using a spot meter as an incident meter!

[Oren Grad]

Well, that's the essence of the matter - whether a incident reading taken using just a few degrees around the vertex of a diffuser dome can ever be of practical use. I don't know, but I can imagine that it might be, and I think that this is a matter to be resolved by testing of the proposed meter setup under the conditions of intended use.

Remember as well that there are a variety of techniques for using an incident meter, not all of which involve taking just a single reading, and not all of which involve pointing the meter at the camera and nowhere else. It is possible that optimal technique for such a "narrow" incident meter will be different from that for a standard incident meter.

You seem to be arguing on theoretical grounds that it could not possibly be of use to anybody under any circumstances. I guess we will just disagree on this.

Cheers...

[Oren Grad]

...an incident reading...

[george jiri loun]

"you seem to be arguing... that it could not possibly be of use to anybody under any circumstances." (Oren) Where did you read this absolute statment in my posts? I cannot find it...

[Oren Grad]

If you think about the geometry, this "narrow area" incident reading off the inside of a diffuser dome should generally give results in between those provided by a full dome reading and those generated with a flat diffuser such as the Sekonic "Lumidisc", but much closer to the result from the flat diffuser. So much for reinventing the wheel...

George - Sorry for the rhetorical overreach. Let's leave it at that.

[george jiri loun]

No hurt feelings, Oren, Cheers!

[Oren Grad]

And good light to all!

[Gene Crumpler]

My, My such a fuss! How about an 18% gray card?

[tim atherton]

"My, My such a fuss! How about an 18% gray card?"

Hold it at 18 different angles and get 18 different readigns... :-)

In addition, many light meters aren't calibrated for 18% grey

[Ken Lee]

How big is the Lumisphere dome ? Its $12 price point is attractive.

Will it fit on a Pentax Digital Spot Meter ?

The smallest ExpoDisc is 52mm, so I guess you could get a step-down ring.

Are there other options ? (This is a 7-year-old thread.)

[BetterSense]

Well, that's the essence of the matter - whether a incident reading taken using just a few degrees around the vertex of a diffuser dome can ever be of practical use.

How would you take such a reading? Not the way you think!

If you think about the geometry, this "narrow area" incident reading off the inside of a diffuser dome

Think about the geometry a bit more. You are having a conceptual failure.

A spot meter is not some type of laser beam. A typical spot meter uses regular thin-lens optics, with a small-area photosensor placed at the focal plane (think of it like a subminiature camera with a sensor instead of film). Focus of the system is set at infinity or hyperfocal. For a given sensor size, there is a fundamental tradeoff between the size of the spot-reading and the intensity at the focal plane (due to numerical aperture), so to maximize both qualities with practical sensor sizes, spot meter designs tend to have fairly large-diameter lenses (low-F-stop). I designed my spot meter with an f/.9 lens.

If you place a dome over the lens, you are not reading "a narrow area" reading off the inside of the dome. Since the dome is so far out-of-focus--it subtends effectively the entire exit pupil of the lens--an area of the dome equal to the exit pupil of the spot meter is registered on the photosensor. So in the special (actually typical) case of thin-lens optics where the dome is the same diameter as the lens, the entire dome is equally weighted by the spot meter's sensor. Assuming a Lambertian dome, there is only cos^4 falloff to account for, which also exists for the conventional incident meter design, and cos^4 falloff is actually worse for the conventional incident meter design due to the near-zero numerical aperture of the typical configuration.

Placing a dome over the lens of a spot meter makes a very suitable incident light meter, which is no worse and possibly better than a conventional incident light meter in omnidirectionality! The only disadvantage compared to a conventional incident light meter design is less sensitivity due to the imperfect transmittance of the lens(es) and the larger numeric aperture of the system. In fact, if you wanted to make an incident meter with improved omnidirectionality compared to the usual incident meter design (something not necessary in practice), and you were willing to trade off size and/or sensitivity, you would do it in exactly this way.

Taking a small-area spot reading off the inside of a diffusing dome--a situation which you keep bringing up--would require a spot meter that could adjust the focus down to the mm range...I have never seen such a device before.