reflective cameras?

[emanresu]

Just watched a NOVA episode on telescopes.  Earlier Galileo-type refractive telescopes used lenses, and they suffered terribly from CA. Then Newton came along and invented reflective telescopes which allowed the aperture size to become bigger and bigger.  Relate this to photography, I wonder why cameras still use lenses these days instead of mirrors.  One costing factor for large aperture lenses is the correction to CA.  But if we design a body that uses mirrors instead, we could get arbitrary large apertures without having to worry about chromatic aberration problems.  Maybe someone has already attempted such an invention?

[Bob Atkins]

It's difficult to make a mirror lens with an adjustable aperture and most designs require a central obstruction which degrades the image quality. You also need extra optics (lenses) to flatten the field over the whole area of the image.

There are dozens of 500mm f8 mirror lenses available if you relly want one, prices from around $100 and up. For example the Opteka 500/8 mirror

See also http://www.bobatkins.com/photography/tutorials/mirror.html

While mirror lenses don't suffer from CA, they do suffer from field curvature, astigmatism, spherical aberration (unless the mirror is aspheric) and coma. It's generally easier to correct these additional aberrations across a sizable image field with a multi element refractive lens than with a lens which uses only mirrors.

[klindup]

I assume that most mirror lenses are based on the Maksutov design and the tricky bit is the corrector plate at the front.  A quality Mak isuch as the Questar is very expensive.  I suppose in theory you could use a Newtonian which is totally reflective but very bulky and heavy.  I agree with Bob lenses are the way to go unless you are looking at specialist applications such as solar and astro photography.

[KeithB]

And, the reasons that telescopes have such lens issues is that they are trying to see *very* faint obects, and have huge magnification factors.  (I saw the same Nova...)  Both of these assumptions do not necessarily hold for most lenses.  Photographers generally have plenty of light and our magnification ratios are generally pretty modest.  This keeps the radius of curvature down, and makes things easier.

[klindup]

That is certainly true.  With telescopes people are often straining to see things so faint that they cannot be seen with the naked eye and with exposures so long (several exposures totalling hours) that you need a computer controlled mount to compensate for the earth's rotation.  But still a refractor gives a better image than a reflector mainly because there are no bits of metal to interfere with the light.  I think flourite and low dispersion glasses first appeared in telescopes to allow the creation of apochromatic lenses.

If you can afford several thousand dollars you could look at the Questar which uses a mix of lenses and mirrors.  I have seen photographs taken with one of these and they are impressive but very expensive.