Your pedantic exercise is useful information but irrelevant to my
particular case. I am well aware of the projection method for
determining the adequacy of a hood. But it only works if you have a
hood in hand to test. I have zero 58mm hoods of any type. I also
understand that the geometry is important but a 57mm slip-on and a 58mm
screw-in aren't that far apart on initial diameter. The Vivitar's front
element is more deeply recessed than the Zuiko's but Walt's info on the
flare point and diameter and total length is very useful info.
I have to order a hood with no other info than its diameters and length.
With data from the Zuiko hood I at least have a starting point to know
whether the proposed hood is likely to provide some protection without
vignetting. I also know that I could use a flexible rubber hood and cut
it down if necessary. Except that I don't like flexible rubber hoods.
I view a hood as a glass protector as well as a lens shade and always
prefer something fairly rigid.
Chuck Norcutt
Moose wrote:
> Chuck Norcutt wrote:
>
>>Anybody have or know where to get a 58mm lens hood for a Vivitar Series
>>1 90/2.5 macro. Or, if not the real hood, anyone know the specs? Inner
>>diameter and length of the hood?
>>
>><snip>
>>Or, all you folks out there with a Zuiko 90mm f/2 or 135 f/4.5 macro and
>>the 108-242 hood. What's the inner diameter and length of that hood?
>>
>
> I held my pedantic self in check with the little thread about hoods for
> 28mm Zuikos, but I don't want to bust a gut...
>
> There is an unwarranted and often incorrect assumption being made here.
> And it is so obvious in this particular example. Has anyone asked
> themself how the same hood can be correct for a 90mm AND a 135mm lens?.
> Using the kind of logic going on here, either the 135 has to vignette or
> the 90 has less than ideal protection, not like Oly at all. :-)
>
> The thing is, the depth and diameter of the hood mean nothing unless you
> know the relationship of the filter thread or slip-on mounting point to
> the front node of the lens. It is obvious that the front node of the
> 90/2 is closer to the hood mounting point than that of the 135/4.5, so
> the same hood length and diameter will subtend different angles for the
> two lenses.
>
> So if you get the dimensions of the Viv I 90/2.5 macro hood, you have
> something useful to work from. The dimensions of the Zuiko hood don't
> mean a thing for the Viv.
>
> The only home technique for dealing with this that I know of that
> doesn't involve shooting lots of images* is to project from the film
> plane through the lens. I put the camera with lens and hood attached on
> its back on a light table. Then I lay a translucent screen of some kind
> across the hood. With my light source and a dark room, all I need is a
> piece of white paper.
>
> Then I can zoom, focus and change the aperture while observing the
> projected form of the film opening to see if it hits the hood. It's
> worked for me so far. With a cheap rubber tele hood, if it vignettes,
> you can just cut it down.
>
> Moose
>
> * A viable option with ditgital, where it wasn't for me with film, but
> still more trouble that the projection technique.
>
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