Re: Lens design questions

Anonymous writes:

I am designing a device that will project a 35mm wide object onto a
7mm camera sensor (3MP). One of the objectives is to keep the distance
between the object and the sensor small (less than 20cm or 8"). I
understand that if I just use a single double concave lens, in theory,
it should work. However, I don't know much about other effects
(aberrations, etc) that should be accounted for to maximize the image
quality. My question is: how does one go about designing a lens system
that will make a 'good' image?

You don't. You buy a ready-made camera lens. You can't possibly design
your own lens better or cheaper than buying such a highly optimized and
mass-produced product.

Elementary optics govern the distance from object to image, and image
size. If you want it closer, you need a shorter focal length (i.e.,
higher power) lens. If you are imaging an actual object (versus re-
imaging an image) and can time-expose, then you don't have to worry
about stops and illumination. Otherwise things get more complicated as
f-numbers and pupils/windows/fields/apertures/stops get involved.

You've specified conjugate planes at 20cm with a 35mm/7mm shrink. This
determines the required focal length. Theory sez minimum object-image
distance is 4x the focal length (yielding 1:1 image:object ratio), so
you need something substantially shorter in focal length than 20cm/4 =
50mm lens. Something like a 28mm wide-angle SLR lens, or even a 12.5mm
C-mount TV lens.

What's the application?

Nowadays you can get superb but obsolete old camera lenses for next to
nothing on eBay. These are the ones with short flange distances that
with a mechanical adapter won't reach into today's digital SLR's. Or C-
mount TV lenses if you need short focal lengths. Or enlarger lenses.
Perfect for experimenting or implementing custom optical applications.
You would use these with what the photographers know as macro extension,
as on a macro bellows or extension tube. Or refit components of a 35mm
SLR slide copier assembly, also obsolete/surplus and cheap on eBay now.

Digital photography has yielded a bonanza of wonderful surplus optics
from obsolete film apparatus. This has never happened in the history of
optics. The closest thing would have been surplus and seconds from WWII
manufacturing.

Look at the http://www.edmundoptics.com/ and http://www.mellesgriot.com/
tutorials where they will teach you how to design exactly what you've
asked. The skill is implementing your design without paying Edmund's
prices for their ready-made but equisitely-priced components.
.

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