This
is the first of a two-part post from guest blogger Spencer Luster.
Spencer runs Toledo-based LIGHT
WORKS, LLC which
specializes in clever optical solutions to machine vision challenges.
If you want to know find out if a telecentric
lens is right
for your application, he’s the man to contact. Now, read on …
Camera detectors continue to get smaller. And bigger.
On the
one hand individual pixels are shrinking, annoyingly so, with the
tiny 2 micron range now being available. Optics people tend to
dislike this trend because as the pixels get smaller, lens
performance must increase.
At the
same time chip makers are producing some models with many more
pixels, for both area and line scan cameras. The result being that
overall detector sizes can get quite large, with some over 60mm long.
Yet most
machine vision lenses aren't designed for these very large formats,
or for good imaging with puny pixels. What's a poor end user to do?
There
are at least two answers, and sometimes they work together
beautifully. We'll discuss the first answer in this article. The
second will come in Part 2.
Photographic
and Enlarger Lenses
The
classic 35mm format photographic camera lens is designed for creating
big images. Not all the way up to 60mm, but they can cover a large
majority of detector sizes. This is true even of the lenses designed
for digital SLR cameras whose formats are usually a bit smaller. They
can be pretty good tools, and adapters for their bayonet-style mounts
are available. If you feel very ambitious you can find a way to
utilize their autofocus/autoiris functions, but I never have.
One
downside is that they are not usually designed for very low
distortion and thus can present a challenge for critical gauging
applications.
If you
need excellent image reproduction, flat field, and low distortion,
then photographic enlarger lenses could be your answer. You do
remember photographic film from the Pleistocene Era, right?
The film from cameras was developed and then projected onto large
pieces of photographic developing paper using enlarger lenses.
Reproduction geometry had to be nearly perfect.
Figure 1
shows the typical use, although the lens-to-paper distance is scaled
much shorter.
Figure
1: Enlarger Lens with Film
Rodenstock
and Schneider
(to name two) still make these lenses and they're terrific for use
with large format detectors substituted for the film. That is,
instead of projecting the film onto paper, you put your camera
detector at the film plane and capture images of whatever you want.
These
lenses typically come in focal lengths of 28mm, 35mm, 50mm, 60mm,
80mm, and 105mm and larger. The longer the focal length, the larger
the film or detector format that can be used. The 28mm fl. lenses are
for use with 18mm x 24mm film/detectors. At the top end focal
lengths, 4" X 5" detectors could be used! (In fact, some of
the old Nikon enlarger lenses could be used with 8" x 10"
film plates! Alas, Nikon no longer builds enlargers.)
Typical
reproduction ratios for the shorter focal length enlargers is 5X to
30X. This means that the film can be accurately projected as being 5
to 30 times larger depending on the film and paper distances. For
machine vision purposes, an inspected object can be imaged onto the
detector with a magnification of 1/5 to 1/30. Longer focal length
lenses operate best at 2X to 10X (1/2 to 1/10)
In order
to use these you will need an adapter to go from the enlarger lens
male thread (39mm x 1/26") to your camera. Rodenstock at least
provides such a beast, and there may be others. Edmund
Optics sells the
Rodenstock model. Be aware that unlike c-mount lenses or even
photographic camera lenses, the back (or flange) focal lengths of
enlargers vary with lens models.
The
primary down side is that enlarger lenses typically don't come with
minimum f-numbers smaller than F/2.8, and usually F/4 to F/5.6. (No
need for a "fast" lens when you can spend as much time as
you want exposing the developing paper.) So if you need to operate
with short exposure times, order up a truckload of photons.
One
other thing: Enlargers don't come with focus rings. You adjust focus
by adding or subtracting spacers, or use a separate accessory such as
Rodenstock's "modular-focus" attachment. Some web links are
embedded in the article.
(NOTE: Edmund sells "Rodagon Large Format" lenses. These are one line of Rodenstock's enlarger lenses. They happen to be very good, 6-element designs.)
Spencer Luster
Posts
No comments:
Post a Comment