Spotmeter the ground glass?

Discussion in 'Large Format Cameras and Accessories' started by BetterSense, Aug 20, 2010.

  1. BetterSense

    BetterSense Member

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    I've been interested in focal-plane metering with LF. Has anyone had any success simply using a spot meter, from the back of the ground glass? It seems to me that if it were to work at all, it would require a very good dark cloth for consistency.
     
  2. Marco B

    Marco B Member

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    Haven't done it myself, but I remember some other active APUG member writing about it, maybe Jason (JBrunner), or Ian Grant?

    Do an APUG Google search with spotmeter and ground glass as keywords, you may be able to find the thread...
     
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  3. RalphLambrecht

    RalphLambrecht Subscriber

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    Haven't tried it, but I'm pretty sure flare will be a huge issue.
     
  4. David Brown

    David Brown Subscriber

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    I am just guessing, but it seems to me that you would not get accurate readings since the amount (intensity) of light has been altered by passing through both the fstop of the lens and the ground glass.

    But, beyond that, my question would be "why?" If you are standing there with a spot meter, why not just meter the scene directly?
     
  5. Ian Grant

    Ian Grant Subscriber

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    I've written about it but only in the context of comparing focus screens. You need the proper probe attachment that fits certain meters and slips under the screen for accurate spot readings from an LF camera itself..

    It's reflections rather than flare that are the main issue, but I guess using a spot meter might be possible, however I'd say why when used direct is so much more accurate, as long as you take the extension factor into account.

    Ian
     
  6. Q.G.

    Q.G. Inactive

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    You can meter on (as opposed to under) the GG too.
    I have used the Gossen fiberoptic probe for that without problems.

    I believe a spotmeter will work too, but, as mentioned, reflections will be something to avoid.
     
  7. chriscrawfordphoto

    chriscrawfordphoto Member

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    Minolta made an accessory for that, called the Booster II. It plugged into a jack on the side of the Flash Meter II, III, IV, and V and Autometer III and IIIf (and maybe the newer meters, I am not sure). Sekonic sells a device that looks identical in every way and is probably made by the same subcontractor for some of their meters (I've tried the minolta one on a Sekonic meter and it works!)
     
  8. Photo Engineer

    Photo Engineer Subscriber

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    There is a spot meter made for LF cameras. It is made by Gossen, and comes in 3 parts.

    1. A special back for the camera with a slit and mounting stage for the probe.
    2. A movable probe that moves anywhere on the GG and can measure light.
    3. A Gossen meter that attaches to the probe and gives the reading for exposure.

    These are excellent units, but IDK if they are still available.

    PE
     
  9. Q.G.

    Q.G. Inactive

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    That's the thing Ian mentioned.
    It was indeed also available to fit Minolta meters.
     
  10. Ian Grant

    Ian Grant Subscriber

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    The problem with metering from the glass side (rear) is calibration as screens can vary in brightness by at least 3 stops, and the coarseness/fineness of the ground side will also play a big part. This is why they are best internal.

    Some people make their own meters, and it would be very easy to make an internal spot version if you were prepared to adapt or make a new LF back. It needs a good light seal around where the probe fits and moves.

    Ian
     
  11. BetterSense

    BetterSense Member

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    Oh, of course. I don't expect the readings given by the spotmeter to actually be something you could set on the camera. I'm only wondering if they would be precise. I use homemade meters so I would just recalibrate.
    Why does the reading, in principle, have to be taken from underneath the ground glass? Since I can calibrate my meter, taking the reading from the top of the ground glass itself seems like it would work fine.

    Focal plane metering would
    -compensate for aperture marking inaccuracies/allow use of unmarked apertures
    -compensate for lens flare and differences in transmission between lenses (which can be substantial)
    -compensate for filter factors
    -compensate for bellows extension

    Heck, given the existence of flash meters, it should be possible to take the shutter speed and aperture settings both into account and obtain a true, integrated focal plane exposure dose. It would be left as an exercise to the photographer to translate those readings into a sensible exposure though....I'm not sure how to get density vs. lux-seconds data for film.

    That sounds very fiddly. I have hard enough time dragging along the meters I have. I'm thinking more of just adding an alternate calibration curve to my spotmeter that allows me to take readings off the ground glass. It seems like reflections would be the biggest enemy, requiring readings to be taken under a good dark cloth. And come to think of it, my spotmeter is not focusable (same with most I imagine), so measuring a small spot on the gg wouldn't be easy. I could make a meter with a seal that I could press directly up against the ground glass to take a spot reading of the glass.
     
  12. Ian C

    Ian C Member

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    Booster II Kit

    The Minolta meters have long had a Booster II Kit that included a photocell on a small diameter probe to be placed directly onto various parts of a ground glass to take readings at selected points.

    The Booster II kit is still available for the Kenko and the older Minolta meters.

    http://www.bhphotovideo.com/c/product/35295-REG/Konica_Minolta_8045.html
     
  13. Q.G.

    Q.G. Inactive

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    Been there, done that, etc. It will.

    Gossen's method (not with the ProfiSelect-TTL, which is the thing that goes under the glass, but with the fiberoptic ProfiFlex) was to put the probe on the glass, and use the time the meter suggested for f/1.

    It's easy enough too to calibrate the process to the particular groundglass. Just compare a reading to what you get metering the subject directly. You then have to take bellows extension compensation into account, but only once, when you figure out how much to compensate for the glass.

    The compensation (if any) needed for the glass is a fixed size entity, so no curve, but a flat line.


    I have used the ProfiSelect TTL too, by the way.
    It also works great, but is a lot to do, hence slow, and needs quite a few parts.
     
  14. jp80874

    jp80874 Subscriber

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    If you are not already familiar with it, look into the metering options offered on the Sinar late P and P2 models. It is the probe mentioned above with rear standard designed to accept it. My Sinar 8x10 late P has it, but I have not used it, preferring to meter the subject itself.

    John Powers
     
  15. Ian Grant

    Ian Grant Subscriber

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    The only issue is that the focus screen tends to drop the dynamic range as well, so that might need to be taken into account.

    Ian
     
  16. steven_e007

    steven_e007 Member

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    On the point about spot meters not being able to close focus...

    Just stick a close up lens on the front. I use a Soligor light-meter to measure light intensities at a distance of 25cm (This is for industrial calibration of equipment, not photographic use) and placing a 3 dioptre lens on the front of the soligor allows this. You would maybe need something stronger to get comfortable under the ground glass... maybe a 5 dioptre, at a guess? This will shift the calibration a little, but since you've got to compensate for losses in the ground glass anyway...

    Another point that might be of interest is that using spot meters in my day job, as I do, we realise that in most meters the circle on the viewing screen that denotes the area you are metering is frequently not very well aligned. Sometimes it is a mile off! Sticking a close up lens on the spot meter and metering up a small area that just fits inside the spot really shows this up badly. It is well worth clamping the meter on a tripod with a ball head (most meters have tripod sockets) and metering up a tiny light source, such as an LED in a dark room. Move the meter around carefully until you get the highest reading - then see how well the light source fits in the circle on the screen. You might be surprised. (You may be horrified!)

    If you make your own meters, here's an idea I had a while back, which is on my "Things I might try one day - if I ever get around to it" list. (It is a very long list...):

    Many photo-transistors and photo-diodes have a 'flat' body shape, with the wires coming out at 90 degrees to the direction they receive light. Stick a small square of steel on the back - and a thin layer of plastic tape on that (so it doesn't scratch the ground glass) and attach very fine, flexible, copper wires. By placing a magnet on the viewing side of your ground glass screen and the diode inside on the ground side - you should (hopefully) be able to move the diode around to any point you like to meter it. When you've finished, pull the magnet off and, pulling the spring loaded back open, pull out the sensor and push in the dark slide. No special backs, no slots or complex probes. Never tried it - just an idea.
     
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  17. BetterSense

    BetterSense Member

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    This is what I do to align my meters' aiming device...I use my LED flashlight positioned across the house. The last meter I made had a relatively fixed aiming device, and it was not possible to see into the body of the meter to adjust the position of the photodiode to match. So, I placed a piece of photopaper where the photodiode goes, then aimed the meter at my LED flashlight for a few seconds. I developed the photopaper in place by swiping a bit of developer across it; the dark spot showed me where I should drill the hole for the photodiode.

    I like it...it reminds me of those aquarium cleaning devices that work in a similar way.
     
  18. cyberjunkie

    cyberjunkie Member

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    I have a SinarSix.
    Bought long ago, and seldom used.
    I think it's a proprietary version of the Gossen meter that has already been mentioned.
    It has an arm, with a reading cell, that is connected to a Lunasix-like meter, and with a fake cut film holder where the arm is housed.
    The reading is done BEFORE the ground glass.
    Once you check the calibration, or recalibrate it (if needed), everything is very simple, with no need to calculate filter or bellows factors (well... with some filter would be better to do the reading without it, and then add the filter factor).
    With this kind of meters it is very easy to calibrate the f-stops scale for shutters that are used to house a new lens.
    After the meter readings are taken, it's even possible to print a sticker of the right shape/size with a computer, for a pleasing result.
    Nowhere near the engraved scales made by S.K. Grimes, but better than a scrap of paper with pencil marks taped to the shutter. Isn't it?

    The only bad thing could be the battery that's used to power the meter.
    I haven't checked, because the original battery, believe it or not, still have some juice after all these years.
    But i am afraid that a mercury cell was used, like in Lunasix meters, so if you need to replace the battery a Wein cell should be purchased

    have fun

    CJ
     
    Last edited by a moderator: Aug 26, 2010
  19. Q.G.

    Q.G. Inactive

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    The Sinarsix (indeed a Lunasix) had a CdS cell, and you had to make sure not to leave it exposed for too long, else you needed to wait quite a while for the readings to be correct again. It also was rather slow to arrive at the 'final verdict' in low light (but it sure was good measuring low light levels!).
    Nice thing, though.