Focal length reducer? / Wide angle converter?

ForenSeil · Joined: 15 Apr 2011 Posts: 2726 Location: Kiel, Germany.

Theoretically it should be not that hard to compress a pic and make a lens with a large coverage faster and give it a larger angle.
I know that these can be bought for telescopes to make F6.6 or F3.3 from an F10 lens if coverage is big enough etc.

I wonder why I've never seen any for lenses!? Or is that a market niche or for some reason not prossible?
Lens for 35mm on 2x crop sensor would mean double speed!
Imagine the speed of a medium format lens which overs 70x70mm Wink

iangreenhalgh1 · Expire: 2014-01-07

What have you been smoking? Very Happy

ForenSeil · Joined: 15 Apr 2011 Posts: 2726 Location: Kiel, Germany.

Problem would be that the register distance would be shortened - but I guess that handeled at least partially because NEX, MFT and co have a short register distance. Right?

sichko · Joined: 20 Jun 2008 Posts: 2475 Location: South West UK

ForenSeil · Joined: 15 Apr 2011 Posts: 2726 Location: Kiel, Germany.

http://homepage.isomedia.com/~cvedeler/scope/focal_reducers.htm for example!
They are readily available but only for decent telescopes till now - they wouldn't work for photographic lenses directly.

An 6x6 F2.8 80mm like Volna etc. should be able give you F1.0 normal angle on MFT with correct converter theoretically! Twisted Evil

DigiChromeEd · Joined: 29 Dec 2009 Posts: 3462 Location: Northern Ireland

Focal reducers are supplied by Celestron and Meade for their catadioptric telescopes. For example, using one of these reducers with an f10 cat reduces it to f6.3.

JohnBar · Joined: 21 Jun 2012 Posts: 581 Location: Liverpool

you might have better luck here

http://www.personal-view.com/talks/discussion/577/using-focal-reducers-with-gh12-and-old-lenses#Item_40

or putting a 1.4x TC on a 10mm cmount 1" lens might be more doable

kds315* · Expire: 2021-03-09

Done that, works, but lacks sufficient IQ.

iangreenhalgh1 · Expire: 2014-01-07

I don't understand how sticking something on the front of a lens can increase it's maximum light transmission. Surely if a lens is, say, f4 then that is a fixed figure determined by the maximum physical diameter of the light path?

I have some experience of wide angle converters, I tried two very expensive high-end ones, a Schneider-Kreuznach Ultrawide Aspheric which has three or four elements, at least two of them aspherical, and that introduced a large amount of lateral CA - severe blue and red fringes on objects, I counted 5px width which is ridiculous as you couldn't even find 1px width CA on the images from the lens without the S-K converter on it. The other one I tried was a Century one intended for broadcast 3-CCD TV cameras, it came attached to a Fujinon lens from a broadcast TV camera. This also introduced unacceptable CA and also some rather unsightly barrel distortion, I expect the distortion wasn't visible on the small CCDs of the TV camera, but on a APS-C sensor it was on the order of 8-10% I estimate which is too much for me.

kds315* · Expire: 2021-03-09

A few examples of what I have experimented with (just ignore that it's also for UV...) using a 3.5/35mm wide angle lens as the main optic plus a self constructed reducer lens.

No focal reducer

full format:

100% crop:

0.86x reducer, making that lens a 30mm lens

full format:

100% crop:

But the speed increase was considerable (and most welcome)...

kds315* · Expire: 2021-03-09

Another example:

without reducer (full format)

with reducer (full format)

Reducing factor:0.41x turning the used f4/84mm lens into a f1.6/34mm

This should simply serve as proof that this concept can be made to work under certain conditions (only).
I won't bother with detail here... Wink

ForenSeil · Joined: 15 Apr 2011 Posts: 2726 Location: Kiel, Germany.

iangreenhalgh1 · Expire: 2014-01-07

Sounds like a crazy idea to me to be honest, obsession with lens speed is not a good thing.

ForenSeil · Joined: 15 Apr 2011 Posts: 2726 Location: Kiel, Germany.

kds315* · Expire: 2021-03-09

iangreenhalgh1 · Expire: 2014-01-07

kds315* · Expire: 2021-03-09

Let's phrase that this way: one should generally not over generalize guys! There are always special needs and projects that the "everyday shooter" hardly ever meets... Wink

Calm down guys. That's a call to order.

danfromm · Joined: 04 Sep 2011 Posts: 595

There have been focal length reducers for TLRs, fixed lens 35 mm cameras, and cine cameras.

I've used Canon converters for the 310XL's 8.5-25.5/1.0 on my 310XLs and for the 814/1014 XL-S on my 6-66/1.8 and 6-70/1.4 Schneider zooms, also Eumig's converter for the Nautica in and out of water. Got very good footage with 310XL + converter and from the Nautica + converter, so-so from the Schneiders with the big Canon converter. The problem with the Schneiders was vignetting; I've always thought that the problem was that the converter couldn't be brought as close to their front elements as on the lenses it was made for.

EKCo made a 25-15 converter for the 25/1.4 Cine Ektar II. I have a couple, one with poor cosmetics and good glass, the other with lovely cosmetics and equally lovely Newton's rings. The 25/1.4 CE II is a superb lens but I never tried mine with the good converter. Kern made similar converters (Aspherons) for some Switars used on Bolexes, Century has made 'em for cine and video cameras, and I'm sure there are many more.

None of these afocal converters increased relative aperture.

Pancolart · Expire: 2013-11-18

Fabian · Joined: 04 Dec 2011 Posts: 254

Metabones on the cheap.

Tried what's discussed in this topic, works better than my previous lens-shifting solution:

Size savings:

Results:

No SpeedBooster quality, but not even close to that pricetag either.
I'll make some same-frame shots later. It's currently 0.58x with one 5mm spacer, further reduction is possible with multiple spacers but then it becomes macro.

std · Joined: 09 Feb 2010 Posts: 1826 Location: Bulgaria

Cool

Can you give us some more details on this reducer you are using?

kds315* · Expire: 2021-03-09

A simple reducer used in astro telescopes. Can be found on ebay or elsewhere...

kds315* · Expire: 2021-03-09

Fabian · Joined: 04 Dec 2011 Posts: 254

sammo · Joined: 04 Jan 2012 Posts: 223 Location: CH and SI

This might be a good idea. But I think it wouldn't work good. If lenses that are specialised to work at short focal lengths (such as, let's say a 50mm f1.2) can't produce sharp image on the edges, there is no way that a 100 mm f2.5 would produce a sharp image when compressed to 50% of the original size that it produces. If this would work, then lens producers must have missed a sharp lens design that works for fast apertures. But I'm sure with all the year's of development they considered also this.

I have some experience with both 0.63x and 0.33x reducers for telescopes. 0.63 should work on full frame (I never tried it) but I doubt of the image quality at the corners, it works well on the APC-S, though it vignettes heavily...and the 0.33x is just crappy, it cannot produce a sharp image even on a sensor with 10mm diagonal.
There are more complex systems, such as Hyperstar, where the secondary mirror is removed and the camera is mounted on the front. It costs 1500$...but then you can have a 700mm f/2 "lens" that produces a sharp image over the APC-S frame.