Metering in red light

[Charles Twist]

Hello,
I have recently taken some piccies at the end of the day in very red light. They have come back a stop or so under-exposed. I am trying to isolate the cause and wondering if I had to compensate for the lack of other light in the scene. I mean: when one puts a red filter on the lens, one needs to compensate - either for the lack of blue and green light or for the partial transmission of the filter in the red, I am not sure. So what happens in the case where the scene is entirely lit by red light (and no filters are used)?
I doubt it's a problem with the sensitivity of the meter as it is a modern Sekonic, and any fault there would have led to over-exposure (the old fault being a lack of sensitivity of the cell to red light).
Either that or I was tired and I plainly got it wrong.
Any advice? Thanks,
Charles

[Brian E]

Charles

You do not say which film - Colour Trans or B & W - As a general rule for sunsets I meter on cloud to one side of the sun (no direct sun hitting the meter sensor) otherwise put the incident dome on the meter and meter the light falling on the scene in front of the camera (ie: point the meter at 180 degrees away from the scene - your back to the camera) I find this also works for a lot of awkward situations.

Plus Bracket - to hell with the expense - its' still cheaper than spending time and money returning to the scene for another go. (IMHO)
hope this helps

Brian

[Keith Tapscott]

Hello,
I have recently taken some piccies at the end of the day in very red light. They have come back a stop or so under-exposed. I am trying to isolate the cause and wondering if I had to compensate for the lack of other light in the scene. I mean: when one puts a red filter on the lens, one needs to compensate - either for the lack of blue and green light or for the partial transmission of the filter in the red, I am not sure. So what happens in the case where the scene is entirely lit by red light (and no filters are used)?
I doubt it's a problem with the sensitivity of the meter as it is a modern Sekonic, and any fault there would have led to over-exposure (the old fault being a lack of sensitivity of the cell to red light).
Either that or I was tired and I plainly got it wrong.
Any advice? Thanks,
Charles

Checking the exposure with a Polaroid or D-SLR are two methods, but are time consuming and an expensive options.
A much cheaper and fun option, is to use a BLACK CAT Extended Exposure Guide, which provides exposure recommendations for a wide range of different scenes and lighting situations. It is very easy to use and is useful for night scene photography.

http://www.retrophotographic.com/blackcat.htm

[masch]

Charles,

Si photodiodes (i.e. the contents of modern lightmeters) do not have an entirely linear response to wavelength, unfortunately. If memory serves (I can look it up if needed), they peak in the red (700-800), and then sensitivity reaches further into the IR than the human eye.

The upshot is, that if your scene is significantly dominated by red light, I would expect the lightmeter to assume our scene is brighter than it really is. Hence the under exposure. These systems are calibrated to expect a reasonable mix of wavelengths (i.e. typical human vision scenes). There are diodes that mimic the human visual response, but they are usually used for specialist applications rather than photography.

Hope this makes sense.

Marc

[Emmanuel Bigler]

For those who can read French, I can recommend this excellent report by Henri Gaud. "Rouge - Une belle journée" (Red, a nice day or : a nice day in red, not really a Study in Scarlet )

http://www.galerie-photo.com/henri-gaud-rouge.html

The red ambiant light is due to modern monochrome red stained windows in a church.
Pictures were taken both with a digital camera (Canon EOS 1dsMK I) and on film (6x8 / Provia)
The digital camera has no real problem with this light. And there are no non-reciprocity corrections with silicon sensors.
Actually this red light is not a monochromatic laser light, is covers quite a broad range of the visible spectrum ; hence, says H. Gaud, metering is manageable for film as well with a hand-held meter.

[Charles Twist]

Merci Emmanuel. Next time, I'll be a bit more careful, as the shadows really are a bit dense. I must say too that the article is very interesting to anyone wishing to exhibit in France: it shows a number of typical questions that the French photographer has to ask him/herself.
On another note, something completely different: does anybody know how much the polarisation of the sky changes with latitude - if at all? I get the feeling that being closer to the equator means the polarising filter has a greater effect, ie darkens a blue sky more. Or am I mistaken?
Thank you,
Charles

[Emmanuel Bigler]

Well, let's go for a digression.
Charles, I've never been closer to the equator then Phoenix, Arizona (and the place was already quite hot for me ) so I'm not qualified for anything really equatorial here.

However what I know from the laws of physics, namely the Rayleigh theory of light scattering by molecules in air, is that the maximum amount of polarised light comes from a direction at 90° from the sun.
More precisely, at 90° from the direction of the sun's incident light rays, scattered light is linearly polarised; looking at any other direction, this scattered light is elliptically polarized, so the darkening effect of a polariser filter (be it "linear" or "circular") is maximum when the scattered light is linearly polarised and the polariser, of course, rotated to the position of the maximum darkening.

At higher latitudes, the circle of maximum polarisation can be quite high in the sky during all day since the sun never raises very high.

At low latitudes the sun travels from E to W through a point not at the zenith but much closer to, than here in Western Europe (Europe now spreads from the Lampedusa Island to Lappland, so we should be careful here ).
So actually the circle of maximum polarisation moves in the sky according to the time of the day; I really do not see immediately the practical consequences except that it should always be at 90° from the sun's direction of incident light.
Say, in the morning, when the sun is already high at 45°, the linearly polarised circle of light is also at 45°.... Does this make the effect more visible ? May be !

A recommended book for everybody regarding atmospheric optical phenomena, very readable with superb pictures and a minimum of maths is:
Prof. Marcel Minnaert's famous textbook "Light and Colour in the Outdoors"
Springer; 2nd printing edition (April 13, 1995) / ISBN-10: 0387979352 / ISBN-13: 978-0387979359

BTW Minnaerts' book emphasizes a physiological phnomenon called : over-estimation of heights.
Ask anybody how high the sun is in the sky in the British Isles in a hot summer, he'll answer ! at the zenith !
In fact our neck cannot easily bend toward the zenith; so may be the explanation that the polarised sky is more obvious at lower latitudes could come from this effect, the fact that we usually see most comfortably objects lower than 45° above the horizon ????