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Re: [OM] Lenshood calculation

Subject: Re: [OM] Lenshood calculation
From: "tOM Trottier" <Tom@xxxxxxxxxxxxx>
Date: Tue, 14 Jan 2003 01:48:02 -0500
Dear Michael ,

Thanks for the tip. I went looking and found a method, though no calcs, 
back on Sep 19:

------- Forwarded message follows -------
From:                   ReinholdLetschert@xxxxxxxxxx
To:                     olympus@xxxxxxxxxxxxxxx
Date sent:              Thu, 19 Sep 2002 00:27:47 +0100
Subject:                [OM] re: lens hoods/suitablity/shapes
Send reply to:          olympus@xxxxxxxxxxxxxxx

Following the discussion on suitability of hoods to lenses and 
shapes of hoods, I remembered some good advice that I got from a 
good friend of mine (a professional photographer) when I was still
a student and always grabbed anything cheap with a M42-mount 
(long before OM) and which never had a suitable hood:

-Get a light box (for sorting slides) and a sheet of grease proof 
paper. 
-Put the camera with back removed on the lightbox (back to glass). 
-Mount whatever you think is a suitable hood to the lens, open the 
lens to the maximum f-stop and put it in infinity position.
-Open the shutter in "B"-position with a cable release. Put the 
grease proof paper (or any suitable matte screen) flat on the hood.
-When you switch on the light box you will see a spot of light 
shining through camera, lens and grease proof paper (you might 
have to cover the rest of the lightbox, which isn't covered by the 
camera, to avoid getting dazzled by the bright light).

- when the front diameter of the hood is slightly bigger (2-3mm or 
1/8") than the projected light figure on the screen your hood is ok

- when the light figure is cut by the hood its angle is too narrow

- when there is plenty of room between the light figure and the rim 
of the hood it is too wide

As to shapes of hoods:
When you experiment with different lenses in the above mentioned 
way on the light box, you will find different figures of projected light:

fully opened:
wide angles tend to a rectangular shape, sometimes also pin 
cushion
tele lenses from about 100mm on (for 35mm) tend to a circular 
shape

when you stop down to f8 you get rectangular shapes from wide 
angle to roughly about 300mm, the projection of longer focal 
lengths still tend to be round

ergo: rectangular is not always better

I never looked at zoom lenses that way, so I don't know anything
about them.
Unfortunately these experiments don't work with digicams :-)

Also: when you use lenses with extension rings or in the bellows, 
the effective opening angle of the lens gets narrower. Thus it might 
be useful to use a hood that is narrower than the standard one. 
That's why some pros use bellows-hoods for macro.  However with 
macro photograhpy and strong side light sometimes the ONLY 
thing that helps is one ore more pieces of dark cardboard to get rid 
of strong reflexions.

---
Reinhold

------- End of forwarded message -------


tOM

On Tuesday, January 14, 2003 at 13:32
Michael <olympus@xxxxxxxxxxxxxxx> wrote:

> 
> ----- Original Message -----
> From: Gary L. Edwards <garyetx@xxxxxxxxx>
> To: <olympus@xxxxxxxxxxxxxxx>
> Sent: Tuesday, January 14, 2003 1:20 PM
> Subject: RE: [OM] Lenshood calculation
> 
> 
> >
> > -----Original Message-----
> > From: owner-olympus@xxxxxxxxxxxxxxx
> > [mailto:owner-olympus@xxxxxxxxxxxxxxx]On Behalf Of John Cwiklinski
> > Sent: Monday, January 13, 2003 4:17 PM
> > To: olympus@xxxxxxxxxxxxxxx
> > Subject: Re: [OM] Lenshood calculation
> >
> >
> > Moose wrote:
> >
> > >>
> > Film plane doesn't work. You need to know where the front node of
> > the lens is relative to film plane or  front of lens or rear of lens
> > mount, etc. Then it would be simple geometry.
> > <<
> >
> > And I would note:  The design also differs depending on your choice of
> > degree of hood performance.
> >
> > Gary Edwards
> >
> 
> Does anyone remember the posting a few weeks back that described a foolproof
> method for testing if a lens hood was suitable for a particular lens?  I
> printed the text out but I did not keep the address.  If it can be found
> again it might help Tom Trottier.
> 
> Michael.
> 
> 
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> 
> 



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