Sorry if I didn't say it well but I thought I was simply adding to what
Winsor said. According to the data that's in the OM3&4 brochure the
spot meter covers the area of the microprism circle in a standard
focusing screen. As Winsor said, that's also the area of the circle in
a 1-4n screen or approximately 2% of the image area. The amount of the
screen covered by the spot meter is constant. But the angle of view
covered by that 2% will vary with the lens in use since their coverage
angles vary.
What I did was compute the area of the screen (actually the total film
image area but the effect is the same). 24x36mm = 864 square mm. 2% of
that area is 17.28 square mm. But that's a circle so divide by pi to
get the value of the radius squared = 5.5mm. Take the square root of
that = 2.35mm and mulitply by 2 to get the diameter of the spot metering
area on the screen = 4.7mm. The lazy man's way to avoid getting up and
actually measuring the diameter of the microprism area on a 1-13 or 2-13
screen.
When the angle of coverage of a lens is given the angle is measured
across the diagonal of the film frame. (instead of a more useful
horizontal of vertical coverage angle) To compute the actual angle of
coverage of the spot meter with any given lens you need to know the
angle of coverage of the lens, the size of the diagonal on the film
frame and the percentage of the diagonal crossed by the spot metering
circle. The diagonal of a 35mm frame is 43.3mm long. Since the
diagonal bisects the spot metering circle over its diameter the spot
metering circle takes up 4.7/43.3 = 0.109 or 10.9% of the length of the
diagonal.
If you apply this percentage to the angular coverage of the lens it will
tell you how many degrees are being measured by the spot meter when it's
using any particular lens. Since a 100mm lens spans an angle of view of
24 degrees and since the spot metering area covers 10.9% of that 24
degrees the spot meter is actually measuring an angle of 24x0.109 = 2.6
degrees. To get a 1 degree coverage area similar to some external spot
meters you'd have to put a 250mm lens on the camera. A 250mm lens has a
10 degree coverage and 10.9% of that is 1.1 degrees.
Hope that clarified rather than further muddying.
Chuck Norcutt
OM4 Land wrote:
> Thanks to both of you for the answers. I'm afraid I'm still a bit
> confused.
>
> I get that with a 50mm lens the spot is roughly just a bit bigger
> than the split image but not as big as the micro prism collar of a
> 2-13 screen.
>
> I know I'm being a bit thick, but I can't tell if what you (Chuck)
> are saying backs up what Winsor said, adds to it, or refutes it.
>
> Are you saying that if I use a 100mm lens, the spot will still be the
> same size relative to the focusing aid or not? I thought it would
> change but now I'm not sure.
>
> Of course I guess I could always just play around with a white circle
> on a black background, move the camera around and see where the meter
> jumps, but I'd love to understand it too.
>
> Thanks again.
>
> Chuck Norcutt <chucknorcutt@xxxxxxxxxxxxxxxx> wrote: And there is no
> direct relationship with the lens other than the fixed 2% of the
> image area is reading a larger or smaller angle depending on the lens
> focal length. The spot area at 2% of the frame should be approx.
> 4.7mm diameter and the diagonal of the film frame is 43mm so the
> linear coverage of the field of view is 4.7/43 = 10.9%. You should be
> able to figure out the angular coverage of the meter given the
> angular coverage of the lens using that percentage. A 50mm sees a
> field of 47 degrees so the meter should see 5.1 degrees.
>
> Chuck Norcutt
>
> Winsor Crosby wrote:
>> It is a true spot meter reading only 2% of the image area. It reads
>> a circle slightly smaller than the focusing aid on the 2-13 screen.
>> I always used the inside split image circle as the best
>> approximation. That is probably not accurate, but close enough. You
>> can seen the sensitive area in the 1-4n screen on this page:
>>
>> http://olympus.dementia.org/eSIF/om-sif/findergroup/focusingscreens.htm
>>
>>
>>
>>
>> Winsor Long Beach, California, USA
>>
>>
>>
>>
>> On May 19, 2007, at 10:37 AM, OM4 Land wrote:
>>
>>> Hi all:
>>>
>>> It seems everywhere I look I find different information regarding
>>> the size of the metered zone of the spot meter. Can someone set
>>> the record straight?
>>>
>>> What is the relationship between the spot and the focus aid of a
>>> 2-13 focusing screen with a Zuiko 50mm f-1.8?
>>>
>>> How about 28mm f-2.8 and 100mm 2.8?
>>>
>>> Thanks
>>
>>
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