Hi, Rod,
Thanks for the reply. Regarding the second question, I
didn't make it very clear in my wording. What I meant is
if two projector lenses of different focus lengths but with
the same f-stop number, e.g. a 50/2 lens and a 100/2 lens,
project the same image source onto the same screen at the
same distance, do their images have the same brightness?
If not, at what f-stop can I get the same brightness from
the 100mm lens as the 50mm lens at f/2?
I'm asking for this because I'm setting up two different
projectors to project images side by side. I know the image
sizes and distances from the images to the two projectors,
the light output in lumens of the two projectors, and the f
stop numbers of the two lenses, and I need to know the
difference in the brightness of their images if the projector
screen is identical, so that I can determine what kinds of
reflective paint with what difference in gain number I should
apply to the projection surfaces to make the two images look
to have the same brightness, or perhaps I can somehow find
a neutral density filter for one of the lenses, or make an
adjustable diaphram, or sort of things like this.
Thanks again!
Yefei
>
> Date: Tue, 14 Feb 2006 12:07:57 +1000
> From: "Rob Studdert" <[EMAIL PROTECTED]>
> To: pentax-discuss@pdml.net
> Subject: Re: OT projector lens questions
> Message-ID: <[EMAIL PROTECTED]>
> Content-type: text/plain; charset=US-ASCII
> Content-transfer-encoding: 7BIT
> Content-description: Mail message body
>
>
> > 2. If two lenses of different focus length have the
> same working
> > aperture, do the images they generate have the same
> brightness on an
> > identical screen at
> > the same distance?
>
> No, because the projected image is spread over a larger
> surface area in the
> case of the shorter FL lens.
>
> > If so, then the lens that generates the larger image
> > passes more light since brightness is amount of light on
> unit area, right?
> > Now this gets me confused assuming my guess to the first question is
> > correct,
> > as for example, a 20mm/2 lens has a theoretical aperture
> size of 10mm, and
> > a 40mm/2 lens has a theoretical aperture size of 20mm, and
> the 20mm lens
> > actually passes more light???
>
> No.
>
> > 3. What I really should ask is that, there is a
> projector with a
> > lens
> > that's f=20.0mm/2.0 to 40.0mm/2.8, and I'd like to know,
> does the image
> > brightness stay the same when I change the focal length
> from 20mm to 40mm
> > and keep the screen at the same place? Obviously the image
> size will change
> > so light spreads out more in one case but does the
> different aperture
> > cancel that out?
>
> You really need to determine the required magnification
> first, there is no
> point getting the fastest lens if the magnification is too
> large or small.
>
>
> Rob Studdert
> HURSTVILLE AUSTRALIA
> Tel +61-2-9554-4110
> UTC(GMT) +10 Hours
> [EMAIL PROTECTED]
> http://members.ozemail.com.au/~distudio/publications/
> Pentax user since 1986, PDMLer since 1998
>
