Gossen vs. Sekonic meters - don't mix em

[robc]

Mark,

I enquired about calibration of my Minolta Spotmeter F sometime ago and this is the response I received. It may be of some use to you but makes no mention of colour temp.

<--

The meter is designed on the assumption that the midpoint of the film
characteristic curve is receiving an exposure level of 0.1 lux.seconds.
The formula used to calculate exposure is :

2^EV= (B*S)/K where

B= luminance in cd/m2
S=ISO sensitivity of film
K=calibration constant (calculated to be 14)

-->

[bglick]

Fred, remember, unless you know what the meters reflective value was set for, as well as the the reflectivity of the gray card, then this test is unreliable. Even Grey cards that claim to be 18% can be way off....and it only takes maybe 4.5% off to equate to .5 stop.

And now that I learned that Gossen calibrates to daylight, as a landscape shooter, I would think these readings would be more reliable in the outdoors. But I refuse to make anymore common sense assumptions.

Kevin, a man with one watch doesn't know the time when it fails. :-)

Leonard... > One meter, one film, do a film test, never have to worry again. This is one of the reason there are so many film speeds for the same film.

I agree, but the purpose of the post was to explain how involved this calibration / test must become, as you must test at all different color temps. It certainly complicates matters.

On the subject of testing light meters, or better said, calibrating them. I once ran a test to check how linear the Seknoic 608 was. I placed the ambient dome behind a LF lens, and sealed up all the areas around the rear of the lens so the dome got all the light coming through the back of the lens. The lens was lit by bright room light from a white wall, no direct rays. I start reading with the lens full open, the continue to stop down one stop and continue to take readings. Assuming each higher f stop is delivering half the light, (as apertures are well designed for this task) then the meter should drop 1 EV per f stop. However, this test also did not go as planned, as the meter was NOT very linear. Linear referring to, one f stop equals 1 EV. At the high EV readings, 10+ EV, very linear. Hower at lower EV readings, < 10 EV, the readings were no longer linear. At some f stops, the meter would miss the f stop by .4 stops, and this became additive as I kept stopping down the lens..... not good....

However, now adding the color temp issue, it's a real pandoras box to get it right.... Kirk was really "ahead of the curve" with his proposed solution.... on the last post, I did not appreciate how eloquent his concept was! OH KIRK.... ARE YOU READY TO TEAM UP AND FIND SOMEONE TO BUILD THIS ULTIMATTE METER? On the last thread i was joking, now I am more serious!

Now, I wonder how I ever got good exposures :-)

[tim atherton]

"Even Grey cards that claim to be 18%"

and doesn't one of the standards for meter calibration involve 12.5 or 13% grey and not 18%?

[ronald moravec]

I don`t think selenium meters have this problem. My Weston Master with incident dome and Seconic Studio Delux match up well.

George at Quality Light Metric has had them both but not at the same time.

I`ll have to check out my Minolta Flash meter IV in ambient-incident mode if I can remember how it is supposed to work. The instruction book is obscenely long.

[tim atherton]

http://www.bythom.com/graycards.htm



http://www.kinesisgear.com/opinion.html#sekonic

[Eric Leppanen]

Bill,

I completely agree that the "two negatives technique" (shooting two duplicate transparencies, with the second being held in reserve for push/pull if necessary) is a pain on a long trip. It can be made semi-tolerable, though, by using one-gallon plastic baggies to segregate each pair of negatives. Since each two-sided film holder by definition holds two exposures of the same image, I can place a pile of exposed holders, a box of baggies, and my exposed film box in my changing tent; put each pair of negs in its own baggie; then store the baggies in the film box. When I get home, each baggie gets transferred to its own film box, and the boxes are dropped off at the E6 lab with instructions to normally process one negative and twin-tag it with its box. I can then separately process any of the duplicate negatives as needed.

This is certainly more hassle than simply dumping the exposed negs into the film box and being done with it. But it can be argued that having a duplicate neg is a good idea for a variety of reasons, especially when shooting 8x10 in the field (more susceptible to wind or other sources of vibration). Plus you might determine after looking at your first neg that better contrast or color separation can be achieved with a push or pull, even if the exposure of the first neg was technically accurate.

I understand, of course, that this system is not for everyone and may exceed one's threshold of pain! I use a similar technique when shooting B&W, as it provides much more control than separately placing exposed film in NOM, +1, -1, etc. film boxes.

[Graeme Hird]

Tim said ""Even Grey cards that claim to be 18%"
and doesn't one of the standards for meter calibration involve 12.5 or 13% grey and not 18%?"

The Gossen is calibrated to 18%, the Sekonic to 12.5%.

I use a Gossen Starlite. I'm a happy camper.

[bglick]

Tim, wonderful links.... once again, just demonstrates how impossible it is to get reliable information from anyone, even the manufacturers. The entire grey card concept is beyond me...

1. What is grey card reflective value? (sometimes this is not even stated, sometimes its stated wrong!) Kodak had a bunch that were not labeled properly years back.

2. What is reflectivity % is the meter calibrated for?

3. Angle of the grey card... almost impossible to get consistent readings.

4. Amount of ambient light sneaking in, altering the spot meter reading?

When this is all tallied up, it would be quite the project to get sensible results.... now add the other items that still apply from above.... color temp of light source, what color temp is meter calibrated to, how linear is the meter..... In my opinion, grey cards are pandoras box indeed....

R. Moravec. I beleive the meters we are discussing have Silicon photo diodes.... not Selenium... but you are correct, the Sekonic 398 Studio Delux does have a Selenium photo cell. Now, what are you suggesting this Selenium photo cell is not vulnerable to, which the Silicon cell is vulnerable to? Interesting, please elaborate....

Eric, I fully grasp your method, and I am quite impressed you can keep all this so organized. For me, changing film in the RV in side film tents, etc.... I could never get all this straight.....in addition, sometimes pushing or pulling more then one stop can cause undesireable effects of the film... but surely a workable solution for some. I would prefer to fine tune the exposure methodology and be confident it is close to "good" :-)

Interesting....just out of curiosity, I checked how Linear the Gossen Luna Pro S is..... I could only check it with low light levels. I used the same technique as described above. Results very poor, just like Sekonic 608. First reading at 5.8 EV. I stopped down the aperture 6 stops, so final reading should be down to 0 EV. But, it's at 2.4 EV. So 2.5 stops off. This is with a reliable relatively new Copal 0 shutter.

The color temp issue I can understand, as I think it's a case of "not enough meters sold", to justify the cost of re designing the meters with modern high tech methods, similar to Kirks reccomendations. But the linear aspect of the meter..... they have had 150 years to fine tune this aspect!

Graeme, most of this discussion is revolving around incident mode, so we have eliminated the % reflectivity issue.

[Graeme Hird]

Bill,

That's not just reflected calibration - that's the whole meter's calibration. Gossen uses 18% to calibrate reflected and makes the incident mode match it. Sekonic does the same, but with 12.5% as their calibration.

That gives about a 2/3 stop difference between the two, incident or reflected.

What I'm saying is that Gossen says 18% is middle grey, Sekonic say 12.5% is. Their meters will read light levels and calculate their results for that basic assumption.

[tim atherton]

then of course their is the story that Kodak decided on the 18% grey for their card because when they sent a Kodak engineer out to measure it into the Kodak car park in Rochester at noon in mid summer, the was the refelctivity of the tarmac...