2 meters, different readings

[Jeffrey]

Hi everyone,

I just got another Pentax Digital spotmeter as a backup for my first one which is Zone6 modified. I see a 1/3 to 1/5 EV difference in my tests. I trust my original one, as my images confirm that. Is there a way to adjust my new one (not zone6'd) to match my trusty one?

 

[Loose Gravel]

Jeffrey

Are they off the same up and down the dynamic range of the meter, say from 5 to 15EV? The Pentax can have lumps and bumps in the transfer function.

There are several ways to fix. Send them both to Quality Light and Metric and tell them to make them match. Or, set the ASA dials differently on the two meters. Or, put an ND on one of them. Or open the meter up and start turning screws and then go back to the first suggestion.

If one is Zoned and the other not, that could be part of the issue. 1/3 stop is not much. You could just let it go. Or try dropping the meter. That usually tweeks them about 1/3 stop.

 

[Jeffrey]

Thanks, Loose.

Hey, you're in SB. Where?

 

[DBP]

I believe I read somewhere that normal tolerances were ± 1/3 stop.

 

[Jim Noel]

It is unusual to have two meters read exactly the same across the range of the scale. SInce one of these is a Zone 6 modification mad apparently the other is not, it would be amazing if they read the same. WHen I first sent ameter to have it altered, I noticed more difference than that when the meter was returned. If your new meter stays linear withtheold one across the entire scale you can really count yourself lucky. ANy deviation of les tha 1/3 stop can be ignored without problems. Most of us make greater areas than that in our metering methods.

Basically, the difference is so slight as to not be a major concern anyway.

 

[vet173]

As Jim said, if it's linear all you have to do is look at the shift to see what the shooting speed for that meter is. Go to the camera store and try to find a matching pair.

 

[Stephen Benskin]

Meter's spectral sensitivity

The major cause for meters to disagree is due to the spectral sensitivity of the photo cell. Zone VI modification uses filters to compensate for the cell's spectral biases. Odds are the differences between the two meters will vary depending on the color temperature of your light source and the color of the subject being metered. If you adjust your unmodified meter to match the modified meter under a given condition, chances they will differ in a different situation.

In the ANSI and ISO standards for calibrating a light meter, the two primary variables in determining a value of K is lens transmittance and the spectral sensitivity of the meter's photo cell.

Anyone remember when film had two speed settings, one for incandescent interior and one for exterior? That was really in response to the sensitivity of the meter and not the sensitivity of the film.

 

[timbo10ca]

Wise man say man with 2 watches always late. Man with one watch know exactly what time it is.

 

[Roger Hicks]

Anyone remember when film had two speed settings, one for incandescent interior and one for exterior? That was really in response to the sensitivity of the meter and not the sensitivity of the film.

Dear Stephen,

Not always, of course. Ortho films are much slower to red light, and I'm pretty sure that some pan films are significantly different too. You'd know more than I about this but I thought I'd better throw it in for lurkers.

Cheers,

R.

 

[Helen B]

If anyone is interested: The current ISO for exposure meters (ISO 2724:1974) suggests that meters are calibrated for light with a correlated colour temperature (CCT) of 4700 K (ie between daylight and tungsten), and that their ‘transmittance’ is measured by comparing the meter’s response to the theoretical photopic response of the eye, between two sources: 4700 K and 2856 K. The transmittance is not used in determining the calibration constant, it is just a value that can be quoted to show how closely the spectral response of the meter is to that of the eye, in that particular CCT comparison. Have you ever seen a meter’s transmittance quoted? The ISO also suggest that a meter’s sensitivity is given as the exposure time in seconds at f/8 with ISO 100 film. Ever seen that?

The ISO is based on light measurement in lumens per square metre or candelas per square metre. That means that it is calibrated to the photopic response of the eye, but it doesn’t mean that the meter has to have the photopic response of the eye. It is up to the meter manufacturer. In practice, both Sekonic and Gossen filter their meters to give a spectral response that is close to, but not exactly, the photopic response. As far as I know both Gossen and Sekonic meters are biased slightly towards red in comparison to the photopic response (which peaks at 550-560 nm) and they have a less sharp peak – ie more sensitivity to blue and red than the eye. The Gossen curve is shown below.

If you want to compare the Gossen curve to the spectral response curve of a film, then you need to be careful about the type of film curve you are using. Kodak show equal energy curves, and these can be compared to the meter response curve. Ilford and Agfa show wedge spectrograms in tungsten light, so these represent a combination of the spectral distribution of the source as well as the spectral response of the film: they will underplay the blue sensitivity and overplay the red.

Unlike the older ANSI standard, the current ISO does not mention lens transmittance at all. A lens transmittance of unity is implied in the formulae for the calibration factors (ie the f-stop is used, not the T-stop). The manufacturer can choose the calibration factor, so you could argue that lens transmittance is included in the choice of calibration factor.

Best,
Helen

 

[Ed Sukach]

Informative article, Helen ... although I had to read it a couple of times.

Given the complexities of the light measuring device and its relation to the spectral response of the film, (adding in differences between "T" Stops and f/stops - and ambient light spectra), it is remarkable that most exposures are as close as they are in real life.

Does anyone wonder why I cringe at the declaration by some that, "I ALWAYS (alternative: "you NEED to") contol your exposures to 1/10th stop"?

I've worked extensively with Cascade Photomultiplier - based instrumentation, and all I can say to those "1/10th stop" types is, "Good luck - you are going to need LOTS of it."

Just thought of a similie: "Like trying to measure a spring made of Jell-o with a rubber ruler".

 

[Chuck_P]

If you get into the ZS method of exposure and development (I assume you do of some sort, maybe this is a bad assumption) and you have determined your personal film speed using the original spot meter, and you have determined your normal development time for the enlarger and paper that you use, should you have the new spot meter adjusted to match the original spot meter to nullify any possible difference, be it 1/3 or whatever. Seems that any variation in the new meter can be reduced to at least match the old one so that your standard EI for the film and its "normal" development time is not adversely affected by any variation in the new meter. I don't know myself, because I have only one meter, the Pentax V, and that's all I use----I'm just asking those more knowledgeable than me.

Not to overthink it too much, as that risk is always present in such discussions, but this keeps going through my head when it is said that 1/3 stop difference is not to be a concern. I ask myself, why is it on the meter scale then?

125PX at EI 64 dev for 5min and 15sec (hc-110 dil 1:63 from concentrate) did not produce the proper gray scale values in Zones VII, VIII and IX when I was testing Plus-X, but a further reduction in development time of about 15% to 4min 30sec nailed those zones properly on the scale without adverse affect in the lower zones.

So, my question is, when it comes to the upper zones of the gray scale, I wonder if a 1/3 stop difference in the direction of increased exposure is a valid concern to one's personal EI and dev time?

I don't think +1/3 stop is a great concern for a short scale subject, owing to lattitude, but for a "normal" SBR, when "normal" dev is planned, a continuous +1/3 difference could squeeze the upper zones where you don't want them, it would seem. I may someday want to get me a backup spot meter. Yes, I'm a zonie, but that is beside the point , so if someone could provide input to my concern, that would be great.

Thanks in advance.
Chuck