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In article <5435@m1.cs.man.ac.uk>, mark-r@spec0.ee.man.ac.uk (Mark Robinson (JO PhD)) writes:
> Hi.
> 
> Quick question about lenses. I want to buy a fixed focus lens
> (fixed focus == less glass == lower distortion ?). I would
> like an object about 15mm diameter to fill an image with the
> object - camera distance somewhere between 40 and 70cm.
> 
> How do I calculate the focal length of the lens?
> 
> Cheers
> Mark
> mark-r@snow_white.ee.man.ac.uk

You need to know the active dimensions of the camera sensor array
(assuming you're using a solid state video camera). Then it's a matter
of simple proportion between the image-height:object-height and
focal-length:object-distance ratios.

--
graham a stephen                        ``The only key to your riddle is
(e-mail: graham@sees.bangor.ac.uk)        to accept the absence of a key.''
                                              --  Laibach, 1992     
Aposiopesis is not really all that it

In article <5435@m1.cs.man.ac.uk>, mark-r@spec0.ee.man.ac.uk (Mark Robinson (JO PhD)) writes:
> Hi.
> 
> Quick question about lenses. I want to buy a fixed focus lens
> (fixed focus == less glass == lower distortion ?). I would
> like an object about 15mm diameter to fill an image with the
> object - camera distance somewhere between 40 and 70cm.
> 
> How do I calculate the focal length of the lens?
> 
> Cheers
> Mark
> mark-r@snow_white.ee.man.ac.uk

A 2/3 " CCD camera has sensor dimensions of 8.8 x 6.6 mm.

Your Object has a diameter of 15 mm, so the magnification ratio is 
approximately 2.5 : 1

1 / f = 1 / 50 + 1 / 20  (Lens equation) u & v may be confused.

=> 1 / f = 0.07

=> f = 14 mm

so use a 16 mm c-mount lens + some extension tubes.

These can both be bougth from Cosmicar, but the lens quality is 
not great as they are for CCTV rather than CCD Macro.

Alternately try Tamron, or get a 35 mm camera lens and convert it or
get a c-mount -> 35 mm camera adaptor.

A cheap way of doing this is to glue (yes) [using superglue or epoxy resin]

a c-mount 5mm extension tube to the back of a 35mm camera lens (if the
rear lens element is small enough).  Old Pentax 35 mm and I assume 28mm
screw thread lenses are suitable if you can find them.  A 28mm lens will
be better quality than a Cosmicar (in my experience) but will give a much
greater standoff than a 16mm lens.

	I hope this is of use,
	
		James Mahon

			Vision and sensor research unit
			Trinity College Dublin,
			Ireland

lenses are quite good.  This (say a 28mm will give longer standoff