Re: [photo-3d] To Zoom or not to Zoom?

["Oleg Vorobyoff" <olegv@xxxxxxxxxxxxx>]

Jay wrote:

>Up until now I've been using a pair of 50mm lenses with my twin Canon
>set-up...but I've been wondering whether I could use zoom lenses or not.

I have been using a pair of 35-105mm zooms with my Canon F1N's with good
success.

>I'm assuming it would be fairly easy to use the extremes...28mm and 90mm, for
>example, with a 28mm-90mm zoom...but how does one make sure both lenses are
>the same for all other lengths?

Test the two lenses on the same camera since viewfinder masks vary widely.  On a
tripod check where objects fall at the left and right edges of the view with
each lens at the extremes of zooming.  I did this using a lens test chart.  Make
sure the lenses are focused to the same distance.  You may find that the two
lenses do not quite register at one or both extremes.  Mark the barrel of the
lens that overreaches at the point it registers with the other lens (I used a
dot of yellow nail polish).  Likewise mark both lenses at the points they
register for intermediate focal lenghts.  I have mine marked at 42, 50, 60, 70
and 85mm and use each those settings as well as the extremes regularly.

>Is it enough to rely on the printing on the
>lens barrel?

No, the printing is not likely to give you adequate registration between the two
lenses.

>I don't see how I could ever be sure that both lenses were set
>exactly the same. How much does it matter?

I have found that up to 1mm of on-film misregistration over the width of the
frame is tolerable.  This is about 1/35 of the full horizontal view.  If you
have the lenses properly marked you should get well within that tolerance on a
consistent basis.  But recheck whether the lenses are on the mark every chance
you get - it is all too easy to bump them off mark or to set them on the wrong
mark.  My telephoto is a one-touch zoom, so zoom setting is push-pull - much
more difficult to set on mark than with a rotating barrel.  If I have time I
check registration between the two lenses using far points on the horizon before
taking my shot.  This is a good habit to get into anyway since it also checks
whether you have your two cameras properly in line.  By the way, you need not
use two identical zooms.  A properly marked zoom lens will work very well with a
fixed lens or a different zoom.  Small coloration and rectilinearization
differences are not apparent when viewing a stereo pair.

Oleg Vorobyoff