18% Grey Card. To use it or not.

[Chuck1]

Last night was a supermoon, I think that's unusually bright

 

[Bill Burk]

Last night was a supermoon, I think that's unusually bright

Thanks!

 

[reddesert]

Colloquially, supermoon means a full moon when the moon is at perigee. This makes it roughly 1.14x larger in diameter than when it's at apogee, and about 1.07x larger diameter than average. (In other words, it's not that super.) You square the diameter to get the ratio of its luminosity, so the Moon at supermoon provides about 1.14x more moonlight than an average full moon. https://en.wikipedia.org/wiki/Supermoon#Appearance

But, the surface brightness of the moon (luminosity per area) is the same at supermoon as at any other full moon. It's the same distance from the light source, which is the Sun. It's only brighter in total because it's bigger in the sky. If you are correctly measuring the surface brightness, you should get the same answer.

A reason the measurement is probably fairly variable, even if you are correctly photometering per area, is that the Moon's surface is non-Lambertian in its reflectance; unlike an ordinary diffuse reflector, it has a peak in its scattering function at 180 deg backscatter. (I mentioned this angular dependance of reflectance in this thread, several pages back.) This is why the full moon is more than 2x the total brightness of a half-illuminated moon (quarter moon).

Edited to add/clarify: IOW, the full moon's surface brightness should be somewhat higher in the center than the edge / limb. The numbers people have been quoting may be averaged over the disk, I don't know.

 

[RalphLambrecht]

A found in a box a 18% grey card and it made me think this:

Nowdays with digital cameras we can do almost everything but with film...things change.

A have a Nikon F100 and I shoot photos using the matrix mode. Until now, it works well for me but, can we improve the technical quality of the photos?

I tried spot metering using the 18% grey card and more than 50% of the takes are 1 stop under or overexposed if I use the matrix mode.

From your knowlegde and experience, does it matter? I mean the fidelity of color, grain, etc... Exposure affects the density of the color layers, so, it must be important, right?

Nikon matrix metering is hard to beat in my experience.

 

[Bill Burk]

Thanks reddesert. I used an SEI Photometer which is a comparison spot photometer, you adjust the brightness of a spot in the field of view until it seems to disappear. The spot is smaller than the moon.

It might be the air was clear. I repeated the measurements twice, and they were identical.

 

[BMbikerider]

I'm with Chan on this one. Gray cards have their usefulness, but with a spot meter??? or Matrix metering???

If you want accurate metering, get an incident meter -- which is not influenced by the reflectance of the subject.

Right from when I started photography in 1963 I have never managed to get on with incident light readings. At work we had a Weston Master 1V Then a Lunasix then another Lunasix. My boss who started me off used reflected light of grass, or the back of his hand and he will never went wrong. By 'eck he was right. He learned his trade as a RAF photo reconnaissance pilot in Spitfires during the war so he had to know what he was doing.

I use a Minolta Autometer 3 now with a 10 degree spot but still the back of my hand when I am with my TLR, but always the matrix metering on my F6 - that does not make many mistakes.

 

[xkaes]

A gray card (used correctly) will give you a correct exposure. It will be the same as an incident meter if both are in the same light. The problem with both is when the subject is in different light. You need to either move the incident meter or gray card into the same light -- or use a spot meter aimed at something that is 18% reflectance under that lighting. Each approach may or may not be easy in any situation. Using the palm of your hand has the same problem.

Plus, the palm of my hand is not the same as a my gray card -- but I can use it because I know how much to adjust (easy to figure out if a reflectance meter is all you have). But if you don't have a incident meter, just place a white Styrofoam cup (preferably empty) over your reflectance meter's cell and point it where the camera's lens will point.

 

[wiltw]

Right from when I started photography in 1963 I have never managed to get on with incident light readings. At work we had a Weston Master 1V Then a Lunasix then another Lunasix. My boss who started me off used reflected light of grass, or the back of his hand and he will never went wrong. By 'eck he was right. He learned his trade as a RAF photo reconnaissance pilot in Spitfires during the war so he had to know what he was doing.

I use a Minolta Autometer 3 now with a 10 degree spot but still the back of my hand when I am with my TLR, but always the matrix metering on my F6 - that does not make many mistakes.

Interesting, your comment about incident metering not working for you.

• The darkness of the back of the hand varies during the year for many (relative to 18% gray as a constant), due to tanning. In fact, even the PALM of my hand, which most photographers cite as much more non-variable and inherently closer to 18% gray in brightness (than the back of the hand)...the plam of my hand is sometimes the same as gray card yet it can also be 0.3EV brighter than a gray card at some times in the year.
• The darkness of green grass varies (relative to 18% gray as a constant) not only due to variety of grass species but also due to the presence of reflective surface moisture on the grass blades

...proven to me by personal testing over time, not merely the claims of others.

My incident meter readings have consistently matched my one-degree spotmeter and my digital camera readings (and the histogram of the resultant shots, even shots of 18% gray test target surfaces). Tests conducted over the period of many decades of photography. I agree with comments by @xkaes

 

[Bill Burk]

xkaes one cannot use a gray card correctly. wiltw knows. It’s very difficult to get repeatable results with one.

But that’s not going to stop me from trying.

I’ve had a two-tone “Minor White” target for years and it was faded badly, so I repainted it.

The two tones are meant to read on an exposure meter as one stop apart.

 

[Bill Burk]

 

[Bill Burk]

The two tones could be anything as long as they read out a stop difference. Copied from a Sekonic test target that has seven patches around 18% in sixth stop differences. The tones I picked are 25.5% and 12.7% reflectance. This is a half stop below and above 18%. In terms of density I was shooting for 0.90 and 0.60 which is obviously 0.30 apart and should meet the requirements.

When the densitometer said I met the specifications the exposure meters disagreed. So I held the 0.90 and increased the reflectance of the lighter side of the target taking half milligrams of black out of the mix, painting, waiting fir it to dry and trying again until the meters showed a full stop.

I had to go down to 0.56 on the light side.

Still, to see a full stop difference I need to aim down to minimize sheen. When I do that the Sekonic L-758DR will show between 0.9 to 1.1 stop difference. Not reliably a full stop.

 

[Bill Burk]

Anecdotally we’ve said that an incident meter would agree with a spotmeter reading from a 12.7% reflectance target.

Even with this target I cannot replicate that story. It’s not agreeing.

 

[BMbikerider]

A gray card (used correctly) will give you a correct exposure. It will be the same as an incident meter if both are in the same light. The problem with both is when the subject is in different light. You need to either move the incident meter or gray card into the same light -- or use a spot meter aimed at something that is 18% reflectance under that lighting. Each approach may or may not be easy in any situation. Using the palm of your hand has the same problem.

Plus, the palm of my hand is not the same as a my gray card -- but I can use it because I know how much to adjust (easy to figure out if a reflectance meter is all you have). But if you don't have a incident meter, just place a white Styrofoam cup (preferably empty) over your reflectance meter's cell and point it where the camera's lens will point.

I think the light levels in UK are pretty well more constant than the area you live in so it is easier to balance. For what it is worth the 'poor mans 18% grey card' can be reproduced from the inside of a breakfast cereal box, but the back of my hand is as good as, and has stood me in good stead for over 60 years. I have no need to change my method, If it ain't broke don't fix it!

An incident meter will give you an average reading, but if there are very bright or dark areas that will be outside of the film latitude range and capability, one has to be sacrificed to record the other. I was brought up with the saying 'Expose for the shadows and let the highlights take care of themselves'

That has definitely not changed. Do you not think some of us as photographers get their proverbial knickers in a twist trying hard to get the perfect exposure when everything is nearly always a lot simpler than that. I have given up worrying and just get on with it.

 

[baachitraka]

The best gray card is the dome of the incident meter. But there is known trick to meter shadows or highlights.

Always works!!!

 

[Bill Burk]

With Sekonic L-758DR in my garage door test location at the northwest side of the house target lit by the sun wall in the shade…

In the shade, spotmeter reading of the light 25.5% target matches the incident reading with dome (3D) which sees more sky when it’s up and 12.7% reflectance matches with dome retracted (flat) because the sky is shaded. Incident reading changes by a full stop depending on on whether the dome is up or retracted.

In the sun, the dome and flat readings are both the same and match the dark 12.7% reflectance target.

 

[Bill Burk]