8mm Forum: VP Zoom Lenses

This is topic VP Zoom Lenses in forum General Yak at 8mm Forum.

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Posted by Paul Adsett (Member # 25) on May 09, 2007, 11:57 AM:

I have noticed that the zoom lens on my VP acts a little differently than the zoom lens on my cine projectors. When you zoom out to a bigger picture with a cine projector, like the GS1200, the brightness of the picture falls off very rapidly, roughly inversely as the area of the projected picture. This does not seem to be the case with my Panasonic 700 VP, which has a 2X zoom lens. When I zoom out the Panny lens to go from a 16:9 picture about 7 ft wide to a Cinemascope 2.35:1 picture about 9ft wide, the brightness does not seem to drop hardly at all. Very strange - but nice! [Confused]

Posted by David Pannell (Member # 300) on May 09, 2007, 01:01 PM:

Hi Paul,

You're quite right. However, the difference between a 16:9 and 2.35:1 is really quite small in terms of area. 16:9 is equivalent to 1.77:1, so your difference in illuminated area is only 2.35/1.77 = 1.32 or one third as much.

Within the laws of optics, the inverse square law is the one which applies, which means that if you double the width (or height) of the picture equally, the illumination will be a quarter overall. Twice the size of picture = one quarter the brightness; not half, as many people think.

Now, however good your lens might be in transmitting a large amount of light, the ratio still applies, but if you start off with a fast lens, it may give the psychological impression that the picture is much brighter, but I can assure you that this isn't the case in reality. Sorry [Frown]

QED. You can't alter the laws of physics! An interesting observation, though, Paul.

Best,

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