Retroreflectivity of the lunar surface

[Eventcone]

I have noticed over the years how many Apollo photographs appear to show the lunar surface as having varying levels of brightness within the same photograph. I guess this can be due to varying levels of surface material brightness and/or colour. However I would like to know how much retroreflectivity comes into play.

I am, of course, referring to the digital scans available within the online Project Apollo Image Gallery and not to some of the poor quality reproductions that have been published down the years.

I have noticed that the surface often appears to be significantly brighter in the 'down-sun' direction. With a horizontal Field of View of 53.5 degrees for the Hasselblad & Zeiss Biogon combination, many single shots can easily capture the surface in directions ranging from 'down-sun' to more than halfway to the 'cross-sun' direction, so this effect can be very noticeable. It can be even more noticeable in a stitched panorama - especially the 'Post Landing Panoramas' where the surface either side of the LM shadow and beyond to the west is very much brighter than the surface to left and right extremities. There is also a tendency for the surface near to the horizon to be less bright - I guess because retroreflected sunlight from the horizon would pass over the camera position.

Referring to the 'Photography' page at ww.clavius.org, I found the following statement interesting:

Shutter speeds were typically 1/125 or 1/250 second. F-stop settings varied from f/5.6 for up-sun photos to f/8 and f/11 for cross-sun and down-sun photos.

When I first read this I thought that it must contain a typo as it suggested that the aperture would be closed down for 'down-sun' shots and opened up for 'up-sun' shots. I felt certain it should be the other way around. However if it is correct it would appear that the surface retroreflectivity was anticipated before even the Apollo 11 landing (not that I would be suprised by that).

I have noted retroreflectivity cited as the reason for the extra brightness of the full moon vs the half-moon etc, and also the 'heiligenschein' effect in photographs where the photographer's shadow appears in the photograph. I guess I am saying here that I know the surface is retroreflective, but I would like some confirmation that this is a good explanation for the apparent variations in the surface's apparent brightness within the same photograph.

Thank you in advance to any who take the time to reply.   

[JayUtah]

The reason for the counter-intuitive camera settings really doesn't have much to do with lunar retroflection.  When you shoot into your light source -- that is, when the subject is backlit -- you'll be photographing the shaded side of the object.  If you want any detail to show up in the shaded side, you'll have to open up the aperture, increasing the exposure.  Of course this means other parts of your image may be so overexposed they lack detail.

[bknight]

From your questions I'm  slightly confused are you suggesting that there is something wrong with the images especially the panoramas?

[smartcooky]

From your questions I'm  slightly confused are you suggesting that there is something wrong with the images especially the panoramas?

I didn't get that impression. I think he just mentioned the panoramas because, as is typical with any wide angled shot, it is very difficult to get even lighting across the whole width of the exposed area, so any problems like that are more likely to show up.

One of the things I used to do in the days before smartphones, was to stitch photos together to make a panorama. It was a really difficult task to get the exposure looking right across the whole width of the image, even if the photographer had been very careful. In my experience, photos taken on dull days or with considerable cloud cover (low contrast) were the easiest, while sunny days with their consequent high contrast were the most difficult. The lunar landscape would be "the sunny day from hell" for anyone attempting to make a stitched panorama.

[Eventcone]

From your questions I'm  slightly confused are you suggesting that there is something wrong with the images especially the panoramas?

I am certainly not suggesting that there is anything wrong with the images - I am no hoax believer. I am merely trying to gain a better understanding of what causes this effect, and I was struck by the fact that it is noticeable within the field of view of a single photograph.

Truth to tell I was prompted to ask the question because I became embroiled with a hoax believer who described this as light 'falloff', and I wanted to be more certain in my answers to him by being better informed. However I felt it more appropriate to post the question in the 'Reality of Apollo' forum. There is no doubt in my mind as to this being a genuine feature of Apollo lunar surface photography, or to the reality of Apollo itself.

[Eventcone]

One of the things I used to do in the days before smartphones, was to stitch photos together to make a panorama. It was a really difficult task to get the exposure looking right across the whole width of the image, even if the photographer had been very careful. In my experience, photos taken on dull days or with considerable cloud cover (low contrast) were the easiest, while sunny days with their consequent high contrast were the most difficult. The lunar landscape would be "the sunny day from hell" for anyone attempting to make a stitched panorama.

Thanks for your reply.

Interesting. So you would say it's not just a feature of lunar photography, but can be of photography in general, if the field of view is wide enough? Do you think that as shadows are naturally hidden in the down-sun direction, and begin to reveal themselves as you move towards cross-sun, this has the effect of reducing the overall light levels being reflected towards the camera from the cross-sun direction?

On a rough, as opposed to smooth surface this effect would be increased would it not? On the moon, even in flat areas the regolith covered surface combined with the highly directional lighting (no diffuse lighting from the sky) would contain a myriad of tiny shadows that would make their own contribution to the surface's overall apparent brightness in the cross sun and up-sun directions - at least with the sun at low angles above the horizon as it was during the Apollo missions. What do you think?

[smartcooky]

One of the things I used to do in the days before smartphones, was to stitch photos together to make a panorama. It was a really difficult task to get the exposure looking right across the whole width of the image, even if the photographer had been very careful. In my experience, photos taken on dull days or with considerable cloud cover (low contrast) were the easiest, while sunny days with their consequent high contrast were the most difficult. The lunar landscape would be "the sunny day from hell" for anyone attempting to make a stitched panorama.

Thanks for your reply.

Interesting. So you would say it's not just a feature of lunar photography, but can be of photography in general, if the field of view is wide enough? Do you think that as shadows are naturally hidden in the down-sun direction, and begin to reveal themselves as you move towards cross-sun, this has the effect of reducing the overall light levels being reflected towards the camera from the cross-sun direction?

On a rough, as opposed to smooth surface this effect would be increased would it not? On the moon, even in flat areas the regolith covered surface combined with the highly directional lighting (no diffuse lighting from the sky) would contain a myriad of tiny shadows that would make their own contribution to the surface's overall apparent brightness in the cross sun and up-sun directions - at least with the sun at low angles above the horizon as it was during the Apollo missions. What do you think?

The wider the angle of the photo, the greater the variation there will be in contrast, desnoty and back-lighting between the left end and the right edge of the image.

Try this... go outside and take three separate photos of a landscape several degrees apart, but so that the edges of the fields of view overlap. Now print them and try to match up the edges - take particular note of the sky. If it is blue, it will be exceedingly difficult to match when you are trying to stitch those three photos together to make a panorama. On a dull, cloudy day, its not so bad because its a few shades of grey that can easily me smudged. A blue sky however has a HUGE tonal range that makes the job much, much harder. But its not just the sky, its the colours and the densities of the landscape as well. You could find that colours and densities of the exact same part of the landscape in the overlap area is different on each photo. I imagine this would be even worse for a lunar landscape, because of the reflectivity of the lunar soil, and the lack of atmosphere (and therefore, the lack of diffusion of light).   

[Eventcone]

Thanks smartcooky - I'll give that a try.
 

[Eventcone]

The reason for the counter-intuitive camera settings really doesn't have much to do with lunar retroflection.  When you shoot into your light source -- that is, when the subject is backlit -- you'll be photographing the shaded side of the object.  If you want any detail to show up in the shaded side, you'll have to open up the aperture, increasing the exposure.  Of course this means other parts of your image may be so overexposed they lack detail.

Thanks Jay. Of course I should have realised (despite my limited knowledge and experience of photography) that goes right back to the basics I was taught with an old box camera - namely photograph your subject with your back to the sun. I was thinking only of landscape photography in this instance and got my wires crossed.

[JayUtah]

I'm glad it made sense to you.  It's actually even more basic than that, I realized after thinking about it.  It's still just basic photography.  Any scene you photograph will have light and dark parts.  You set the exposure so that whatever part you want to look best will land in the best part of the film's response curve.  It literally doesn't matter why the photograph has light and dark parts.  It matters only that you know how to set your camera to properly expose whatever area in the viewfinder is why you're taking the picture.  In terms of general photography, we used to words like "opening up the shadows" to describe increasing the exposure for shaded and shadowed areas.

I would like to mention that this is very important for Ektachrome film.  It's a high-contrast film.  If you don't "open up the shadows," you stand a very good chance of getting entirely black shadow or shaded areas.  There is narrower range of useful exposure than in other films.  Also consider that the toning and grading you see in various Apollo photos around the web are inconsistent and not very good.  The scans I've seen from the original transparency are much better graded than scans of dupe masters.  So you're seeing an exaggeration of what the limitations of that particular film stock already imposes.

Photographic textured surfaces is very rewarding when you get to mess with the phase angle in artistically appealing ways.  But what Smartcooky says is spot-on.  With the Zeiss Biogon lens you have a 45-degree field of view.  That means your phase angle will change 45 degrees from the left to the right of any particular picture.  It can produce some interesting effects, especially when the difference between light and dark is so stark.  It even sort of works on Earth, if you've taken the classic picture of your own shadow cast onto, say, grass.  If you do it right, you get a nice halo around your head.