Re: Fun!!! Large Format Lens Question. Low tech answer

----- Original Message -----
From: "Michael Briggs" <MichaelBriggs@earthlink.net>
To: <alt-photo-process-l@sask.usask.ca>
Sent: Saturday, February 26, 2005 5:49 PM
Subject: Re: Fun!!! Large Format Lens Question. Low tech
answer

>
> On 26-Feb-2005 J. Lehmus wrote:
>
>> I understand from the following that it might be possible
>> to obtain
>> quality images with a homemade box camera with a lens
>> from a simple
>> magnifying glass. Am I correct?
>>
>
> It depends on what you mean by "quality". Some high
> quality telescopes use
> simple achromats, but are slow (high f-number) and have a
> very small angular
> field, of order 1 degree. There are reasons that quality
> photo lenses
> typically use 4 or more glass elements. Single glass
> element lenses have only
> been used on the very cheapest cameras for more than a
> century. Even a
> two-element achromat is a dramatic improvement.
> Experiment with a using a
> magnifying glass as a photo lens, but don't expect the
> same performance as a
> modern photographic lens.
>
> --Michael

    It is surprizing how good the quality of a box camera
lens can be. But, these are not simply magnifying glasses
with a stop. The shape of the lens and the position of the
stop are very important. Some text books on lens design have
graphs showing the variation of aberrations of a simple lens
with its shape, and the variation of the optimum shape when
combined with a stop. The best shape for a box camera lens
turns out to be a meniscus with the concave sides facing the
subject and the stop in front. Many box cameras were built
with the reverse arrangement because it resulted in a
shorter camera and protected the shutter. By making some
adjustments the quality is not much worse than the optimum
set up.
    At some early point Kodak tried putting achromatized
lenses in very cheap cameras. They found most people could
not see the difference, so they reverted to simple lenses.
Box cameras typically operate at around f/11, maybe smaller.
At this small stop much of the stop dependant aberrations
are minimised. It is undesirable in a fixed focus camera to
correct the spherical aberration completely because it has
the effect of extending the depth of field. Nonetheless, box
cameras can give surprizingly good looking images especially
when used with orthochromatic film, as they were intended to
be, because the lack of red sensitivity reduces the effect
of chromatic aberration significantly.
   For experimental purposes a desk magnifier can be used
although it is not the optimum shape for a single lens
either with the stop at the lens or with a distant stop. If
one makes a series of stops of heavy opaque paper one can
discover the zonal nature of the aberrations, particularly
spherical. The stops should be a centered hole and a series
of concentric rings. The simplest arrangement is to have a
centered hole and a centeral obscuring stop. If these are
proportioned to leave the same area of the lens uncovered
the images will be of equal brightness. It is istructive to
note the difference in the position of the lens for best
focus between the two. This is BTW, a good way to get a
rough idea of the amount of spherical aberration in any
lens.
   Since old spectacle lenses are generally meniscus in
shape they can make good experimental lenses. Watch out for
astigmatism correction, it will make the lens hard to focus
but can produce some interesting special effects. Of course,
the lenses must be positive and not the negative lenses used
to correct near-sightedness. Some drug-store reading glasses
are of good quality and are useful for lens experiments.

---
Richard Knoppow
Los Angeles, CA, USA
dickburk@ix.netcom.com