Inverse square law, sort of...

Steve Smith · Apr 10, 2014

markbarendt said:
When I say incident meter in full sun I mean exactly that during daytime pointing the dome at the sun.

Incident metering is pointing the meter at the camera, not the light source.

Steve.

markbarendt · Apr 10, 2014

Steve Smith said:
Incident metering is pointing the meter at the camera, not the light source.

Steve.

Incident metering is measuring the light that is falling on the subject to get the reference point(s) you need to make good decisions about setting the camera. It is not always about pointing the meter at the camera.

Pointing the incident meter at the camera is the classic/normal/stereotypical way to incident meter. Used this way an incident meter typically provides a camera setting that will render a scene to look "normal"; middle grey as middle grey and so on.

There are other ways to use it though.

One example is duplexing is where the meter is used in the classic manner and then a second reading is taken with the meter pointed directly at the brightest light and the two readings are averaged to find the camera setting. This works well in backlit situations where both subject and background detail are important. BTZS uses a variant of this type of metering.

Yet another is highlight pegging which is the method I described for finding the setting to take a picture of the moon.

Steve Smith · Apr 10, 2014

If I remember correctly, duplex metering is for meters with a flat fronted diffusion disc. Incident meters with a dome will integrate the light coming from all angles and should be pointed towards the camera, ideally from the subject and in the same light.

I have the Exposure Manual book by Dunn and Wakefield which goes into a lot of detail about duplex metering and is a book worth buying if one comes up for sale.

Steve.

markbarendt · Apr 10, 2014

jcc said:
"Sunny 16" vs. "roughly sunny 16" difference is roughly the percentile difference between Sun-to-Earth distance vs. Sun-to-Moon-to-Earth distance that the light has traveled.

No, the wiggle room I was leaving there is for the difference of your metering methods vs mine and atmospheric issues, not inverse square.

The distance a single "ray" of light travels isn't relevant, distance doesn't diminish it's luminance.

Inverse square measures the luminance of a given area, say the size of the meters sensor. As you move the meter away from the light source fewer rays hit the sensor. The source "looks" smaller to the meter as the distance increases.

That doesn't mean the subject matter/source actually looks dimmer, just smaller.

All the rays that reach the film have to come straight from the subject to your camera. Subjects like the bright side of moon normally appear small in a composition but their luminance is still as if in full sun.

markbarendt · Apr 10, 2014

Steve Smith said:
If I remember correctly, duplex metering is for meters with a flat fronted diffusion disc. Incident meters with a dome will integrate the light coming from all angles and should be pointed towards the camera, ideally from the subject and in the same light.

I have the Exposure Manual book by Dunn and Wakefield which goes into a lot of detail about duplex metering and is a book worth buying if one comes up for sale.

Steve.

You are correct Steve. Flat or retracted dome is what Dunn and Wakefield suggest.

Extended dome readings where the dome is fixed do work fine also though. It is simply a matter of getting used to the tool.

jcc · Apr 10, 2014

markbarendt said:
The distance a single "ray" of light travels isn't relevant, distance doesn't diminish it's luminance.

Inverse square measures the luminance of a given area, say the size of the meters sensor. As you move the meter away from the light source fewer rays hit the sensor. The source "looks" smaller to the meter as the distance increases.

That doesn't mean the subject matter/source actually looks dimmer, just smaller.

You can't compare a "ray" to luminance, since luminance is a measurement of the flux per area. Those two are related but not directly comparable.

Fewer rays hitting a unit area is the very definition of less luminance, i.e, looks dimmer.

Farther objects look smaller because of geometric optics (think similar triangles), which is a whole other topic.

snapguy · Apr 10, 2014

go figure

Math makes my head hurt. Shoot it at f11 and soup the devil out of the negatives.

markbarendt · Apr 10, 2014

jcc said:
You can't compare a "ray" to luminance, since luminance is a measurement of the flux per area. Those two are related but not directly comparable.

Fewer rays hitting a unit area is the very definition of less luminance, i.e, looks dimmer.

Farther objects look smaller because of geometric optics (think similar triangles), which is a whole other topic.

Pick any specific subject and test this. Maybe a face for a portrait or a still life of a bowl of fruit.

Meter at the subject with an incident meter or stand back and spot meter the face; doesn't matter which.

Set the camera and lens.

Take a shot at 5', 10', 20'.

The subject you metered for will be exposed the same in all three.

Move back to 100' and the subject you metered for will still be exposed the same.

The rest of the frame may be poorly exposed but the original subject, no matter how small it looks in the composition, will be exposed the same.

fotch · Apr 10, 2014

Classic example of over thinking.

jcc · Apr 10, 2014

markbarendt said:
Pick any specific subject and test this. Maybe a face for a portrait or a still life of a bowl of fruit.

Meter at the subject with an incident meter or stand back and spot meter the face; doesn't matter which.

Set the camera and lens.

Take a shot at 5', 10', 20'.

The subject you metered for will be exposed the same in all three.

Move back to 100' and the subject you metered for will still be exposed the same.

The rest of the frame may be poorly exposed but the original subject, no matter how small it looks in the composition, will be exposed the same.

This is the same phenomenon from the original post... "Changing the subject-to-camera distance by x/2, x, or 2x does not require a stop-compensation."

markbarendt · Apr 10, 2014

jcc said:
This is the same phenomenon from the original post... "Changing the subject-to-camera distance by x/2, x, or 2x does not require a stop-compensation."

Exactly the same as the moon examples too. No loss for distance.

Inverse square law, sort of...

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