re: 116 vs. 716 (part II)

[P3D Gabriel Jacob <jacob@xxxxxxxxxxxxxx>]

Dr. George A. Themelis writes

>Regarding lenses, image size and brightness:
>
>How about brightness?  Well, image brightness is inversely proportional to
>image size.  FL of lenses has nothing to do with brightness. 

Oh yes it does. The light collection and consequently the brightness of
the image varies with the f-number. The smaller the f-number (or faster
speed) the greater the light output thru the projector. The f-number of
course, is aproximated by the focal length/diameter of the lens.

>Sounds reasonable, however after a brief discussion with Colin Clay
>I am not sure if the aperture ratings of the projection lenses have
>the same interpretation as those of camera lenses, i.e. a f/2.8 lens
>transmits twice as much light as the f/4, etc...

I can't see why not. The aperture ratings alone should be the same
as in cameras. Of course f-numbers alone don't tell the whole story
thou. Practically low f-numbers sound great but the problem with that
is poorer focusing of the beam and aberrations. Also the lamp filament
area is another factor. Sometimes a higher wattage lamp will yield
dimmer projection if the filament area is not matched to the condenser
optics. This last case is especially true if the light is to be focussed
in a small area. I am not saying that is the case with your setup, since
the proof is in the pudding and your setup definitely results in brighter
images. As you mentioned to compare fairly, the two projections should
be the same size. 

Gabriel, going to convert his viewers into projectors.
 


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