Spot meter & ZS vs. BTZS & incident light

There are, for sure, different interpretations of 'specular', especially when applied in different situations. My previous post was about reflecting surfaces ('specular reflections'), and Claire's post is about print values ('specular highlights') - '...a specular highlight is direct sunlight hitting polished chrome or other metals or glass and it is intended to be printed paper white in the print...'

For Claire's definition, with which I have no quarrel, there is indeed no reason to meter for the 'specular highlights'.

But don't we need some way to describe the bright, directional mirror-like reflections from non-mirror surfaces when we want them to record detail? The practical importance of the apparently academic distinction is a result of the assumptions made about the limits to the range of surface reflective index when using an incident meter. 'Specular' reflections may lie outside this range, and we may wish to record detail in them. If we wish to record detail in them an incident meter cannot, on its own, indicate the exposure required because the ratio of incident to reflected light is unknown.

How does that sound?

Best,
Helen

PS. "Lambertian". That's the name of a totally diffuse surface with equal intensity reflections over a hemisphere.

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Originally Posted by Helen B

. . . But don't we need some way to describe the bright, directional mirror-like reflections from non-mirror surfaces when we want them to record detail? The practical importance of the apparently academic distinction is a result of the assumptions made about the limits to the range of surface reflective index when using an incident meter. 'Specular' reflections may lie outside this range, and we may wish to record detail in them. If we wish to record detail in them an incident meter cannot, on its own, indicate the exposure required because the ratio of incident to reflected light is unknown.

How does that sound?

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Yes, I think so, and sounds great. ;)

I think part of the issue here is the technique used in taking the incident reading. The usually-suggested method of pointing the incident dome at the camera tends to under-measure the intensity of light that creates the specular reflections, whereas pointing the incident dome at the light source (i.e. metering the light itself) takes that into account. It's rather like being aware of the solar disc in your peripheral vision versus looking directly at the sun.

The practical problem is that if the light source is the sun, the under-measurement resulting from the usual incident reading technique is extreme, and the light intensity is such that the highlight on film tends to spread/bleed into the surrounding area of the negative due, I believe, to failure of the anti-halation backing. As the print has an absolute maximum white value, the medium can't deal with the excess negative density, so all that is seen is the highlight and its bleed into the surrounding areas.

Although I haven't studied/tried the BTZS metering method, it seems to me that in situations (similar to that Noseoil confronted) that fall outside the usual range anticipated by the method, an additional meter reading needs to be added to the decision matrix - either a spot reading of the highlight, or an icident reading with the dome pointed at the light source (e.g. the sun in this case). Then, how one balances the exposure/development probably depends on which end of the luminence range one wishes to give precedence.

I suggest doing a comparison between the reading given by an incident meter pointed directly at the sun, and a spot meter reading of the specular reflection from an aluminium-painted roof, for example. As soon as the sun comes out I'll do it.

Best,
Helen

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Originally Posted by jdef

a specular reflection is of the same intensity as the source, so an incident reading in direct sunlight should be the same as a spot reading of a specular reflection. Jay

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Not it you have the light integrating sphere on your meter.

If the dome is still on the incident meter, then you have just attenuated the light striking the light meter sensor.

If you removed the dome, then you have just turned your incident meter into a reflection meter. Some meters allow you to do this - but you need to have special reflectance attachments to get accurate numbers and also to know the field of view.

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Originally Posted by Claire Senft

My idea of a specular highlight is direct sunlight hitting polished chrome or other metals or glass and it is intended to be printed paper white in the print.

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Don't forget reflections of the sun on water are commons source of specular as well.

(edited) Ooops, I see you did get around to mentioning this.

Helen, it sounds like you have a very well thought out approach to an interesting problem.

My concern would be pointing any type of meter directly at the sun. I'm not sure if the circuitry involved in any meter would be able to deal with a gain of this magnitude. Reflected sunlight from a roof sounds ok, but I would hate to see you ruin a good meter on a purely academic question. It seems to me that metering an incident specular reflection and then an incident value of full sunlight would make more sense than trying to deal with the sun's full illumination.

Since we (typically) don't take pictures of the sun itself, an incident sunlight reading and a specular reading would suffice.

In any event, please post your findings as I'm curious about the results. tim

The BTSZ incident system is not even intended to deal with any SPECIFIC reflectivity. If you read the manual on it and try it I firmly believe that you will find reflections that should show detail are accomodated very handily by the system. What may be difficult for one steeped in traditional zone system lore is the fact that you are not trying to place any object on any zone. You are merely providing an exposure and developing time to take care of all the tone the film can handle ..if they exist in the scene or not is immaterial. Everything from jet black in shadow to extreme white in sunshine is covered. This is a very firmly grounded system based on sensitometry. Read the manual, think about it and try it. It works very nicely.

'Since we (typically) don't take pictures of the sun itself, an incident sunlight reading and a specular reading would suffice.'

Tim,

That's what I intended to do, but may not have made it clear! I wouldn't point a spotmeter at the sun.

Best,
Helen

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Originally Posted by Helen B

'Since we (typically) don't take pictures of the sun itself, an incident sunlight reading and a specular reading would suffice.'

Tim,

That's what I intended to do, but may not have made it clear! I wouldn't point a spotmeter at the sun.

Best,
Helen

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APX 100, Lee IR filter, 4 x 1/2 second at f:22, developed in Windisch' Compensating Pyrocatechin:

Exposure was pure guesswork, the filter almost opaque, and the developer chosen for the extreme compensation. And then they stopped making APX100 sheet film...

OK, the sun came out, but there were no aluminium-painted roofs handy, so I had to make do with wet blacktop.

Here are the readings to interpret as you wish:
Incident meter directed straight at the sun: f/11½ (no surprise!)
Spot meter reading off wet blacktop: f/64½

No matter which way I pointed the incident meter, the highest reading I could get was f/11½.

Meter set at EI 100, 1/125 second. Clear sky, slight haze. 2 pm, New York.

Best,
Helen