Following the discussion on suitabilty 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 postion.
-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 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
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
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