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Finally covering half the lens (two stops; 1/4 transmission) produced much darkening!
...Hmmm, not only students struggle, apparently.
You are invited to critique this approach:
The focal ratio (typically called the F stop) is the ratio of entrance
pupil diameter
to focal length. It is expressed in reciprocal form as f /x for example
f /8 meaning 1/8.
Usually, the entrance pupil is approximated by the lens diameter, though
its
edges are not always well illuminated.
Covering half the entrance pupil’s area halves the available light flux.
This gives much the same effect as a round entrance pupil of half the
entrance area or 0.7 the original diameter. (diffraction effects differ
of course, as long base interferometers demonstrate)
Such a reduced entrance pupil would be represented by a focal ratio of
0.7/x which is expressed as f /1.4x.
Devices with adjustable aperture lenses usually mark them in steps of
halving/doubling light flux, such as f /2 f /2.8 f /4 f /5.6 f /8 which are
relative pupil diameters of 0.7 the previous value.
For this scheme of marking lenses, two f stops represent a doubling
or halving of the pupil diameter, and a quadrupling or quartering of the
flux.
Brian W
Bernard Cleyet wrote:
/snip/... ray tracing. Is it too much of a leap from the previous to have a good idea what happens? /snip/
p.s. I covered the top half of a projection lens (Kodak diapositive projector) and the field did not darken uniformly, but one would expect uniformity only in the position where one places a stop. The final lens was set back in the barrel, also. I tried another lens w/ the last lens at the mouth of the barrel; rather good uniformity. Finally covering half the lens (two stops; 1/4 transmission) produced much darkening! One quarter was much less, but noticeable.
On 2010, May 19, , at 15:14, Anthony Lapinski wrote:
/snip/ "If you cover the top
half of a lens, how will the image change?" He struggled with this and
said that his teacher never discussed this in class. To me, it's a
fundamental idea of optics.*
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