(stupid question?) Less than 1.0x teleconverter?

Started Jan 14, 2010 | Discussions thread
Joseph S Wisniewski Forum Pro • Posts: 34,493
Blocked by a Kodak patent...

Andrew dB wrote:

But I've mainly seen them used (and used well) with telescopes. It's quite common in astro imaging to use focal reducers/telecompressors (as they're known) with a multiplying effect of 0.6x or whatever. They fit on the rear of the scope, unlike wide angle adaptors seen in general photography which usually fit on the front of a lens.

The ones used for astrophotography are typically four lens devices, symmetrical pairs of positive achromats. This configuration has low distortion and good color correction, but it also reduces the telescope's backfocus, the distance from the mount to the camera. It's not typically a problem, because most telescopes have a generous surplus of back focus. Regular camera lenses have only as much back focus as they need to mount the lens. A converter that reduces backfocus won't work.

They widen the field of view (at the expense of the image circle) and speed up the optical system (from say f10 to f6 r lower) and are often used to correct other aberrations such as field curvature. Typically a model will be designed for a particular scope so they don't offer as much flexability as a teleconverter and good ones can be very expensive.

I don't think I've ever heard of a focal reducer being used in 'normal' photography.

They have to be designed differently than normal "focal reducers". Regular photography has the "teleconverter", a variation on the astronomy "Barlow tube". That's pretty easy, because they're based on negative lenses and increase back focus, so the lenses compensate for the extra length of the tube.

To turn a "focal reducer" into a functioning "wideconverter", you have to get crazy with the optics, you actually have to have a little telescope, a positive lens system (multiple lens elements, to correct the color and distortion) then a negative system to compensate for the reduced back focus.

The big problem is Kodak. They have a very broad patent on the wide converter concept. US Patent 5,499,069, and parallel filings in other countries. Claim 1 is the kicker: it covers anything focal reducer that maintains the backfocus of an existing lens.

There are other problems with the wide converter:

  • First, it's an easy design on a telescope. A fully positive focal reducer is a fairly simple devices, but it tends to increase curvature of field. In astrophotography, this is actually a "good thing" (tm) because telescopes use spherical mirrors and tend to have negative curvature of field. The focal reducer's curvature of field cancels out that of the mirror. So the device is also referred to as a "field flattener". Photographic lenses tend to have slightly positive curvature of field, and a "mostly positive" wide converter makes this worse, if you use one of the simpler designs.

  • Second, symmetrical designs cancel a lot of aberrations, so even the simple 4 element converters that are normally used in astro work very well. Make it asymmetrical, and you have to make it more complex to decrease aberrations. The number of elements increases, exotic glasses are often needed, and using aspherical lenses makes the solutions easier.

  • Third, teleconverters are negative devices, and the rear element can be smaller than the front element, making them fairly easy to construct. Wide converters have larger rear elements than front elements. This restriction on the optical path puts an absolute limit on wideconverter speed, regardless of how complex and exotic the wide converter design is. The wide converter that Nikon and Fuji built into the infamous "E2" limited you to f5.6 lens settings, with an f2.8 output. The 0.5x wide converter that Oly built into their 35-100mm f2.0 and 14-35mm f2.0 to turn common 70-200mm f2.8 and 28-70mm f2.8 designs did a little better, an f2.0 output, but it still throttled the f1.4 that they should have gotten out of those lenses.

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Rahon Klavanian 1912-2008.

Armenian genocide survivor, amazing cook, scrabble master, and loving grandmother. You will be missed.

Ciao! Joseph

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