When do cameras have lenses instead of mirrors?
Thursday, May 13th, 2010 at
12:23 am
When do cameras have lenses instead of mirrors? Almost all telescopes use mirrors instead of lenses because mirrors work better. So, why don't cameras use mirrors?
Sorry, it was a type. I obviously meant WHY, not WHEN.
I meant using the mirrors to focus the image.
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My D-SLR Canon camera has a mirror in the camera body – if i take off the lens i can see the mirror in front of the imaging censor.
I am only aware of one lens that uses a mirror for normal use, eg Minolta 500mm.
Mirrors are used to shorten the length of the lens by reflecting the light inside on the mirrors until it is focused on the receptor. This brings its own problems to the party. You could not manage one on a zoom. Small fixed lenses do not need to be shorter so nothing would be achieved. The manufactures have settled on a fixed light path through the lens. If it the lenses could be made cheaper and better by using mirrors they would have done it by now.
Regards.
Most telescopes use mirrors for a few reasons. The first reason is that mirrors don’t suffer from chromatic aberration like lenses do. The other big reason is that in order to get a high-power refractor telescope (the kind with lenses) it needs to be very long. Because the main weight of the telescope (the lens) is at the end of a long body this can cause big structural problems. The problem with mirrors is that they’re expensive to make because they need to be flawless.
Cameras don’t really need mirrors because they’re not required to be big enough that the structural issues become a problem. Also, chromatic aberration isn’t as much of an issue at the short focal lengths of camera lenses. Also, there’s the issue that a mirror requires the sensor to either be backwards facing or have a complicated system of more mirrors to reflect the light to the sensor. In a small camera body this is simply not feasible.
Technically cameras can use mirrors. Reflective (mirror) lenses are typically really wide lenses that are relatively lightweight and, despite how far they go, they also can do macro photography. The use of mirrors is cheaper for production as well.
So why aren’t they used normally? There’s a few reasons.
-Light. Reflective lenses require more light to get good shots most of the time. Whereas a refractive (glass) lens of the same length is usually made for f/5.6 or faster, I’ve only seen one mirror lens that went faster than f/8.
-Focus. Mirror lenses are cheaper to make but also lack the particular things that offer a creamy focused shot of the subject. Normal photos will have a sharp shot of the subject in view with a background that’s fuzzed out. Reflective photos… well, you have to see it to believe it (check source site).
-It’s a telephoto! In all honesty, not a lot of people use telephotos to begin with. The only reason you’d use one is for distance anyway is because a 50mm mirror lens would be slow and not offer you the depth of field you’d like for portraiture as a result anyway!
In all honesty, the best reflective mirror lenses out there for photography use the least mirror elements possible (Tamron’s SP 500mm, for instance). They’re interesting to use, but in all honesty? Not useful enough stop people from lugging 10 pounds of high-quality glass.