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.
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.
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.
You need the proper probe attachment that fits certain meters and slips under the screen for accurate spot readings
Focal plane metering would
But, beyond that, my question would be "why?" If you are standing there with a spot meter, why not just meter the scene directly?
-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.
There is a spot meter made for LF cameras. It is made by Gossen, and comes in 3 parts.
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.
Been there, done that, etc. It will.
Originally Posted by BetterSense
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.
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.
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.
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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.
Last edited by steven_e007; 08-21-2010 at 05:59 AM. Click to view previous post history.
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.
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!)
I like it...it reminds me of those aquarium cleaning devices that work in a similar way.
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.
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
Last edited by cyberjunkie; 08-25-2010 at 11:25 PM. Click to view previous post history.
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.