'S done this before"...a little search and VOILA:

http://www.cctv-information.co.uk/i/Light_Transmission_Through_Lenses

so f/1 gives 20%...20% is 2 1/3 stops less than what you'd meter outside. THE F/1 "trick" takes care of the f number of the lens though--it compensates for that....

Using Light Meter inside Camera Obscura

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