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[OM] re: lens hoods/suitablity/shapes - 2nd & hopefully last

Subject: [OM] re: lens hoods/suitablity/shapes - 2nd & hopefully last
From: ReinholdLetschert@xxxxxxxxxx
Date: Fri, 20 Sep 2002 03:49:40 +0100
Hi there,

thanks for the positive comments to my lens hood
posting. 

I have only a short real addition: Once I got the 
opportunity, to ask a Rodenstock sales support 
engineer about the performance of lens hoods. 
His answer:

-The hood should be as long as suitably possible 
and the lens opening angle should be matched as 
closely as possible.
-Too short a hood has little effect and too wide
an angle drastically reduces performance as well.

====

This was  all news, the rest tries to answer Winsor's
questions.

All what this method does, is to show a cross sectional
cut and the intensity distribution of the light-ray bundle
that crosses at the plane spanned by
the front rim of the lens hood. 

And the common theory goes, that this cross sectional
cut would have the shape of the film format.
Close enough to the lens this obviously isn't
always the case.
Then, with rectangular formats (instead of 
square ones) a round hood theoretically always
gives away a fraction of possible performance 
as it always has to support the longer diagonal
line formed by a square built of the longer side
of the picture format rectangle. 
The diagonal of the 24mm x 36mm picture
is 43.3mm, the one for 36mm x 36 mm is 50.9mm. 

This is the main difference we are talking about. 
The wider the angle of the lens the more it could 
make sense to use a rectangular hood, the narrower 
the angle (tele lens), the smaller the difference 
becomes. 
A hood for an ultra wide angle lens can't be long 
or its outer diameter becomes soon not practically 
usable, hoods for standard and tele lenses are no
problem here.

A rectangular hood that is EXACTLY matching the 
opening angle of the lens is ALWAYS suitable, 
no doubt about that! It is however much more 
expensive to produce, it fits only exactly one 
type of lens AND it has drawbacks in handling
(Julian had the remark about polarising filters.
I can only second that).

Both good PRACTICAL and economical reasons against
their use with standard and tele lenses.

NO OPTICAL reason against it! And as the experiment
with tele lenses has shown, unless the hood is much 
longer than usually produced, there might even be no
benefit at all from a rectangular hood because the 
shape of the ray bundle at the plane in the 
"usual hood distance" in front of the lens is still
round. 
Or looked at it the other way: in this case the
difference of opening angle between the longer and
shorter sides of the 35mm format is that small, that
the rays at this distance in front of the lens
still don't form a rectangle. Therefore a (standard)
rectangular hood would in THIS CASE represent a lesser 
performance than a round one as there would be 
a give away in angle at the longer side. Practically
I doubt if there could a difference be spotted, given 
the hood is long enough and doesn't vignette.

Whenever you can see any form of vignetting it 
simply means, that the hood is not suitable. In the
experiment that's where the light projection gets
sharply cut by the front of the hood. You can get
this with any shape of hood. 

The only practial use of the described method (with
the light box) that I see:

-it facilitates the matching of hoods to lenses,
where there was no hood when the lens was bought

-it might be a way to find several hoods for zoom
lenses with big angle differences. 

If the latter however is practical, might be disputable.

Sorry it got that long! This wasn't intended.

BTW, I once did get some really sharp pictures (without
reflexions) with an old Super Ikonta 6cm x 6cm with
Tessar (no coating) and no lens hood attached. 
Inspite of side light and me holding the camera with 
only one hand, because I needed the other one to 
form a kind of a hood. And, as far as I know, I wasn't 
even dead then.

---
Reinhold


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