Long Lens Apertures

[KeithB]

You touched on this in your article about the Pro-Optic mirror lens (I'll keep my Sigma 150-500, thank you!) but why are long telephotos so slow? I understand that the Aperture would have to really widen to get an f/1, but is there an optical reason for this, or just keeping size, cost and weight down? I am just guessing, but I wonder if the image circle would be pretty big for a 500mm f/1 lens.

[Bob Atkins]

Cost, size and weight.

A 600mm f4 requires an objective lens (front element) which is 150mm in diameter (about 6"). A 1200/4 would requires a front element that is 12" in diameter. Weight goes up by at least the square of the diameter, but more likely the cube since not only would it be wider, but also thicker.

They do make such lenses, but they are used in things called telescopes and 12" refractors are pretty much professional instruments. They are also likely to be slow because you don't need a fast telescope for most things and aberrations are much easier to control in slow lenses (f8 to f12 might be typical).

They do make 400mm f2.8 lenses and they have made a 300mm f2 lens (Nikon), but that's as fast as they get. Canon had a 200/1.8 but it's a 200/2 now. Past that and the cost of attempting to correct the spherical aberration would be "astronomical". I think the Nikon 300/2 was listed at around $30,000 back in the early 1980s, so just imagine what the cost might be today.

If the Canon 50mm f1 was $4000, just imagine the cost of a 500mm f1. If it could be made it wouldn't be very sharp wide open and my guess would be a cost of at least $100,000, maybe closer to $250,000, maybe more. That's a 20" diameter front element, probably made of exotic glass or fluorite and it would probably need to be aspherical. There's not much call for a soft focus 500mm portrait lens though.

Just as a price reference, a 12" f12 two element achromat (just the lens elements) currently sells for around $8500. That's just the glass and it's only 12" in diameter and it's slow (i.e. easy to figure). Add on another $1000 if you want it coated!

They do make very large, fast mirror scopes, but the really fast (f1) ones don't have a flat film plane.  You can however buy a 305mm f3.8 Astrograph for around $18000. The 305mm is clear aperture, so that's an 1160mm f3.8 lens. It's flat field and even covers a 60mm image circle so there's no vignetting on 35mm and you could even put a medium format camera on it. It's designed for infinity focus so performance might drop slightly when focused closer. It only weighs 58lbs, so you can lift it, though shooting handheld might be a bit of a problem.

Image circle isn't really a function of speed.

There is a ZEISS Apo Sonnar T* 4/1700. That 1700mm f4. It weights over 550lbs. I don't know what the price is. It was custom built, but if you ask nicely they'd probably make one for you.

[klindup]

If I understand things correctly there is another reason that limits the long focus apertures, at least as it applies to telescopes.  I use a Televue NP101 as a long focus lens.  It gives me a 540mm f5.4 lens.  I believe that the front two elements equate to a 500mm f2 or thereabouts but the two elements at the rear that ensure that the field is flat take it to f5.4.  There are limitations using it as a lens in that it is a fixed aperture of 5.4.  It also requires a pretty hefty tripod to support it.  That said I like it as a camera lens, the images are sharp and free of chromatic aberation and of course it is a great telescope.  It is not cheap, I would guess that you are looking at $6000.

Given the improvements in sensor technology that mean that the later cameras give noise free images at higher ISO settings, are the f numbers a problem?
Ken Lindup

[Bob Atkins]

Actually I think it's the other way around. I have the Televue Genesis, which was the predecessor to the NP101 if I remember right. It's 500mm f5.  I think both scopes are a Petzval lens design in which there is a doublet objective of long focal length and then a positive doublet corrector (with a fluorite element in the Genesis) some distance along the optical axis which effectively shortens the focal length and increases the speed. I assume the rear group also flattens the field to some extent. My understanding is that the doublet objective is actually something like a 1000mm f10.

This is different from telephoto designs which use a negative rear element group to effectively act as a multiplier and increase the focal length (which decreases the speed).

The Televue is actually physically longer than its focal length, whereas the definition of a telephoto design is that it's physically shorter than its focal length.

Current price on the Televue NP101is OTA (optical tube assemby) is around $4200, so it's certainly not a low cost route to a telephoto lens, especially considering that it's manual focus, has no aperture stops and is bigger and heavier than something like the Sigma 500/4.5 APO which sells for sround $5000. It may be optically better though.