Camera Basics

Whether you already own a camera or are thinking of buying one for astrophotography, these are some things that you should be aware of. The vast array of digital cameras out there may or may not allow you to control the parameters below, but the more control you have over them, the more versatile your astrophotography will be. That is, the more subjects and types of astrophotography you'll be able to do with your camera. So, what are these important camera things you should know?

ISO: This setting dates back to film photography, where ISO was also known as film "speed". Each film was characterized by a certain speed or ISO, which today can be set to a range of values on many digital cameras. Most ISO's range between 100 and 1600 and even up to 3200. To understand what these numbers mean, you need to realize that for every doubling in ISO, the exposure time required will be reduced in half. That is if you need a 10 second exposure at ISO 100, you can practically get the same picture with a 5 second exposure at ISO 200, or 2.5 seconds at ISO 400. The thing to realize with ISO is that although a higher setting will allow you to take shorter exposures (which can be very useful when photographing things that require many minutes or even hours of exposure time), the higher ISO settings will increase the noise in your images. At the end, it's a balance between how long you are willing (or can) expose and how much noise you're willing to tolerate in your pictures. There are tricks to reduce noise, and these will be covered in future posts.

Focal Length: The focal length of a camera lens or telescope is how far light needs to travel inside the optics before it forms an image on your camera sensor. The significance of focal length is magnification and field of view, such that higher focal lengths will produce higher magnifications (and smaller fields of view). Most telescopes, as you can guess are large focal length instruments, and when a camera is attached to them, the image produced will be magnified quite significantly. This is good for close-up imaging of objects such as the moon or planets and certain deep-sky objects. Camera lenses can either have a fixed focal length or be so-called "zoom" lenses which can vary the focal length over a range. Typical focal lengths for camera lenses are 18mm, 50mm, 100mm, etc. For wide-field photography or large portions of the sky such as constellations or the Milky Way, you'll probably want a focal length of 50mm or less. Nebulae and other deep-sky subjects will require large focal length camera lenses if you want to capture them up close.

With DSLR cameras and others, focal length can be varied by using different lenses (or one zoom lens). Of course, if you can attach your camera to a telescope, you'll be able to access much higher focal lengths than most camera lenses can offer.

F-Ratio: Short for Focal-ratio, "f-ratio" is basically the ratio of the focal length of the lens or telescope attached to your camera to the aperture of the lens or telescope. So what does this mean? Well, if the aperture of the lens or telescope is large, then more light will be collected and the image will appear brighter. Thus, at a fixed focal length, smaller f-ratios will produce brighter images. The advantage of this is, like high ISO, is shorter exposure times. The disadvantage of a low f-ratio is that the quality of the image that a lens will produce at very low f-ratios is not optimal in terms of contrast. At higher f-ratios, the image will appear sharper, and things like stars and star-trails will look nicer. Thus, as with ISO, the f-ratio you choose will depend on how long you can expose. The image will look better at a higher f-ratio, however, a smaller one will allow for shorter exposures. DSLR camera lenses (and perhaps others) will allow you to choose from a variety of f-ratios. The f-ratio of a camera attached to a telescope depends on the type of attachment you use (different methods of attaching a camera to a telescope will be covered later). Typical f-ratios (written as f/#) are f/1.4, f/2.8, f/5.6, and f/8, etc.

Exposure Time: I've alluded to exposure time above, and it's basically what it sounds like: the time an image is exposed. In other words, exposure time is how long light continues to fall and be picked up by the camera's sensor. In astrophotography, exposure times can be much longer than a normal photographer will usually encounter. Such long exposure times are mainly encountered in deep-sky work and other astrophotography which involves photographing dim objects such as the Milky Way. This is why it is useful to have a "bulb" or "B" setting on your camera, which allows you to make indefinitely long exposures. Exposure times can also be varied to fixed values, which are stored in the camera's software.

Cable Releases and Tripods: Two useful accessories to have with a camera are a cable release and tripod. A cable release is basically a real cable or electronic device which allows you to release the camera's shutter without pressing the shutter button directly. This will reduce vibration of the camera when taking exposures that are more than a few fractions of a second. A tripod is another accessory on which the camera is mounted that is used to keep the camera steady, again for exposures that are longer than a few fractions of a second. Holding the camera by hand or pressing the cable release button manually will cause the image to turn out blurry. I will talk about astrophotography that can be done with just a camera, tripod and cable release in later posts.

So, now hopefully you are armed with some knowledge about how cameras and astrophotography works. Being familiar with the things discussed above is a good first step. A DSLR camera will usually give you the option of controlling all which has been discussed above, and is thus the most versatile of the non-astrophotography dedicated cameras to use for astrophotography. But you don't need a DSLR to do astrophotography, and there are many other DSLR-like cameras out there and that you can use, and there are also astrophotography methods that involve special types of cameras which can be attached to telescopes (like webcams).

Here are photos of two camears I have used for astrophotography. The first is an SLR film camera (an Olympus OM-1), the second a DSLR (Digital SLR), a Canon XS (1000D) which I currently use.

Thanks for reading :)

and clear skies!