| article: focal reducer photography |
The name says it all: a focal reducer is reducing the overall focal length of an imagaing system.
It is also called Shapley Lens.
When the focal length is reduced the f-ratio of the system gets faster.
This is proportional to the factor written on the focal reducer, i.e. 0.63x or 0.5x.
In theory the image size gets bigger too so one could think that objects of bigger size now could be imaged.
Unfortunately this is rarely the case because the outer parts are very often vignetted.
Vignetting comes from stops in the telescope, very often the focuser and is darkening the outer parts of the picture.
Unlike Barlow Lenses the Shapley Lenses *must* be calculated for a special optical system not to give unacceptable aberrations or curvature of field.
Together with the huge amount of back focus space needed they are mostly used (and needed) by SC users.
On the other hand it is very easy for Newtonian telescopes to reach fast focal ratios without using Shapley Lenses.
Same is true for APO refractors.
Having only refractors and Newtonians I never used a Shapley Lens so far.
Focal reducing can be looked at to be the opposite of eyepiece projection.
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