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The "dome" of incident light exposure meters

Doug Kerr

Well-known member
Theory_section_notice-2.gif

Today, as for many years, essentially all "serious" incident light photographic exposure meters feature a prominent dome-like (hemispherical) translucent "collector" over their actual photosensitive element. Here is a currently-available, but classical, example, a Sekonic L-398A exposure meter:

ND_L-398A_Front_F42515-S400.jpg


Douglas A. Kerr: Sekonic L-398A exposure meter

This arrangement was invented by Donald W. Norwood in the late 1930s, and since then has been acclaimed as bringing to incident light exposure meters the ability to consistently give, over a range of lighting situations, a photographic exposure recommendation that would lead to an "appropriate" exposure result.

A while ago I became intrigued by this scheme. Just exactly what might we mean by an "appropriate" exposure result? And how does the use of a hemispherical collector make the meter consistently deliver an exposure recommendation leading to such a result?

My quest for an understanding of these issues was lengthy and at times frustrating. My conclusions are covered in detail in a new technical paper, "Norwood’s dome: a revolution in incident‑light photographic exposure metering", available here:

http://dougkerr.net/Pumpkin/index.htm#NorwoodsDome

Best regards,

Doug
 

Doug Kerr

Well-known member
Theory_section_notice-2.gif

By this statement:

Today, as for many years, essentially all "serious" incident light photographic exposure meters feature a prominent dome-like (hemispherical) translucent "collector" over their actual photosensitive element.​

I did not mean to disregard the Weston Master line of exposure meters, basically reflected-light meters, which can be put into the incident light metering mode by attaching an optional collector that is not dome-shaped (the infamous Weston "Invercone").

But in fact this collector (in its later version) very cleverly emulates the directivity of a dome collector.

Best regards,

Doug
 

Asher Kelman

OPF Owner/Editor-in-Chief
Nice post, Doug, but why did you make me twist my eyeball sockets to read your upside-down text?

I usually "get" humor, but this one escaped me!

Asher
 

Tom dinning

Registrant*
Hi, Asher,



It's upside-down so it will read rightside-up in Australia, where it is most needed.

Best regards,

Doug

I would be offended if I could be offended, Doug. Alas, I spotted the humour instantly and was appropriated amused.
As for the dome. I have a current Sekonic with sliding dome which I use in the studio mostly and occasionally outside when I don't believe what the camera is telling me. My old Weston is around here somewhere (sans dome).
As for 'appropriate exposure results' , what the **** does that mean?
 

Doug Kerr

Well-known member
Hi, Tom,

I would be offended if I could be offended, Doug. Alas, I spotted the humour instantly and was appropriated amused.

As for the dome. I have a current Sekonic with sliding dome which I use in the studio mostly and occasionally outside when I don't believe what the camera is telling me.
Yes, those are nice.

My old Weston is around here somewhere (sans dome).
As for 'appropriate exposure results' , what the **** does that mean?

Indeed. I discuss that conundrum at length in my paper.

Thanks.

Best regards,

Doug
 

Tom dinning

Registrant*
Hi, Tom,


Yes, those are nice.



Indeed. I discuss that conundrum at length in my paper.

Thanks.

Best regards,

Doug

I read it, as usual. Wouldn't have missed it. I now look at my dome in a new light, incidentally.

Even better regards, Doug. Most days I wish I could understand even half of what you say. Holding you in high regard is an understatement.
Keep safe during the winter and unwrap your xmas presents carefully.

Cheers
Tom
 

Doug Kerr

Well-known member
Hi, Tom,

I read it, as usual. Wouldn't have missed it. I now look at my dome in a new light, incidentally.

Even better regards, Doug. Most days I wish I could understand even half of what you say.
I suspect you do far better than that!

Holding you in high regard is an understatement.
Thank you so much. That is particularly gratifying from you, for whom I have great respect as a photographer and philosopher (among other talents).

Keep safe during the winter and unwrap your xmas presents carefully.

You and Christine as well.

Warmest regards,

Doug
 
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