Shooting the Moon

[gwarrellow]

The exposure's going to depend on the magnification.

Ray,

I know this is counter-intuitive but I believe the magnification doesn't affect the exposure when photographing an object that is illuminated by reflected light, such as the moon.  Imagine a terrestrial object such as a brick wall, then the correct exposure for the wall is say, 1/250 sec f8, whether you use a 50 mm lens or a 1000 mm lens, or you move 10x closer to the wall, for example.

The dimming effect you see in a telescope when viewing at higher magnifications is due to other effects of the optical system.

It's curious I know!
Regards,
Graham

[Bill]

A great time of year to shoot the moon (at least I found) is during the fall full moon (I imagine there is a correpsonding time in the spring too).  

More precisely, for reference, was the full moon around October 12th, 2003.

At that time of year the moon rises just after the sun sets.  There is still ample light being reflected in the atmosphere.  The moon is also bright.

[gwarrellow]

Oh! I forgot to add, if it's just the moon you're after, this is one of those occasions when both the 10D and SD10 will outperform the 1Ds. Just thought I'd slip that one in, to clear the waters.  

What makes you think that!  Just curious  

[Ray]

Hmnn. Here we go again on pixel sizes... People keep forgetting that at the end of the day it's always the lens quality that matters, not really the pixels, especially when it comes to tele capabilities. You can always add tele-extenders (or for the astro-buffs eyepiece projection) to magnify the lens' image such that all its limitations become patently obvious. Let me explain. Any Canon supertele will have its job stretched quite beyond its "razor-sharp" capabilities if we use stacked 2x + 1.4x converters, on both the 10D or the 1Ds. Were this not the case then Canon would have offered us 3x or 4x extenders...

Well, I've not forgotten this.   . I think your bringing in other issues here; diffraction and the Rayleigh's limit.

I'm talking about ordinary (but expensive) telephoto lenses which are used at apertures well above the Rayleigh's limit, not astronomical telescopes

Of course, at the end of the day it's always the lens quality that matters. My comparison of the three cameras are in relation to the same lens. If the lens is lousy enough, you probably won't notice much difference. If the lens is absolutely superb, the 1Ds might have the edge with a 2x converter. The Canon 1200mm, which costs as much as a house, might just trump the SD10 if used with a converter, but I'm not certain, and I don't expect anyone to show me some comparisons.  

[Ray]

Okay! I'm glad we sorted that  . As a matter of interest, running the eyedropper over default TIF conversions of the moon at various exposures and 100 ISO, and checking the info palette in PS, I get the following.

(1) F8 and 1/90th, maximum highlight value 240.

(2) F8 and 1/125th, maximum value 230.

(3) F8 and 1/180th, maximum value 209.

[Howard Smith]

Scott, when you say f/8, was that f/8 on the lens setting or f/8 after adding a stop for the extender?

The image looks pretty good.  It is lkely just a personal perference, but I would like a stop or so less exposure to make the seas a darker and reduce the highlights to less than white with detail.

[Exegeter]

When I saw the clouds creeping in front of the moon on my way home tonight I almost ran a stop sign trying to get home.  I ran to my tripod leaning by the door and set everything up on the roof.  One last look with my light to make sure everything was level, a few shakes to see if I'd be shopping for a new body and lens, and I was off...  

A missed opportunity but a learning experience none the less.  I learned a lot about setting up on the roof and got a few ideas to try implimenting before next time.  

How do you shoot the moon?  How do you pick up all the gradients  and contrasts?  All the craters?  And still pick the clouds up?  

[Exegeter]

Thanks a lot!  I'll be using a 70-200 2.8 on a Sigma SD9.  

It's disapointing that I need to essentially choose between moon and clouds.  I'm not sure how I'd blend them very well with the composition I'm after.

[b.e.wilson]

The exposure's going to depend on the magnification.

Ray,

You are quite correct, exposure does depend on magnification, especially if you use some external exposure rule instead of in-camera metering.

For example, using a 2x extender means you must increase the exposure two stops more than the rule dictates. This is because the extender is taking only one-fourth of the full image circle and making that 25% the new image circle.

The principle involved is the lens-to-film distance. The farther away the lens is, the wider the image circle it will project on the film. If the film is a fixed size, then it will 'capture' a smaller amount of the total light. Macro photographers (who use extention tubes or bellows to increase magnification) know this rule all to well. So do kids burning leaves with a magnifying glass. The farther you get from the lens the less heat you project onto the leaf.

This effect is seen in consumer zoom lenses also. I have a Canon 75-300 lens that is f/4 at 75mm, but only f/5.6 at 300mm, even though the iris is completely out of the light path. As the magnification increases, the passage of light through the lens decreases. Gives you a clue why constant-f-stop lenses (like the Canon 'L' line) are so expensive.

[Ray]

Assuming one is using a camera telephoto lens and not a telescope.  

[samirkharusi]

Hmnn. Here we go again on pixel sizes... People keep forgetting that at the end of the day it's always the lens quality that matters, not really the pixels, especially when it comes to tele capabilities. You can always add tele-extenders (or for the astro-buffs eyepiece projection) to magnify the lens' image such that all its limitations become patently obvious. Let me explain. Any Canon supertele will have its job stretched quite beyond its "razor-sharp" capabilities if we use stacked 2x + 1.4x converters, on both the 10D or the 1Ds. Were this not the case then Canon would have offered us 3x or 4x extenders... With stacked extenders it becomes a moot point, IMHO, as to whether you use 7.5 or 8.8micron pixels. Both images will look "stretched" as on a balloon. I did some calculations recently, plus some field testing, to educate myself (one does get distracted into irrelevancies by web noise)  to verify whether there's any merit in "extending" a diffraction-limited scope well beyond its theoretical diffraction limit. Here are my results, showing graphically what the various definitions of diffraction limit mean, what happens when you sample the intensity profiles at Nyquist or otherwise, and finally shooting Saturn at 4600mm f23 and 10,000mm f50 with 5.6micron pixels:
http://www.geocities.com/ultimaoptix/sampling_saturn.html
For the field testing I used a 2x and a 5x extender and then resized the images to an identical size to compare them.

[Howard Smith]

An error.  Adding 1 1/3 stops would give about f/11 @ 1/50, not 1/80.  So that is about a stop more exposure than BJL suggested and 2 stops more than sunny-16.

[scott kirkpatrick]

f/8 after correcting for the one stop increase in focal length due to the extender.  The E-1 is really pretty smart about this stuff.  As far as it is concerned, you may have set f/5.6 before, but you're at f/8 now, so that is what it displays, and what the EXIF shows.  To check that this is happening, you notice that the largest opening available is one stop less bright after installing the 1.4X extender.  

I'll go back and check the darker shots to see what I can get from them after developing the RAW files in Viewer or PS-CS.

scott

[b.e.wilson]

The moon is about 25% reflective, and it's illuminated by the sun. The Sunny-16 rule usually works (f/16 and 1/ISO for the shutter speed).

[Ray]

Last time I shot the moon, I used my 100-400 IS zoom with 1.4x extender attached to my D60 producing an effective 35mm focal length of about 900mm. (Just wanted to see how sharp I could get those craters  ). I tried a number of exposures, but the one that brought out the most detail was something like 1/125th at F11 and ISO 100.

The exposure's going to depend on the magnification. If you want some foreground as well, and/or visible clouds, you'll need to take 2 exposures and blend them.

[Ray]

I know this is counter-intuitive but I believe the magnification doesn't affect the exposure when photographing an object that is illuminated by reflected light, such as the moon.

Quite right!    Whether the object is big or small within the frame, it needs the same exposure to bring out the detail. What was I thinking of!  

[gwarrellow]

The exposure's going to depend on the magnification.

Ray,

You are quite correct, exposure does depend on magnification, especially if you use some external exposure rule instead of in-camera metering.

For example, using a 2x extender means you must increase the exposure two stops more than the rule dictates. This is because the extender is taking only one-fourth of the full image circle and making that 25% the new image circle.

The principle involved is the lens-to-film distance. The farther away the lens is, the wider the image circle it will project on the film. If the film is a fixed size, then it will 'capture' a smaller amount of the total light. Macro photographers (who use extention tubes or bellows to increase magnification) know this rule all to well. So do kids burning leaves with a magnifying glass. The farther you get from the lens the less heat you project onto the leaf.

This effect is seen in consumer zoom lenses also. I have a Canon 75-300 lens that is f/4 at 75mm, but only f/5.6 at 300mm, even though the iris is completely out of the light path. As the magnification increases, the passage of light through the lens decreases. Gives you a clue why constant-f-stop lenses (like the Canon 'L' line) are so expensive.

Bruce,

I think we may be talking about different things here.  If there is no change in the f-ratio of your optical system (e.g. by adding an extender or reducer) then for an object illuminated by reflected light (e.g.) moon) then there is no change in exposure.

In the examples you cited you are either changing the effective f-ratio (adding extension tubes) or using a source (the Sun) of light that is not a point source (as you have with e.g. a star).

I told you it was counter-intuitive!
regards,
Graham

[gwarrellow]

I have been out in to the cold the last two full moons and taken a number of shots

<snip>

Minus 32 C

Minus 32 C!  Minus 32 C! That's not cold, that's brass monkey weather  

Where on earth do you live?

BTW Nice full moon with plenty of detail.
Regards,
Graham

[b.e.wilson]

Re you getting "bellows extension factor" mixed up with magnification by focal length?

Is there a difference?

M = image height/subject height

M = (lens-to-film distance - focal length/focal length)

M = focal length/(lens-to-subject - focal length)

In macro shots, where the bellows factor is significant, the magnification is high enough to matter to the exposure. With the "magnification by focal length" I presume we are speaking of distant large objects where the magnification is extremely low, low enough not to affect the exposure.

Except when extension lenses are used. That's changing the focal length without changing the aperture, and that changes the f-stop (f-stop = focal length/aperture).

It's been fun going through the old equations again.

[Ray]

I've just been checking the actual exposures I used for my moon shots. From memory I think I wrote F11 at 1/125 with 1.4x extender. I see this is not correct. I did take some underexposed shots at f8 and 1/125th without extender, but the best exposure was 1/90th at f8 and 100 ISO (400mm without extender) of a full moon. This was a very slight underexposure with the histogram about 1/3 of a stop away from the right.

On a different night I shot a 3/4 moon using the 1.4x extender with the 100-400 zoom. Best exposure was 1/30th at f11 with histogram just touching the right.

I'd say it's more like a sunny F8 rule that would apply.  

What I did find surprising was the noticeable improvement I got with the 1.4x extender; slightly more detail and slightly contrastier detail with the craters appearing more 3-dimensional. Well, you might say, this is to be expected. However, previous experience with this combination of 100-400 IS zoom and 1.4x extender had led me to believe there was no advantage when the subject was at or close to infinity - something I'd puzzled over and attributed to dust in the atmosphere blocking out or reducing the contrast of all fine detail, and the fact that only the best lenses produce useful results with converters.