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[OM] re: lens hoods/suitablity/shapes

Subject: [OM] re: lens hoods/suitablity/shapes
From: Winsor Crosby <wincros@xxxxxxxxxxxxx>
Date: Wed, 18 Sep 2002 19:06:18 -0700

-snip


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

-snip

.

---
Reinhold


That is very interesting.  Thanks for sharing, Reinhold.

I do wonder if your teacher was correct though. I admit that I am a complete dunce about optics. However I wonder whether shining light through the back of the lens is really highly related to light coming through it from the direction it was designed for. I was always the kid that looked through the wrong end of the binoculars and said, "Oooh. Everything is so small."

The shapes of the projections you mentioned were thought provoking, especially the round projection for a telephoto which would seem to indicate that a rectangular cross section would not be appropriate for a hood. Out of curiosity I took an OM 135/2.8, extended the built in hood, opened the aperture all the way, and focused on infinity. I then cut a mask with a rectangular opening whose corners would rest on the front of the hood. Looking at an evenly lit twilight sky I could detect no vignetting on the focussing screen, nor could I detect any difference alternating between masked and unmasked.

So it would appear that no light to the film plane is blocked by a rectangular mask in spite of the fact that it would appear that the opposite would be true when projecting light through the back of the lens. Then you wonder whether the rest of it holds water.

Just thinking out loud.

Winsor
--
Winsor Crosby
Long Beach, California


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