I'm trying to find a hood I can use on my 55mm mamiya lenses.
I had a pair which looked ok in the viewfinder (and on the
ground-glass I held at the film plane) but which vignetted
when I actually used them.

I have several hoods from which to choose, so rather than
burn film with every hood, I thought I'd just do some math.

That turned out to be harder than I expected. Different folks
offer different field of views (FOV) for this lens, and then
I think the hood design is going to depend on aperture being
used (not just FOV). It seems logical to me that a smaller
aperture can use a tighter hood than a large aperture. Then
there is the question of from where the FOV is measured. Is
it from the optical center of the lens or the surface of the
front element? Ugh.

So, since I don't seem to know enough to do the math, can
someone share with me me a good way to empirically determine
the correct hood dimensions? I picked the first ones by
studying the film-plane with a loupe. It didn't give
satisfactory results when I actually burned film with them.
Any other ideas?
________________________________________
John Thurston
Juneau, Alaska
http://stereo.thurstons.us

John Thurston wrote:

> So, since I don't seem to know enough to do the math, can
> someone share with me me a good way to empirically determine
> the correct hood dimensions? I picked the first ones by
> studying the film-plane with a loupe. It didn't give
> satisfactory results when I actually burned film with them.
> Any other ideas?

I use a compendium bellows on my 4x5 camera. In order to ensure that
I am not causing vignetting I stop the lens down to the shooting
aperture and then look through the corners of the ground glass. So
long as I can see the full circle (or oval if I'm using extreme
movements) of the aperture from each of the corners I know the shade
is not vignetting.

You can do something similar by opening the back of the TL-120 with
the 55mm lenses mounted, setting the shooting aperture you're most
likely to use, and looking from the corners of the film gate towards
the lens.

--
Brian Reynolds | "It's just like flying a spaceship.
reynolds@panix.com | You push some buttons and see
http://www.panix.com/~reynolds/ | what happens." -- Zapp Brannigan
NAR# 54438 |

Brian Reynolds wrote:

> John Thurston wrote:
>> . . . can someone share with me me a good way to
>> empirically determine the correct hood dimensions?

> . . . opening the back of the TL-120 with the 55mm lenses
> mounted, setting the shooting aperture you're most likely
> to use, and looking from the corners of the film gate
> towards the lens.

That's exactly what I was looking for, Brian.
And now that you have reminded me, I know I did that same
thing when I was working on my rolleidoscop a couple years
ago. Thank you for the guidance.
--
John Thurston
Juneau Alaska
http://stereo.thurstons.us