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  1. #1
    Bosaiya's Avatar
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    Calibrating L-718 for G.G. measuring

    I just picked up a Sekonic L-718 with metering attachment to measure the ground glass for macro work. Normally what I do is use my old Sekonic L-398, take an incident reading, find the equivalent reading for the bellows length, and set the lens length to that reading. This works almost flawlessly, but the meter is limited to 60seconds (most of my exposures are longer) and there is a lot of twisting of dials involved. I figured I'd get a meter with a G.G. attachment and just press a button.

    The manual for the G.G. attachment says to multiply the ISO of your film by four with an example of a 100speed film being set to 400. I use 25 so I set it to 80. It says to then adjust for the darkness of the G.G. itself by measuring a lightbox with the probe and on the G.G., in their example they ended up using 500 for the film speed. So I bumped the ISO on the meter up to 100 just for a quick test. Then I blocked out the stray light as best I could (I could see the image bright and clear on the G.G.) and pushed the button with the probe firmly on the G.G. My lens was open all the way (5.6).

    To say that the measurement was off would be a major understatement. On my old Sekonic a correct exposure for a 3.5" lens at 27" of bellows under the lighting conditions was 1 second. The new Sekonic measuring on the G.G. gave four minutes(!) of recommended exposure. The Polaroid test I did agreed with my old meter perfectly.

    I compared the two meters just using the incident attachments and found them to be in agreement within a stop. Then I figured Id see just what ISO setting I would need to get anequivalentt reading on the new meter, figuring Id just set the new meter to that. To get the same reading I need to set it to an ISO of 4000!

    Clearly I am doing something incredibly wrong - like perhaps trying to use it at all. I've read plenty of people complain that this is not a reliable method of metering, but I've also read a lot of people who wrote that it worked well. It seems unlikely that the meter would work that poorly on its own without my intervention, I can only imagine that I've messed something up along the way.

    Any ideas?

  2. #2
    David A. Goldfarb's Avatar
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    You should be comparing a reflective reading of a uniformly lit surface with the lens focused at infinity (the surface doesn't need to be in focus) and wide open to the groundglass reading of the same surface.

    Then when you go to meter a real subject, you should meter with the lens wide open and stop down for the exposure.

    On my Minolta Booster II probe, the probe has a dial to boost the signal for calibration, so it isn't necessary to change the ISO seting on the meter. Does your GG attachment have something like that?

    [I moved my response from the original thread, and I'll delete the original misplaced thread].
    flickr--http://www.flickr.com/photos/davidagoldfarb/
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  3. #3
    Bosaiya's Avatar
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    Thanks for the quick reply.

    Quote Originally Posted by David A. Goldfarb
    You should be comparing a reflective reading of a uniformly lit surface with the lens focused at infinity (the surface doesn't need to be in focus) and wide open to the groundglass reading of the same surface.
    I've only got the incident dome attachment available. Are you saying I should try to get the non-G.G. reading to match with the G.G. reading at 1:1? Or do you mean something else about focusing to infinity? I almost never shoot 1:1 so I'm not sure how that would help. Or is that just to get a baseline?

    Quote Originally Posted by David A. Goldfarb
    Then when you go to meter a real subject, you should meter with the lens wide open and stop down for the exposure.
    Yep, tried that, but perhaps I missed the first part by too much.

    Quote Originally Posted by David A. Goldfarb
    On my Minolta Booster II probe, the probe has a dial to boost the signal for calibration, so it isn't necessary to change the ISO seting on the meter. Does your GG attachment have something like that?
    I'm not using a G.G. attachment other than the little thing that replaces the incident dome on the meter itself. It's just a fiber-optic attachment, it's not electronic or anything. I looked at getting an actual probe for my Sinar but the cost was prohibitive.

    According to the manual that came with the attachment, which is sketchy to begin with, you just need to compensate for the loss of light (the "times four" plus G.G. darkness factor).

    Quote Originally Posted by David A. Goldfarb
    [I moved my response from the original thread, and I'll delete the original misplaced thread].
    Thanks, sorry about that.

  4. #4
    Bosaiya's Avatar
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    I just tried doing some tests in my dark studio. I set up a 1K hotlight with a diffuser on it as the only light source and pointed the lens at it. The G.G. reader and the calculations with the old manual one matched, more or less. I'm guessing that there's just a problem with using it in strong daylight no matter how much I try to block out the stray light. Still seems a bit funky, but I don't mind limitations as long as I'm aware of them.

  5. #5
    David A. Goldfarb's Avatar
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    Quote Originally Posted by Bosaiya
    I've only got the incident dome attachment available. Are you saying I should try to get the non-G.G. reading to match with the G.G. reading at 1:1? Or do you mean something else about focusing to infinity? I almost never shoot 1:1 so I'm not sure how that would help. Or is that just to get a baseline?
    I didn't say anything about a reading at 1:1. This is just calibration.

    When you're taking a groundglass reading, you are taking a reflective light reading. In theory, you could focus the camera at infinity, aim the camera without refocusing at a uniformly lit 18% grey card (the card will be out of focus), take an incident reading at the position of the grey card with the dome pointed toward the lens, and then take a reading with the GG probe and adjust the ISO setting so that the reading with the probe matches the incident reading. The potential problem with this is that you may have an error in reading the grey card, so I like to be sure and compare a reflective reading to a reflective reading.

    I think after this step, your procedure sounds okay. After calibrating at full aperture, you're taking readings from the groundglass at full aperture and then stopping down. One potential issue if the light level is very low is that you could be in the non-linear range of the meter, and that's something you may not be able to avoid without adding more light.
    flickr--http://www.flickr.com/photos/davidagoldfarb/
    Photography (not as up to date as the flickr site)--http://www.davidagoldfarb.com/photo
    Academic (Slavic and Comparative Literature)--http://www.davidagoldfarb.com



 

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