These are simplified answers to some of the commonly asked questions concerning photographs of the sky...whether it's of the sun, moons, comets, stars, or whatever else is up there, these photographic principles remain true.
FAQ: Camera's, photography, astro-photography, etc
Q: How does a camera work?
A: Cameras work by focusing light through a series of lenses onto either film or a tiny bank of light sensing mechanical "eyes". The most rudimentary form of camera, though, doesn’t have a lens, it has a pinprick in a sheet of foil which projects the light into the back of the camera box onto the film (called a pinhole camera, you can
make one yourself with a box, some aluminum foil, and some black-and-white photo paper)
Q: How do photographers get clear pictures of the night sky?
A: This is a tricky one, because there's almost as many ways to do it as there are photographers doing it. There are certain things that you
must have though:
- A sturdy tripod. This is a 3-legged stand, which the camera attaches to with a large screw. Turn over your camera, if it has a threaded hole, that's for a tripod. If you have nothing else on this list, you
must have a tripod.
- A "cable release". This is a cable that goes from the camera picture-taking button (shutter release) to a little button you push and makes it so you don’t actually have to touch the camera to take the picture. If you've seen the old cowboy movies where they photographer is standing under the hood of a big camera, and he's got the flash in one hand, and a black trigger thing in the other, the trigger thing is a cable release.
- Bracket exposures. This means you don’t just take one snap-shot of the sky. You take several pictures of the same thing using different settings on the camera, like the time the lens is open. What this does is makes sure you have at least one good picture, even if the camera's light meter system is fooled by the dark sky. The amount of pictures someone brackets directly relates to the amount of experience they have taking night-sky pictures. It can be as few as 3, or as many as the entire roll of film, or more.
- Fast film. This means film that is sensitive to light. The way you can tell how fast a film is (how fast it can record light) is by the ISO or ASA number on it. ISO 200 film isn’t very fast, it's used for daylight or flash photography. ISO 1600, the fastest color film on the market right now, is extremely fast and should not be used for daylight pictures at all. The problem is, the faster a film is, the more the little grains of color can be seen, so many astro-photographers actually use slow film which has been chemically treated with a gas to make it react faster to light, which gives the film a "hyper" designation.
- Time and patience. The longer the shutter on a camera is open, the more light gets captured by the film. The spectacular shots you've seen of galaxies are usually exposures of 30 minutes or more. For exposures that long, the photographers have special motorized tripods, which can follow the sky in order to keep the stars from blurring with motion.
Q: How does a photographer take a picture through a telescope?
A: There are several methods of doing this as well, but I'll talk about the one that is the most reliable. The first things you need are: a camera with changeable lenses and a device called a T-Ring adapter. The T-Ring adapter fits onto your camera where the lenses normally would. The next thing you need is an adapter tube for your brand of telescope, or a generic one that matches the measurements of the lens opening in the telescope. The adapter tube screws onto the T-Ring adapter, then the other end of the adapter tube fits into the lens opening on the viewing end of your telescope. Now, you can look through the camera right through the telescope as if the telescope were a giant zoom lens. After that, it's just a matter of experience learning how to find and focus on objects this way. More advanced camera tubes allow you to put magnifying lenses between the telescope and the camera, which enlarges the images, but also makes it much more difficult to find and focus on those objects. (You could also go to your telescope dealer and get a device called a CCD, which is a digital imaging device, and it fits directly on the telescope's lens opening. CCD's feed digital images directly into a computer, making it so you don’t need a camera, film, developing, etc, but they are also very expensive.)
Q: What causes <insert strange photographic anomaly here>?
A: I'll tackle this by categories:
- Circular blobs with very faint centers but very defined edge outlines (like a circular line with fog inside it): usually only on film cameras. This is caused by a bubble getting on the negative when it was in the developing process. As that bubble sat on the negative, it prevented the developing chemicals from reaching the film properly, causing the ring with the smoky center.
- Streaks that look like sparks: Coming from pictures that don’t have very bright objects in them, in film cameras, these are caused by static electricity striking the film while it was in the camera or being handled for development. Most cameras have a metal plate, which touches the film in order to absorb any static, but it doesn’t always work. If the streaks come from a bright object, and are in the form of a cross, or a 6-point star, they are called Diffraction Spikes and are caused by irregularities in the lens surface (like dirt, scratches, fingerprints, or the way the lens was ground into shape when it was manufactured).
- Small splotches or dots on digital images: These are either a) one of the sensor "eyes" in the camera got overloaded by light and reported a false color or brightness or b) some little bit of data got messed up when the picture was being recorded by the camera or being transferred from the camera to a computer. You can tell these glitches by the way they are regular colored (no shading like a 3D object would have) and they don’t seem to match up with what you saw with your eyes.
- Circular blobs with even color and without defined edge outlines: These are lens flares. They can appear circular, or as hexagons. They can be almost any color, but are usually between yellow and red. There can be as few as 1, or as many as 20, depending on how many pieces of glass are in the lens (most cameras have 3 lenses, but telephoto zoom lenses can have 7 to 15 or more pieces of glass inside them). They are caused by the camera being pointed at or near a bright light and are the most commonly mistaken camera artifact that I know of.
Q: What causes a lens flare?
A: Lens flare is caused by a few factors:
- Bright light enters the front lens of the camera
- A portion of that bright light is reflected off an internal part of the camera
- That little piece of light is reflected again off the inside of the lens
- The light ends up as a small, faint circle on the picture, quite often shaped like a hexagon
As you can see, it takes all 4 of these things to make the lens flare appear on the picture. Usually, the only way for all the reflections to line up the right way, the source of bright light has to be either right in front of the lens, or almost 90 degrees to the side of it. If the source is right in front, there is only 1 flare spot. If the source is to the side, there can be as many spots as there are pieces of glass in the camera lens.
Above is a diagram I did showing a very simplified view of a lens flare happening. The yellow area is the entire field of view, the red and blue lines are light from a very bright object. The blue line is the light from that object as it travels to the film, the red line is a portion of that light that gets reflected and becomes a lens flare.
Here is an illustration I did, similar to the one above, which deals specifically with webcam reflections within Plexiglas housings. Recently there were several questions about how a reflection of a bright object can be reflected in duplicate on the same picture. The blue line is a bright object, such as the moon, the red line is the reflection off the lens, the enclosure, and back to the film at an offset location.
Notice their similar causes? They are beasts of the same breed, so-to-speak. (Pictures not to scale nor technically accurate concerning the path light takes through a lens...merely representative)
Q: How do I get rid of lens flares?
A: A photographer who knows he/she is going to be in a situation with harsh light sources takes precautions. They make sure they have lenses which come with coated optics. That means the pieces of glass in the lens have chemical coatings on them which don’t let light bounce off of them. The photographer also has a variety of hoods to use, which are attachments for the lens, and act like the brim of a hat does when you wear it to block sun from your eyes. Lately, a lot of outdoor photographers are trying to make use of lens flare as a dynamic effect that makes your eyes follow through a picture. You can even add false lens flare, with programs like PhotoShop, to pictures that don’t have one.
Q: Why did I get lens flare on my picture when I didnt see it in the viewfinder with my point-and-shoot camera?
A: Your camera has a viewfinder that does not "look" through the same lens that puts an image on the film. The viewfinders of point-and-shoot (PaS cameras are the kind that dont have the ability to remove the lens) cameras only have two lenses, one sets your field of view to the maximum that your camera can use, the other focuses the first lens. These two lenses are too far apart for lens flare to occur, and are also usually just a little bit recessed into the camera body which protects them further from lens flares. The focus lens system is also offset from the main lens by sometimes more than an inch. That offset is often enough to make one set have a flare and the other not. Even on cameras with zoom capabilities, where your viewfinder zooms to match what you see on the film, this is the usually the case.
Q: Why is it so hard to get a picture of the Moon that shows the "face"?
A: Well, most cameras are set up to automatically measure the amount of light entering them so that they can properly open the shutter for the perfect amount of time. This meter inside the camera takes a measurement of everything you can see through the lens, then averages that measurement, which is usually fine for daytime or flash photography. When you're shooting the Moon, though, the meter is fooled into an average that is too low for the Moon (making it over-exposed, or just a white blob) or too high for the surrounding sky (washing out the sky and making it look gray instead of black). Cameras with a "Spot Meter" system are more appropriate for shooting bright objects against a dark background because they don’t average the entire scene, they only measure what is in the middle of the picture. If you don’t have the ability to spot meter, then you have to manually adjust for the camera's mistake by taking several pictures of different exposure settings (like shoot one at the camera's automatic reading, then another with a faster shutter speed, then another with an even faster speed, and so on) this is called bracketing.
Q: How do you get the 1 hour folks at Wal-Mart to actually develop all your shots?
A: Ideally, you should take your film to a professional developer with some experience with astro-photography developing. You should also take a few reference shots at the beginning of the roll of film, 1 or 2 shots of the box the film came in, using normal light or a flash. That allows the developer to have a starting point. If you must take your film to the 1 hour developers, then it's a bit more of a hit-or-miss proposition. I generally tell the teenagers working there this basic speech "Ok, this roll of film has some astro-photography on it. That means I took pictures of stars. Your machine is going to think that the black frames with a white dot in them are mis-shot pictures where the flash didnt fire or the lens cap was on. I want you to override the machine and force it to print
every frame even if it's just pure black." And then I hope for something good. At least, even if they dont make a good print, it's difficult to mess up the negatives. So if they cant print something right, I just take the negative to my local camera shop, rent a dark room, and print my own.
Q: This sounds fun, what camera should I buy to get started?
A: I cant really answer this question for you. The best thing to do is actually go to camera stores and handle the cameras they have for display. Find one that has these minimum functions:
- Tripod attachment hole
- Spot metering capability
- Ability to attach a cable release
- A manual function that includes a "Bulb" setting, which is what you will use for exposures over the maximum available on your camera. You can lock the cable release in the shutter-open position and set a stopwatch, then unlock it when your exposure is complete.
- The ability to switch lenses, and for those lenses to be set to manual focus
There are many $150-300 cameras that meet these minimum requirements, and you should choose the one that meets your budget and comfort levels. I use a professional quality
Canon Elan 7E for all my work, of which astro-photography is only a fraction. I also have an old Canon AE-1, which I use to teach my teenage step-sons about photography. (good suggestion
g99)
Post questions and I will try and answer them, as long as they are about taking pictures of the sky...
edit 1: added film developing and camera choice questions
edit 2: added flare on picture vs. no flare in viewfinder question