Understanding our Light Meters

R

ruben

Guest
Given the fact that many of us own several and different cameras and hand light meter, I think it would help us a lot if any of our knowledge gifted member could describe the weak sides of the different photo cells.

I understand that there are three types of light meters, either inside the camera or hand meters, sellenium, cds and sillicon. And I am not sure if the sillicon is the one used for digital light meters.

So given the three types working at their best, at which situations each one should not be trusted for accuracy ? Are digital light meters perfect ?

Thanks,
Ruben

PS, the question is not about what type is best.
 
I have only used meters with CDS cells. I've always considered Sunny 16 to be at least as accurate as Sellenium. I also tend to use Sunny 16 to periodically check whether a meter is accurate ... easy with indicent. With reflected and in-camera meters, point it at a cloudless northern sky on a sunny day (in the southern hemisphere, point at the southern sky).
 
ruben said:
...it would help us a lot if any of our knowledge gifted member could describe the weak sides of the different photo cells.
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Knowledge-gifted. Right then... :)

Selenium - doesn't work well as it gets old.

CdS (cadmium sulfide) Ah that would be a direct bandgap semiconductor. It's a light sensitive resistor. It works - but the response is not as linear as that of the

Silicon photodiode. Accurate. What we use these days.

So given the three types working at their best, at which situations each one should not be trusted for accuracy ? Are digital light meters perfect ?
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If they're all working at their best - then they're all OK, surely? Oh hang on - do you mean, if you're doing extreme IR or UV photography, or something like that?

Yes, digital meters are perfect. Everything digital is perfect.

colin
 
colinh said:
Selenium - doesn't work well as it gets old.
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Also isn't particularly sensitive, so you need big cells for low-light usability, and has linearity problems at least at the lower end. BUT it doen't need a battery.

colinh said:
CdS (cadmium sulfide) Ah that would be a direct bandgap semiconductor. It's a light sensitive resistor. It works - but the response is not as linear as that of the
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Also has memory effect problems. When I take a measurement with a CdS meter in bright sunlight and then in a dim room, measurements are likely to be off; the meter takes some time to adjust (with my Weimarlux CdS on the order of a couple of minutes). And a CdS meter is a bit slower than silicon photodiodes. And CdS cells need a constant voltage, and most CdS meters were designed at a time when the most straightforward way to get this was a mercury battery, which means that many CdS meters nowadays have battery problems.

Philipp
 
Hi All,
Not so important the type of the cells, because, not linear response can be calibrated easy and is always done. Also I am not sure we need linear metering all the way form darkness to full white. More important is to distinguish and be aware of reflected or incident type of the meter we use. As known in studio the pro's use reflected type of measuring the light. Usally we use reflected type of meters in the street or landscape shots. Those must be calibrated relative to digitla sensor or film.
 
Selenium: Needs no battery since it's photovotaic (generates electricity) but needs large cells since it's not very sensitive. Cell aging can be a problem. The response curve is reasonably close to that of film.

CdS: Fairly sensitive, small cells can be used. Suffers from "lag" so it's response is a bit slow. If a bridge circuit is used there are no issues on supply voltage but this often wasn't done on older meters with mercury-cell supplies. Cameras like the Pentax Spotmatic, where the needle centres for a correct reading, are usually a bridge circuit.

Silicon: Fast response and sensitive. The response has to be modified to match that of film (hence the "blue" diodes).

Worth remembering that a meter that measures very low light is of limited use anyway, since reciprocity failure is a far greater factor than meter accuracy.
 
Selenium cell produces voltage. The voltage is then shown by a voltmeter (usually needle). Selenium cells degrade in humid environment, as well as when exposed to light for very long time.

CdS cells are just variable resistors and their resistance changes with light. They have quite slow response (or so called memory effect) so when exposed to strong light (sky) only after about 30s or more they will meter darkness exactly.
Also since they are just resistors - they need battery to supply voltage, and then the cell causes voltage drop. The repaining voltage is then usually shown by a voltmeter (needle).
If the circuit is simply connected in series (battery - CdS cell - voltmeter) the battery must have very flat discharge curve - like for example the mercury cells or silver oxide cells. Alkaline cells have sloppy discharge curve and shouldn't be used in this type of meter.
But there are other types of circuit with "bridges" that can work with any type of battery.

Silicon cell produces voltage depending on light. It has very quick response (unlike CdS). These are usually used in digital meters. There is a A/D converter which changes the voltage from the cell to digital value (number). This can be easily converted to LV and then to exposure values shutter speed and aperture. Digital meters can also compensate for non-linearity of the cell, so they are perhaps the most precise.

OTOH every meter can only be calibrated for one reflectance of the subject (when talking about reflected light metering) - this is the middle gray. So it's up to the photographer to understand the (simplified) zone system and compensate exposure for subjects with different reflectance (black or white subjects).
 
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I remember reading that some guy used solar cells to replace selenium cells in old lightmeters, but the only thing I can dig up at the moment is a thread at photo.net.
 
wolves3012 said:
Worth remembering that a meter that measures very low light is of limited use anyway, since reciprocity failure is a far greater factor than meter accuracy.
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But since reciprocicity failure is known (either through manufacturer's data sheets on the Schwarzschild factor, or through experimentation), a meter that is sensible in low still helps in getting accurate exporures at night.

Philipp
 
ruben said:
.....So given the three types working at their best, at which situations each one should not be trusted for accuracy ? ............Thanks,
Ruben

PS, the question is not about what type is best.
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Still waiting friends,

:angel:
 
They're all accurate. You just have to learn each one's quirks and limitations.

I guess the modern silicon stuff has fewer quirks.

I frankly don't know what's inside the guts of my tiny Sekonic incident meter I bought last year. It's problem is that I can handhold below it's EV threshhold ... I can shoot 1/8 at f/1.4, and it can't measure below 1/15 at f/1.4 with ISO 400. So in really low, I just shoot for as long a shutter speed as I dare.
 
Silicon is generally the most accurate and reliable.

However, how the meter is used is much more important than its absolute accuracy. Usually the correct usage for a meter is to determine the exposure needed for a middle grey. This may be done either by taking a reading of the incident light or of the light reflected from a middle grey area. Meters built into the camera are the most likely to be misused (by mindlessly basing exposure on the center of the field of view or a center weighted average of the field of view). Fortunately most negative film is forgiving of incorrect exposure, and the digital darkroom can do wonders with badly exposed images.

Richard
 
rxmd said:
I remember reading that some guy used solar cells to replace selenium cells in old lightmeters, but the only thing I can dig up at the moment is a thread at photo.net.
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Yes,

there's for example the new Sekonic L-398A Studio Deluxe III LightMeter, which uses an amorphous silicon cell and is still battery free. It replaces the old Studio Deluxe II light meter with its selen cell.

~ HD ~
 
ruben said:
So given the three types working at their best, at which situations each one should not be trusted for accuracy?
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OK, now to answer your original question, assuming that the meter is working and that the user knows how to use a meter:
  • Selenium: Don't trust in low light; don't trust after sharp bumps because the needle instruments have to be very sensitive for selenium cells; don't trust it when the meter is very old or was stored in a damp closet or so; verify against a known good meter on purchase and maybe once a year.
  • CdS: Don't trust in very low light; don't trust for some time after intense changes of brightness; give it time; don't trust that you can use them with any battery.
  • Silicon/SBC: Can mostly be trusted.

ruben said:
Are digital light meters perfect?
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Not any more or any less than a good SBC meter.

Many have an interface I don't like; I prefer directly seeing speed/f-stop combinations on the meter dial from having to click through combinations. OTOH, the e.g. Gossen Digisix displays EV numbers which you can use neatly if you have a camera with an EV-based shutter and aperture system, such as the Rolleiflex T or the Iskra-2.

Philipp
 
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I think CdS cells (origionally) unless thay are (were) filtered are (were) more red sensitive then eye or colour film [a bit film like a M8]... o.k. if you have a super pan film IR sensitive) in tungsten light maybe.

Noel
 
THANK YOU PHILIPP,

Anything to say about sunlit bright situations (beyond the compensation stuff)?

Cheers,
Ruben
 
The one eternal problem I have on this subject is how to check or assure the accuracy of a light meter, whether in the camera or a handheld meter. There seems to be no reliable and convenient method of doing this.

For about every other physical measurement we have something that's easy and convenient and more than accurate for everyday use. For length we have rulers, for volume we have measuring cups, for mass/weight we have gram weights, for temperature, we have the freezing/boiling point of H2O, for time we can measure the high point of the sun if we have to.

For luminance or illuminance, we really don't seem to have anything more precise available to the lay person other than "Sunny 16" which I think has too many variables and gotchas.

There really isn't anything like a standard candle or anything traceable to a known standard.

I have several cameras, and I know they are somewhere in the ballpark, but for some reason it frustrates me that all I can do is compare one with another.

Any comments on this angle?
 
I guess the lay person doesn't really need much more precise information than what you need to for slide film. If you've got a meter that is precise enough to do that, use this as a reference for all your other meters. That should really be all the lay person needs. Maybe a gray card, and maybe a step wedge, too.

Incidentally, Sunny 16 is actually about as precise as using the high point of the sun for time :)
 
Okay...I'm just here to fill in one small gap, however trivial (what, me, trivial?):

Pentax, with the introduction of the MX SLR, made a big deal of using a Galliam Arsenide Silicon Photocell (uh-huh, GASP...bet they were dying to use that acronym), and the cell's alleged superiority to SBCs in terms of accuracy, by which I presumed they meant linearity, as well as not needing to be filtered, as was the case with SBCs. Probably not entirely hot air, but also likely nothing to lose sleep over if your camera had an "ordinary" SBC. I liked my MX when I had it, but no, its metering wasn't staggeringly better than anything else I shot with.

I think Pentax, Sekonic and/or Gossen used GASP cells in one or more handheld meters, but I don't recall which.
 
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