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Light metering

 
 
alertjean@rediffmail.com
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      02-27-2007
Hello there,
I waw wondering how camera is actually doing light metering and
adjusting proper exposure.

In TTL metering light from the scene will be entering the camera
through the lens.
Once the light hit the light meter (or metering points/area) ,it will
measure light intensity.
Now based upon the light intensity it LOOKS UP (Or is it calculating
something on the fly) the corresponding exposure value which need to
be executed.
When we use exposure compensation it uses the exposure value lying
above or below that of the one it arrived at.
If that is the case how is the first look up table calculated ? By
humans through trial and error ?
And it need to know only the light intensity to do all these ?

Is this correct ?

 
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ASAAR
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Posts: n/a
 
      02-27-2007
On 27 Feb 2007 03:38:49 -0800, http://www.velocityreviews.com/forums/(E-Mail Removed) wrote:

> I waw wondering how camera is actually doing light metering and
> adjusting proper exposure.
>
> In TTL metering light from the scene will be entering the camera
> through the lens.
> Once the light hit the light meter (or metering points/area) ,it will
> measure light intensity.
> Now based upon the light intensity it LOOKS UP (Or is it calculating
> something on the fly) the corresponding exposure value which need to
> be executed.
> When we use exposure compensation it uses the exposure value lying
> above or below that of the one it arrived at.
> If that is the case how is the first look up table calculated ? By
> humans through trial and error ?
> And it need to know only the light intensity to do all these ?
>
> Is this correct ?


See dpreview's Glossary page for Metering. It describes several
types of metering, and the description for the one that you seem to
be asking about (Matrix or Evaluative Metering) is this:

> This is probably the most complex metering mode, offering
> the best exposure in most circumstances. Essentially, the scene
> is split up into a matrix of metering zones which are evaluated
> individually. The overall exposure is based on an algorithm
> specific to that camera, the details of which are closely guarded
> by the manufacturer. Often they are based on comparing the
> measurements to the exposure of typical scenes.


http://www.dpreview.com/learn/?/Glos...etering_01.htm

 
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David J. Littleboy
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Posts: n/a
 
      02-27-2007

<(E-Mail Removed)> wrote:
> Hello there,
> I waw wondering how camera is actually doing light metering and
> adjusting proper exposure.
>
> In TTL metering light from the scene will be entering the camera
> through the lens.
> Once the light hit the light meter (or metering points/area) ,it will
> measure light intensity.
> Now based upon the light intensity it LOOKS UP (Or is it calculating
> something on the fly) the corresponding exposure value which need to
> be executed.
> When we use exposure compensation it uses the exposure value lying
> above or below that of the one it arrived at.
> If that is the case how is the first look up table calculated ? By
> humans through trial and error ?
> And it need to know only the light intensity to do all these ?
>
> Is this correct ?


There are at least two ways to measure light _correctly_.

1. Incident metering. This measures the light falling on the subject and
produces a theoretically/technically correct reading. (A gray card will turn
your TTL meter into an incident meter.) In even lighting with "normal"
subjects and no need for artistic intervention, incident metering is the
right thing.

2. Spot metering. This measures the intensity of specific areas of the
subject and can be used (if you know what you are doing) to determined a
subjectively (artistically) correct exposure. It can also be used to
determine compromise exposures in uneven lighting. (You don't need a gray
card, but you need to know what you are doing; this is the metering method
used in zone exposure systems.)

The books advertised at this site are good introductions to spot metering.

http://www.spotmetering.com/

All the other ways I know of off hand (e.g. center weighting, matrix)
produce what are essentially random guesses.

In real life, most people use one of the random guess methods, looks at the
histogram, and then reshoot with an appropriate exposure compensation dialed
in.

David J. Littleboy
Tokyo, Japan


 
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Alan Browne
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Posts: n/a
 
      02-27-2007
(E-Mail Removed) wrote:
> Hello there,
> I waw wondering how camera is actually doing light metering and
> adjusting proper exposure.
>
> In TTL metering light from the scene will be entering the camera
> through the lens.
> Once the light hit the light meter (or metering points/area) ,it will
> measure light intensity.
> Now based upon the light intensity it LOOKS UP (Or is it calculating
> something on the fly) the corresponding exposure value which need to
> be executed.
> When we use exposure compensation it uses the exposure value lying
> above or below that of the one it arrived at.
> If that is the case how is the first look up table calculated ? By
> humans through trial and error ?
> And it need to know only the light intensity to do all these ?


There are various simple to complex metering schemes. The most basic
TTL is to reflect the scene light off the mirror up to a meter under the
prism (or on a prism surface).

Some meter sensors are on the mirror (backside) with a semi silvered
section to allow the light through.

From there, it can be as simple as measuring the light and calculating
the EV (whether done in an analog circuit, dedicated digital circuit or
soft/firmware), and thence exposure time or aperture (A or S modes) or
select a P mode pair according to the overall light, ISO and possibly
sub-sets of the P-mode. The user can select (in P-mode) different A/S
pairs in "Program shift" or offset the exposure with exposure compensation.

Meters are weighted from none (spot metering) to full matrix with
various weights in between. Most up to date metering systems have
several modes (spot, center, scene). RTFM is quite important to be sure
you know which mode your camera is in and how that affects the metering.

When "matrix" metering comes in, then each manufacturer has their own
approach to the problem. Some meters weight for the "bottom" of the
scene (Leica R8, R9) and even detect if the camera is held in "portrait
mode" (and which side) or landscape (Minolta and others).

Nikon has one of the most advanced high resolution and color sensitive
metering systems that began with (IIRC) the F5. Looking at 1005 points,
and evaluating color (not just light level) and using a stored
"template" memory, the camera would find a best fit and then calculate
the exposure from that fit and actual measured light.

Compensation is simply an offset from whatever exposure value is arrived
at. A few simple guidlines will help you determine the compensation for
most situations. Chimping allows on the fly compensation with DSLR's.

Canon are PITA's for amateurs as they only include spot metering in
their high end (pro) cameras. Nikon, Minolta, Pentax and others include
spot metering in their mid class and some entry level class cameras.

TTL Flash metering with digital is a problem, as the Off-the-film method
does not work with digital sensors. So the camera will "pre-flash" the
scene with a weak blip, measure the return with the available light
metering system and then set the flash duration from that measurement,
then fire the shot. Again, each manufacturer has their own twists on
this. For example, Minolta (Sony Alpha) cameras with Distance
Integration (focus distance) lenses will weight their flash throw more
to the focus distance than to the pre-flash TTL measurement.

Cheers,
Alan

--
-- r.p.e.35mm user resource: http://www.aliasimages.com/rpe35mmur.htm
-- r.p.d.slr-systems: http://www.aliasimages.com/rpdslrsysur.htm
-- [SI] gallery & rulz: http://www.pbase.com/shootin
-- e-meil: Remove FreeLunch.
 
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ASAAR
Guest
Posts: n/a
 
      02-27-2007
On Tue, 27 Feb 2007 21:46:17 +0900, David J. Littleboy wrote:

> All the other ways I know of off hand (e.g. center weighting, matrix)
> produce what are essentially random guesses.


Informed guesses subject to some randomness. If matrix metering
utilized truly random guesses even Ken Rockwell would avoid using it
as his preferred D200 metering mode. Center weighting is more
likely to produce bad results in the hands of a poor or unskilled
photographer. You know, the kind of shooter that would be better
off with a $150 P&S than a DSLR.

 
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Joseph Meehan
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Posts: n/a
 
      02-27-2007
David J. Littleboy wrote:
> <(E-Mail Removed)> wrote:
>> Hello there,
>> I waw wondering how camera is actually doing light metering and
>> adjusting proper exposure.
>>
>> In TTL metering light from the scene will be entering the camera
>> through the lens.
>> Once the light hit the light meter (or metering points/area) ,it will
>> measure light intensity.
>> Now based upon the light intensity it LOOKS UP (Or is it calculating
>> something on the fly) the corresponding exposure value which need to
>> be executed.
>> When we use exposure compensation it uses the exposure value lying
>> above or below that of the one it arrived at.
>> If that is the case how is the first look up table calculated ? By
>> humans through trial and error ?
>> And it need to know only the light intensity to do all these ?
>>
>> Is this correct ?

>
> There are at least two ways to measure light _correctly_.
>
> 1. Incident metering. This measures the light falling on the subject
> and produces a theoretically/technically correct reading. (A gray
> card will turn your TTL meter into an incident meter.) In even
> lighting with "normal" subjects and no need for artistic
> intervention, incident metering is the right thing.
>
> 2. Spot metering. This measures the intensity of specific areas of the
> subject and can be used (if you know what you are doing) to
> determined a subjectively (artistically) correct exposure. It can
> also be used to determine compromise exposures in uneven lighting.
> (You don't need a gray card, but you need to know what you are doing;
> this is the metering method used in zone exposure systems.)
>
> The books advertised at this site are good introductions to spot
> metering.
> http://www.spotmetering.com/
>
> All the other ways I know of off hand (e.g. center weighting, matrix)
> produce what are essentially random guesses.
>
> In real life, most people use one of the random guess methods, looks
> at the histogram, and then reshoot with an appropriate exposure
> compensation dialed in.


You have been at this too long. In real live most people don't have
any idea and have never heard of the word histogram.

Like with most things the answer really is "It depends."

The exposure the camera comes up with is good enough for most people.
Most people are not likely to read this newsgroup. Around here there are
mostly more advanced photographers.

For anyone who has to ask, I would suggest first following the basic
instructions that came with your camera. Work with that first. Next try
some adjusting. Get a feel for what works for you and if you and get a feel
for what those adjustments do to the final results. If you find you do want
to do some adjusting, then as suggested look up information on histograms,
they can be a very good tool. If you are experiencing a specific problem,
then stop by and ask about that specific issue.

Photography has a lot of general rules and they are good, but they are
all made for certain levels of photographers and they all need to be broken
from time to time.



>
> David J. Littleboy
> Tokyo, Japan


--
Joseph Meehan

Dia 's Muire duit



 
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Charles Schuler
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Posts: n/a
 
      02-27-2007

<(E-Mail Removed)> wrote in message
news:(E-Mail Removed) oups.com...
> Hello there,
> I waw wondering how camera is actually doing light metering and
> adjusting proper exposure.
>
> In TTL metering light from the scene will be entering the camera
> through the lens.
> Once the light hit the light meter (or metering points/area) ,it will
> measure light intensity.
> Now based upon the light intensity it LOOKS UP (Or is it calculating
> something on the fly) the corresponding exposure value which need to
> be executed.
> When we use exposure compensation it uses the exposure value lying
> above or below that of the one it arrived at.
> If that is the case how is the first look up table calculated ? By
> humans through trial and error ?
> And it need to know only the light intensity to do all these ?


Center weighted average, spot, and so on are only one part of sorting this
all out. The other is how "middle gray" bears on this topic (the zone
system). Some feel that Ansel Adam's zone system is dead, but it is
actually the basis for the algorithms used in modern digital cameras.

This is a topic that you will not conquer quickly and, due to different
design philosophies, never exhaust completely.


 
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