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Studio, Portrait, Still Life, Lighting Equipment and Technique Continuous and Strobe Lighting. (The Sun is considered continuous!) Great ideas are really ten a penny! Technique in setting up the subject is, of course, essential. However, the ability to bring out form, texture, tonality and color is where the skill in lighting provides all the keys to engraving one's ideas on the delivered picture.

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Old July 31st, 2014, 08:32 AM
Doug Kerr Doug Kerr is offline
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Default Expsure metering for "side only" lighting

Those of you who have ventured into our "theory" room may know that for a week or so I have been struggling to develop a satisfying model of why incident light metering with a "hemispherical receptor" instrument (perhaps the most common type of "serious" incident light meter) should yield an exposure recommendation that is considered to be "very appropriate" over a range of different lighting situations.

I have here brought the discussion out of that musty chamber to seek insight as to one practical impact of the matter.

Don Norwood, the father of the "hemispherical receptor" meter, says, in his definitive patent on the scheme:
One of the particular objects of the invention is to provide an exposure meter which is substantially uniformly responsive to light incident upon the photographic subject from practically all directions which would result in the refection of light to the camera or other photographic register.
At first reading, that sound eminently reasonable. But upon further analysis, it does not really do the trick. In the complex matter of getting the "exposure result" we want with a given lighting situation, it is hard to see why the "total amount of light" that lands on the entire camera-facing surface of the subject should lead to a determination of the optimal exposure. We are after all concerned with the exposure result on individual elements of the subject.

But an interesting glimpse into Norwood's vision is given in an important paper he wrote in 1941 [J SMPE 1941, 36:389-402.] In one discussion, he considers two "clean" lighting situations:

A. Only a single source (floodlamp, perhaps) from alongside the camera.
B. Only a single such source from one aide of the subject.

He notes that the working of a hemispherical-receptor incident light exposure meter (with the receptor always facing the camera, a fundamental tenet of such technique) will, in situation B, recommend an exposure (in the sense of shutter speed and aperture) twice that as in situation A.
This is in fact exactly what theory would predict.
He then notes that experienced photographers would, by "rule of thumb", often arrive at the same exposure decision: essentially giving the exposure a "one-stop bump" (based on what the light source would otherwise suggest, for a consistent distance from the subject) when the light source is directed from the side.

I would be interested as to how this resonates with the experience of those of you who do use incident light metering for exposure planning.

Thanks.

Best regards,

Doug
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Old July 31st, 2014, 07:50 PM
Doug Kerr Doug Kerr is offline
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I have recently discovered an important 1950 paper by Don Norwood ("Light Measurement for Exposure Control", J SMPTE 1950, 54:585-602) in which (among other things) he describes the results of subjective tests intended to ascertain the relative impact of a key light placed at various angles to the camera axis (and accompanied by fill light at a ratio of 8:1, key to fill).

I am just reading the paper now, and should have some observations on it shortly. (It's a good thing Norwood's writings are useful - his papers cost me $20.00 each from SMPTE!)

In any case, jumping the story a bit, it seems that one conclusion is that a key light of a certain "potency" directed from a 90 angle has 50% the impact on determination of the "desirable" photographic exposure that it would have if directed from the camera position.

This is of course exactly consistent with the theoretical "weighting" that a Norton-concept hemispherical incident light meter accords light arriving from an angle 90 to the camera position.

Fancy that.

The relative impact of the key light for angles of 45% and 135% were observed to be 75% and 25%, respectively. (The theoretical weights for those angles accorded by a Norton-concept meter are 85% and 15%, respectively.)

More later.

Best regards,

Doug
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Old July 31st, 2014, 07:54 PM
Maris Rusis Maris Rusis is offline
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I use both incident light metering and spot metering for exposure determination. My Sekonic L-758D lightmeter offers for incident light a hemispherical diffuser over the light sensor cell and the hemisphere can be retracted to approximate a flat sensor. This meter also offers one degree reflected light spot-metering.

Beyond the technical specifications of the meter everything else is a triumph (?) of empiricism. In front of my house I have at mid-morning a standard front-lit landscape with houses, trees, grass, sky, clouds, and a neutral grey tar macadam road across the front. When I take up a new film I photograph this scene at a wide variety of exposures. Then I take 20 steps into my darkroom and develop the film immediately. Inspecting the negatives gives me a known correct exposure without reference to the light meter. Minutes later, before the light has changed, I step into the scene and set a film speed on the meter and do the metering "dance" again and again until the meter indicates the exposure I already know is correct.

A similar procedure is followed for back-lit, side-lit, and top-lit subjects using both incident and reflected light meter readings. I build up a mental repertoire of metering strategies for all kinds of subjects in all kinds of lighting and I rehearse it often. In all cases I work from known correct exposures to calibrate what I do.

The photographic materials I use, black and white films, have considerable exposure latitude in the more exposure direction rather than the less exposure direction. So I tend towards to the maximum useable (not maximum possible!) exposure in order to obtain a negative with the maximum information content. This gives me more options when making the positive.
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Old July 31st, 2014, 08:09 PM
Doug Kerr Doug Kerr is offline
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Hi, Maris,

Quote:
Originally Posted by Maris Rusis View Post
I use both incident light metering and spot metering for exposure determination. My Sekonic L-758D lightmeter offers for incident light a hemispherical diffuser over the light sensor cell and the hemisphere can be retracted to approximate a flat sensor. This meter also offers one degree reflected light spot-metering.

Beyond the technical specifications of the meter everything else is a triumph (?) of empiricism. In front of my house I have at mid-morning a standard front-lit landscape with houses, trees, grass, sky, clouds, and a neutral grey tar macadam road across the front. When I take up a new film I photograph this scene at a wide variety of exposures. Then I take 20 steps into my darkroom and develop the film immediately. Inspecting the negatives gives me a known correct exposure without reference to the light meter. Minutes later, before the light has changed, I step into the scene and set a film speed on the meter and do the metering "dance" again and again until the meter indicates the exposure I already know is correct.
Ah yes, "teaching the meter"! Before we can learn, we often must teach.

Quote:
A similar procedure is followed for back-lit, side-lit, and top-lit subjects using both incident and reflected light meter readings. I build up a mental repertoire of metering strategies for all kinds of subjects in all kinds of lighting and I rehearse it often. In all cases I work from known correct exposures to calibrate what I do.

The photographic materials I use, black and white films, have considerable exposure latitude in the more exposure direction rather than the less exposure direction. So I tend towards to the maximum useable (not maximum possible!) exposure in order to obtain a negative with the maximum information content. This gives me more options when making the positive.
Indeed, you final "exposure result" objectives are attained during the positive stage!

Thanks so much for that very illuminating (!) discussion.

Best regards,

Doug
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