Comment on Apollo Moon Landing Hoax – Photographic Evidence by jfb.
From the article:
“But there is a very reasonable explanation, the cameras that the Apollo mission used, had manual light filters, (a bit like the pupil of your eye). The Sun reflects off of the moons surface and the white suits of the astronauts. Making both exceptionally bright, while the stars (because they are so far away) are quite dim.”
This dances around the truth a bit, and I’d like to expand on it if I may.
It all comes down to the limitations of photographic film, which has a non-linear response to light. Below a certain level of exposure (expressed in lux-seconds), the film simply doesn’t record an image; after processing, that area of film is blank (pure black on a print). Above a certain level of exposure, the film is as dense as it can get, and no more information can be recorded; after processing, that area is totally opaque (pure white on a print). Between these two levels, the response is more or less linear, and you get a steady gradation of tones (black, very dark grey, dark grey, grey, light grey, very light grey, white). If you check the data sheet for the film, such as the one for Kodak Tri-X:
you’ll find a series of charts labeled “Characteristic Curves”, which shows this non-linear behavior for various developers, processes, and development times.
It turns out that the range of exposure levels between pure white and pure black is roughly 1000:1 (depending on the specific film and processing method), meaning the brightest area in the scene can’t be more than 1000 times brighter than the dimmest area if you want to record both in the same frame with any detail. If the two areas fall outside of that range, then you can either capture the dim areas and overexpose the bright ones, or you can capture the bright areas and not record the dim ones. This is why pictures taken on a sunny day have such stark shadows; the range between the shadows and the highlights is approaching the limits of what the film can handle, so the shadows wind up looking extremely dark.
So film can’t capture a dynamic range outside of 1000:1. Unfortunately, the sunlit surface of the Moon is more than 1000 times brighter than the brightest stars. As seen from Earth, a full moon is more than *30,000* times brighter than the stars (see the page http://en.wikipedia.org/wiki/Apparent_magnitude for an explanation of how that number was computed).
There’s no way to expose for the lunar surface *and* the stars in the same image; it is simply beyond the capability of the medium to record. If they had exposed for the stars, the lunar surface would have been a detail-free blob of pure white. By exposing for the surface, the stars simply don’t register on the film.
I’m limiting this discussion to film since that’s what was used on the Apollo missions, but sensors in modern digital cameras are similarly limited; they can’t capture a much better dynamic range than film.
As I pointed out in another comment, this is something you can test for yourself. Grab a camera with manual exposure controls. If film, use ISO-100 speed film; if digital, set the ISO level to 100. Go outside on the next clear, sunny day in the late morning or early afternoon. Set your shutter speed (Tv) to 125 (1/125 seconds) and your aperture (Av) to f16 and take a picture of the landscape. Now set Tv to 30″ (thirty seconds) and take another picture of the same scene (you will want to put the camera on a tripod for that). The first picture will be properly exposed. The second will be horribly overexposed and show up as a blank white frame.
Now, go outside on the next clear, moonless night and take pictures of the stars using the same exposure settings. At Tv=125, you should have a black frame. At Tv=30″ (and a tripod), you should see at least the brightest stars.
Another way of thinking about this is to realize that the Moon is clearly visible during daylight hours right around its first quarter phase, whereas the stars are swamped by the light scattered by the atmosphere; again, the Moon is just many thousands of times brighter than the stars.