True or false: sunlight = sunlight

David Lyga · Apr 28, 2013

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

HTF III · Apr 28, 2013

I'd need a light meter and an appropriate day to contest that. Today it is cloudy, bleak, a little cold, and miserable out there. April 28, and hasn't felt like spring yet.

Bill Burk · Apr 28, 2013

Agree, the white clouds add fill light to scene 2.

Not only because the entire scene may be "brighter"... but the illumination in the shadows themselves will be lifted by the clouds.

HTF III · Apr 28, 2013

But a whole stop??

Bill Burk · Apr 28, 2013

Sure.

Think of scene 1 as Sunny-16 and scene 2 like snow/sand... can be f/22

David Lyga · Apr 28, 2013

But, Bill, if we do not take shadows into consideration, I still maintain that the SUNLIT parts are brighter with the white clouds. And I am not the only one to believe that.

Yes, HTF, a whole stop.

This is a situation that is rarely discussed, but appropriately relevant. - David Lyga

MattKing · Apr 28, 2013

The shadows are critical when you are determining exposure of negative films.

More diffusing and reflecting clouds means more light in the shadows.

The suggestions that David refers to don't really apply to transparencies (or digital).

Doremus Scudder · Apr 28, 2013

I'd think that the clouds would fill the shadows a bit, making the total subject brightness range less. If you base your exposure on the shadows, then scenario 1, complete blue sky, would likely be more exposure and less development. The same scene with lots of white clouds filling in the shadows would be less exposure but likely a bit more development. I think I end up with N-1 in the first case a lot more than in the second, especially if I'm using any blue-subtracting filters.

Best,

Doremus

David Lyga · Apr 28, 2013

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

cliveh · Apr 28, 2013

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

For my money there would be no difference in exposure. In both cases (if I'm reading it correctly), the scene (that to be photographed) is lit by full sun, so if the surrounding illumination is blue sky or diffuse, it would have little influence over direct sunlight.

kintatsu · Apr 28, 2013

I would imagine that the same amount of light would be falling on both, as they are directly lit by the sun. Giving panchromatic b emulsion as the film, the exposure should stay the same, perhaps up to 1/3 less. That is entirely without taking into account the actual values.

I have found that in similar cases, the sunlit values don't change much. It's when filters and other values within the scene are accounted for, there can be quite a difference.

HTF III · Apr 28, 2013

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?

baachitraka · Apr 28, 2013

When I incident meter(loosly BTZS for roll film) I may prefer to have scene 2 every where.

David Lyga · Apr 28, 2013

It is admittedly difficult to test this definitively: one would have to wait days on end for the perfect situation. I think that the masterful Focal Encyclopedia of Photography parses this issue though and agrees with what I have inferred.

Like others here, I originally thought that an 'elegant' answer such as 'it is still lit by the same light' would trump all because of the logic that it seems to rest upon, but there really might be a difference. We'll wait for more replies to this simple but confusing quandary.

I will not be at another computer until Monday. - David Lyga

Bill Burk · Apr 28, 2013

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?

They followed instructions like these...

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

Bill Burk · Apr 28, 2013

David Lyga said:
It is admittedly difficult to test this definitively: one would have to wait days on end for the perfect situation.

We wait days on end for this situation anyway... Beautiful light.

HTF III · Apr 28, 2013

Thanks Bill. I mean on the moon in 1969. EVERYTHING was a chance and a risk then. Can you imagine the wonder and worry in the minds of all the NASA people then? I'm betting they used Sunny f/16 on the side of the LEM facing the sun. How could they really know?

bernard_L · Apr 28, 2013

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

@ David. Agree. But definitely by not one full stop. That (1 full stop) would be the case if the clouds had only forward scattering; then they would distribute on the ground a flux equal to the incoming solar flux. But as we know from air travel, clouds also reflect a lot of sunlight towards deep space. Which can be confirmed without flying: under a cloud cover the light flux is less than under direct sun. Closer to 1/4 (2 stops from sunny 16). So, in the most favorable case (under a small clear hole in a sky otherwise covered with nice white cumulus) one might hope for 1+1/4=1.25=+1/3stop; rough, quick estimate.

@ other readers. If you don't agree, I won't go into one of those sterile forum controversies; just use your light meter under comparable conditions (same period of year + same daytime = same sun elevation).

Measurement worth thousand theories. Confucius. (or, was it Galileo? or Groucho?)

ntenny · Apr 28, 2013

In the last analysis I think it varies from scene to scene. Certainly cloud cover can reflect light---city skyglow on a cloudy night will attest to that---but there are so many variables that I would be greatly surprised to find a usable general rule (other than "when in doubt, meter").

-NT

Bill Burk · Apr 28, 2013

Yesterday, out on the dressage court with my sister-in-law Patience, I was a stop over Sunny-16. No clouds. This was light sand, so reflecting up to the subject.

Same light, different scene, the ranch house, no sand. I felt that the shaded side of the building had to be at least Zone IV, so I gave more exposure. The white pickup truck in full sun will be a challenge to print.

Will have to wait until the clouds come out to test bernard_L's theory. For now, it's a sunny day with blue skies...

pentaxuser · Apr 28, 2013

Is there a minimum percentage of the sky that has to be filled with white clouds to make a difference and might it depend on latitude and time of year? Even in high summer here in the U.K. and around noon shadows can tend to be soft even in an unbroken blue sky.

pentaxuser

benjiboy · Apr 28, 2013

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

That's why God invented light meters.

fotch · Apr 28, 2013

benjiboy said:
That's why God invented light meters.

Arvee · Apr 28, 2013

I find it hard to believe that the contribution of the reflected light by the white clouds would be a stop brighter. When I think about the intensity of the light from the white clouds compared to unobstructed sunlight, it has to pale in comparison. Therefore, I refute your hypothesis.

Edit: Actually, I have done this. Near noon I have taken incident readings with a completely blue sky. On successive days I have taken incident readings with lots of puffy white clouds but not obstructing the sun. The readings were virtually the same; the worst case was perhaps down by 1/6 stop (most likely water vapor in atmosphere). (my secondary hobby is collecting/testing light meters.)

Ian Grant · Apr 28, 2013

Interesting, when I shoot in Turkey/Greece my meter's near maximum when it's blue sky (the norm most of the year), and a few clouds drop this because there's normally some atmospheric haze.

I definite don't need to stop down or use a faster shutter speed when there's clouds.

Ian

True or false: sunlight = sunlight

Moderator

Privacy & Transparency

Privacy & Transparency