“Telecompressor”的意思、由来-开放百科全书

A telecompressor or focal reducer is an optical element used to reduce focal length, increase lens speed, and in some instances improve optical transfer function (OTF) performance. Popular applications include photography, videography, and astrophotography. In astrophotography, these qualities are most desirable when taking pictures of nearby large objects, such as nebulae. The effects and uses of the telecompressor are largely opposite to those of the teleconverter or Barlow lens. A combined system of a lens and a focal reducer has smaller back focus than the lens alone; this places restrictions on lenses and cameras that focal reducer might be used with.

Calculating focal reduction

For a refractor telescope or simple camera lens, the new effective focal length f_n is given by:^[5]^[6]

For a reflecting telescope, the calculation is the same. However, since the telecompressor increases the field of view, there could be vignetting in the image, depending on the sizes of the secondary mirror and the telescope tube.^[7]

For a catadioptric system that has a combination of mirror and lens, the determination of reduction is more complicated, due to the fact that the telescope has a variable focal length, where the imaging plane can move along the axis of the imaging system. As the addition of the telecompressor will increase the necessary back focus, the original focal length will increase by a certain amount, and then this new focal length would be used in the above formula.

Keplerian (relay) telecompressors

Telecompressors were used in early digital SLR systems like the Minolta RD-175 and the Nikon E series. The technology of the time used relatively small sensor sizes, so lenses designed for 35 mm film could not be used with their native field of view without additional optics used. Implementing a telecompressor helped to mitigate these limitations. One effect of a telecompressor is that it reduces the diameter of the image circle, which means that a lens meant for a larger format can be used on a smaller sensor with a reduced crop factor.^[8]

See also

References

1. ^{{Cite news|url=http://www.mirrorlessjourney.com/blog/2014/9/focal-reducer|title=Focal Reducer|work=Mirrorless Journey|access-date=2018-05-31|language=en-CA}}
2. ^{{cite web|url=http://blog.uniquephoto.com/metabones-speedbooster-tele-compressor-speeds-up-mirrorless-cameras/|title=Metabones Speedbooster Tele-Compressor Speeds Up Mirrorless Cameras - Unique Photo Blog Unique Photo Blog|author=|date=|publisher=|access-date=22 September 2016}}
3. ^http://www.adorama.com/brands/kipon
4. ^{{cite web|url=http://www.adorama.com/g/kipon-main|title=Kipon Adapter Finder|author=|date=|publisher=|access-date=22 September 2016}}
5. ^ {{cite book |year=2000 |title= Practical Astrophotography |last= Charles |first= Jeffrey R. |publisher=Springer Publishing |isbn= 978-1-4471-0503-9}}
6. ^ {{cite web |url= http://www.pk3.org/Astro/index.htm?scopes_barlow_focal_reducer.htm |author= Peter Katreniak |year=2001 |title= K3's Astronomy - Telescopes}}
7. ^ {{cite web |url= https://www.cloudynights.com/topic/36383-focal-reducer-with-fast-newtonian/ |author= "Jarad" |year=2005 |title= Focal Reducer with Fast Newtonian? |website= Cloudy Nights}}
8. ^{{cite web|url=http://www.lensrentals.com/blog/2013/01/metabones-magic|title=Metabones Magic?|first=Lens|last=Rentals|date=22 January 2013|publisher=|access-date=22 September 2016}}