At 04:55 PM 1/3/01, Ken wrote:
>I would say that my understanding of how a spot meter works is pretty
>rudimentary, so an explanation would not be out of line. I
>understanhd it does no averaging, and only measures a very small area,
>but that's about it. And I could be wrong there!
Roger's explanation of how it works is extremely good but this also may be
helpful.
You probably know all of this, but it never hurts to review the basics as
the camera meter and a spot meter are more alike than different. Your
camera (and a spot meter) will give perfect exposure if the scene is
reflecting 18% of the ambient light. The story is that the engineers
determined many years ago that the average scene reflected 18% so meters
are set for that average scene (which probably means the story isn't true).
Of course in nature nothing is really average, which is why meters have a
complex scheme of measuring different parts of the scene and then weighting
it.
Some Caucasian skin (for example the palm of my hand) reflects 36% of the
light hitting it, so I can meter on my hand (if the light hitting the
subject and my hand is the same) and then open one stop or the camera will
try to expose the skin as if it were reflecting 18% of the light and
underexpose the scene by one stop. That's why the common advice is to open
up two or more stops at the beach as white sand reflects almost 90% of the
ambient light. An external spot meter typically reads a 1 degree area
(allowing you to focus on someone's face for example at a good distance).
The Canon A2 internal spot metering mode is 3.5 degrees (and the measured
area is not shown in the camera viewfinder per the Magic Lantern Guide).
Thus you have to guess the area its measuring and also guess as to how much
light the other areas of the subject are reflecting. B&H has a good web
page on the various EOS models currently sold and the scope of their spot
modes. While I don't sell the Magic Lantern Guides, they are very helpful
in understanding your camera and how it works.
http://www02.bhphotovideo.com/FrameWork/charts/comp_canslr_1.html
Outdoors, the in camera meter works very well and you can easily learn the
scenes that fool the meter with practice once you understand what its
looking for. For precise metering in unusual conditions, a spot meter is
very helpful. For example, in a stage presentation, there are several means
you can use to obtain proper exposure. You can guess at the proper lighting
or actually meter it ahead of time by going to tech rehearsal and metering
off someone's face with the actual lights (and opening one stop for
Caucasian skin or closing down one stop for black skin as Roger noted). But
the lights are changing during many productions. An A2's spot mode is not
too useful in that context for me because it is too big and I do not know
exactly what the 3.5 degrees is in the viewfinder. Admittedly that's not a
common situation. You can always bracket and I have done it many times when
caught without my Pentax Spotmeter. I have not used a EOS 3 or EOS 1, so if
you have one of those cameras the situation may be different as they have
much more sophisticated spot modes. One of the big complaints about the EOS
7 was that its ten degree spot is too big to be useful.
Roger's point that the spot meter slows you down is well taken. I don't
shot birds but I can see how that would be a problem. The EOS 3 or 1v is
probably the best solution. My solution is to hang the spotmeter around my
neck.
Bill White
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