Petal Shaped Lens Hoods

Aren't round hoods usually for lenses with focus rings on the front?

--
Joan
http://www.flickr.com/photos/joan-in-manly

"Ed" wrote in message
...
: Just out of interest, what is the purpose of petal shaped lens
hoods?
:
: Is it so that you can still access filters and lens cap with the
hood
: installed, or is it because they are designed to let more light to
the
: corners of the photo? Or maybe something else?
:
:

"Charles Schuler" wrote in message
. ..

"Ed" wrote in message
...
Just out of interest, what is the purpose of petal shaped lens hoods?

Is it so that you can still access filters and lens cap with the hood
installed, or is it because they are designed to let more light to the
corners of the photo? Or maybe something else?

Petal shaped hoods control flare better than round types because they take
into account the aspect ratio of the camera's sensor, so the angle of view
is greater in one direction than the other.


Actually, Charles, I don't think that's the reason. The cutouts are there
to avoid vignetting on the corners of wide angle lenses, wide zooms when set
wide, and even some short to long tele zooms, like the 70-200 f2.8L IS
Canon. Notice that lenses like the 100-400 IS Canon don't have petal shaped
hoods, nor does the 70-200 f4L IS.
--
Skip Middleton
www.shadowcatcherimagery.com
www.pbase.com/skipm

"ASAAR" wrote in message
...
On Sat, 17 Mar 2007 22:31:00 -0000, Geoff. Hayward wrote:

Is it not to stop vignetting in the corners of wider angle lenses?

No. It's possible for a lens hood to increase vignetting, but
they're not used to prevent it. Their purpose is to prevent the
wrong light rays from hitting the lens, the ones that produce
unwanted reflections and flare. Think of how people sometimes put
their hands up to their eyes to help eliminate glare it they're
looking in a sunwards direction. It's not to reduce vignetting. g


He means isn't the cutout there to prevent vignetting, I think...

--
Skip Middleton
www.shadowcatcherimagery.com
www.pbase.com/skipm

On Sat, 17 Mar 2007 21:03:50 -0700, Skip wrote:

Is it not to stop vignetting in the corners of wider angle lenses?

No. It's possible for a lens hood to increase vignetting, but
they're not used to prevent it. Their purpose is to prevent the
wrong light rays from hitting the lens, the ones that produce
unwanted reflections and flare. Think of how people sometimes put
their hands up to their eyes to help eliminate glare it they're
looking in a sunwards direction. It's not to reduce vignetting. g

He means isn't the cutout there to prevent vignetting, I think...

You may be right, maybe even probably. g But it was an odd way
to put it, since if there's no vignetting without a lens hood,
adding a lens hood designed for a particular lens should add no
vignetting whatsoever, whether the hood is round or has
petals/cutouts. If a round lens hood extends as far as possible
beyond the lens so that to extend it any further would initiate
vignetting, if the hood was extended with petals in the proper
locations, vignetting wouldn't be introduced. But more potentially
flare producing light would be blocked.

I should amend my first sentence. Since vignetting is one of the
easier image problems for software to correct, there might be a use
for super-hoods, designed to block "too much" light. This would be
useful for lenses that are notorious flare producers, or even for
good, relatively flare-free lenses if they are needed to take
pictures of objects near very bright lights, even if the lights
aren't within the frame. It would a worthwhile tradeoff for some
situations, eliminating the more problematical flare at the expense
of vignetting that can be corrected.

Skip wrote:
: "ASAAR" wrote in message
: ...
: On Sat, 17 Mar 2007 22:31:00 -0000, Geoff. Hayward wrote:
:
: Is it not to stop vignetting in the corners of wider angle lenses?
:
: No. It's possible for a lens hood to increase vignetting, but
: they're not used to prevent it. Their purpose is to prevent the
: wrong light rays from hitting the lens, the ones that produce
: unwanted reflections and flare. Think of how people sometimes put
: their hands up to their eyes to help eliminate glare it they're
: looking in a sunwards direction. It's not to reduce vignetting. g
:

: He means isn't the cutout there to prevent vignetting, I think...

A lens hood with no cutouts can only be of a length that is just short of
causing vignetting. But with a petal shape, the hood can be longer with
the cuts fitting the corners of the FOV. Where the corners of the image
would extend to the circle of the lens, the middle of the sides do not
extend as far out, and thus the petals can extend further to provide more
protection. Notice that lenses that extend more into the wide angle end
have deeper cuts and more rounded petals. While more tele lenses the cuts
are much shallower (or not there at all) and the petals can be more
squared off. That's why a lens hood must be for a particular lens. The
shape and depth of the hood must match the Field of View of the widest
setting of the lens it will be used with. A mismatched hood could cause
the edges of the hood to intrude onto the image at the widest angle, which
is what vignetting is, afterall.

Randy

==========
Randy Berbaum
Champaign,IL

ASAAR wrote:

: I should amend my first sentence. Since vignetting is one of the
: easier image problems for software to correct, there might be a use
: for super-hoods, designed to block "too much" light. This would be
: useful for lenses that are notorious flare producers, or even for
: good, relatively flare-free lenses if they are needed to take
: pictures of objects near very bright lights, even if the lights
: aren't within the frame. It would a worthwhile tradeoff for some
: situations, eliminating the more problematical flare at the expense
: of vignetting that can be corrected.

Maybe for a "super hood" we need a hood that changes length and shape to
reflect the current zoom setting of the lens. This way the hood would
always block all possible off-axis light while stopping just short of
vigneting.

(Inventors, start your drafting tools.)

Randy

==========
Randy Berbaum
Champaign, IL

On Sun, 18 Mar 2007 04:51:41 +0000 (UTC), Randy Berbaum wrote:

Maybe for a "super hood" we need a hood that changes length and shape to
reflect the current zoom setting of the lens. This way the hood would
always block all possible off-axis light while stopping just short of
vigneting.

(Inventors, start your drafting tools.)

Good idea! The first generation would be "dumb", similar to the
automatic zooming done by some flash units. In a few years when
enough DSLRs are using EVFs, the "live view" can be tapped to
dynamically operate the hoods for best performance. (DOH!)

"Randy Berbaum" wrote in message
...
ASAAR wrote:

: I should amend my first sentence. Since vignetting is one of the
: easier image problems for software to correct, there might be a use
: for super-hoods, designed to block "too much" light. This would be
: useful for lenses that are notorious flare producers, or even for
: good, relatively flare-free lenses if they are needed to take
: pictures of objects near very bright lights, even if the lights
: aren't within the frame. It would a worthwhile tradeoff for some
: situations, eliminating the more problematical flare at the expense
: of vignetting that can be corrected.

Maybe for a "super hood" we need a hood that changes length and shape to
reflect the current zoom setting of the lens. This way the hood would
always block all possible off-axis light while stopping just short of
vigneting.

(Inventors, start your drafting tools.)

Already been invented, and long ago -- motion-picture cameramen have used
such devices for most of the last century at least. But nobody would want
one on a DSLR.

Neil

Randy Berbaum wrote:
ASAAR wrote:

: I should amend my first sentence. Since vignetting is one of the
: easier image problems for software to correct, there might be a use
: for super-hoods, designed to block "too much" light. This would be
: useful for lenses that are notorious flare producers, or even for
: good, relatively flare-free lenses if they are needed to take
: pictures of objects near very bright lights, even if the lights
: aren't within the frame. It would a worthwhile tradeoff for some
: situations, eliminating the more problematical flare at the expense
: of vignetting that can be corrected.

Maybe for a "super hood" we need a hood that changes length and shape to
reflect the current zoom setting of the lens. This way the hood would
always block all possible off-axis light while stopping just short of
vigneting.

(Inventors, start your drafting tools.)

Randy

==========
Randy Berbaum
Champaign, IL

In a halfway there solution Hoya makes a a multi-angle rubber hood
filter you can see what it looks like at:

http://www.2filter.com/prices/products/hoyamulti.html

It still does not solve the problem that a petal shape hood attempts to
fix by having less hood at the corners to accomodate the wider field of
view at the diagonal. And one thing that has not been noted, on a lens
that rotates the front element while focusing, the petal hood had better
be adjustable like a polarizer filter. Otherwise, those cutouts will
rotate and be in the wrong postion.

JD

In article , JD
wrote:

Randy Berbaum wrote:
ASAAR wrote:

: I should amend my first sentence. Since vignetting is one of the
: easier image problems for software to correct, there might be a use
: for super-hoods, designed to block "too much" light. This would be
: useful for lenses that are notorious flare producers, or even for
: good, relatively flare-free lenses if they are needed to take
: pictures of objects near very bright lights, even if the lights
: aren't within the frame. It would a worthwhile tradeoff for some
: situations, eliminating the more problematical flare at the expense
: of vignetting that can be corrected.

Maybe for a "super hood" we need a hood that changes length and shape to
reflect the current zoom setting of the lens. This way the hood would
always block all possible off-axis light while stopping just short of
vigneting.

(Inventors, start your drafting tools.)

Randy

==========
Randy Berbaum
Champaign, IL

In a halfway there solution Hoya makes a a multi-angle rubber hood
filter you can see what it looks like at:

http://www.2filter.com/prices/products/hoyamulti.html

It still does not solve the problem that a petal shape hood attempts to
fix by having less hood at the corners to accomodate the wider field of
view at the diagonal. And one thing that has not been noted, on a lens
that rotates the front element while focusing, the petal hood had better
be adjustable like a polarizer filter. Otherwise, those cutouts will
rotate and be in the wrong postion.

Uhm... the hood attaches to the front - if it rotates, the hood rotates
with it.

--
You need only reflect that one of the best ways to get yourself a
reputation as a dangerous citizen these days is to go about repeating
the very phrases which our founding fathers used in the struggle for
independence.
-- Charles A. Beard