Lightmeters adding to confusion?

[Dan Fromm]

Patrik, I have a couple of hand-held meters. They don't agree, no surprise. The two that have been adjusted/recalibrated fairly recently both give good advice. The third was fine until something electrical inside failed.

I bought all three used. All three functioned on receipt and gave results consistent with my Nikons' on-board meters in broad daylight. All three were very nonlinear and basically not safe to use until they'd been overhauled. After that, safe to use.

From which I conclude that all used meters with unknown history are broken until repaired.

[Colin Robertson]

For years I've used a Sekonic selenium meter, mostly incident. I've been very happy with it, but it's not very sensitive in low light. So, thirty minutes ago I took delivery of a Capital SP1 spotmeter. I think some kind of Soligor clone (???). Anyhow, straight out of the box it disagrees with my Sekonic by almost one full stop against a grey card. I expected this since I've seen such discussion elsewhere on this forum. Now, since I have calculated EI's for my two favourite films using the Sekonic, my plan is simply to calibrate the spot meter to match the sekonic. Yes, I expect the two meters to be set to a different ASA setting, but as long as they deliver readings which are in agreement, then surely they become interchangable depending on the situation. Or am I missing something . . . .

[robc]

one thing to remember is that photographic light meters are not just meters. If they were they would tell you how much light is reaching the meter sensor in lumens or candellas or some other measurement which is meaningless to a photographer.

The meter does a transformation of the reading it takes based on certain criteria one of which is the sensitivity of the film (iso film speed or your EI) and the others being manufacturer derived such as the meter being designed for use with positive film or negative film which have different dynamic scale as regards exposure. Then different film manufacturers films of the same type have different scales and the meter manufacturers do tests to work out what they think is optimum for calibrating their meter. They all end up with different results and and the meter is given what is called a K factor which is put into the calculation to give you shutter speed and aperture readout. Different manufacturers have different K Factors so two different makes of meter are never likely to give the same readout. Differing amounts of lens flare and construction of the meter will also cause differences. Different light sensors. Different electronics. And finally incident and reflective meters have always used different criteria to do the calculations.


There is an article at this website about it. (Recommended for insomniacs...)

http://www.largeformatphotography.in...-meter-cal.pdf


Now if light meters only gave a readout of how much light there was, then you could rightly expect them to all read the same. That would of course make it all too simple and uniform.

[EDIT]That is one of the reasons why you should do film speed tests![/EDIT]

[Ralph Barker]

Simple and uniform? You're dreaming, eh, Rob?

While we're in that lovely REM state, how about an ISO standard for a compact protocol for the RF transmission of film spectral sensitivity curves (FSSC)? Then, we could tack that onto the GPS signals, and have it available everywhere. Then, light meters could have little FSSC receivers, measure the light by spectral bands, and give readings according to the film we've dialed in. (The FSSC signals would, of course, automatically update the meter's list of available films.) Oh, wait. We's also need adjustment factors for developer and optimal CI for the target printing method.

Oh, darn. The alarm is going off. Time to wake up, I suppose.

[Jim Rhoades]

My digital Pentax reads the same as four others I've tested it with. Most other meters have a screw on the back to adjust to null. I would then check on a sunny 16 day. With the same meter reading different, are you using a gray card when testing reflective? And the card has to be really big for an accurate read.

[Leonard Evens]

I have a Pentax Digital Spotmeter and a Sekonic FlashmateL308. For the latter, in normal sunlight, incident mode, getting a general reflective mode reading from a scene, and getting a reflective mode reading from a gray card agree pretty well. The reading I get using the spotmeter on a gray card also gives essentially the same reading. However, I did notice some discrepencies, perhaps as high as 2/3 stop, in readings at lower light levels between the Pentax and the Flashmate.

In making gray card readings, the anlge you hold the card can be crucial. It should be perpendicular to the lens axis.

On the other hand, when I use the Zone System, usually by placing important shadow values on Zone III, I find that I give as much as a stop additional exposure compared to the general readings. That would place the reading from the card closer to zone VI. Of course, if you use negative film, exposure isn't critical, except in special cases, because of film latitude.

[Slade Zumhofe]

I have a motto--"I don't care if it is wrong--as long as it is consistantly wrong". Pick one of the meters and stick with it--you can compensate. I bet if tested your lenses they don't all fire exactly 1" either but as long as it is always off the same you have tested it and compensated for this. Of course this is assuming that whatever is off isn't too far off.

[Patrik Roseen]

...I bought all three used. All three functioned on receipt and gave results consistent with my Nikons' on-board meters in broad daylight. All three were very nonlinear and basically not safe to use until they'd been overhauled. After that, safe to use.

From which I conclude that all used meters with unknown history are broken until repaired.

Dan, I think you are right...the meters I am referring to are as you put it 'non-linear'. If one is giving a higher reading in bright situations it could give lower readings in darker situations. And this is adding to the 'confusion' or rather giving me an 'unsafe feeling'.
I certainly do not want to hand over the uncertainty to my friend.

...Now, since I have calculated EI's for my two favourite films using the Sekonic, my plan is simply to calibrate the spot meter to match the sekonic. Yes, I expect the two meters to be set to a different ASA setting, but as long as they deliver readings which are in agreement, then surely they become interchangable depending on the situation. Or am I missing something . . .

Colin, I tried this with these meters, but since they seem to be non-linear just changing the ISO did not help...what works in on low light does not match the high light in my case.

...Then different film manufacturers films of the same type have different scales and the meter manufacturers do tests to work out what they think is optimum for calibrating their meter. They all end up with different results and and the meter is given what is called a K factor which is put into the calculation to give you shutter speed and aperture readout. Different manufacturers have different K Factors so two different makes of meter are never likely to give the same readout. Differing amounts of lens flare and construction of the meter will also cause differences. Different light sensors. Different electronics. And finally incident and reflective meters have always used different criteria to do the calculations.

robc, thanks for your clarification...I never thought of this before that there could be dependencies between meters and film like you suggest.
Regarding what you say about different K factors for incident and reflective readings??? The meters I have now are all the type where the only difference between incident and reflective is that the white dome is pushed over the CDS-sensor (But it could be that there is something happening on the inside but I doubt it). so I assume they only include one and the same K-factor. Mybe this has changed in the modern 'state of the art' light meters including both incident and reflective mode.

Thank you all for your time and willingness to share you knowledge with me, Patrik.

[Dan Fromm]

Patrik, linearity can be restored. Here in the US, the preferred meter repair shop is probably Quality Light Metric. Sorry, can't suggest repairmen closer to Stockholm.

Cheers,

Dan

[David_Senesac]

I'd expect that comparing incident and reflective sensors on the same meter needs to be done quite carefully in order to arrive at a valid reading. Each manufacturer is likely to have their own methods of calibration unless there is a common ISO or other optical technical standard to do so. For any of the more prominent lightmeter manufacturers given the cost of these instruments and the modest amount of units produced, one would expect products to be optically tested. Accuracy might just be to a few percent but one ought to expect it to be considered reasonable for the intended use.

I have a Polaris Dual-5 digital spot/incident meter that years ago I compared to my old OM-4T sensor after I received it. The readings tracked reasonably enough across dim to bright targets well enough that I was able to continue to expose film with the same strategies without going through a period of trial and errror. Since I expose for narrow latitude Provia transparency film and rarely bracket, I depend on consistent results on readings that I then interpret with other factors to select exposure. ...David