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  1. #21
    RPC
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    Quote Originally Posted by fotch View Post
    The card is only reading a small part of the scene, the dome is reading the entire scene.
    The dome is used to read the light illuminating the scene, not the scene itself. The gray card is used to represent a typical scene, not be a part of one unless you put it in the scene and spot read it. In any case the meter should be calibrated to give the same results, or at least close.

  2. #22

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    Quote Originally Posted by Bill Burk View Post
    You might need to get in the habit of using Tungsten speed ratings that are lower than Daylight speed ratings. Sensitivity of selenium cell to near infra-red makes the needle jump higher under Tungsten than it should.

    ---

    By the way 1 stop higher reading of 18% gray card seems perfectly reasonable.
    Bill, your comment about Tungsten speed rating is interesting. I keep asking myself why Tri-X Super 8 Reversal is rated at 160ASA for Tungsten and 200ASA for daylight. I never thought about b&w Tri X 35mm having two different ratings as I thought it was panchromatic. Is this related to the selenium meter sensitivity that you mention (which I guess many Super 8 cameras, like mine have) or another factor?

    Thanks for showing us the graph. It looks fascinating and satisfyingly methodical. Your description kind of makes sense but I'm afraid I still find it hard to read it without the background knowledge...

    You comments on the Weston make me think that mine is working within acceptable limits and I will carry on with the 18% grey difference in mind when metering but probably rely on the incident reading until my results tell me differently. I'm testing with a Stouffer zone wedge step as my guide.
    Last edited by mr.datsun; 02-23-2013 at 06:14 AM. Click to view previous post history.

  3. #23
    fotch's Avatar
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    Quote Originally Posted by RPC View Post
    The dome is used to read the light illuminating the scene, not the scene itself. The gray card is used to represent a typical scene, not be a part of one unless you put it in the scene and spot read it. In any case the meter should be calibrated to give the same results, or at least close.
    Not necessarily, it depends on the where and how the light falls, and where the gray card is for the reading. Example: wooded area in bright sun. If say, outdoors, in a field, nothing to obstruct or alter the light, then, in theory, should be close to the same.
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  4. #24
    fotch's Avatar
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    Quote Originally Posted by mark View Post
    Yep. But I would word it as the dome is an average. the reflected is a small part.
    Whatever.
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    The dome isn't an average measurement of the scene but rather a measurement of light falling on the dome which is a small area. (if you call it average, it averages within the area of the dome only not the scene.). The spot meter measures light reflected of a small area (depending on the how far you position the meter). So neither the spot meter nor incident meter is an average reading.

  6. #26
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    The grey card method is prone to mistakes mainly due to the fact that you have to keep it a 45° between the light source and your subject, and you don't have to project your shadow or the camera shadow over it (very easy to project a camera shadow), and that you should avoid flare on the card, and that, I suppose, also on the vertical plane the card should be at 45° between light source and subject.

    You basically need three or four arms. It's extremely unlikely you have more than 2 arms.

    A grey card is a cheap substitute or an emergency substitute for an incident light meter.

    Nowadays one can find relatively cheap and good incident light meters so grey cards are something that I learned to disregard since years.
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  7. #27
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    Quote Originally Posted by Chan Tran View Post
    The dome isn't an average measurement of the scene but rather a measurement of light falling on the dome ...
    That is a great description Chan Tran. Thank you.
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  8. #28
    RPC
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    In practice, the dome is used to integrate the light illuminating the scene. It is normally held somewhere in the scene and pointed toward the camera. Used in this manner it will integrate the the light coming from the light source (sun, flash, etc.) and the light reflected off of the environment in front of the scene, all which illuminate the scene. The meter reads this integrated light and gives you a camera setting that, based on a given ISO should place an 18% gray card in the scene, or any light level, at its proper place on the curve. This is a general description and actual results may be affected by where exactly the meter is pointed but pointing it at the camera is known to give good results.

  9. #29
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    Quote Originally Posted by mr.datsun View Post
    Bill, your comment about Tungsten speed rating is interesting. I keep asking myself why Tri-X Super 8 Reversal is rated at 160ASA for Tungsten and 200ASA for daylight. I never thought about b&w Tri X 35mm having two different ratings as I thought it was panchromatic. Is this related to the selenium meter sensitivity that you mention (which I guess many Super 8 cameras, like mine have) or another factor?
    I'd known of the different speed ratings in darkroom dataguides for years, and never thought too deeply of why until recently...

    I picked up a (new to me, but I'd had one before) Weston Master II recently, and noticed the needle really jumps when aimed at a regular light bulb. At the same time I was reading LP Clerc. Section 402. Daylight and Artificial Light Speeds. Explained several factors: "spectral distribution of the artificial light, colour sensitivity of the film and spectral response of the exposure meter." "Photo-sensitive -- and especially photo-conductive -- cells tend to respond more to the longer wavelength bands than an average panchromatic emulsion." "This tungsten speed rating is thus an equivalent rating rather than an actual emulsion speed"

    (p.s. CdS cells are the photo-conductive cells).

  10. #30
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    Some time in the 1960s the color temperature of the calibration light source was changed from 2800 K to 4700 K. It was suggested that the change would allow the discontinuation of requiring a daylight and tungsten EI for films.

    The spectral sensitivity of the photo cell has always played an important role with meter accuracy. Two meters could disagree simply because of the difference in sensitivity to the color temperature of the light source. Also, most photo cells are sensitive to infrared. If the material reflects a lot of infrared it will effect the reflective meter while the incident meter is unaffected. A simple test for infrared sensitivity can be done with any remote. Point it at the meter and monitor any changes.

    Chan Tran was right about how a reflective exposure meter has a greater chance of matching an incident meter when the incident meter is used with the disk. They should be about 1/2 stop difference when using the dome.

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