Photographing the Full Moon with a Digital Camera

posted: 3828 days ago, on Tuesday, 2007 Jan 30 at 11:54
tags: astronomy, outreach, Moon, astrophotography.

About once a month, it's Full Moon, and our nearest celestial neighbour rises at sunset and remains visible throughout the night, setting at daybreak the next morning.

Popular wisdom has it that crazy people ("lunatics") go even crazier at the time of Full Moon, and werewolves skulk in the shadows, pouncing on and devouring unsuspecting astronomers as they peer through their telescopes. In the real world, however, reliable reports of such events are rare.

The most obvious change the Moon undergoes during the 28 day lunar month is the sequence of phases, from a slender crescent to a half Moon, swelling to the bright full phase, and then waning to a crescent again.

A more subtle change, that requires a bit more effort to notice, is the change in the angular size of the Moon. Of course, the Moon's actual size doesn't change: it's about a 10,900 km round-trip if you took a walk around its equator. The angular size (how large a patch of sky it covers) as seen from Earth, does change, because the Moon isn't always at the same distance from us. By photographing the Full Moon at different times during the year you can see for yourself how its angular size changes.

The diameter of the Full Moon varies from around 29.5 arcmin to 33.5 arcmin. The diameter thus increases by about 13% from minimum to maximum, but in area, this is a 29% increase!

Using a digicam to photograph the Moon

Check out Carol Botha's article "Moon Photography Tips" for more lunar digital photography lessons.

DISCLAIMER: There are many, many makes and models of digital cameras on the market, and their capabilities differ markedly. Some cameras work only in fully automatic mode, and do not allow the user to change settings. This article, however, focuses on those cameras that do allow changes to be made to exposure time and aperture. Usually, such cameras will have a dial (as in the photo below) or an option that can be selected from a menu.

Nevertheless, the basics of photography remain the same irrespective of a particular camera's features. Play with whatever settings your camera offers, and don't be disappointed if the images aren't as good as what the guys at SALT or the Hubble are taking. Have fun!

Accessories

Modern digital cameras are excellent for photographing the Moon. You don't need a telescope, either in fact all you need is a tripod. The tripod doesn't need to be an elaborate affair as the accompanying image shows, they can be quite humble; this one cost me R40 at my local camera shop (match not included).

In a pinch, you could also try using a detachable tripod head (or scavange one from a cheap tripod) and fix it to something sturdy, like a fence post. Keep in mind, though, that a good sturdy tripod (something you could use to bash down a door, say) is nevertheless important the cute mini-tripod should be used for emergency photography only.

Photography basics

In a traditional camera, the detector is film, a strip of plastic coated with light-sensitive chemicals. In a digital camera, the detector is a CCD, a light-sensitive electronics component (a distant cousin of those gadgets that automatically turn on an outdoor light when it gets dark). A CCD consists of a grid of tiny sensors, like the compound eye of an insect, each capable of sensing a small portion (a pixel, or "picture element") of the image. Each pixel converts light into electrons; the electrons are counted ("digitized") and the values stored in a data file. The data file can then be transferred to a computer for display.

When taking a photograph (film or digital), there are basically only two variables you can control: how much light the lens lets through, and for how long it falls on the detector.

After discussing these basic ideas, I will briefly summarize a few other concepts and, with a dose of practical tips, you'll be on your way.

Aperture setting (F-number, Av)

The first variable, how much light you allow to pass through the lens to the CCD, is the aperture setting (or lens aperture, F-number, focal ratio, Av). It is numerically expressed as an f-ratio, with a value from, say, 1.6 up to, say, 22. The bigger the number, the more light is blocked. A lens set to f/16 gives an image four times dimmer than a lens set to f/8. For astrophotography, because things are generally faint, it is best to "open up" the lens, i.e. use a smaller f/number. The Moon, on the other hand, is bright enough so you can safely try a range of f/stops.

Exposure time (shutter speed, S, Tv)

The second variable is the exposure time (shutter speed, Tv), which is the duration, in seconds, that the CCD is exposed. Longer exposures means more light gets through, so fainter objects are recorded. Exposure times range from many seconds, to thousands of a second. For astrophotos, long exposures are typical, because most astronomical objects are faint.

The trick in any photo is to know how to match these two: size of opening and exposure time. Too small an aperture, or too short an exposure time, may let through too little light. Too much light, and the image is over-exposed.

Fortunately, cameras have a built-in light meter which balances the relationship between aperture and exposure. However, light meters can be unreliable when you're taking astrophotos they are mostly designed for "normal" daylight photography.

The joy of a digital camera, though, is that it is easy to get it right just take a photo, look at the result, and decide if it needs more or less light. Look at the following three images:

All were taken with the lens set to f/8. The first image, exposed for a thirtieth of a second, is obviously overexposed.

The last image was only exposed for 1/250th of a second, 8 times shorter than the first. It looks a bit dull, the whites appear gray, so is a bit underexposed. The middle one looks a bit better. It would be a simple matter to have taken another image, at 1/60th of a second, to see if it was an improvement. This technique of taking multiple shots with different exposures is called bracketing, and it can be your best friend!

What is the correct setting? That's up to you to decide. Look at these two images:

The left one shows the sunlight portion of the Moon well, so that you can make out craters and lunar seas. The second image, exposed almost 200 hundred times longer, heavily over-exposes the sunlight portion, but allows detail to be seen in the shadowed portion. The "correct" exposure thus depends on what you want to show. In general, a well-balanced exposure will show white as white, and black as black, with a full range of grays in-between.

Exposure guidelines

Table 1. Exposure guidelines for the Moon (assuming ISO 400 at f/8)

Lunar aspectExposure
 
 
thin crescent Moon1/60th
wide crescent1/125th
quarter phase1/250th
gibbous Moon1/500th
Full Moon1/1000th
bright features on the terminator1/250th
dim features on the terminator1/125th
partial lunar eclipsehalf-second
total lunar eclipse at maximum6 to 135 seconds
 

The exposure times in Table 1 are for a camera set at f/8 and ISO 400.

For different aperture and/or ISO settings, use the following rules:

Each time you halve the ISO, then double the exposure time, and vice versa: for each doubling of the ISO, halve the exposure time. (Full Moon example: [f/8, ISO 400, 1/1000th] = [f/8, ISO 200, 1/500th] = [f/8, ISO 800, 1/2000th]).

Each time you halve the f/ratio, then divide the exposure time by four; double the f/ratio and you need to expose four times longer. (Full Moon example: [f/8, ISO 400, 1/1000th] = [f/4, ISO 400, 1/4000th] = [f/16, ISO 400, 1/250th]).

When changing both ISO and f/ratio, apply the first rule, then the second. (Full Moon example: [f/8, ISO 400, 1/1000th] = [f/4, ISO 200, 1/2000th]).

The bottom line: bracket your exposures, examining each image after you've taken it to see if things looks good. When in doubt, take another image.

Dealing with "automatic" exposure settings

Digicams that don't give you direct control over exposure time, need a slightly different approach. In addition to the usual "Auto" exposure mode (which is fully automatic; you just point and shoot), other modes are usually available, often with creative names, such as "High Sensitivity", "Twilight", "Soft Snap", "Beach", "Fireworks", and so on. Try all the modes your camera offers and note which work. Dr Philip van Heerden, for example, discovered that "High Speed Shutter" mode on his camera worked perfectly (see example images).

Another tactic is to fool the camera's light meter into using a different exposure time than what it thinks is necessary. Hans van der Merwe has had success by shining a bright torch into the camera lens while pressing the shutter release trigger half-way, to prompt the camera to take a light reading. The bright torch-light tricked the camera into choosing an exposure time that worked perfectly for the Moon. Willie Koorts has pointed out that (crazy as it sounds) forcing the camera to use its flash can also do the trick. The flash isn't used to illuminate the Moon in flash-mode, a camera will almost always fix the exposure time at 1/60th of a second (some models may use 1/125th). To avoid air-borne dust from being picked up by the flash, cover the flash with your hand (and close your eyes!).

ISO ("film speed")

With film cameras, you can use film rated at different speeds, or light sensitivity. Numerically this is expressed as an ISO number: ISO 100 is "slow", while ISO 1600 is "fast". The bigger the number, the more light-sensitive the film is.

The same is true for digital cameras. If you can change the ISO setting on your camera, you can select how light-sensitive it will be. If your camera has a fixed ISO setting, then things just got a whole lot simpler!

It seems obvious that one would always want to set your camera at its highest sensitivity. This makes good sense, but there's a hitch. Look at these two images:

Both images were taken with the same aperture setting (f/2.7) and exposure time (15 seconds). The left-hand image, though, was taken with the camera set to ISO 200, while the right-hand one was at ISO 1600.

Although fainter stars can be seen in the right-hand image, there's also a massive amount of background dots, or "noise". Instead of a nice black night sky like in the left-hand image, the high-speed image shows random speckles, almost swamping the stars. This noise is an unwanted side-effect of the high-sensitivity setting.

Different digicams have different ways of dealing with noise; some cameras are good, others are bad, at high ISO settings. High-end digital cameras, for example, often have a useful "noise reduction" setting. Experiment with your camera and choose a speed that works for you. As a rule, if the object is bright, use a slower setting because it will have less noise. There is always noise you can't always see it, though.

Focus

A fundamental thing that a lens has to do, is focus the light. Some cameras have a "fixed-focus" lens, which means you can't change the focus, and that's that. Fixed-focus lenses are common in low-end cameras.

The majority of digicams have an auto-focus setting, in which the camera does the focusing for you. Sometimes you have to try a few times before the camera gets it right. Some cameras struggle to focus in low-light conditions, so it's best to go to "manual focus" mode.

With manual focus selected, you are in charge of focusing the image. For astronomy, this simply means setting the lens to be focused at "infinity". Fiddle with the setting, before taking the image, until the view on the display is sharp. You're set.

Zoom

All compact digital cameras today have a built-in zoom lens, allowing you to magnify the image, or zoom out for a wider view. The amount of zoom is numerically given as the focal length (usually in millimetres), where a big number means a closer zoom.

Digicams have two kinds of zoom: "optical" and "digital". Using "optical zoom" is like changing eyepieces in your telescope.

Unlike film cameras, digicams also have "digital zoom". If your camera allows you to download a "raw" image, then digital zoom is a gimmick, and you'll achieve the same result by just sitting closer to you computer monitor or pressing the "magnify/zoom" icon in your graphics program.

However, if you can't download "raw" images, then you will get slightly better results using digital zoom (rather than enlarging the image on your computer). The two images below show the magnificent crater Langrenus. The right-hand image was taken with 48x zoom (12x optical plus 4x digital). The left-hand image was taken at 12x optical zoom, and then enlarged in Paint Shop Pro. The blocky appearance is caused by the image compression used by the camera. If the right-hand image is enlarged, it too will be seen to have a blocky appearance. A "raw" setting does not create this artefact.

When photographing the Moon, by all means use as much zoom as you have. Experiment with the "digital zoom" and see how it works for your camera.

Image resolution

This is the big number quoted on the box, the megapixels. An image which is 3,000 pixels wide, and 2,000 pixels high, has 3,000 x 2,000 = 6,000,000 pixels, or 6 Mega pixels. The more the megapixels, the more the detail. And the more hard drive space you need to eventually store all those megapixels. If you can change the image resolution on your camera, choose the highest setting (mega mostpixels).

Steady as she goes

When you press the shutter release button to take the picture, you may slightly bump or nudge the camera, causing the image to jitter. To avoid this, some cameras use a remote control (on film cameras, called a "cable release"). Fortunately, there is an easy solution if you don't have such a cable. Simply use the camera's self-timer function, which waits for a number of seconds after you press the shutter release button before taking the image. All this atop a tripod, remember.

Climate control

Sometimes, no matter what you do, despite your best efforts, the image looks bleary. The craters aren't sharp, and the edges of lunar seas are fuzzy. If you've made sure that the camera is focusing properly, the most probable cause is the atmosphere. The two images below, taken a mere fraction of a second apart, shows this.

If the skies are hazy, or there is (unseen) cloud, your images will be correspondingly blurry. Sometimes, particularly when you notice the stars twinkling rapidly, it helps to change your settings so that the exposure time is as brief as possible, hopefully catching a sharp image at the instant the flickering is frozen. Mostly, blurry images mean a blurry atmosphere, and there isn't much you can do about it.

Do keep in mind, though, that the higher above the horizon your target is, the clearer the image will be (because you're looking through less murky atmosphere).

Also consider local factors that could be disturbing your view. Turbulence caused by heating can be minimised if you set up on a grassy patch (a lawn, rugby field, etc), as opposed to a tar road, parking lot or too near a building. And wind, too, can give you the jitters.

Take note

Having taken your photos and transferred them to your computer for viewing, sort the good from the bad, and make a note of which settings worked best for you.

When the Full Moon comes around again, use the same settings to image it in particular, pay attention to the degree of zoom (focal length) you used, because this determines the size of the Full Moon on your image.

Conclusion

Digital cameras afford a luxury unheard of to conventional photographers: you can instantly review the image you've just taken, and if you're not happy, change settings, and try again. The Moon is a patient subject, and will hang around sedately while you change settings, compose the shot, or search for your spare set of batteries in the dark. "Experiment" is the key word; try, and try again it only costs you a few photons & electrons.

10 Things To Do

To get cool Full Moon pics with your compact digitical camera:

  1. Use a tripod.
  2. Use the self-timer (or a remote control)
  3. Fast shutter speed (small Tv number)
  4. Low sensitivity (small ISO value, try 200)
  5. Use a tripod.
  6. Focus manually (unless the autofocus is good)
  7. Use maximum optical zoom (try digital zoom anyway, though)
  8. Use a tripod.
  9. Use image size with most pixels
  10. Try various ISO / aperture / exposure / white-balance settings!
  11. Use a tripod.

Share your photos

If you've tried to photograph the Full Moon, I'd love to hear from you. By sharing your experiences, advice and photos, others who are just starting out can learn from you. My contact details are on the "Contact" page.

Updates

2007 Jan 31 @ 10:54, via Chris; Jan 31 @ 16:22; Feb 02 @ 21:00 via Willie; Feb 04 @ 02:27, via Hans & Carol; Feb 06 @ 19:12 via Pat; Oct 31 @ 10:32.

Footnotes

2007 October 31: The largest Full Moon for 2007 has come and gone; now you can compare the April and October Full Moon sizes here.

2007 February 05: The February Full Moon has come and gone. Carol Botha (Cape Town), Dr Philip van Heerden (Cape Town) and Lerika Cross (Johannesburg) have kindly sent in their Full Moon images, which are presented, along with my image, in the gallery. Note that the images haven't been manipulated or fiddled with in any way, other than to crop away the surrounding negative space to reduce file size.

2007 Feburary 07: Chris Stewart has provided some comments on Lerika's Full Moon image.

Links

  1. Moon photography tips, from me to you. by Carol Botha.
  2. ASSA Imaging Section homepage

nothing more to see. please move along.