Wednesday, January 17, 2007
Guide to Choosing a Lens: From Macro to Astrophotography
Posted by Suhit Gupta in "ARTICLE" @ 07:00 AM
Understanding The Lens
There are several concepts that you need to understand before you can go and buy a lens. I encountered this problem especially when I was about to buy a telescope for some astrophotography.
- ISO - A camera's ISO function sets the light sensitivity of the camera's image sensor (this is similar to the speed rating of FILM). ISO settings are often rated at 100, 200, or 400 but go as high as 800, 1600, and even 3200 on some advanced models. A lower ISO setting is used when capturing overly bright scenes, since it reduces the light sensitivy of the image sensor. But realize that the higher the ISO setting in a photograph, the granier the image.
- White Balance - Our own eyes are an amazing judge of colors because we can (from what I understand) judge colors based on, among other things, comparison to the color white and we can judge white based on temperatures. Cameras need to do the same thing, however they need a reference point to what white is. So cameras have settings for allow the user the specify the dominant light in the space where the picture is being taken so that the colors in the image a appropriately callibrated.
- Aperture - The lens you will be buying is simply a device that will take light from the world (at least the direction in which it is pointing) and pass it along to the camera. The aperture of the camera is the "entrance" that controls the amount of light that is let into the camera.
- Auto-focus - This is a feature that a lot of people mistake to be a function of the camera but it is actually a function of the lens, even though it is the camera that controls/activates it. Lenses have a zoom function and a focus function. For a given focus, there is a correct focus value that will produce an optimal image. Most cameras will have Auto-Focus (AF) points that will be used to auto-focus an camera. When taking a picture, the camera will try and focus as many AF points as it can. In fact, if you keep trying to tell the camera to auto-focus, it wil usually try and cycle the focus setting to focus on a different combination of AF points so as to give you the option of picking the setting you would like to shoot with.
- Image Stabilization - This is another setting in the lens. Most lenses are just a bunch of pieces of glass, where you can control the distance between the various pieces of glass (i.e. move them along the X axis of the lens) in order to zoom and focus the image. However, some lenses (usually much more expensive) have some gyroscopes built in that further control the pieces of glass in the lens along the Y and Z axes. Since most photos are taken by people holding cameras in their hands and since there is always some jitter when we hold or cameras, no matter how steady we think our hands are, the photographs we take can be blurry. The blur is even more pronounced when we take photos of action shots or are in moving vehicles. Image Stabilization gyroscopes are built to counter that motion and provide a stability to the components within the lens, regardless of the motion outside.
- Focal length - The definition of focal length is the distance from the lens to focal point, i.e. the distance from the center of a lens or the surface of a mirror to the point at which light passing through the lens or which is reflected from the mirror is focused. A system with a shorter focal length has greater optical power than one with a longer focal length.
- Focal Ratio - The focal ratio is the relationship between the aperture and focal length. The focal ratio is defined as the focal length divided by the aperture. For example, a refractor with a focal length of 800mm and an aperture of 100mm has a focal ratio of 800/100 = 8 or f/8. The focal ration gives the relative "speed" of the optical system. This is important for recording extended objects . A faster focal ratio will record an image faster (with a shorter exposure).
- Other Attributes - Of course, there are a number of other features a swell. Optical quality is just as important as the above mentioned items. You can have a gigantic aperture and if the optical quality of the lens is not good, the light won't be very well focused, and the images produced won't be very good. Weight and cost of the lens are also important criteria. If either of them are too high, well, you know the rest.
