What F/stop approximates no lens at all?

BetterSense · May 26, 2010

I'm enlarging 35mm slides to Tmax sheet film with an enlarger.

I know the speed of the film well. I should be able to project the image of the slide onto the enlarger baseboard, and use my incident meter to take a reading. Then I need to interpret the reading however. The trick is that a photographic meter gives recommendations for how to expose for light falling on the scene. What I'm measuring here amounts to the light falling on the 'film plane' of the camera. The question then is, at what f/stop is the light falling on the film plane of the camera, equal to the light falling on the subject? F/1?

I know that what I really need is a luxmeter or something, but the question itself interests me anyway.

richard ide · May 26, 2010

Could you not take a meter reading of the slide by removing the lens and pointing your meter at the slide? Then calculate your exposure taking aperature and enlargement factor into account. That should get you pretty close. A 21 step transparent grey scale is a really useful tool for this type of application. The best results will sometimes necessitate making a contrast control mask. In any event I think a lower contrast negative will retain the most detail.

BetterSense · May 26, 2010

Could you not take a meter reading of the slide by removing the lens and pointing your meter at the slide?

I'm not sure. I think that would work if I had a spot meter. I'm not sure how to accurately measure the brightness of a 35mm slide with an incident meter considering it's lit by transmitted light and not incident light.

Rick A · May 26, 2010

I'm not sure I understand what you are trying to accomplish here, but I have tried to take meter readings of negatives enlarged before. I thought it would help me become consistant printing, to no avail. I now use an Ilford EM-10to calibrate my negatives/paper, but I dont think thats what you are after. Are you trying to ascertain a reading for printing purposes? Are you trying to use the zone system for printing? If so I dont think that will work, not the way you are thinking. Hmmm... I think this is one for someone who has the time to extrapolate the datum, I'm getting ready to run out the door to go turkey hunting with my kid(its 4AM)

2F/2F · May 26, 2010

How 'bout doing a test strip?

Athiril · May 26, 2010

If it helps I just used my evil camera (30D) with prime lens @ f/2.8 on spot meter at a evenly lit wall, it reads 1/50th to 1/60th, taking the lens off, it reads 1/160th.

This is 1.42 to 1.68 stops difference away from f/2.8

This is f/1.57 to f/1.71, would need testing with other cameras seeing what they meter, as well as some light meters.

This is in a indoor (so low light) tungsten lit room.

Incident meter reading is around 1/8th (same f-stop, ISO) from that area.

BetterSense · May 26, 2010

How 'bout doing a test strip?

I could, but that would be a waste of expensive Tmax film, and 12 minutes of developing time, just so that I can trial-and-error the exposure. Do I do a test strip in the field? Of course not. I meter. Why shouldn't I be able to meter in the darkroom just as easily? The trouble is that metering the 'scene' is hard when it is a slide chucked in an enlarger. It's easier to measure the light at the film plane in this case. I can make a perfectly exposed negative in the field in one try by using a light meter, regardless of the scene. I should be able to make a perfectly exposed internegative in one try, I just need to know how to correlate incident light readings taken at the film plane with incident light readings taken at the scene.

I'm not sure I understand what you are trying to accomplish here

I want to know what's the correlation between a light meter reading taken at the camera's film plane, and a light meter reading at the scene. There must be one, because metering the scene always gives consistent results regarding film exposure.

I can't just take off my ground glass and take a reading of the light coming through the lens with my camera meter because obviously, with any given scene brightness, the brightness at the film plane varies with the camera's f/stop. What I need to know in order to take film-plane readings is what theoretical f/stop results in a film-plane light reading equal to the scene reading.

richard ide · May 26, 2010

In the method I suggested, You would want an average reading not spot. You are going to have to do tests anyway. You will need some very serious ND filtration as well.
I have just given this a serious rethink. Using sheet film, I assume you have a 4 x 5 camera. It will be far easier to photograph your slides (backlit) as the lens shutter has the fast speeds you will need. As I suggested, you will probably need to make contrast control masks as well.

outwest · May 26, 2010

Film has a speed, paper has a speed. Once you get a good exposure on paper, translate it to the film's speed????

bblhed · May 26, 2010

As said in the first post, remove the lens and meter the illuminated slide. You know the f/stop of your lens, set the exposure time based on the light meter and film speed at the f/stop of your enlarger lens and then you can expose your film. While you are at it meter the projected light and figure out what F/stop gives the same time reading at your film speed, that f/stop is the one for your particular set up only. You will have to redo the whole calibration if you change anything, the light bulb, lens, film speed, the light meter, anything. Good luck and I hope that helped.

anikin · May 26, 2010

Sounds like you are doing internegative. Here's how I do it. I don't have a light meter, but I have Nikon FE2 camera. I just set internegative film speed on the camera ISO dial, take the lens off and put the camera under enlarger with lens opening up. Open enlarger's lens up all the way, lock exposure on the camera (stop-down metering mode) and read the time from the camera's meter. After that, multiply the time by 2 for each stop you are going to close the enlarger lens, and you have your exposure time.
Works like a charm.

RalphLambrecht · May 26, 2010

BetterSense said:
... The question then is, at what f/stop is the light falling on the film plane of the camera, equal to the light falling on the subject? F/1? ...

Yes, f/1!

You can prove it to yourself by taking a camera with a buit-in lightmeter. Mount a wide-angle lens (to mimic the wide angle of view without a lens) and hold it close to your evenly-lit monitor. Note the proposed exposure. Take off the lens and do it again. Now compare. I did and this is what I got:

with 20mm lens: 1/15s @ f/2.8
without the lens: 1/125s

Now, if I'm not mistaken:

1/15...1/30...1/60...1/125
f/2.8...f/2.0...f/1.4...f/1.0 !

Christopher Walrath · May 26, 2010

Posted wirelessly..

Two things pop into the head here. 1st, the light coming through the slide will probably be similar in intensity to natural light, within a couple of stops. You'll need the capability to control-expose the sheet film for a fraction of a second. 2nd, without a FOCUSING lens, wouldn't it kind of negate the ability to obtain a sharp latent image?

What F/stop approximates no lens at all?

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