How do spotmeters work?

BetterSense · May 6, 2009

I'm not talking about the sensors or the calibration, but the optics. I've never seen a spotmeter so I'm wondering how they pull off a 1-7* spot, while letting in enough light. Silicon photodiodes are very small. In priciple a pinhole placed in a plane fairly far forward of a small-area sensor would work, but I doubt enough light would be admitted. Another way would be to use a simple lens with longish focal length and place a small sensor at the focal plane. Has anyone ever dissected a spotmeter or can tell how they work by looking at one?

Kirk Keyes · May 6, 2009

BetterSense said:
Another way would be to use a simple lens with longish focal length and place a small sensor at the focal plane.

You answered your own question.

ic-racer · May 6, 2009

The narrowest you can get by just shielding a conventional sensor from extraneous light is about six degrees. Think Sekonic L-206; but its not really a 'spot meter,' they call it a 'view meter.' As indicated, the true 'spot meters' us a lens.

Q.G. · May 6, 2009

You could reach much smaller angles of view with a tube only. Just make it long enough.
But it would also reduce the light reaching the cell quite a bit.
Hence the glass.

BetterSense · May 6, 2009

How big are the lenses typically? I was looking at this lens but I don't know the focal length.

http://www.allelectronics.com/make-a-store/item/LNS-49/TINY-PLANO-CONVEX-LENS/1.html

DannL · May 6, 2009

I love my Pentax Spotmeter V. It's the cat's meow.

Spec's on page 27.
http://www.cameramanuals.org/flashes_meters/pentax_spotmeter_v.pdf

Uses a pentaprism and probably a beam-splitter configuration. I think the lens is a normal 50mm design, though not addressed specifically in the spec's. The narrow degree spot angle is very sensitive.

BetterSense · May 6, 2009

Hm. I didn't think the lenses were that big. I bought a bunch of lenses but they are all about 15mm in diameter. Oh well.

nicefor88 · May 6, 2009

QG,
That's an interesting contribution.

ic-racer · May 6, 2009

Q.G. said:
You could reach much smaller angles of view with a tube only. Just make it long enough.
But it would also reduce the light reaching the cell quite a bit.
Hence the glass.

Yes indeed. I once converted a L206 to a one-degree with a toilet paper tube and a disk with a small hole on the end. This made it un-usable, because the low zones now were off the scale

If one were constructing their own circuit, using a very sensitive cell, higher voltage or something other than a standard galvometer movement, then a shielding approach may be practical. The SEI meter only is 1.3 x in its viewfinder.

How do spotmeters work?

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