Light Meter Sensor - Angle of View

[chris_chen]

I know about the "little" white spot on the shutter curtain from which the light is metered. I have two questions:

 

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1. Why not put another spot on the second curtain like the CLE, such that one could meter w/o the shutter being cocked. No big complaint, but why not?

 

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2. What is the angle of view of the lensed sensor? If it reads more than the spot, Leica, in the future may be able to alter the curtain markings - and calibration, of course - for averaging (again like CLE), center weighting, etc. Adding more reflecting to the curtain may increase low light sensitivity after calibration(?).

 

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Another topic: Does the M7 measure exposure off the film in AE mode a la CLE? I too want to try one.

[jackflesher]

The "angle of view" for the meter in the M6/M6TTL/M7 is totally

dependent on which lens you have mounted on the camera at the time.

The white metering spot represents approximately 13% (1/9th) of the

total <i>image</i> area.

 

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:-),

[chris_chen]

Jack,

 

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I know what your saying. But, I don't think you know what i'm trying

to say.

 

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Take the lens off; look inside; do you see the little light sensor in

the body cavity; there's a lens over it. What portion of the

shutter curtain does this sensor "see"? Is it greater (certainly not

less) than the 12 mm white spot? How much greater? Does it see the

whole frame? If Leica changed the little lens (cheap) and the

pattern on the curtain (cheap) and reprogramed the software (cheap)

would any one want that?

[tim_franklin]

The clip-on MR type meters are said to meter from an area

similar to the 90mm lens field of view. Would it not be a safe bet

to think the meter in the M6 and M7 is similar?

[chris_chen]

Com'on Folks,

 

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Do I have to stop this car and get the flame throwers out? 8^0

 

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PLEASE read the question.

[olliesteiner]

It seems to me that eveyone who responded above is taking time and

trouble to try to be helpful to you. Your response is unkind. If

they did not give an answer that satisfies you, you might try

rephrasing the question in an attempt to make it clearer.

[chris_chen]

But Ollie,

 

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I did try to put a BIG smillie w/my comment, I just can't get my

keyboard to emulate teeth. Rereading, I think the question is pretty

clear. 8*)

 

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See post about self timers for an insite about taking the time

to...I'm guilty.

[andy_piper2]

Sorry folks, but Chris is right - what you're answering is NOT what

he's asking. Forget clip-on meters and forget the white spot.

 

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What is the angle of view of the light-sensitive metering cell combined

with it's little lens (assume you've taken it out of the camera and are

pointing it at a testing device of some kind).

 

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Chris - I don't know. Here's a possible test scenario.

 

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Take off the camera lens. Make sure the camera is cocked (so the meter

will come on). Put something white inside the camera covering the

entire film gate (white paper may be enough, or a low-tack white sticky

label) as a 'full-frame' substitute for the white dot. Put ANY lens on

the camera (except those that block the meter or have substantial fall-

off). I'll come back to focal length choice.

 

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Find a dark place with a single bright light source (light bulb, e.g.)

Point the camera at the light source and take a bunch of meter readings

with the light in different parts of the frame-lines (not the whole

viewfinder) (zero the indicators and record the shutter speed at each

point). Record the various readings and chart them on a rectangle

representing the film area/framelines.

 

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This will give you a rough idea of what the cell 'sees' all by itself -

if the shutter speed needs to be real long with the light source out

near the corners, then you've gone past the point where the light

registers on the meter cell anymore. Maybe the cell sees all the way to

the corners, but I'd EXPECT a circular pattern larger than the white

dot's area, falling off at the edges. You'll find out (maybe) how MUCH

larger than the dot.

 

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Lens choice: A 35 will give the finest, most discreet readings, since

the image of the light bulb projected will be smaller. But a 50 (NOT

the fall-off prone Nocti) may have less fall-off. A 90 may have even

less fall-off, but the image of your light source may get too big for

fine determination of the meter's viewing angle.

[chris_chen]

Thanks again Andy,

 

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I'll try it sometime; though, it's not on my to-do list yet, as it

seems a bit OTT. I was hoping someone already knew the answer.

 

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Cheers

[john_layton]

Chris, you've given me an idea. Leica could have a rotating knob

or sliding switch that would change the existing sensor lens with

another for true spot readings. It would be easy for them to

design the lenses with different densities so the calibration

would be the same. Thus an M-6/7 (M-6/7sp?) could have both

spot and the existing (effectively center-weighted) metering. I

find both types of metering to be extremely useful and often

complimentary for a given situation. Leica could also have the

spot coincide with the rangefinder patch to make this more

predictable, but they'd probably have to build the spot-meter cell

into the rangefinder to do this.

[john_layton]

Chris, you've given me an idea. Leica could have a rotating knob

or sliding switch that would change the existing sensor lens with

another for true spot readings. It would be easy for them to

design the lenses with different densities so the calibration

would be the same. Thus an M-6/7 (M-6/7sp?) could have both

spot and the existing (effectively center-weighted) metering. I

find both types of metering to be extremely useful and often

complimentary for a given situation. Leica could also have the

spot coincide with the rangefinder patch to make this more

predictable, but they'd probably have to build the spot-meter cell

into the rangefinder to do this. By the way, I'll be curious to know

how you resolve the bugeye lens recalibration issue!

[jerry_pfile]

Guess there were at least three questions here.

 

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First on meter activation. Making it necessary to have the shutter

cocked for the meter to operate, warns the shooter if the shutter

isn't cocked. I'm sure that was more important to the Leica

engineers than giving one the ability to meter a scene without being

able to shoot it without then cocking the shutter. You could get

into escoteric things like increased battery drain while the camera

jostled around in the bag while walking to but the first was probably

enough for them.

 

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Secondly, having a pronounced spherical lens would seem to indicate

that the sensor has a wide angle of view, certainly greater than the

spot itself. How wide? I would suspect greater than 90 degrees, but

it really doesn't matter, as the angle of "read" is only whats

important, and that's the width of the spot.

 

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And here is where the use of the "spot" that has most of the posters

hung up somewhat with words. Using the 135mm lens, with an 18 degree

angle of view (diagonally) will still give one a larger metered area

than any 1-5 degree "spot" meter. For the bulk of lens usage of

lesser focal length, the area metered is in effect more of the center

weighted variety than any spot meter. Simply because you have a

larger center to meter.

 

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As to the question as to whether or not increasing the amount of

white on the shutter curtain will increase the sensitivity of the

meter. No. Take grey card and reflector meter (spot or otherwise)

and hold it up to the card and take a reading. Now move it back,

making certain you don't exceed the "field angle" of your meter.

Same reading even though you are metering a larger area. Meters

don't read the amount of light reflected, rather the strength of that

reflected light.

 

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Lastly I suspect the M7 in AE does measure light off the film plane

with the second sensor devoted to that purpose, but only when you

have the proper flash attached to make use of the TTL. I don't have

an M7, but I can't imagine it's ability to shoot daylight fill flash

in any other fashion and still leave a properly exposed scene.

 

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How about hearing from new M7 owners on that one?

 

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Best,

 

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Jerry

[ray_moth]

Chris,

 

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I agree with your first point, which sounds practical. I don't like

to have to cock the shutter just to be able to take a meter reading.

 

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As to the second point, being able to read the whole shutter curtain

surface instead of just the central "spot" area would enable centre-

weighted average metering in the M7, which would be a very useful

option in AE mode because it would save having to lock the exposure

and recompose for every shot. The R camera designers have always

understood this but I suspect they don't communicate with the M

designers!

 

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Incidentally, it doesn't matter what the angle of view of the

photocell used for "spot" metering is: the camera could simply switch

to the other photo cell for full-field average metering, the one that

already exists to control TTL flash. That cell reads from the whole

area of the film but it could also be made to read from the shutter

curtain if the curtain was suitably patterned. All Leica has to do is

to use a "checkerboard" pattern on the shutter curtain (as in Olympus

OM), instead of the white spot, and adjust the sensitivity of the

spot-metering cell accordingly.

 

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In answer to your final question, no, the M7 doesn't measure OTF in

AE mode and it couldn't do so, if you think about it. At speeds

faster than 1/50 sec. the shutter is never fully open, so the camera

would have to meter from the curtains and/or the film, as with

Olympus OM cameras. The M7 meters exactly the same as the M6 TTL,

i.e. from the white spot on the shutter curtain for non-flash and OTF

(full field average metering) for TTL flash.

[jerry_pfile]

Ray,

 

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I still think for the vast bulk of shooters, having the meter

inoperable unless the shutter is cocked is far more rational than

having the meter operable whether it is cocked or not. If one has

camera in hand and takes the trouble to "meter a scene" the obvious

presumption is that one is going to take a photo, either at that

moment or fairly soon. If you do choose to take the photo at that

very moment, its nice to know if the camera is ready to comply

instantly when you press the release. If not at that very moment,

chances are more than great that you will expose a frame soon....and

your camera will be ready.

 

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I'm not familiar with OMs you mention but am curious regarding the

ability of a full shutter, geometric "checkerboard" arrangement on

the shutter curtain, to yield a center weighted metering result for

exposure. It would seem that the strength of the reflected light

passed to the sensor would again be determined by the strength of the

total light from the whole of the white (or whatever color as long as

it isn't black) checkerboard squares on the curtain. That certainly

wouldn't appear to be "center weighted", but equally weighted across

the whole of the frame.

 

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I would think that if one wished to have a weighted (in any fashion)

reading from the whole of the frame, you would accomplish that either

in the design of the sensor itself, or to forego the sensor

altogether, and have the shutter curtain itself wired to yield what

ever pattern chosen to serve as the only sensor. Certainly don't

mind hearing if I'm wrong, but wasn't the latter the way the CLE

computed exposure?

 

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Again as I noted before, all I know about the M7 is what I've read.

However, apparantly it does have the ability to synch flash at speeds

greater than 1/50 (1/125 through 1/1000 with only the pulsating Metz

model for the moment). I'd certainly be more than curious to know

how it determines not the rapidity of the pulses, but their strength

without reading not off the shutter curtain, but the film surface

itself through use of its flash sensor. Without using the AE mode

and first sensor to determine shutter speeds greater than 1/50 for a

daylight scene, and the second sensor reading the portion of the film

open to its view thoughout slit travel to determine the proper pulse

strength for fill, how would automated daylight fill flash be

possible? Understand the M6 TTL can't even do this at 1/50 without

manually manipulated flash strength.

 

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Inquiring minds...etc., etc.

 

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Best,

 

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Jerry

[ray_moth]

Jerry,

 

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What I called a "checkerboard" pattern (for want of a better

description) is actually a "random" pattern of very small white

rectangular dots on the OM's black cloth shutter curtains, which

gives a pseudo-gray area from which to meter. You could build the

weighting into the distribution of the dots on the curtain or you

could achieve it through the design of the sensor; the latter is the

way Olympus does it.

 

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My point was, mainly, that both the M6 TTL and the M7 ALREADY have a

secondary photocell that is used for TTL flash control. It reads from

the entire film surface but I'm not sure if it's centre weighted. It

probably should be. Whatever, it wouldn't take a genius to arrange

for that cell to be selectable, as an alternative to the one used

for "spot metering", to provide the option of pre-exposure full-field

average metering, provided that the shutter curtain has the necessary

pattern for such metering. I suppose this could be in addition to the

white spot or instead of it. Leica could experiment with a paint-

brush to get the best results!

 

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It would mean, though, that the photocell used for "spot" mode would

have to have a narrow angle of view, restricted to the size of the

spot; with the present M6/M7 design, it doesn't matter because the

white spot's surrounded by black. If it were surrounded by a pseudo

grey area, it would be important. That was one of Chris's questions.

 

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As I understand it, the M7 can control the flash using TTL metering

only at or below the synch. speed of 1/50 sec, with an SCA 3000

compatible flash unit. For higher flash synch. speeds of 1/250 sec or

faster, with a Metz 50 type unit, the flash is used in strobe mode

WITHOUT TTL control. Sounds a bit basic to me!

[andy_piper2]

Jerry:

The 'checkerboard pattern' of light squares in the OM cameras is

center-weighted - there are more light-colored squares per surface area

near the center than at the edges. Take a real checkerboard, randomly

paint a few of the black squares near the center red, and a few of the

red squares near the edge black, and you'll get the idea.

 

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This "weights' the amount of light reflected to the metering cell,

which 'sees' the whole curtain more or less evenly. A bright highlight

near the edge of the frame, projected onto cloth that is mostly black,

will not not send much light on to the meter - the same highlight

projected onto a portion of the shutter curtain with LOTS of bright

spots (the center) to reflect it will have more influence (weight) on

the exposure (hence centerweighted).

 

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The CLE worked the same way - the pattern on the shutter was NOT light

sensitive itself - it was just a reflector to send a weighted amount of

light from different parts of the shutter curtain on to the metering

cell. The only difference between the OM-2/CLE metering and the Leica

M6 (or Contax G or Hexar RF for that matter) is that the latter three

use a solid shape (circle/rectangle) of gray/white for the reflector,

while the 1970/80's cameras used a scatter pattern of smaller dots to

modify the metering pattern and give SOME weight to the outer portions

of the frame.

 

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"I'd certainly be more than curious to know how it [the M7 High Speed

Sync flash] determines not the rapidity of the pulses, but their

strength..."

 

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The M7's HSS sync mode is NOT automatic

 

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8^(

 

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but fully manual - you personally have to pick an aperture based on

subject distance. The flash will show you different scales based on the

shutter speed in use so it does some of the calculation - but it does

not vary the flash power to suit your aperture - you must vary the

aperture by hand to suit the flash's output. Therefore the M7/Metz

combo does NO flash metering in the HSS mode - it just puts out a

constant stream of light of a factory-determined amount.

 

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Auto-metered flash will not work in the HSS mode regardless of how/

where it's metered. Autoflash works by shutting down the flash once the

sensors have 'seen' the correct amount of light. But with the HSS -

shutting down the flash this way will leave part of the frame unexposed

- the flash turns off before the curtain has moved all the way across

(just as with 'normal' flash sync). And the early parts of the frame

will probably be overexposed - there is no way for the flash to go back

in time and reduce the exposure for film that is already under the

second shutter curtain.

 

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..which leads us back to Chris' last question - no, the M7 doesn't

meter ambient light off the film. In fact (as you noted) it quits

metering once the first shutter curtain and dot have disappeared. The

second cell (as with the M6TTL) DOES meter off the film, but only at

the normal 1/50th sync speed.

[andy_piper2]

Ray - what cosmic connection allows us to say exactly the same thing at

exactly the same time from 8000 miles (10000 Km) apart?

[ray_moth]

Andy,

 

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LOL! Actually, it's more than 8000 miles because Internet uses

satellites. We're not saying exactly the same thing, however.

Olympus, in the initial OM-2 model, did, as you say, print a pattern

on the shutter curtains that was weighted toward the centre. The

pattern was designed to have the same reflectance, overall, as a

typical film but had more white dots in the middle than at the edges.

Interestingly enough. Olympus found that the reflectance of different

types and makes of slide, colour negative and black-and-white films

was pretty consistent.

 

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However, because OM cameras (in AE mode) do real-time metering, they

have to use a combination of both off-the-film and off-the-curtain

measurement, the proportion of light measured from each being

dependent upon the shutter speed the camera chooses. Slow shutter

speeds measure mostly from the film, while high shutter speeds

measure mostly from the shutter curtain.

 

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The problem was that, because a film has, of course, a uniform

reflectance over its entire surface, it appears differently to the

photocell than a curtain having non-uniform reflectance. This led to

light metering inconsistencies in the early OM-2 and Olympus had to

change to an even, unweighted shutter curtain pattern in subsequent

models to be consistent with film reflectance. they built the

weighting into the design of the photocell.

[tom_tong]

"However, because OM cameras (in AE mode) do real-time metering, they

have to use a combination of both off-the-film and off-the-curtain

measurement, the proportion of light measured from each being the

shutter speed the camera chooses. Slow shutter speeds measure mostly

from the film, while high shutter speeds measure mostly from the

shutter curtain."

In the OM2, when the shutter curtain is open to take exposure, light

is measured directly from the film, disregard high or slow shutter

sppeds. The readings taken off the surface of the shutter curtain is

to provide (approx.)exposure information in the viewfinder prior to

you fire the shutter. So the actual exposure measurement is not a

combination of reading taken from the shutter curtain and off the

film. Regards,

[jerry_pfile]

Thanks for the explanation guys. However, within that explanation

rises another question for me at least.

 

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I have no equipment that meters off the film in either flash or

regular mode. In that various film emulsions would appear to have a

range of reflectance, how does the OTF sensor compensate for that?

 

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Perhaps the question is worthy of a thread of its own but it arouses

my curiosity.

 

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Thanks,

 

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Jerry