Sekonic L508 dome vs spot inconsistency

[Bob Gentile]

"... the incident dome absorbs light so that the amount of light reaching the cell is the same as would be reflected from an 18% grey card..."

You're right, of course. I guess I was looking right past that angle cuz I don't think in terms of a grey card when I meter. Or maybe my problem is I think in terms of my trusy old (analog!) Pentax spot meter. I'm not a Zone Zealot, but I do tend to think loosely in their terms. I'll point my spot meter at a shadow area and the meter will tell me where to place Zone III. That's "what I'm thinking" when I work out an exposure. I'm not really conscious that this is putting Zone V at 18%.

I made a little Zone scale that I printed onto a peel-off mailing label. Cut it to shape and stuck it over my meter scale. I saw some similar templates on the net, but they were all for the newer high-speed, low-drag digital version of the meter. So I figgered it was cheaper to make myself a new sticker for my old analog meter than buy a new digital meter and use the existng templates.

Anyway... now, when I get the EV reading of the shadow area from the viewfinder, I just line the EV number up with the "III" on my Zone scale and... Voila! What could be easier! I'll usually poke around the scene a little bit, hoping there's nothing important going on out at Zone XVII... cuz if there is, I've got some decisions to make. But otherwise, once I'm satisfied that I can capture all the detal I want -- from both the shadows and the highlights -- I let the meter figure out how to get me to Zone V at 18%!

Guess that's just how my mind likes to do things.

[bglick]

This matter has been discussed many times on this forum....

There is a simple answer to this and you rarely find it in writing anywhere.... the short version is this....

For starters, most light meters are not calibrated to 18% reflectance value. Sekonics vary between 12 - 13%, Minolta a bit higher. Whats worse, some makers have changed their calibration standard through the years on different models.

Although I have not see any 12% grey cards, many of the 18% cards are not a true 18%, as there is differences between luminance and reflective. Most cards are set to a luminance value....with a computed reflectance value. Luminance refers to a certain amount of light energy that is measured directly, while reflectance refers to light as it is seen after bouncing off an object.

Now, the simple fact the card is reflecting less light then the meter was calibrated for can represent the difference in readings. Without doing the math, the difference between 18% vs. 12% is a tad over a stop. Then toss in the "true" value of the reflectance as they age over time and of course, the angle of the light vs. the angle of the card... its amazing they get within a stop... the safest bet is calibrate it to the film you use...

Remember, all these meters are very sensitive to color temp also, and some very sensitive to IR light. So no matter how sophisticated the meter, lots of traps....

[Helen Bach]

"Now, the simple fact the card is reflecting less light then the meter was calibrated for can represent the difference in readings."

If the meter is calibrated for 12% and the card is 18%, the card is reflecting more light than the meter was calibrated for.

"Without doing the math, the difference between 18% vs. 12% is a tad over a stop."

Well, the maths isn't hard to do. 12% is one stop darker than 24%. 18% is one stop lighter than 9%.

12½% is half a stop darker than 18%.

"Although I have not see any 12% grey cards, many of the 18% cards are not a true 18%, as there is differences between luminance and reflective. Most cards are set to a luminance value....with a computed reflectance value. Luminance refers to a certain amount of light energy that is measured directly, while reflectance refers to light as it is seen after bouncing off an object."

Are you confusing 'luminosity' with 'luminance'? A grey card cannot have inherent luminance, but it can have luminosity (lightness), as in L*a*b* for example.

Some mid grey cards, like the 'neutral 5' of the Macbeth Colorchecker are set to a Munsell Value of 5 (ie a perceived middle grey) or a CIELAB lightness/luminosity of 50%. That is slightly lighter than 18% reflectance - it's about 20% - but close enough.

That aside, my L508 has always read exactly the same for a grey card reading and an incident reading in both tungsten and daylight. I use a 'Kodak Gray Card Plus' (3%, 18%, 90% reflectance) for comparison. The spectral response of the meter should have no effect on the comparison between incident and grey card readings if both the grey card and the incident dome are neutral, of course.

Best,
Helen

[bglick]

Helen, yep, good catch, the card is reflecting more light then the meter was calibrated to, not less... the OP is off in the wrong direction, not good. Check color temp of light first, if this is accurate, have the meter re calibrated.

The issue regarding luminance vs. reflectance....its been too long since I dealt this issue, I can't offer more at this time...it was explained by a Sekonic rep to me many years ago.

Another interesting test regarding meter linearality.... put incident meter inside bellows, set aperture at widest. Take indident reading, record.... then stop down one f stop, take reading, all the way through the f stop range.... we know f stops on camera lenses are pretty damn accurate, cutting 1/2 the light at each f stop. Yet this test always fails miserably, over a 9 stop range, most my meters are off 1.5 - 2.5 stops....... hard to figure this one out to...

The more you dig, the more issues you learn about photographic light meters.... frustrating... specially the IR and color temp issues.

[Helen Bach]

"Another interesting test regarding meter linearality.... put incident meter inside bellows, set aperture at widest. Take indident reading, record.... then stop down one f stop, take reading, all the way through the f stop range.... we know f stops on camera lenses are pretty damn accurate, cutting 1/2 the light at each f stop. Yet this test always fails miserably, over a 9 stop range, most my meters are off 1.5 - 2.5 stops....... hard to figure this one out to..."

When you say incident, do you mean a meter with a dome, an invercone or some other integrator? If so, then that is the likely reason for the apparent non-linearity.

Best,
Helen

[bglick]

yes, incident, ambient, dome, etc. I even used the spot meter portion of the 608 and had the same issue. I used a long lens also to keep the front of the meter far from the film. A report I read stated that very few meters are linear. But, as usual, very hard to ascertain if its the non linear or the premise of the test is flawed somehow.

[Helen Bach]

Mulling it over, I realised that I took it for granted that the comparison was between a grey card reading and an incident reading with a lowered diffuser. Maybe that’s worth mentioning, as is the fact that I’m calling two readings ‘the same’ when they are less than a third of a stop apart.

Best,
Helen