Re: How would lens optics/engineering drive toward an optimal sensor/film size?
- From: Rich <none@xxxxxxxx>
- Date: Wed, 05 Apr 2006 17:56:15 -0400
On Wed, 5 Apr 2006 16:09:03 +0100, David Littlewood
<david@xxxxxxxxxxxxxxxxxx> wrote:
In article <d7IYf.40274$ty4.7072@xxxxxxxxxxxxxxxxxxxxx>, Tom
<Tom@xxxxxxxxxxxxxxxxxx> writes
Tom wrote:
[sweet talk]
The related question I have is, if we started with some goals for the
glass and worked toward a sensor what kind of system would we end up
with?
The prior posts are mostly about sensors...is that because we are
sensor-centric or...what?
Is a 50mm 1.8 35mm lens a great lens optically, and for moderate
cost, because its in the sweet spot for the 'physics of glass' or is
just the most engineered / largest selling lens and market size
reduced the cost?
Still interested in physics of the "Normal Lens".
As a D50 owner, a fast 35mm lens will currently cost me 2x - 3x the
cost of a 1.8 50mm.
Will we see fast, good, cheap "Normal" lenses for the 1.5X format?
For wide angle for a given field of view, is a 24m easier to get
'good' (distortion, flair, etc) vs an 18mm lens? So body and sensor
wars aside, if we could start over and pick a format driven by optics
would it be?
Same here, just a matter of time for good 18mm glass at 24mm prices...or no?
Yes, I noticed everyone leapt in on the sensor question and rather
ignored the optics part.
To some extent the answer to your question is that 50mm on 35mm
represents the easiest specification for lens design. The angle of view
- typically 46 degrees diagonal on 35mm film - is about the least
challenging to perfect. The face that, historically, they tended to be
the most popular may or may not have resulted in earlier perfection of
designs, and cheapness through larger production runs, but after several
decades of sophisticated computer design and of the almost complete
dominance of zooms in the mass market I would be surprised if this is
still a major factor.
Wider lenses tend to suffer from a rapid increase in some design
problems: vignetting, curvilinear distortion (i.e. barrel or
pincushion), coma and astigmatism all get worse as FoV increases. The
solutions include: restricting maximum aperture, using aspherical
elements and floating elements, and in any event many *more* elements;
all this results in lenses which are larger than you would expect, and
much more expensive.
Longer lenses tend to suffer a different array of problems. Chromatic
aberration, especially lateral CA, gets worse as relative focal length
increases.
No, it gets worse as lens speed increases. The way colour error
happens is due to the edges of the lenses acting like prism wedges.
The faster the lens, the steeper the curves, the greater the prismatic
effect. A 1000mm lens at f16 has virtually zero colour but a 1000mm
lens at f6 has noticeable colour, provided both designs use the same
glass.
This can be reduced or eliminated by the use of (very
expensive) fluorite or extra-low dispersion glass, and more complex
design. Also, temperature effects start to become significant (focus
point shifts as the lens barrel expands). And of course the size of
glass elements required to give a decent f-number goes up as f goes up.
To illustrate this, just look at a few specifications. These are from a
Canon book as that's what I happen to have, but I have no doubt a
similar story exists in other makers' lists.
50mm f/1.8 --- 6 elements in 5 groups
50mm f/1.4 --- 7 elements in 6 groups
That is basically "old school" lens design using all spherical
surfaces. Molded aspherics (at least in shorter lenses where the
optical errors of such products can be tolerated) allow for a decrease
in the number of spherical elements to be used. In fact, some
specialized, highly corrected and flat field lenses suitable for
medium format film coverage have only 3-4 elements. What this does is
eliminate (lens surfaces) and the contrast and light loss associated
with them. However, where lenses need higher optical correction
(telephotos) molded aspherics would be impossible to use owing to the
problems of making error-free large molded elements. The only
solution in this case is the old spherical solutions or very expensive
ground aspherical elements.
Aspherics also eliminate some of the "trade off" abberation control e
that characterizes all spherical lens designs where you would put up
with (for e.g.) 5 waves of spherical aberration in order to control a
wave of chromatic aberration, lesser of two evils.
The invention of multicoatings has (IMO) hampered lens design
because it has kept the cheap to fabricate spherical lenses as the
mainstay for camera lens design by eliminating much of the problem
with multiple element design which are internal reflections and light
loss.
-Rich
.
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