Different readings, flashmeter in reflected/incident modes

[elerion]

I'm getting different reading for flash measurements in reflected and incident modes.
I use an old Lunasix F, in flash mode. I use a manual flash too (Mecablitz 36 C2).
I never used a flashmeter before, until very reciently.

The first tests I've done were against a wall, at different distances (ranging form 1,5 to 3,5 meters).
Flash is pointing at 0º, towards the front.

When I measure in reflected mode, the readings seem to be about right (according to flash guide number and estimated aperture, given in a scale at the back of the flash unit).
When I measure in incident mode (putting the meter's diffuser dome in place) I get like 2 stops lower f-number, meaning that the meter detects much less light power.

Does it make sense?
Could I be doing something wrong?
Thank you

 

[jeffreyg]

When you read in reflected mode you are reading the light reflected off the surface along with any ambient light that enters the sensor. With an incident reading you are reading the light that falls on the subject. The value or brightness of the reflecting surface will vary accordingly to what you are reading off. While I rarely use flash I have always used my UltraPro with the incident dome over the sensor opening facing the camera to read the light falling on the subject. I've gotten good results that way but testing your equipment and technique that gives you the best results would be the way to go.

http://www.jeffreyglasser.com/

 

[mdarnton]

(there was a url link here which no longer exists)

 

[M Carter]

Over simplified, but - with a reflected reading, the meter is telling you what exposure is correct to render that surface at 18% gray. Incident is telling you the general exposure for the scene. Do a reflected reading of a white wall, and you might get F11, where an incident reading would be F5.6 - and 5.6 would be the correct exposure.

The other problem with slide-away incident dome meters - what's the thing actually reading? What's the coverage of the little lens in there, and how do you know what it's seeing? Even being fairly close to a solid wall, is there falloff in the area the meter is reading?

I've owned dual meters like this for 20-some years, and I don't think I've ever slid the dome away - I never felt I'd get info I could use.

 

[Mr Bill]

Hi, as indicated, for the readings to match, there is some average scene reflectivity that is needed. So if your test scene isn't exactly there, then no perfect match. But a two stop difference sounds like a lot. Two possibilities I would suggest are: perhaps your reflective scene is getting a large specular reflection (a glare spot) making it appear brighter than it should be, or possibly the incident dome has discolored with age, or needs to have the inside cleaned.

I'd be inclined to test the meter outdoors, in lighting where you already know roughly what the exposure ought to be, then see how the two meter modes compare (in ambient mode). (Actually, I'd compare with other meters, but I'm presuming you don't have any.)

 

[chassis]

Have a look at the reference/tutorial information on Sekonic's website. Good stuff there. Incident and reflected readings are generally not comparable. There are photos included in the Sekonic reference material that illustrate this.

 

[Leigh B]

It's extremely difficult to get truly accurate reflected readings.

You need a good-quality grey card of 18% reflectance, properly positioned in the field of view.
The angle of incidence of the light, and the angle of the meter to the card surface, are critical.

Use a good incident meter and believe it unless the subject is highly unusual.

- Leigh

 

[AgX]

The first tests I've done were against a wall, at different distances (ranging form 1,5 to 3,5 meters).


When I measure in incident mode (putting the meter's diffuser dome in place) I get like 2 stops lower f-number, meaning that the meter detects much less light power.

That wall must have a 18% reflection. What it likely has not not.
Use a grey-card instead.
Chose it big enough to fill the angle-of-view of the meter. Or locate the meter near enough, but without it putting a shadow on the card or be angled steep.

 

[mdarnton]

You folks who are hung up on the 18% card idea all need to read the thread I linked to at the beginning! Your're perpetuating an old myth, and steering the man wrong.

 

[Chan Tran]

You folks who are hung up on the 18% card idea all need to read the thread I linked to at the beginning! Your're perpetuating an old myth, and steering the man wrong.

While you can say meters are not calibrated to the gray card which they aren't. But the conversion from incident to reflected or vice versa based on 18% reflectance.

 

[Leigh B]

You folks who are hung up on the 18% card idea all need to read the thread I linked to at the beginning! Your're perpetuating an old myth, and steering the man wrong.

Don't need to read your very long thread.

Reflectance light meters are CALIBRATED using an 18% reflectance gray card.
That's what we used when repairing and re-calibrating them.

Proper use of a gray card is not as simple as it might appear.
The angle of incidence of the light and the angle of the meter to the card both affect the reading.

- Leigh

 

[darkroommike]

You folks who are hung up on the 18% card idea all need to read the thread I linked to at the beginning! Your're perpetuating an old myth, and steering the man wrong.

I have heard other reflectance standards suggested and I don't think all brands of meters are calibrated to the same standard but the standard for reflected light is some value between say 10% and 20% where a white wall will reflect somewhere in the neighborhood of 85-90%, or, about 2-3 f-stops difference. The Luna-Pro F is a fine machine, I have one, and trust mine implicitly just don't loan it to a "butterfingers" (that's was an expensive repair).

 

[Ian C]

Kodak Gray Card instructions in pdf form:

http://www.silverbird24.com/shop/media/files_public/trwtjbbcr/1903061.pdf

The last page has a paragraph labeled "For Calibrating Exposure Meters."

 

[Leigh B]

Kodak Gray Card instructions in pdf form:
http://www.silverbird24.com/shop/media/files_public/trwtjbbcr/1903061.pdf
The last page has a paragraph labeled "For Calibrating Exposure Meters."

Thanks for posting that.

My comment about the angle of the card is explained by the drawings at the bottom of page 2.

- Leigh

 

[wiltw]

You folks who are hung up on the 18% card idea all need to read the thread I linked to at the beginning! Your're perpetuating an old myth, and steering the man wrong.

I have participated in lots of past discussions about meter calibration, and can state that while the gray card might be disputed, it certainly can never be blamed for a 2EV discrepancy as mentioned in OP! It might account for about 0.33EV of underexposure, compared to incident reading.

Yet I also know that when I take a 18% grey card reading (Minolta Spotmeter F) and compare it against an incident light reading (Minolta Autometer Vf), in my case the readings tend to match. So the principle the OP should really take away from this is that it is necessary to point a reflected light meter flash reading at a mid-tone colored area and not a white wall, and compare reading that against the incident light flash reading.

 

[RalphLambrecht]

While you can say meters are not calibrated to the gray card which they aren't. But the conversion from incident to reflected or vice versa based on 18% reflectance.

for sake of this conversation it is a fairly accurate assumption.

 

[Bill Burk]

...paragraph labeled "For Calibrating Exposure Meters."

Which merely states the obvious, that for calibrating exposure meters, the card is 18%

How you relate 18% to the calibration of the exposure meter... is an exercise left to the reader.

And that is not simply explained.

 

[Chan Tran]

For a flat subject like the wall, the incident reading should be made with the flat diffuser. If it's made with the dome the reading the reading is about .4 stop low. If the wall is 54% reflectance I would expect 2 stop different from reflected and incident.
By the way Metz recommend to test their GN with a 50% reflectance surface.

 

[Bill Burk]

I have participated in lots of past discussions about meter calibration, and can state that while the gray card might be disputed, it certainly can never be blamed for a 2EV discrepancy as mentioned in OP! It might account for about 0.33EV of underexposure, compared to incident reading.

wiltw has given us a lot of good information about the gray card.

And our discussions about how far 18% gray card differs from the calibration of meters, is on the order of 0.33 EV.

Two stops is a significant difference, so it sure sounds like there is a problem with the test OP is doing.

 

[darkroommike]

For the sake of the discussion can we agree that a reflected light meter is calibrated to some sort of gray card or target, maybe not the Kodak 18% Gray Card but some gray card. And that the incident mode on the meter is calibrated to give an exposure that is very close to the value we get when the G.C. is metered correctly in the work shop. Then, if the OP meters with the incident dome in the field it is metering a surrogate G.C. but when metering off an off white wall the OP has to get a different value. The only time the two readings would be close would be if he had painted the wall 18% gray.

 

[Bill Burk]

Hey darkroommike,

Meters are calibrated to something like this... which makes a more stable and reliable test setup... than trying to get the reflected light off a gray card to come out the same every time. wiltw has demonstrated how hard that is.

If I were to have my vote for purposes of discussion, a reading off the gray wall, if it were about 12.7% gray, would possibly correspond to the incident light meter reading.

But there are a lot of discussions about what the percentage is, why and what brand of meters that pertains to.

All within 1/3 EV, so just discussion points. If you want something practical, pick one or the other and run with it.

 

[Mr Bill]

For the sake of the discussion can we agree that a reflected light meter is calibrated to some sort of gray card or target

No, I have a hard time with this since they're not - they're actually calibrated to a source of known luminance.

I'm happy with a slight rewording, to the effect that reflection meters are calibrated such that they give correct exposure on reading a mid-tone area, of roughly 12% to 18% (or whatever) reflectivity.

And the incident meter reading should substantially match IF the scene is mostly frontally lit.

I "learned" a number of things incorrectly in my younger days which were impediments to future learning; so I'm sort of finicky about this sort of thing.

 

[wiltw]

Irregardless of constant light or flash illumination, the metered results will be similar in relative terms...in this case the photo shows the two meters used, and the ISO 400 1/30 f/stop measured with each meter type. In the case of the reflective reading it measured with -0.3EV of the incident reading whereas (per the great perpetual debate about 18% vs. 12.5% gray card) the reflective light reading should have measured more light, not less.
(And the blue cloth upholstery of the loveseat happens to be the same brightness as a gray card.)

As stated already by a number of us, the results of the two meter types are largely 'in the ballpark' of one another, and are absolutely not 2EV different as observed by the OP.

 

[Leigh B]

Which merely states the obvious, that for calibrating exposure meters, the card is 18%
How you relate 18% to the calibration of the exposure meter... is an exercise left to the reader.
And that is not simply explained.

The idea of using 18% reflectance for meter calibration...

Is only imprecise if you fail to understand how meters are calibrated.
It's a very precise exercise. I used to do it in my repair business.

- Leigh

 

[Leigh B]

Irregardless of constant light or flash illumination, the metered results will be similar in relative terms...
(And the blue cloth upholstery of the loveseat happens to be the same brightness as a gray card.)

Irregardless is not a word. Regardless is a word.

Similarly, a blue loveseat is NOT the same brightness as a gray card by any wild flight of imagination.

Gray cards are carefully crafted to provide uniform reflectance at all wavelengths of light.
That is impossible with a "blue" surface by definition.
Using a blue reference surface makes the readings highly variable based on the meter sensors and filtering.

- Leigh