Ask Ethan: Why don’t we build a telescope without mirrors or lenses?
“Why do we need a lens and a mirror to make a telescope now that we have CCD sensors? Instead of having a 10m mirror and lens that focus the light on a small sensor, why not have a 10m sensor instead?”
Every time you shine light through a lens or reflect it off of a mirror, no matter how good it is, a portion of your light gets lost. Today’s largest, most powerful telescopes don’t even simply have a primary mirror, but secondary, tertiary, even quaternary or higher mirrors, and each of those reflections means less light to derive your data from. As CCDs and other digital devices are far more efficient than anything else, why couldn’t we simply replace the primary mirror with a CCD array to collect and measure the light? It seems like a brilliant idea on the surface, and it would, in fact, gather significantly more light over the same collecting area. True, CCDs are more expensive, and there are technical challenges as far as applying filters and aligning the array properly. But there’s a fundamental problem if you don’t use a mirror or lens at all that may turn out to be a dealbreaker: CCDs without lenses or mirrors are incapable of measuring the direction light is coming from. A star or galaxy would appear equally on all portions of your CCD array at once, giving you just a bright, white-light image on every single CCD pixel.
How do you know the best lens to use for a shot? Different lenses have different focal points. The focal length of the lens is the distance between the lens and the image sensor when the subject is in focus.
Below is a quick overview of the different lenses and how they look on screen.
Zoom Lenses- have a range as their focal length. Common zoom lenses may be 18-55mm. These are nice because they are flexible and can capture photos or videos at varying focal lengths.
Prime Lenses- have a fixed focal length. Although not as versatile as zoom lenses, these lenses tend to have better quality, wider maximum aperture, and are smaller and lighter. These are most common in professional filming and photography.
Focal lengths of prime lenses: the higher the focal length (mm), the higher the zoom.
-Wide angle lenses (14-35mm)- These are often used for wide shots, landscapes, group photos, or interiors. Distorts images especially when close to the camera.
(15mm, high light, landscape)
-Standard lenses (50-60mm)- These are considered the “normal” focal length because it is close to how the human eye sees. Minimal distortion and used for portraits (close ups in film) and nature.
(50mm, low-light, portrait)
-Telephoto lenses (70-300mm)- Use these when you want to get close to a subject but can’t. They tend to distort images and are susceptible to camera shake the higher the focal length.
(200mm, average light, action)
-Super-telephoto lenses (300mm-600mm)- same as telephoto lenses but closer. Good for capturing subjects from a large distance.
(400mm, average light, wildlife)
-Macro lenses (60, 105, 200mm)- used for close ups that can be reproduced life-sized. Used for extreme close ups in film and images of flowers and insects in nature.