Lunar exposure time @ f10/ISO 100

[raucousimages]

I am helping on a technical project involving photographing the moon. The telescope being used is a fixed f10. The film body (camera) is adjustable in 1/3 stops for shutter speed but once a roll is started I may not be able to adjust or bracket the shutter speed. So given IOS 100 film and f10 what would I use as a shutter speed? Useing the "Sunny 16 Rule " for reflected light I come up with 1/250, am I close?

Thanks for any help.

John

 

[raucousimages]

BTW. I am helping on this because digital is not giving enough detail in the shadows from the lunar mountains. Good old T-max 100 works better!

 

[Lee L]

From Covington's Astrophotography for the Amateur
Appendix A.5

f:10 and 100 ISO

thin crescent 1/8
wide crescent 1/15
quarter moon 1/30
gibbous 1/60
full 1/125

So about a stop more exposure than f:16 sunny, most likely because of atmospheric extinction and variations. Bracketing a little is also a good idea.

Other important considerations are to balance the scope/camera combination on the mount carefully and use the mirror lockup if using an SLR. I've seen some instances in which making the scope slightly tail-heavy can help damp vibrations, mostly when the camera shutter/mirror excites the system's resonant frequency. If it's f:10, there's a good chance it's a Schmidt-Cass and you often can't adjust balance on those common fork mounts very effectively without a slide bar and some weights.

Lee

 

[Tom A]

Sunny rule 16 will work. And for technical use it will give useful pictures.

But I'm not sure how much details in the shadows, you will get with this exposure time. As the shadows apparently are important for your project, I would consider one or two stops overexposure.

Tom

 

[Jim Jones]

I agree with Lee and Tom. The surface of the moon is actually a fairly dark grey. Yet, we percieve the full moon to be quite bright. Instead of the Sunny 16 rule, I suggest the Looney 11 or, better yet, the Looney 8 rule. More exposure is usually required when the moon is close to the horizon.

The full moon lacks contrast. A good B&W film to compensate for this was Kodak Tech Pan developed for fairly high contrast. This will emphasize detail. The contrast can always be reduced when printing, if desired. I always bracketed when using Tech Pan.

 

[David A. Goldfarb]

Looney 11 is what I use for the full moon.

 

[glennfromwy]

Full moon - 125 at f/8 has always done it for me.

 

[c6h6o3]

So given IOS 100 film and f10 what would I use as a shutter speed?

Ansel Adams describes in The Making of 40 Photographs how he computed the correct exposure for the moon in his "Moonrise, Hernandez, NM" photograph. He couldn't find his meter and a cloud was about to obscure the moon so he had to think fast.

He knew the luminance of the moon to be 250 c/sq.ft. At a shutter speed equal to the luminance of a subject in c/sq. ft. the correct aperture is equal to the square root of the film speed. In your case, that would be the square root of 100, or 10. Therefore f10 at 1/250 sec. will give you a Zone V moon. Adams wanted a Zone VII moon, so you need to open it up two stops, yielding a final exposure of f10 at 1/60th.

Of course this doesn't take into account any filters you may be using. The phases of the moon are irrelevant as its luminance is uniform over the illuminated surface, no matter how much illuminated surface there happens to be.

 

[Lee L]

The phases of the moon are irrelevant as its luminance is uniform over the illuminated surface, no matter how much illuminated surface there happens to be.

This page contains a rather technical explanation of why this is not the case:
Lunar Albedo

Albedo is the general term for percent reflectance by an object.

Albedo is given in a variety of definitions, and the blacktop analogy is the result of the abuse of a couple of such definitions. Without knowing the definition that is used, its impossible to be sure you are comparing apples to apples. The simplest version of albedo is the Lambert albedo. A Lambert surface is one which scatters light isotropically - in other words, an equal intensity of light is scattered in all directions; it doesn't matter whether you measure it from directly above the surface or off to the side. The photometer will give you the same reading.

For a lambert planetary surface, the illumination effects are entirely geometric. The brightest illumination is directly below the sun, and the amount of light reflected diminishes the farther you get from this point, simply because the sunlight is played along a greater arc of the surface. The illumination isophotes will be round. Unfortunately, the moon is not a Lambert surface.

For one thing, the subsolar point does not provide the brightest reflection - the limb does. And the phase curve has a sharp peak in brightness during full moon - the moon is extra reflective at full compared to first quarter. Attempts were once made to explain this in terms of a Lambert surface with various kinds of topography, but this does not work out.

It is now known that this departure from a Lambert surface is caused by the very porous first few millimeters of the lunar regolith. Sunlight can penetrate the surface and illuminate subsurface grains, the scattered light from which can make its way back out in any direction. At full phase, all such grains cover their own shadows; the dark shadows being covered by bright grains, the surface is brighter than normal.

This is why all well written astrophotography books suggest longer exposures for partial phases.

Further reading on "Moonrise, Hernandez, NM" will also reveal that the negative was one or two stops underexposed given what Adams intended, and it took him several years to coax the print he wanted out of it.

Lee

 

[raucousimages]

Thanks for the help. Everything is ready to go. Telescope, camera (gave up on the film body, it won't allow bracketing), tripod. Now if I can just get the clouds to help. Utah has about 60 heavy overcast days a year and it is looking like this year they are all this winter.

 

[Sirius Glass]

Estimating the exposure for the Moon

Back to first principles:

The Moon and the Earth are illuminated by the same source and the distance from the source is approximately the same.

The albedo of the Moon [surface reflection] is approximately 0.5.

Therefore the exposure is one f/number smaller [read bigger aperature] for a full moon. Two f/numbers for a half moon. Interpolate for the rest of the phases. The shutter speed is 1/ASA film speed.

ASA 100 => 1/100 second [1/125 second]
ASA 400 => 1/400 second [1/500 second]
​

For a telescope with an f/10 aperature and ASA 100 the exposure would be:

Full moon => 1/125 second
Half moon => 1/60 second​

Also the Moon moves 15 degrees per hour. For a 50mm lens on a 35mm camera the longest exposure without motion blurring is 1/25 of a second or 2*1/focal length in sconds. For a 6x6 MF camera an 80mm lens would also be 1/25 of a second. The tradeoff that arises from using a telescope is that the longer the focal length of the telescope, the shorter the exposure must be EXCEPT THAT an Equatorial motor mount will take this motion out.

*** John, are you using an Equatorial motor mount to eliminate the relative motion of the Moon? ***