Re: Lens with fixed focal

From: Antonio Martos (antoniomartos_at_terra.es)
Date: 09/29/04 

Date: 29 Sep 2004 09:00:56 GMT

Helge Nareid <hn.ii04@nareid.demon.co.uk> wrote in
news:g8ajl0tufnm3cfn56porka816l80fnaeid@4ax.com:

> Well, some of us do have some interest in both.

I agree that the question was made in a way that could fit better at
rec.photo, but let me reformulate the problem to fit this grupo better
and ask a few basic cuestions by the way. My apologies if this still
don't fit here, i will not insist if there is no interest.

Forget about particular models and makers, I will need to find it
comercially but my problem first is to understand what do I need exactly
and how to ask for it, and need help even in the definition of the
problem.

I have formation in physics, and work in development of photogrammetric
software and tecniques for close range aplications (that is, not aerial).
In this field real photos are modeled as point proyections from world
into CCD, with a model for compensating radial and decentering
distortion.

The ideal lens for this is a pinhole, but is too impractical, you need
too much light and exposure time. I need a real lens that resembles a
pinhole at long distances so focal length does not change. Distortion is
not a problem, can be calibrated and corrected. The optics should be as
fixed as possible, nothing should move between taking two pictures.

In practice I can model some tolerance for variations in focal length,
distortion, and position of the optic axis between photos compared with
calibration, but the final results are better when this parameters change
the little.

Also there is a problem with calibration charts. I can take images from a
know 30 cm card placed at 1 m distance and get precise calibration values
for this parameters. But when the model is 30 meters long at 100 m
distance (ie, a building) distortion and optic axis positions doesn't
move too much (and have little impact in final solution) but focal uses
to change a lot and this is important.

> To be fair to the original poster, he is asking a question which is
> outside the field of interest for most photography enthusiasts. Using
> standard serial production equipment is often a good way to get
> precision optical components at a good price.

Anyway I asked at rec.photo.digital and checked forums that optiker
recomends, but I think I it is not something what architectural
photographers or fuji customers uses to know, but some guidelines related
to lens design and/or/for photogrammetry. I'm not even sure what to ask
for exactly, remember, but is something of quite technical nature.

> The answer to the original question is that this is a specification
> which is normally not given explicitly in the manufacturer's technical
> documentation. Most, but not all, fixed focus length camera lenses

True.

> focus by moving the entire optical assembly. The two main exceptions
> are internal focusing (IF) lenses (mostly expensive long focal length
> lenses) and lenses with close range correction by moving groups of
> elements (mostly wide angle retrofocus lenses such as the Nikkor
> 2.8/24mm).

Most lens design clues today are in getting silent, small, with no
distortion and luminous lenses. That's secondary for me.

I'm even thinking in triying to design my own lens and building it but
I'm pretty sure that I will be reinventing wheel and would take a lot of
work for me.

> IMHO, the best way to ensure that the lens focuses entirely by moving
> the optical assembly is to use something like a bellows unit for
> focusing. and a suitable lens, possibly one without a focusing mount
> (such as the long discontinued 100mm Bellows-Nikkor)

Probably that is not a very practical solution, because lens should be
compact and with as few mobile parts as possible. But a very simple lens
with just too groups would work fine.

> As a side issue I'm somewhat intrigued by the assertion that the
> Nikkor 1.8/50mm changes focal length by 0.2mm over its focusing range.
> Where does that number come from and is that figure really larger than
> the measurement error?

I wrote 0.2 mm as an aproximate typical value in my projects, not over
all his focusing range, but from the usual distances, say 5 meters, to
near infinity. I can calculate focal length of the lens using calibration
cards with known sizes and with lots of ligth. The exact focal length
value is not known, but the ratio focal length / format size, being
format size fixed.

Focal length should be estimated at each picture with a 1/5000 error, but
can change more between pictures to some unknown extent because bundle
adjustment method could recalculate it, but the more variation the more
chances for bundle not to work.
In my experience a variation of until 1/100 is tolerable, but should also
be reduced if possible to improve final results accuracy.

The lens has a limited depth of field. But with enough ligth I can close
diafragm a lot and focus at hyperfocal distance so when I take a picture
all object are in focus and measure image sizes of known objects at known
distances to check how ratio varies (acounting for distortion)

I'm thinking that another aproach to solve my problem is to look for
fixed lenses with small hyperfocal distance and lock focus ring forever,
but will need lots of ligth...

Thank you for your attention.

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