Why so few fast ultra-wides/teles?

[squirrel$$$bandit]

Considering what lenses are available and their cost, it seems as though it must be easiest to manufacture fast primes around 50mm. There are very few really fast ultra-wides or telephotos.

Why is 50mm the "sweet spot" for being able to make fast glass affordably?

 

[squirrel$$$bandit]

OK, but what is it specifically about the 50mm focal length that requires less correction than longer or shorter lenses? And, with that in mind, is there a precise "perfect" focal length, in terms of the lack of need for corrections? And if so why?

 

[squirrel$$$bandit]

Ah OK, I think I get it...

 

[Al Kaplan]

Fast long lenses need to be huge. A 500mm f/2 would need an aperture that can open to a diameter of 250mm (about 10 inches). The mount needs to be able to focus smoothly with all that weight. It also needs to be bigger around than the aperture.

With wides you have to assure that the center isn't lots brighter than the corners, and barrel distortion also becomes a major bugaboo for the designer. It's amazing what you can do with a 15mm lens that only opens up to f/4.5.

 

[squirrel$$$bandit]

It's amazing what you can do with a 15mm lens that only opens up to f/4.5.

You'll get no argument with me there!

 

[jordanstarr]

I think it has a lot to do with lens curvature and physics. It makes sense as the further you get away from the 50mm (which is generally how we see the world) it gets harder to control the image based on lens curvature, focal points, light transmission, etc. I'm really just stated what my grade 12 physics teacher taught me 8 years ago, so I'm not sure how accurate that info is, but I suspect there's some truth to it.

In terms of "fast" lenses, I think that because most people who shoot wide angle rarely drop below f5.6 except in extreme situations -there's no point. At 5.6 many wide-ange equivalents (such as the Zeiss 28mm f2.0 and 28mm f2.8) are the same quality at a certain f-stop (I think it's f5.6, but it could be 8.0). It doesn't make sense to manufacture a 20mm f1.2 because it isn't nearly as marketable and I doubt many people would actually shoot it at f1.2.

 

[bmattock]

There are fast wide lenses. A number of manufacturers make, or have made, 35mm and 28mm lenses for 35mm film that open as wide as f/1.4, which would still be considered 'fast' by the standards of a 50mm lens. However, they are seldom seen and very expensive.

http://en.wikipedia.org/wiki/Photographic_lens_design#Types_of_lenses

http://en.wikipedia.org/wiki/Wide_angle_lens

One of the major reasons is the lens designs themselves. The single most popular lens design for a 50mm (and around that area) prime lens is the 'Double-Gauss' design. This lens design does not lend itself to telephoto or wide-angle designs, but it is generally a good design for 50mm and thereabouts, and the design itself is amenable to apertures to f/1.4 (can go faster, generally sacrificing some quality or demanding much higher manufacturing tolerances).

The lens design formula is generally the limiting factor with prime lenses. You may notice that Tessar designs, thought by some to be superior to Double-Gauss for a 50mm prime lens, are limited to f/2.8. You won't find any faster than that - the quality would drop so dramatically that no one would want one.

http://en.wikipedia.org/wiki/Double_Gauss

http://en.wikipedia.org/wiki/Tessar

As the makers of telescopes know, it is hard to make things big and accurate. Wide-angle retrofocus design lenses typically have a very large front element. A big lens is hard to keep aberration-free. Faster lenses would have to be bigger yet to let in more light proportionally.

 

[35mmdelux]

Economics plays a big part as well. There is a big demand for standard lenses, lowering R&D costs. Generally, the 50mm is the mfg standard bearer -- witness Leica's 50 Summicron & the 50 Summilux ASPH. This is where they must excel.

 

[ferider]

Think about it this way:

The longer a lens is the more glass/diameter you need for speed.

The shorter the lens is, the more elements you need for optical correction. For example 8 or more elements for the typical slow 21mm. On the other hand, unless retro-focal, these elements need to be packed closer to the film plan, since the focal distance is much shorter.

35-50 is a nice in between, where good quality can be achieved with 5-7 elements and enough distance from the film plane for lots of glass.

I'm not a lens designer, but I would believe the sweet spot is around 40mm, given all the tiny pancake lenses.

Cheers,

Roland.

 

[venchka]

They are out there

They are out there

There are 24mm 1.4 lenses around too. On a rangefinder, size matters. What good would a 135/2.0 lens be if it blocked the rangefinder patch?

On the other hand, there are quite a few 1.4 and 2.0 lenses between 24mm and 100mm for rangefinders. There are a few SLR lenses that are faster and/or longer. Canon's 85/1.4 and200/2.0 come to mind. Given the quality of ASA 400 film today, even a 2.8 lens is plenty fast. Unless you are The Night Stalker. 😀

 

[bmattock]

To me, fast lenses are for more than just shooting available light. Intentional DoF is yet another way to creatively control your photographs.