# Differences between incidence and reflected light meters.

Discussion in 'Lighting Equipment' started by timarmes, Nov 18, 2005.

1. ### timarmes

Hi all,

I hoping that someone here will be able to explain something to me
that's been playing on my mind for a while - the difference in usage
between incident and reflacted light meters.

I understand that an incidence meter mesures the amount of light
falling on a subject, whereas the reflected light meter meters the
amount of light being reflected back off the subject, but I don't
understand the following:

1) How is the incident meter useful? The reflected light from a
metal sheet will be far higher than that off black cloth, even if the
light source is the same. Since the incident meter knows nothing of
the material, how can it possible give you an exposure level for the
object?

2) Why so you have to face the indicence meter at the camera rather
than the source when taking a reading?

3) When would you use one type of meter over the other? Why?

http://www.ephotozine.com/techniques/viewtechnique.cfm?recid=195

Now, the author says:

"... what you want is a situation where the background is 2.5 f/stops
brighter than the subject. If your meter will allow you to do this,
the very best way is to take a reflected light reading of the
background and an incident reading of the subject. The reason for
this is that the reflected reading takes into account the colour of
the background while the incident reading of the subject is
independent of colour and reflectivity."

What does the colour of the subject matter when calculating the
exposure? How does this achieve the 2.5stop difference? I don't get
that at all. He goes on to explain:

"...if your meter does not allow you to take reflected light
readings, use it in incident mode, both at the background position
and at the subject position and adjust diffuser and light until you
get the same reading at both positions."

Same reading? How will this achieve the 2.5stop difference?

"...If you can only take reflected readings use the meter directly at
the background, onto an 18 percent grey card at the subject position
and aim for the same reading at each position. Starting with a white
(ish) background, you will now have a true white background for your
images."

I'm still lost. The background meters x eV to be rendered 18% grey.
The grey card also needs x eV to be rendered 18% grey. Take the
photo - grey card against grey background. What am I missing?

Thanks for you help,

Tim

2. ### kelly_flanigan|1

The incident meter will give a far better exposure for a snow scene; beach; or hockey rink surface. A refective meter will tend to underexposue; and give you grey snow; a greyish beach; a grey ice rink floor. Reflective meters are will give a decent exposure with a grey object. With a white obeject they underexpose; with a black object they over expose. Many of us old timers jsut still use a refective meter most of the time for casual images; and just bias up or down the exposure; based on experience with white and dark objects. With a refective meter; a dark; grey; and white object will tend to all come out greyish; since you are not using a proper reference in 2 of the 3 cases.

3. ### kelly_flanigan|1

There can also be NO correct exposure for a "snapshot"; since the lighting ratio is too broad. Here filling the shadows; moving the lights; setting up reflectors will make the scene/photo manageable. In movies a HUGE amount of effort is placed in lighting; and understanding lighting ratios. I feeel folks who only do still photos sometimes get caught up in the "grey card debate" and metering and ignore lighting. The incident meter is common in movie work; copying; product photography; etc to gain control of lighting.

4. ### timarmes

Hi,

Thanks for your responses. I'm so used to the idea of compensating for a reflective meter bases on the subject that I grasped the fact that you don't need to compensate for an incidence meter, good point.

Now, how does that work, exactly? The meter knows exactly how much light is needed to produce a correct exposure for a given ISO, and returns a suggested aperture/shutter speed to expose a 18% subject being subjected to the given amount of light?

5. ### kelly_flanigan|1

The grey card evolved from copying ; process work; and graphic arts before WW2. The white side of the card was a 90 percent reflectance; the reverse on FIFTH the reflectance. This is where the 18 percent number comes from. With weak wimpy early meters; many times the needle barely moved; and reversing the card would move the meter. Thus one could get a reading; and increase the timed copy shot by 5 times. Most shots with these graphic arts cameras are times with a timer; turning off the lights on and off for the exposure. The grey card/white card was used with graphic arts work to get trial exposures and was used in the graphics arts photo industry when FDR was president. The whole saga of "If meters where or where not calibrated for 18% grey cards" didnt existl; there wasnt even a ISO or ASA system before WW2. There was a different system for each meter maker; one for GE; one for Weston meters etc. The ASA sytem was developed for the military during WW2; to cut thru the crap 3 or 4 film speed systems. Your "Defender" film might be Weston 10; Ge 13; Kodak speed 40.

6. ### kelly_flanigan|1

The white dome of an incident meter is pointed towards the light; it measures the light level. Then magically this gives the expsoure level. If the suns output dropped in half; the meter will know. If you read with a reflective meter; the dark uniforms and white uniforms will give different meter readings at a basketball game. If a shot has both sides players; do you expose to make the black team grey; or the white team grey; or make them look correct? Which shot would look best?

7. ### kelly_flanigan|1

With an old copy camera; the exposure was measured; dialed in; and the first negs seen. Then the SAME exposure might be most of the time used for newspaper; blueprints; bluelines; business cards; flyers; since lighting was the same; controlled with a voltage regulator. With a reflective meter you can be chasing your own tail; each object gives a different exposure; with the lighting fixed.

Folks use incident light meters to gain control; reduce scrap; have better yields. In move work the burn rate is many thousands of dollars per hour; gaining control keeps costs in check. Understanding lighting and metering is "KEY" to helping with these costs.

There maybe NO exposure that is correct for a scene; because the lighting is toooooo harse.

No amount of farting around with 18% grey cards and incident meters will move the lights. The lights and reflectors are moved by a human. Metering is just a tool; changing the lighting requires timing of an outside shot; fill lights; fill flash; or extra lights and reflectors; or moving the subject.

Camera makers have sold the public on their XYZ cameras meter will make the killer babe shot with crappy lighting. Lighting requires experimenting; stuff folks rarely do much anymore. Many folks want to "buy" the camera that ABC uses; and ignore the lighting stuff. Even in the 1960's/1970's SLRs were marketed' "its fully automatic; its fully you" - Konica?

8. ### gaius|1

Aye, the most useful reflective meter is a spot meter. You meter something, decide how many stops above or below the mid-grey you want it, and dial that number in, not the number the meter reads. You would only transcribe a spot meter directly if you pointed it at a grey card (IMHO).

9. ### jmf

I won't repeat all the good reasons to own an incident meter, but a spot meter is very handy for understanding the dynamic range of your scene. If, after testing with your digi cam/workflow or profiling your film/development process, you know you have say +2.5 and -3 stops of range (center being the 18% mark), then you can predict if you can fit your scene to your process. It also can tell you if you need to bias the exposure up or down to hold shadows or keep highlights from blocking/blowing out.

I have two seperate meters, but Sekonic makes a combo.

11. ### jonathan_brewer|1

Kelly has hit the 'nail on the head', there's exposure and there's lighting, one is incorporated in the consideration of establishing the other, the masters of duplication/establishing a many times naturalistic setting in scene, whether it was Cinematographer Conrad Hall, or Rembrandt, considered where they were going in what they did with their lighting.

It's not JUST exposure, but an effort to SUGGEST/SAY something, to show this mood, this time of day, this effect, and in doing that, ultimately you can say there's no such thing as the CORRECT exposure, THERE IS 'looks right', 'looks very interesting', 'I like that effect, how did you do it',....and so forth.

See a movie called the 'Conformist', a lighting masterpiece, when watching this movie, consider that most movies are shot w/tungsten balanced film, sensible because when shooting inside on the set, traditionally, the scenes were lit w/tungsten lights, and when the crew moves outside on exteriors, the Asst. Cameraman doesn't have to be told to slap on the 85B in front of the lens, a warm filters which balances the film for the additional blue hitting the scene from the sun.

In the 'Conformist', you'll see a then innovative technique, one interior scene is lit w/tungsten lighting, you can see through the windows that it's snowing, it's also blue, because the DP didn't correct for the light coming through the windows, and it looks blue and cold, it looks NATURAL, the shot inside is lit w/tungsten lights, exposed on tungsten balanced film, and the daylight streaming in registers a blue hue, which because it's snowing, accentuates the effect of 'it's cold outside', IT LOOKS RIGHT, because the DP DOESN'T correct for the mismatch between daylight and a tungsten balance film.

There's no right or wrong, or correct, only did what you do work, and why you did it.

12. ### mydarkroom

Heres the old exaple were did in school. (If you can get a hold of a 4x5 camera and some b&w polaroid film it helps this example) You have a guy with a black shirt standing infront a black wall. You take a refective meter reading. Its taking a reading of all that black with a little skin and it is make a reading based of 18% grey. The picture is obviously much darker than 18%. So, what the picture is underexposed and the man face is white and lacking detail. The reverse is true also. White shirt, white wall and the mans face is overexposed and very dark.
By using the incedent light meter will only read the light that is falling on the subject. Reading the light at 18% grey and giving you the correct exposure. So it reads all the light hitting the subjuct, where that relective meter only picks up the light reflecting off teh subject. Incednet light meter are great in situation where you can get close to a subject. reflective are great for landscapes etc. The best is learn how to use you meters togeather for "proper exposure". I hope this makes since my typing dosent always work like my brain. I hoped it helped.

13. ### ben_holland

Heller, brilliant. I was thinking of the following comparison (I have it in a book),

http://sekonic.com/BenefitsOfIncident.html

there. The most concise demonstration between incident and reflective I've found yet.

14. ### timarmes

Thank you all for your answers. I now understand from you answers and posted links that the incidence meter effectively acts as a neutral grey subject. You face it towards the camera (and NOT the ligt source and someone else suggested) and it essential tells you the exposition that would be needed to expose a neutral grey subject at that point as a neutral grey subject on the film. With the exposure set, all other objects shoould fall into place - white will be white, black will be black, etc. By facing it at the camera you're considering the light that would be reflected back to the camera from the subject. It's all very simple when the principles are explained.

That all said, the other questions that I posed regarding the studio lighting on a budget link still don't make much sense to me.

I now understand why the incidence reading should be the same for the foreground and background, but I still don't think that you should aim for the same readings when using a reflected light meter.

And I'm still confused by his "very best" method of using an incidence reading on the subject and a reflectance reading on the background. He doesn't actually explain what he's aiming for with each reading (same value or 2.5stops difference)

Thanks,

Tim

15. ### jimnorwood

Thanks for this very interesting thread. I'm using a Polaris light meter + Leica M2 and was recently told that my last black and white film was over exposed. I was using the meter in incidence mode where possible. I've been brushing up on how I use the light meter to make sure this wasn't the source of the problem.

I used to use a Nikon FE2 which I thought metered very well. I presumed the relected light meter in the FE2 had some evaluative metering which allowed it to suggest the right settings for images that were mostly all white or black.

Yesterday as a test I metered a white wall with the FE2 and got a matching reflected reading with my light meter. Incidence reading was different.

Same test grey wall all three meter the same.

So does this show that both the FE2 and the relected light meter presume the white wall is grey and do not provide the right setting ? This would lead to an underexposure ? Only an incidence reading would be correct?

I was intetested in what Tim stated above. If the incidence reading presumes a neutral grey subject then if your subject is very bright or dark then you have the same problem and need to adjust the suggested exposure?

Are there typical mistakes you can make using a light meter taking incidence readings that can lead to over exposure ? I'm lost !!!!

One other question I still don't understand why for and incidence reading you need to point the meter at the camera. You are measuring the light falling on the subject which is not linked to camera position?

Sorry for the silly questions!

Last edited: Jan 16, 2022
16. ### John Seaman

Be aware this is an ancient thread!

An incident reading measures the intensity of light falling on the subject from the direction of the camera. That's not necessarily the same as the intensity coming directly from the light source. The simple thing to remember is that the incident reading is completely independent of the reflectivity of the subject. So yes, if it's a mid grey subject, the incident reading will be the same as a reflected reading.

If the subject is lighter that mid grey, the film will record it correctly, similarly if it is darker. If you try to change the settings of the camera from the reflected reading (as you seem to be suggesting), for example increasing the exposure for a black subject, the subject will just appear grey in the final picture, not black.

jimnorwood likes this.
17. ### JDMvW

a reasonably up-to-date discussion newer than 2005 at

https://blog.pond5.com/7066-perfecting-exposure-how-and-when-to-use-a-light-meter/

Unless you are trying to channel Ansel Adams or some such, modern digital cameras are pretty sophisticated even on "auto". Plus, if you shoot RAW, there is so much information in the file, that post-processing allows one to not worry so much about what the jpg looks like.

One of the few things left for metering is if you are somehow able to find and shoot transparency film.

Last edited: Jan 16, 2022
sjmurray likes this.
18. ### AJG

Photographers have used gray cards for years to insure accurate readings since very light or dark subjects can lead to exposure errors with reflected meters. Reflective light meters are designed to assume that what they are pointed at will average out to roughly 18% gray, which in the real world works well most of the time. Many scenes that people photograph will have a range of tones from light to dark that will average out to that 18%. There are lots of exceptions, of course, such as backlit scenes, black or white walls, etc., where a reflective meter will give a bad answer to the question of a correct exposure. The fact that incident meters effectively simulate using a gray card explains their popularity with pros and studio photographers. As for pointing the dome of an incident meter at the camera, the angle of the light source matters in some situations and pointing the dome at the camera prevents some bad readings.

jimnorwood likes this.