# Convert camera meter to incident meter?

Discussion in 'Photography' started by Martin, Oct 2, 2005.

1. ### Colin DGuest

What??? I said, read the card with a *reflected-light* meter, and read
the incoming light with an *incident* meter. Who said anything about
using the *same* meter? And the difference is *not* 3+ stops, it is 2½
stops between 18% gray and white. The Kodak gray card has a white side
as well - go look up the difference on the Kodak website.

So far in this thread, you have had *nobody* agree with you, and about
20 people disagreeing with you. Doesn't that tell you something?

FWIW, the 18% figure is derived from an estimated print brightness range
of five stops, that is 2^5, or 32:1. The geometrical mean of 1 and 32
is sqrt(32), or 5.66, and (5.66/32) x 100 is 17.7%, rounded to 18%.

The 12.5% figure is obtained from a brightness range of six stops, or
64:1, where the geometrical mean is 8, and (8/64) x 100 is exactly 12.5%

In both cases the gray is set at the geometric mean between black and
white, but the difference in level is in the number of stops estimated
to cover the range.

Colin D.

Colin D, Oct 5, 2005

2. ### Floyd DavidsonGuest

Nobody has said that you can do that. They've said that if you
point take a *reflected* meter reading from a gray card it will
be within 1/2 stop of what you get with an *incident* meter
reading of the light that illuminates the card.

Two *different* meters.

(I suggested earlier that you were perhaps using an incident
meter to make measurements of a gray card, and thinking the
results were a "reflected" reading. But what you describe above
suggests that you are using a *reflected meter* to attempt
getting an incident measurement of a light source. Same
problem, same error... and not one that anybody else in this
discussion is making.)

What you apparently are not comprehending about an 18% gray card
is *what* *it* *actually* *does*! It is nothing more than a
light diffuser. When you use a light sensitive device to
measure the light from an 18% gray card it is *precisely* the
same as using any other form of diffusion that allows 18% of the
light from a source to be measured by the metering device.

What is an "incident" light meter???? Well, it is a light
sensitive device with a diffusor in front of it.

What is a gray card and a "reflected" light meter???? Well, it
is an incident light meter! (By definition.)

And if you have two "incident light meters" (one with a little
white translucent dome over the sensor, and one that focuses on
an 18% gray card) that read different by 2 stops, you can rest
assured that at least one of them is calibrated wrong or being
mis-used.

Floyd Davidson, Oct 5, 2005

3. ### McLeodGuest

Because you're not taking a reflected meter reading of the light
source, you're taking an incident reading. With my Gossen you have a
dome that slides over the photo cell, with the Minolta you have an
accessory dome that goes on the photo cell.

McLeod, Oct 5, 2005
4. ### WhiskersGuest

snip

Perhaps his camera is set to 'auto-exposure' and the readings he gets with
his hand-held meters are irrelevant to what his camera actualy does?

Whiskers, Oct 5, 2005
5. ### MartinGuest

Christ! It seemed fairly friendly when I last posted.

Martin, Oct 6, 2005
6. ### McLeodGuest

Have you looked in your clues closet lately?

McLeod, Oct 6, 2005
7. ### ColynGuest

I'm going to ask the question again and hope you don't go off chasing

How do you take a reflected light reading of a gray card (18%) then
turn around and take a reading of the light falling on that card in
incident light mode whether it is that same meter or another and come
up with only a 1/2 stop difference.
I said nothing about nor did I imply reading a white card.
Nothing more than mid tones or middle gray which gives unpleasant
results.

When using reflected light I always open up 1 stop from the metered
f/stop. Gives beautiful skin tones especially with B&W.

Colyn, Oct 6, 2005
8. ### ColynGuest

In this statement I should have said both meters not both methods. My
fault here.
I did the tests with a Weston Ranger 9 and a Minolta Autometer Pro
The reflected mode shots were done by metering off her face and the
brightest and darkest part of the scene. I then averaged the 3
The incident shots were full length portraits as above. Exposure
reading was then compared to the above which indicated a 2 stop
difference in the 2 tests.
I do have to admit the term "perfect" is subjective. and since there
is no such thing as "perfect" I probably should have picked a
different word.

I picked up a GE PR-1 meter a few months ago at a yard sale for 50
cents and while I can't guaratee its accuracy I did a few spot tests
with it this afternoon around the house and outside using it both in
reflected mode and incident mode. This meter gave me just over a 2
stop difference between incident and reflected readings just as the
others did.

My arguement has been how do you point a reflected meter at a subject
that is reflecting 18% of the light falling on it and then measure the
light that is illuminating the subject in incident mode and only come
up with a 1/2 stop difference as everybody claims...

Afterall the light illuminating the subject is brighter than the light
being reflected off the subject..

Colyn, Oct 6, 2005
9. ### ColynGuest

Neither...
I took a reflected reading of a gray card in reflected mode by
pointing the meter at the card. I then set the meter to incident mode
and measured the light source illuminating the card by pointing the
meter at the light source.
I understand that.
Most subjects are considered by most to reflect 18%..however incident
meters read the light illuminating the subject as I'm sure you know
not the light being reflected off the subject and since this light
source is brighter than the reflected light then it stands to reason
that you will get different readings.
No standards or mathamatics can change this as others have suggested..

Colyn, Oct 6, 2005
10. ### ColynGuest

Somewhat convoluted explainations which gave no real answer..
I meant to say reflected.
When metering in reflected mode I always open 1 stop from the
indicated meter reading. I find the 1 stop difference gives better
skin tones.
In incident mode I shoot based on the indicated meter reading..

Colyn, Oct 6, 2005
11. ### ColynGuest

Since when are you supposed to take a reflected reading of the light
source??
Reflected light is just that.. light being reflected off the subject.
Incident light is the light illuminating the subject and is supposed
to be measured in incident mode with the meter pointed at the camera
position..

You also have to turn a switch to incident or reflected mode whichever
mode you are using.
This meter has a 10 degree attachment with viewing prism for precise
viewing or a 40 degree attachment for reflected light and a dome for
incident light.

My Ranger 9 is changed to incident mode by placing a dome over the
light receptor.

Colyn, Oct 6, 2005
12. ### Floyd DavidsonGuest

Read them again. They *do* provide the answers. But if some
parts don't makes sense, try doing web searches on the key words
involved to find basic tutorials on those specifics. Or you can

But I would suggest that you cease claiming that your
understanding is correct, and accept the obvious fact that you
need to work on the basics. It isn't that big a deal to learn,
and it will make your photography more fun and more productive.

But stubbornly making contradictory statements with each post is
not productive...
Here is your entire statement, and your correction is nonsense:

With incident metering you don't measure the 18%
reflectance of a gray card. You read the light falling on
the gray card.

Reflected metering gives an inaccurate measurement if you
rely on its 18%. It yeilds middle gray (muddy gray) in
B&W and desaturated mid-tones in color. Incident metering
does not read 18% reflectance it reads the whole light
falling on the subject. However if you allow the meter to
give a reading based on 18% it to will be inaccurate
too. You should use 36% with incident light..

What you said before is not accurate, and your correction
doesn't change that, but it does give me a clue about where the
confusion is. And perhaps we can understand what is actually
happening.
If you were to take a reflected reading from an 18% gray card,
you'd get it right. That would be approximately (within the 1/2
stop that I've previously mentioned) the same as using an
incident light meter.

But if you take a reflected reading from the "skin tones"
portion of the scene, what you will get is the "middle gray
(muddy gray) in B&W and desaturated mid-tones in color" that
you've described. The reason is because *skin* *tones* *are*
*not* *18%* *grey*, for which the meter is calibrated. Your 36%
figure is probably a lot closer for most Caucasian skin tones,
but it depends of course on the particular shade of skin and
will vary from one person to another.

So yes, because your meter is calibrated for correct readings
from an 18% gray card, if you take readings (particularly spot
readings) from skin tones, you will have to increase the
exposure by about 1 stop to get correct exposure. Otherwise the
skin tones will be exposed to provide 18% grey.

The trick to understanding this is knowing that the reflected
light meter is correct *only* if the metered scene has equal
parts of darkness and brightness, which works out to an 18% grey
average. The problem with using a reflected meter reading is
that the user has to determine if the scene meets that average
18% value, or not; and if not the user has to estimate the
correction factor.

None of that is necessary with an incident light meter.

But you are getting tripped up by such things as your 1 stop
adjustment when metering skin tones instead of metering a gray
card. Your observation of the results is correct, your fudge
factor is correct, and your description of what happens to cause
it is not correct.
Previously you have said your film hasn't got the latitude to
handle 1 stop over or under. You also said you use a pair of
meters that give 2 stops difference, and both result in correct
exposures. Then you claimed to have an incident light meter
calibrated to match the reflected reading from a 36% percent
grey scene. Now you say it is actually a reflected meter that
is calibrated for 36%, and then say you have to give a roughly
36% grey area a 1 stop adjustment to get proper exposure.

Do you see the conflicts in all of these statements? Every
other one *has* to be wrong.

Floyd Davidson, Oct 6, 2005
13. ### Floyd DavidsonGuest

Lets be quite clear on this. *If* you did that and got
significantly different readings, either 1) you did it wrong,
e.g. by shading the meter or the light or the card, or 2) one or
both of your meters is defective.

It could be that you "set the meter to incident mode" but did
not attach the diffuser, as one example of a faulty proceedure.
Then why don't you understand what is wrong with the many
conflicting statements that you make?
WRONG. Get it through you head, THAT IS WRONG.

The meters are *calibrated* *differently* for incident and reflected
readings, and the incident reading has a light diffuser in front
of the sensor that is not there for reflected metering.

If you take an incident reading of the light that illuminates
and 18% gray card and then take a reflected reading from the 18%
grey card, the *sensor* in the meter will see a different level
of illumination, but the *readings* *for* *aperture* *and*
*shutter* *speed* will be identical.
But the proper calibration of the meter will *absolutely* change
it, as others have pointed out to you several times. What do
you think happens when you "set the meter to incident mode"????
If what you say were true (and if the readings you've claimed
are what you actually saw) then there is *no* difference in
calibration for the meter when used for incident and reflected.
You've just used, for example, the same incident or reflected
meter to read both the light and the card.

It *won't* work. The numbers you have provided suggest that is
exactly what you are doing. The description you give of what
you think a light meter should read suggests the same thing.

Yet you deny it and continue to claim you do understand how they
work... and post results that deny it!

Floyd Davidson, Oct 6, 2005
14. ### Floyd DavidsonGuest

So explain how a 2 stop difference can give allow both to be right,
when you've also stated that your film has a latitude of only 1 stop,
plus or minus?
So you *didn't* take a reflected reading from an 18% grey card and
get a two stop difference from the indicent reading. Instead you took
2 meaningless (brightest and darkest) readings, and averaged them with
something that is probably about 1 stop too bright.
You are saying the exposure was right in both cases, and even
though it was 2 stops different you got good results?

On film that has a 1 stop latitude????
Yes, like an "impossible contradiction".
Three meters... we can conclude from this that it is almost certain
that you are using them incorrectly.
It's easy. Inside the meter there are two adjustments for
"sensitivity". One of them is for sensitivity in "incident"
mode and one of them is for sensitivity in "reflected" mode.

The actual calibration is done by measuring light from a known
source in incident mode and adjusting the appropriate control
for the appropriate reading. The meter is then switched to
"reflected mode", and the appropriate control for "reflected
sensitivity" is adjusted to give a *correct* reading (which is
*not* the same as the reading for incident mode). Using the
exact same source of light, if the meter is calibrated for 18%
grey, the reflected meter readings will indicate 5.6 times
brighter than the incident readings.

Then, when *you* point the incident meter at a light source
first, and then the reflected meter at a grey card illuminated
by the same source, the two readings *will* be exactly the same.

The only way *you* will see that 2 1/2 stop difference is if you
*don't* properly switch the meter between incident and reflected
modes.
And if you don't switch between incident and reflected, *you*
will measure that difference instead of measuring a proper
exposure level.

To learn how to use your tools you need to learn what they do.

Floyd Davidson, Oct 6, 2005
15. ### Floyd DavidsonGuest

There is no way you can do it and come up with anything else.

You are assuming that if the exact same amount of light falls
on the meter's sensor in both modes the meter readings will
be the same. But that is not true. The reflected light meter
is calibrated to be about 5.5 times more sensitive to light.

The meter is *calibrated* to give identical reading in incident
and reflected mode when the reflected mode is reading a gray
area illuminated by the same light, with a specified percent of
reflectance (which we are assuming is 18%, but might actually be
10 to 18% depending on the meter).

The only way you *won't* get almost identical readings is if you
don't properly use your equipment (for example, if you don't
properly get the meter switched between incident and
reflectance, or if you take a reflectance reading from something
that is not 18% gray without adjusting for the difference in
reflectance).
The values you give suggest that you metered the white card, not
the grey card.

The above should be highlighted and repeated! Those six lines
put a huge amount of other light metering information into
perspective.
Which suggests that you don't understand the significance of 18%
gray, plus you have metered the skin tones rather than an
average 18% grey area.

Floyd Davidson, Oct 6, 2005
16. ### McLeodGuest

Are you being intentionally obtuse? You wrote the first para of the
snipped portion above, not me, so you should be explaining this to
yourself, not me. I and other people can't explain this any more or
any simpler to you so good day and good luck.

McLeod, Oct 6, 2005

That isn't an argument, it is a question. (Even if that were the
argument, I'm arguing approximately equal, not 1/2 stop).

The answer is that you simply aren't using your meter(s) properly.

On any meter I have seen that is designed for both reflected and
incident readings there will be "reference point" indicators to be lined
up with whatever number is indicated by the needle after taking a
reading. Alternatively, the procedure might to be to line up with the
dial and use a special reference point to read the exposure. The
metering mode determines which indicator to use. Some meters might have
some sort of auto switching between modes, don't know.

Those reference points might be "R" and "I" for Reflected and Incident
or some other indication. In the case of the Gossen LunaPro they green
(reflected) and yellow (incident). Can't tell you about the PR-1, too
many years. In the case of the latter, there will also be another
indicator mark to use when using the large, accessory selenium cell for
very low light. Never used the Minolta, can't tell you. Ditto the
Weston.

You need to get out the manuals for your meters and read them.

--
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dadiOH, Oct 6, 2005

You are being exceedingly dense here, Colyn. As people keep telling
you, the incident attachment - the little white gizmo - *absorbs* some
of the light.

Additionally, if you are using a meter that does both types of readings,
there are probably two *different* reference points to use in reading
exposure: one is for reflected readings, the other for incident.

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dadiOH, Oct 6, 2005

Here's more confusion for you: the second part of your statement above
isn't necessarily true.

Most incident meters have a white hemisphere to collect the light. That
hemisphere is collecting *all* light - front, top, bottom, left side,
right side - in the hemisphere perpendicular to the axis in which you
point the meter. If you point the meter at the subject you are
averaging all light falling on the subject. It's doing the same thing a
reflected meter does when you point thatmeter at a subject with
"average" reflectance.

If you point the meter at the light *source* you are weighting the
reading toward the actual amount of light from that source. If you do
that *and* shade the hemisphere with your hand to exclude other than
source light, you can get a pretty accurate reading of the actual light
source. A better way is to use a meter with a flat "white thing"...it
measures only frontal light (the source if pointed at it) and is very
handy for measuring light ratios and shadow/highlight ratios.

Somewhere in this thread you mentioned that you had seen "supposed pros"
using an incident meter pointed at the subject...an incorrect useage in
your opinion. However, there is nothing wrong with that...it is done to
measure the *back* light...to determine lighting ratios. The supposed
pro knew what he was doing.

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____________________________

....a help file of info about MP3s, recording from
LP/cassette and tips & tricks on this and that.
Get it at http://mysite.verizon.net/xico

dadiOH, Oct 6, 2005

After a while you have to allow for a Frustration Factor