Sekonic L508 dome vs spot inconsistency

[Andrew Ito]

I've got a Sekonic L508 that is exhibiting some strange behaviour. When I take an incident reading with the dome the exposure is typically 1-1.5 stops brighter than when I use the spot meter to read something approximately 18% grey. Is this something others have experienced? I made sure to cover the spot metering viewfinder when taking the incident reading. I guess I could be off in what I judged as 18% grey but my exposures taken using the spot readings are right on.

On a side note, when shooting in the studio with strobes, the readings were right on with my digital exposures.

Any ideas? Should I take the meter in for calibration?

[Henry Ambrose]

Andrew, if you spot meter a gray card from different angles you'll get different readings. There may be nothing wrong with your meter at all. Its not uncommon to experience varying readings especially in the field with a subject that is "something approximately 18% grey" and at who knows what angle to the meter. The subject may look 18% gray but the light it reflects to you may well be something different than what you expect. I usually use multiple spot meterings to determine if the scene will fit in my film's range and figure exposure from there.

Anyway, unless your readings used to agree and now don't, I would not worry about the meter.

[Robert Oliver]

are you sure you are switching the meter from incident to spot meter mode?

I made that mistake once. cost me some images.

[Andrew Ito]

are you sure you are switching the meter from incident to spot meter mode?

I made that mistake once. cost me some images.

I'm fairly certain that I'm switching modes properly.

As far as what I'm metering being "approximately 18% grey", that is probably the problem. I have heard that spotmetering an 18% grey card will not always be the proper technique when you have to consider light reflecting off of it. Perhaps what I think is 18% grey is actually off of that.

[Aender Brepsom]

I often get the same thing with my Sekonic L-608. I always attributed these variations to my misjudging of what is an area of 18% grey. That is why I always take multiple spot meter readings and use an average shutter speeed/f-stop combination from all of my meterings.

regrads

[Bob Gentile]

I don't use an incident meter, but I've always wondered about this. Maybe those more knowledgeable can clear it up for me.

It seems to me that this is to be expected. An incident meter will read all the light that's falling on a subject. The subject will absorb some of that light and reflect the rest. A reflected meter (eg, a spot meter) will read the reflected light minus the absorbed light. So it should show a lower light reading.

No?

[Ralph Barker]

. . . So it should show a lower light reading.

No?

Yes, maybe, but not so much for that reason. The reflective meter is giving you an exposure that will, in theory, render the metered area as a middle gray. Thus, the reading depends on both the tone of the metered area and its surface characteristics - i.e. whether the reflected light being metered also includes spectral reflections. So, the reflective reading needs to be interpreted in order to arrive at the appropriate actual exposure.

[Helen Bach]

I don't use an incident meter, but I've always wondered about this. Maybe those more knowledgeable can clear it up for me.

It seems to me that this is to be expected. An incident meter will read all the light that's falling on a subject. The subject will absorb some of that light and reflect the rest. A reflected meter (eg, a spot meter) will read the reflected light minus the absorbed light. So it should show a lower light reading.

No?

Another slant on Ralph's answer: 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, if the incident meter is designed to give an 18% grey reading.

I have an L-508C and find that the incident meter agrees very well with the spot meter used with an 18% grey card.

Best,
Helen

[Colin Robertson]

Hi Andrew. I suppose the first question is, 'what do we think the meter is doing?' As I understand it, when I take an incident reading the meter should be calibrated to give a setting which will record an 18% grey card as zove V. That is, based on the strength of the illumination not the reflectivity of the subject. With a reflected light meter I should get a setting which records the subject as zone V, regardless of the actual brightness of that subject. That is, a reflected meter will try to make a black subject zone V. And a white subject, and a grey subject . . SO, perhaps the next step is to meter something which is not 'nearly' 18% grey, but actualy 18% grey. As I understand it, an incident reading (which is what I tend to use) and a reflected reading of a Kodak grey card should be almost exactly the same, barring angle of reflection on the card. There was a thread about this once before, I think, which got a bit heated. Sorry I can't recall who posted it. I'll try and find it for you.

[Bob Gentile]

Yes, maybe, but not so much for that reason. The reflective meter is giving you an exposure that will, in theory, render the metered area as a middle gray. Thus, the reading depends on both the tone of the metered area and its surface characteristics - i.e. whether the reflected light being metered also includes spectral reflections. So, the reflective reading needs to be interpreted in order to arrive at the appropriate actual exposure.
Sure. I think that's what I was trying to say. Thanks.

An incident meter would tell me how to expose for the light that's falling on the entire scene. But when I point my reflective meter at the breast pocket on Uncle Louie's navy blue suit, it tells me, "Whoa! Open up, Bobby. I don't see much light comin' back at me. You gotta open up a few stops if you wanna make this suit come out middle grey!" Heh. Heh. Sure. Uncle Louie would like that, eh?

So, with that in mind, a reflected meter can't show more light than an incident meter reports, can it? (I mean... under most normal circumstances.) A subject can't reflect more light than what's falling on it -- it can only reflect less. (And it's up to the shooter to understand the difference in the measurements and adjust accordingly.)

Are we saying the same thing, Ralph?

[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