How Does Reflectance Affect Exposure?

[Ysarex]

Hmm, I see. Thanks for this comprehensive reply.

One last thing though and then I think I got it:

You say that in the photo with the white geranium the assumption applies that the light meter is taking into account the light reflecting off the geranium's surface--that a mixture of the less reflective objects in the photo (i.e. the green leaves in the shade) with the more reflective geranium, averages out the overall reflectance to about 18%. (I suppose given that the camera is in an evaluative metering mode).

Yes. The brighter areas averaged with the darker areas should bring us to an overall average near 18% reflectance. This we've substantiated empirically over time. Long before you were born photographers and camera engineers took a bazillion different meter readings of THE WORLD as we see and and low and behold when they averaged them all together out came 18% reflectance. So each time we go to take a photo we assume that average because our machine has been adjusted to that average. In that flower photo the camera meter is set to weighted average. It's averaging the whole frame but placing extra weight on the center say 60%.

Machines can't evaluate squat. One of my camera's full auto settings is labeled "SMART" -- damn thing is dumb as a 2x4. You can't buy anything today that the marketing people haven't polluted in one way or another. If you let them influence you you'll get confused. A machine can be programed with pretty sophisticated software algorithms that make it appear as if the machine were, in the words of Arthur Clarke, magic. One unexpected variable and they fall face down. You're smart. You can evaluate.

Now, say we fill the entire frame with white geraniums, to the point that we no longer see shade. Averaged out, let's say that boosts the photo's overall reflectance to 50%. Does that mean we must stop down to compensate for the added light? The incident light hasn't changed, but more light has been introduced with more reflective white geraniums. And if this is true, doesn't that mean when one shoots a snowy landscape on a clear day, that the overall reflectance is also higher (making the difference between EV15 and EV16)?

No. Backwards. You're using the camera meter to determine the exposure. When the meter sees a subject that is brighter than the expected average it responds by reducing exposure but you in fact want to photograph that brighter object as it really is.

Let's start over with the flower photo and assume an average 18% reflectance was correct for the scene. The scene is sunlit and so let's assume the camera at ISO 100 came up with an exposure of 1/500 sec. f/8. Now add in the extra flowers without changing the light and the scene reflectance goes up. The camera meter will respond to the increase in light only. It's doesn't know you've altered the overall scene reflectance average -- it just measures brighter light and it changes the exposure to 1/500 sec. f/11. That's going to make the white flowers get dingy and darken. You compensate by forcing the change from 1/500 sec. f/11 back to 1/500 sec. f/8.

Joe

 

[Dave442]

I suggest reading this section, Subject Luminance Distribution,
on page 2 of this PDF article. I think it covers some of your questions.
In that section he notes white at 90% and black at 4% luminance, I have other texts that use 96% for white and 3% for black.
http://www.largeformatphotography.info/articles/conrad-meter-cal.pdf

 

[andytakeone]

Hmm, I see. Thanks for this comprehensive reply.

One last thing though and then I think I got it:

You say that in the photo with the white geranium the assumption applies that the light meter is taking into account the light reflecting off the geranium's surface--that a mixture of the less reflective objects in the photo (i.e. the green leaves in the shade) with the more reflective geranium, averages out the overall reflectance to about 18%. (I suppose given that the camera is in an evaluative metering mode).

Yes. The brighter areas averaged with the darker areas should bring us to an overall average near 18% reflectance. This we've substantiated empirically over time. Long before you were born photographers and camera engineers took a bazillion different meter readings of THE WORLD as we see and and low and behold when they averaged them all together out came 18% reflectance. So each time we go to take a photo we assume that average because our machine has been adjusted to that average. In that flower photo the camera meter is set to weighted average. It's averaging the whole frame but placing extra weight on the center say 60%.

Machines can't evaluate squat. One of my camera's full auto settings is labeled "SMART" -- damn thing is dumb as a 2x4. You can't buy anything today that the marketing people haven't polluted in one way or another. If you let them influence you you'll get confused. A machine can be programed with pretty sophisticated software algorithms that make it appear as if the machine were, in the words of Arthur Clarke, magic. One unexpected variable and they fall face down. You're smart. You can evaluate.

Now, say we fill the entire frame with white geraniums, to the point that we no longer see shade. Averaged out, let's say that boosts the photo's overall reflectance to 50%. Does that mean we must stop down to compensate for the added light? The incident light hasn't changed, but more light has been introduced with more reflective white geraniums. And if this is true, doesn't that mean when one shoots a snowy landscape on a clear day, that the overall reflectance is also higher (making the difference between EV15 and EV16)?

No. Backwards. You're using the camera meter to determine the exposure. When the meter sees a subject that is brighter than the expected average it responds by reducing exposure but you in fact want to photograph that brighter object as it really is.

Let's start over with the flower photo and assume an average 18% reflectance was correct for the scene. The scene is sunlit and so let's assume the camera at ISO 100 came up with an exposure of 1/500 sec. f/8. Now add in the extra flowers without changing the light and the scene reflectance goes up. The camera meter will respond to the increase in light only. It's doesn't know you've altered the overall scene reflectance average -- it just measures brighter light and it changes the exposure to 1/500 sec. f/11. That's going to make the white flowers get dingy and darken. You compensate by forcing the change from 1/500 sec. f/11 back to 1/500 sec. f/8.

Joe

Ahhh~ I see.
So in a way, the camera meter itself is simply always assuming 18% reflectance and adjusting the picture to (no pun intended) reflect that. And in that sense, we must work the around this by compensating at times when there is more or less reflectance.

 

[andytakeone]

I suggest reading this section, Subject Luminance Distribution,
on page 2 of this PDF article. I think it covers some of your questions.
In that section he notes white at 90% and black at 4% luminance, I have other texts that use 96% for white and 3% for black.
http://www.largeformatphotography.info/articles/conrad-meter-cal.pdf

Ah, thanks for this. Will read all the way through.
I've already found something in it that might've confused me before but that I finally understand now:
"In most cases, an incident-light meter will cause a medium tone to be rendered as a medium tone, and a reflected-light meter will cause whatever is metered to be rendered as a medium tone."


Thanks again all for the replies.

 

[Ysarex]

Hmm, I see. Thanks for this comprehensive reply.

One last thing though and then I think I got it:

You say that in the photo with the white geranium the assumption applies that the light meter is taking into account the light reflecting off the geranium's surface--that a mixture of the less reflective objects in the photo (i.e. the green leaves in the shade) with the more reflective geranium, averages out the overall reflectance to about 18%. (I suppose given that the camera is in an evaluative metering mode).

Yes. The brighter areas averaged with the darker areas should bring us to an overall average near 18% reflectance. This we've substantiated empirically over time. Long before you were born photographers and camera engineers took a bazillion different meter readings of THE WORLD as we see and and low and behold when they averaged them all together out came 18% reflectance. So each time we go to take a photo we assume that average because our machine has been adjusted to that average. In that flower photo the camera meter is set to weighted average. It's averaging the whole frame but placing extra weight on the center say 60%.

Machines can't evaluate squat. One of my camera's full auto settings is labeled "SMART" -- damn thing is dumb as a 2x4. You can't buy anything today that the marketing people haven't polluted in one way or another. If you let them influence you you'll get confused. A machine can be programed with pretty sophisticated software algorithms that make it appear as if the machine were, in the words of Arthur Clarke, magic. One unexpected variable and they fall face down. You're smart. You can evaluate.

Now, say we fill the entire frame with white geraniums, to the point that we no longer see shade. Averaged out, let's say that boosts the photo's overall reflectance to 50%. Does that mean we must stop down to compensate for the added light? The incident light hasn't changed, but more light has been introduced with more reflective white geraniums. And if this is true, doesn't that mean when one shoots a snowy landscape on a clear day, that the overall reflectance is also higher (making the difference between EV15 and EV16)?

No. Backwards. You're using the camera meter to determine the exposure. When the meter sees a subject that is brighter than the expected average it responds by reducing exposure but you in fact want to photograph that brighter object as it really is.

Let's start over with the flower photo and assume an average 18% reflectance was correct for the scene. The scene is sunlit and so let's assume the camera at ISO 100 came up with an exposure of 1/500 sec. f/8. Now add in the extra flowers without changing the light and the scene reflectance goes up. The camera meter will respond to the increase in light only. It's doesn't know you've altered the overall scene reflectance average -- it just measures brighter light and it changes the exposure to 1/500 sec. f/11. That's going to make the white flowers get dingy and darken. You compensate by forcing the change from 1/500 sec. f/11 back to 1/500 sec. f/8.

Joe

Ahhh~ I see.
So in a way, the camera meter itself is simply always assuming 18% reflectance and adjusting the picture to (no pun intended) reflect that. And in that sense, we must work the around this by compensating at times when there is more or less reflectance.

Yes.

This has always been the "trick" to using a reflected light meter. It doesn't measure the light intensity directly but rather indirectly such that the reflection rate of the object (percentage of light reflected) becomes a factor. Back to the two sedans photo: An incident light meter held in your hand would be used to measure the light intensity directly by being pointed at the light source. The reflectance rates of the objects then are ignored. The incident meter reading will be accurate for the light source. Because the 18% reflectance average that we have determined empirically is pretty accurate we expect that a reflected meter reading of the entire scene will give us the same exposure result as the known-to-be-accurate incident meter determination. Then, understanding how an reflected light meter works we expect that a close up of just the silver door panel gives us a reflected meter reading that determines less exposure than the incident reading and a close up of the charcoal door panel gives us a reflected meter reading that determines more exposure than the incident reading.

The camera engineers of course know this. They have over the years done numerous things to try and improve the performance of reflected light meters. As a result we have our modern "matrix" or "evaluative" meter modes on modern cameras. These are algorithms that attempt to analyze
content variations over the metered area. For example they apply more weight to the central portion of the metered area. They determine that an extreme variation may indicate for example a light source in the metered area and calculate it as anomalous. The engineers have done the best they can and for the most part our modern camera meters give us very serviceable results, but there will always be a "fudge factor" in play when using a reflected light meter (variable object reflectance). Managing that "fudge factor" is one of the skills that a photographer has to develop in order to produce better quality results -- or you could just bracket.

Joe

 

[dennybeall]

If I understand:
the information about reflected light provided applies very much if I use the "Matrix Metering" option (the entire scene is metered), it applies less if I use "Center Weighted Metering" ( just the area in the center of the scene is metered) and it applies hardly at all if I use "Spot Metering" since that only meters a small spot.

 

[Ysarex]

If I understand:
the information about reflected light provided applies very much if I use the "Matrix Metering" option (the entire scene is metered), it applies less if I use "Center Weighted Metering" ( just the area in the center of the scene is metered) and it applies hardly at all if I use "Spot Metering" since that only meters a small spot.

The reflected meter's response bias to assume 18% reflectance applies equally in all three modes. How you as the photographer apply each mode is up to you.

In both the matrix and center weighted modes the meter is averaging the metered area. Both modes weight the averaging with software algorithms. The matrix mode algorithm is more sophisticated but essentially doing the same thing.

The spot mode still in effect averages the area metered but if the spot area is discrete enough then we can assume the reflectance rate of the metered area is uniform. The exposure calculation returned is still predicated on the 18% reflectance bias.

Joe