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Phillip Mosness

What goes into making a fast lens?

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I've had a little trouble finding info on what is required to manufacture faster lenses.

Is it quality of glass? Less elements? More? Diameter of the elements?

Curious because we've all read about the F 0.7 Zeiss lenses from Barry Lyndon , but what did it take to make them?

It seems like most get to the 1.4 range but not much past that, so what's the hurdle?

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I suspect it's the cost and a limited market, the F0.7 lenses were made for NASA and also with increased sensitivity there's less need for lens speed. There are a number of f0.95 lenses around.

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Mostly physical limitations.

 

The f number is a ratio of the focal length of the lens to the size of the aperture. For example, to make a 100mm f1.0 lens, you need a 100 mm aperture. So the back of the lens has to be more that 4" across, to accommodate it.

 

To make that happen, you need extremely good glass, and a flange mount free of obstructions. And extremely fine adjustments in the manufacturing process.

 

I want to say that the Zeiss f0.7 lenses were originally meant for a Hasselblad, then adapted to Mitchell BNC.

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Yep, the .7s were certainly an adaptation from a stills lens, and I often wonder what "T" they were. Sure they were an F.7, but I'd not be surprised if they were more like a T1.2

I should've checked them out over @ LACMA, but I didn't really want to be around people in a museum. I strongly dislike people at a museum.

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The f number is a ratio of the focal length of the lens to the size of the aperture. For example, to make a 100mm f1.0 lens, you need a 100 mm aperture. So the back of the lens has to be more that 4" across, to accommodate it.

 

That's not quite correct Zac, the f number is the ratio of the focal length to the diameter of the entrance pupil, which is not the same as the actual aperture size. It's basically the size of the aperture as seen through the front of the lens. A lot of lenses have front groups that act like magnifying glasses, so the aperture looks much larger than it is. For example, I'm currently working on a 180mm Cooke S4 which is T2 (lets say f/2 for simplicity). So the entrance pupil is around 90mm diameter (180/2), but the actual iris aperture itself is only about 40mm across. It just looks a lot bigger through the front.

 

This is how a lot of zooms can maintain a constant f stop as the focal length changes - the magnification of the aperture changes so that the focal length to entrance pupil ratio remains the same.

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I've had a little trouble finding info on what is required to manufacture faster lenses.

Is it quality of glass? Less elements? More? Diameter of the elements?

Curious because we've all read about the F 0.7 Zeiss lenses from Barry Lyndon , but what did it take to make them?

It seems like most get to the 1.4 range but not much past that, so what's the hurdle?

 

A few things about fast apertures that might be relevant:

 

The larger the relative aperture the more aberrations that are introduced, so very fast lenses need more correction to produce an acceptable image. They had f/1.5 lenses back in the 30s but they weren't particularly great wide open. Back then more elements meant more light loss and less contrast due to internal reflections so they kept the number of elements to a minimum. These days multicoating, modern glasses and aspheric elements means better correction can be done without compromising speed, but it requires expensive and precise manufacturing. Zeiss managed to make some pretty compact f/1.2 lenses a few decades back in the form of Super Speeds, the modern versions are Master Primes which aren't much faster but are a lot bigger - the difference is mainly in better correction of aberrations wide open (plus better mechanics).

 

The main limiting factors in lens speed tend to be the angle of view and the size of the format that needs to be covered. So you'll find plenty of fast medium focal lengths for a particular format but not so many very fast wide angles or telephotos, and the larger the format the slower (and bigger) the lenses tend to be. Using Super Speeds as an example again, they are generally compact except for the 18mm wide angle which needed a lot more glass to achieve an acceptable image at f/1.2 and the series ends with the 85mm, whose front element only just fits in the barrel.

 

Very fast lenses are often made for particular tasks, such as radiology, aerial/space photography or reproduction, but for general photography and particularly cinematography the problems of a very very shallow depth of field make them almost unusable. I don't think too many people actually want lenses faster than Master Primes, certainly not focus pullers!

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The f number is a ratio of the focal length of the lens to the size of the aperture. For example, to make a 100mm f1.0 lens, you need a 100 mm aperture. So the back of the lens has to be more that 4" across, to accommodate it.

 

Sorry, I should add that Zac is right in terms of fast lenses and long focal lengths needing to be large, only it's the front element that needs to accommodate the entrance pupil. In the above example a 100mm f/1.0 lens needs at least a 100mm front element.

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Dom said the magic words - lens aberrations. Historically, these have been the chief obstacle to speed.

Visualize the cone of light passing through a lens with spherical surfaces. (most lenses)

It is an optical fact of life that the rays at the outer edges of the cone do not focus at the same point as

the rays nearer the center. This gives rise to a number of image defects that lens designers have had

to battle for more than a century. The larger the light cone, the harder it is to get all the rays of all the

colors to come to nice sharp points in a flat image plane.

This is why photographers know, or should know, that stopping down improves the performance of
any lens. We are narrowing the cone of light, eliminating the rays that contribute the most to
aberrations.

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