Discrepancy between Minolta Spotmeter F and Pentax Digital Spotmeter

[Jim Galli]

For many years I've happily used the Spotmeter F at home for my personal work and my inherited Pentax Digital Spot Meter at work. I get along fine with either one, but truth be known I like the Minolta best.

So I brought the Minolta to work and both meters side by side and discovered that with the Minolta set at ISO 400 a reading of 1/30th @ f22 gives EV 14. So if I set the Pentax front ring at 400 and set the 30th at f22, I look at the EV scale and it says 12.

So what gives? I can get a good exposure from either meter but that's a perceived 2 stop difference.

[Arne Croell]

For many years I've happily used the Spotmeter F at home for my personal work and my inherited Pentax Digital Spot Meter at work. I get along fine with either one, but truth be known I like the Minolta best.

So I brought the Minolta to work and both meters side by side and discovered that with the Minolta set at ISO 400 a reading of 1/30th @ f22 gives EV 14. So if I set the Pentax front ring at 400 and set the 30th at f22, I look at the EV scale and it says 12.

So what gives? I can get a good exposure from either meter but that's a perceived 2 stop difference.

The Pentax doesn't care what you set the ISO at, it gives you an "absolute" EV, probably based on ISO 100. The Minolta probably changes the EV according to your ISO input.
I don't have a Minolta, but I do have a Sekonic, a Gossen, and the Pentax. The former two change EV with the ISO setting, the Pentax of course doesn't, because there is no coupling whatsoever between the ISO/EV dial and the EV value that is measured.

[Drew Wiley]

When I had both these meters the actual exposure readings were almost identical.
Arne above has already explained how the EV number itself is affected. The Pentax
meter was originally engineered to match IRE values, typically used on movie sets.
Somewhere back in a pile of rubble, I have an old mfg tearsheet concerning this.

[ki6mf]

I have a similar issue with a Pentax V and a Sekonic 758. My fix was to measure a gray card and adjust the ISO on the Sekonic to get the same reading as with the Pentax. On the Sekonic I shoot at ISO 200 while on the Pentax the ISO is 300. I calibrated the Pentax several years ago, do annual testing to check my settings, and Pentax V is my every day meter. I wanted to make sure the backup meter, the Sekonic, read the same way my Pentax does so it fits my work flow!

[Nathan Potter]

The problem with all consumer light meters is that there is no convenient calibration method unless one sends it off to the usual suspects for calibration. But sending it off is fine - although I'd really like to know their calibration method and if it is traceable to the NBS.

OTOH you don't really need an absolute calibration as long as what you have is stabile over time. But how do you know it's stabile? For years I used the sun - the sunny 16 principle. Of course the caveat is that the sun may not be stabile. So I checked that out for a few months - a short enough time that I could rely on stability of the Pentax spot meter. I used an unblemished 18% grey card in the bright sun with the sun at 45 degrees to the face of the card. In addition, the sun is about 45 degrees above the horizon on a clear day with virtually no haze. Meter reading is 16.3 EV at asa 100 (f/16, 1/100 sec.) I then paste a small arrow on the dial to satisfy these conditions. Over time I have to change the position of the arrow slightly with intermitant checks. This is not quite perfection, but turns out to yield highly consistent negative density over time, fine for precise zone system repeatability. I will say one of the Pentax meters in the 1990s' drifted 1.5 stops over 10 years but it was heavily abused. The sun seems to be surprisingly accurate in intensity at 232,000 lux (16.5 EV).

Nate Potter, Austin TX.

[Nathan Potter]

It should be pointed out that due to a variation in the suns distance from the earth (from Jan to about Jul) the suns irradiance varies about 8% (1.32 to 1.41 W/m^2); but this is really insufficient to significantly affect the measurements described above.
Over periods of years the stability is highly stabile, changing only on the order of 0.1 to 0.2%.

Nate Potter, Austin TX.

Note that the value in W/m^2 above includes energy beyond the visible range.

[Steve Barber]

As has already been said, the problem is that the Pentax does not give a directly usable EV number, even when the ASA is correctly set, for any ASA setting other than ASA 100. The Minolta gives a usable EV for the ASA number used, as do the Gossen meters I am familiar with, the Luna-Pro and Sixtomat, all of which will agree with the readings of a Hasselblad prism meter when it is properly set to the correct ASA and lens speed setting for the lens being used..

When you use the Pentax, for any ASA other than ASA 100, you have to use the f stop and shutter speed combinations it gives to set the exposure. If you are using a Hasselblad with ASA 100 film and you get a Pentax meter, you can set the EV using the Pentax readings, directly, with no problem. However, the day you change from ASA 100 to ASA 400 film, you are going to have a problem. As Mr. Galli has noticed, you are going to be exactly two stops off on the exposure. Go ahead, ask me how I know.

[Chris Strobel]

Jim I have the Spotmeter F.If you open the battery door, in the upper left there is a calibration screw made for the user to adjust per the instruction manual.And I too like my F better than the Zone 6 modified Pentax I bought when first starting this LF adventure several years ago.Its just less confusing to my low IQ :D

[Jim Galli]

Thanks for all the good answers. Very helpful, as always. What a great bunch of folks! :)

[dmmartindale]

The Pentax doesn't care what you set the ISO at, it gives you an "absolute" EV, probably based on ISO 100. The Minolta probably changes the EV according to your ISO input.
I don't have a Minolta, but I do have a Sekonic, a Gossen, and the Pentax. The former two change EV with the ISO setting, the Pentax of course doesn't, because there is no coupling whatsoever between the ISO/EV dial and the EV value that is measured.

That is correct. What the Pentax reads is not EV, it is "EV at ISO 100". Some writers call this reading "LV" for Light Value, though that term is not standardized. Essentially, the Pentax measures absolute luminance directly, and its reading can be converted directly into candelas per square meter (nits) or foot-Lamberts (there is even a conversion table inside some versions of the Digital Spotmeter manual). To convert the Pentax reading into an actual EV (aperture/shutter speed combination) you need to use the calculator around the Spotmeter's lens, and that is where the ISO setting combines with the meter reading to calculate true EV.

In comparison, the Minolta takes the film ISO as an input, and factors it into the reading. So if you put the Minolta into EV-reading mode, it shows you the true EV as EV is defined. If you change the ISO by a factor of 4, then the EV readout changes by 2 stops. (This is based on the older Minolta Spotmeter M; I assume the Spotmeter F behaves similarly).

In other words: the Minolta, Sekonic, and Gossen all read actual EV, with the film ISO already factored in. The Pentax reads "EV at ISO 100", with the effect of ISO being added in the calculator. So the Pentax will agree with the other meters only when the others are set to ISO 100.

- Dave