Re: How would lens optics/engineering drive toward an optimal sensor/film size?
- From: David Littlewood <david@xxxxxxxxxxxxxxxxxx>
- Date: Wed, 5 Apr 2006 16:09:03 +0100
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. 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
This shows that even at 50mm you have to use a more complex design to get good results at a 0.5 stop wider aperture.
28mm f/2.8 --- 5 elements in 5 groups
Similar to the 50/1.8 - but notice we lost 1.5 stops to allow simplification
28mm f/1.8 --- 10 elements in 9 groups
To get the wide aperture back we have to have a much more complex design
14mm f/2.8L --- 14 elements in 10 groups (including 1 aspherical element)
About as bad as it gets at the wide end..
And going the other way:
100mm f/2.0 --- 8 elements in six groups
Bit more complex, smaller aperture.
200mm f/2.8 --- 9 elements in 7 groups
More complex and a stop down
400mm f/5.6 --- 7 elements in 6 groups
Small aperture keeps it fairly simple (though still requires 2 ULD elements). But now see:
400mm f/2.8L --- 17 elements in 13 groups (including 1 fluorite and 2 ULD elements)
I was quite surprised, looking this up, to see just how complex the design of these lenses are.
If you look at the price lists you will see a similar (but more extreme) story - and again, probably not because of scale of production as the 50mm fixed focal length has fallen out of favour.
When I had my first SLR, a 400mm lens typically had 4 or 5 elements - no fancy glass - but the performance was very poor, low contrast and low resolution, for the reasons I mentioned above. Way below the simple 58mm f/2 Helios I had as a standard lens. The 17/13 sophistication (and £4000+ price) is required just to get performance back to the same level as the £80 Canon 50mm f/1.8 - and still at 1.5 stops less aperture.
Obviously if you change the film/sensor format then the focal lengths change - it is (mostly) the FoV that affects design, apart from mechanical things like material strength and size of controls, which as someone else pointed out will limit the benefits of a linear scaling down. There is no obvious reason why a 35mm f/1.8, or even a 17mm f/1.8, should not be available given a format which requires a 40-55 degree FoV at that focal length. Indeed, look at cine/video cameras and you can see it has been done.
David
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David Littlewood
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