True or false: sunlight = sunlight

The intensity of a blue sky varies with the seasons; strongest in Spring-Summer, weakest in autumn-winter, in the southern latitudes. In equatorial regions, the light remains fairly constant and predictable, even with sporadic clouds. A meter reading will only be affected if a large number of clouds fill the sky, thus reflecting light. Where clouds cover the sun either in hazy to overcast light, the light changes from point to diffuse, and an entirely different meter reading again will be obtained. In a nutshell there should not be a change in a meter reading just because of a few clouds, but the more clouds, the more chance of light being reflected and the greater the variation in a metered reading out in the open.

David Lyga said:

Assume midday, with unrestricted sunlight. In neither case is the actual sun obscured by clouds. Now take these two scenarios:

1) Scene fully lit under completely blue sky

2) Scene fully lit under sky with many white clouds (but, again, sun, itself, is not obscured by clouds)

Do the scenes require the SAME exposure or does the scene with the completely blue sky demand one stop more exposure? I think that the 'blue sky' scene does require more exposure because there is no benefit of the white clouds 'filling' in with more light. And, there are (British) photo books that attest to my assumption.

Agree or refute: I do not bite. - David Lyga

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May be but I am sure that it's not a whole stop. May be not even a 1/3 stop.

benjiboy said:

That's why God invented light meters.

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God didn't! You can't blame God for just about everything.

Just overexposed two stops, and call it bullet proof.

David Lyga said:

Roll film does not allow N-1 (unless the scenes are the same type). Thus, Ansel Adams does not come to the rescue here.

Try to forget shadows here: we all know that fill light lightens them. I am talking about the SUNLIT portions of the scene and to simplify, let's focus upon ONLY a grey scale fully lit by sun. Would exposures in either case be identical or different. I think different. - David Lyga

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I would think that the difference in fill light provided by blue sky vs. clouds would be minimal in relation to the strength of the direct sunlight in the sunlit areas of your scene. Negligible enough that your grey scales in direct sunlight would be, for all practical purposes, identical. It's only the shadows that would show a significant difference in your scene.

Two different scenarios:

1: A subject in direct flat sunlight from a cloudless sky coming from behind the camera position with no shadows at all except maybe for limb effects.

2. A landscape with significant important shadows (hillsides, tree shadows, etc.) lit by direct sunlight from a cloudless sky.

Now, add clouds to the above. Scenario one will not change appreciably; the predominant light source is and remains the sun alone. The light added from the clouds is negligible. Scenario 2, however, is going to have more light in the shadows; perhaps significantly more if the blue sky is very dark, as it is at high altitudes, and if the clouds are positioned and illuminated so that they provide significant fill light to the shadows. And the color temperature of the light illuminating the shadows is going to be different as well, affecting possible filter choice. I can imagine up to a stop difference in the shadows in extreme cases.

(...and, BTW, you can develop roll film to N-1 if you like, or use a different contrast grade paper to achieve basically the same thing. That's not the issue here, and certainly has no bearing on a subject in flat sunlight.)

Best,

Doremus

Well, the amount of change in intensity might well be off.

Actually this discussion might be at least somewhat analogous to a situation whereby flash exposure depends upon the reflectance of surroundings. Take, for example, the following: you wish to photograph a grey scale that is exactly 10 feet from the camera. The environment is in complete darkness. With flash on manual, you judge the proper aperture and fire. Now, if you are in a small room that has white walls and white ceiling, you are going to get a different rendition of the grey scale than if you were outside, at night in an open field with nothing to reflect upon. The grey scale in that outdoor instance would be considerably underexposed.

The following attachment shows part of page 416 of my (desk edition) FOCAL Encyclopedia of Photography under the category of 'exposure'. - David Lyga

Chan Tran said:

God didn't! You can't blame God for just about everything.

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Why not Chan, a few years ago a recently constructed ultra modern local church blew down in a severe winter storm and when the church commissioners tried to claim from their insurance company they refused the claim on the grounds that it was excluded under the exclusion clause "war, civil disobedience, and acts of God".

JBrunner said:

Just overexposed two stops, and call it bullet proof.

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same here

HTF III said:

Not to sidetrack the discussion--but I always wondered how NASA pre-planned exposure for the astronauts. The sky on the moon was black. Did they use Sunny f/16?

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didn't you see the movie "capricorn 1" starring orenthal james simpson
even though that was about mars it was based on the hidden truth behind the lunar landings.
http://en.wikipedia.org/wiki/Apollo_Moon_Landing_hoax_conspiracy_theories

Bill Burk said:

They followed instructions like these...

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

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i keep forgetting about time travel

thanks bill

john
john

David Lyga said:

Well, the amount of change in intensity might well be off.

Actually this discussion might be at least somewhat analogous to a situation whereby flash exposure depends upon the reflectance of surroundings. Take, for example, the following: you wish to photograph a grey scale that is exactly 10 feet from the camera. The environment is in complete darkness. With flash on manual, you judge the proper aperture and fire. Now, if you are in a small room that has white walls and white ceiling, you are going to get a different rendition of the grey scale than if you were outside, at night in an open field with nothing to reflect upon. The grey scale in that outdoor instance would be considerably underexposed.

The following attachment shows part of page 416 of my (desk edition) FOCAL Encyclopedia of Photography under the category of 'exposure'. - David Lyga

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Aha, "Lightening the shadows". That's the main reason.

David Lyga said:

Well, the amount of change in intensity might well be off.

Actually this discussion might be at least somewhat analogous to a situation whereby flash exposure depends upon the reflectance of surroundings. Take, for example, the following: you wish to photograph a grey scale that is exactly 10 feet from the camera. The environment is in complete darkness. With flash on manual, you judge the proper aperture and fire. Now, if you are in a small room that has white walls and white ceiling, you are going to get a different rendition of the grey scale than if you were outside, at night in an open field with nothing to reflect upon. The grey scale in that outdoor instance would be considerably underexposed.

The following attachment shows part of page 416 of my (desk edition) FOCAL Encyclopedia of Photography under the category of 'exposure'. - David Lyga

Click to expand...

Hi David,

Yes, this is fairly self-evident. One factor that you haven't touched on is that the spectral composition of sunlight is also influenced by Rayleigh scattering of light, and the frequency affected by scattering depends on the particle size of the matter doing the scattering. The blue colour of the open sky is caused by light scattering by oxygen and nitrogen molecules. We are actually seeing the light scattered away from the direction it is moving in. In early or late hours, the distance the light travels through air is sufficient to deplete the blue spectrum enough to turn the light substantially yellow, hence the "golden hour". Golden hour is not quite relevant to your discussion, but it is worth noting that latitude has a marked effect on spectral composition for the same reason. Smoke, dust and mist in the higher layers of the atmosphere may have similar effects on spectral composition, and may vary due to changes in weather, volcanic erruptions, seasons etc. All things considered, "Sunny-16" is a guideline with a narrow set of parameters, and probably never was intended as a rule or a law.

ANSI - Exposure Guide's first adjustment for clouds is with a haze over the sun. That was basically the same thing I found in Jones' paper (which the standard was based). It appears that an unobscured sun with or without clouds surrouding it has approximately the same illuminance.

BTW, the ISO speed standard's color temperature exposure changed in 1960 from skylight to daylight. This prompted a small change in the proposed speed constant (adjust for slower resulting speeds).