[MrEspie]

Ok, you're all going to think I'm off my rocker...

I think Pentax would be very wise to (if optically feasible) develop a 0.67x TC or, even better, AF adapter. A lot of Pentaxians have, are buying, or will buy legacy Pentax glass. Pentax keeps maintaining backward-compatibility with said glass. Naturally, this must cut into their lens sales a bit. Offering the aforementioned adapter would allow Pentax to benefit from this demographic.

"Wait!" you exclaim, "why would legacy glass-owners possibly want such a contraption?" 0.67 happens to be the reciprocal of 1.5 - the crop factor of the APS-C format relative to 35mm full frame. Simply put, this would make a full frame lens behave like a full frame lens on a crop body.

As I mentioned earlier, this would be even more valuable if it was like the F 1.7x AF Adapter, adding AF capabilities to K, M and A series glass. The AF adapter had the limitation of only being compatible with these three series of glass but I believe, with some added clutching and electronics, it could be useful across the board.

So, am I off to the looney bin yet?

[RawheaD]

Well first of all, they wouldn't be called TCs (tele-converters) but rather WCs (wide-converters), and they do exist. The ones out there will really degrade the image, as you put them on the front of the lens rather than the back of the lens. It's probably very difficult, if not impossible to do it without severe distortion and degradation of the image.

[RawheaD]

Rear wide-converters probably won't work with certain focal lenghts, period, because if you have, say a 50mm lens, and you want to wide-convert that to, say, 32mm, the WC needs to suddently make a standard lens into a retro-focal lens. A retro-focal lens (that can give you a focal length that is shorter than the flange focal length) has a fundamentally different design internally than, say, a regular Planar (double gauss) design 50mm lens. I don't think there's anything you can do at the exit-pupil end to change the fundamental design.


With tele-lenses, it's probably possible: I actually have exactly what you explain for my telescope. It's a 0.67x "reducer" for my Celestron. It takes the f=1200mm scope and turns it into an f=800mm scope, with pretty good results (I paid good money for it). But I bet that only works because we're talking about a "super-tele" if you will. And, for what you're explaining, reducing tele lenses isn't really useful :-D

[Rory]

Yeah, it's pretty much covered. I imagine they would be huge and expensive. I don't think it's something people would spend too much money on it, even if it didn't degrade IQ.

[SOldBear]

I don't think rear-mounted WCs can be designed at all.

Rear-mounted TCs work by taking the center part of the image projected by the main lens, and enlarging that part before projected it on the sensor/film. Essentially TCs work as magnifying lenses.

Rear-mounted WCs need more image than the one projected by the main lens. That "extra" part of the image is just not there in the first place.

[JeffJS]

Nice idea in theory but I think a manufacturer, particularly Pentax, could go a whole lot further introducing a FF (35mm no crop) Digital SLR.

[Wheatfield]

I think it's more to the point for people to learn what focal lengths will do on the APS-C format and buy their glass appropriately.
This whole crop factor silliness is a pointless waste of time.

[Lowell Goudge]

I think it's more to the point for people to learn what focal lengths will do on the APS-C format and buy their glass appropriately.
This whole crop factor silliness is a pointless waste of time.

How true.

[Canada_Rockies]

I think it's more to the point for people to learn what focal lengths will do on the APS-C format and buy their glass appropriately.
This whole crop factor silliness is a pointless waste of time.

On the mark, as usual, Wheatfield!

[Clinton]

I dunno, I rather like the idea. Since I've got FF glass that could have it's image compressed onto my APS-C sensor. I expect it would also come with a similar adjustment for aperture, where my 50/1.2 becomes a 35/0.95 or something similarly silly.

I'm a bit surprised we haven't seen a rear version appear from some generic vendor out of hong kong, as it would seem this idea is possible.

[Clinton]

\
Rear-mounted WCs need more image than the one projected by the main lens. That "extra" part of the image is just not there in the first place.

On APS-C with full frame glass, we do have the additional circle area that could be compressed onto the sensor.

[Boris]

Given a complete optical formula (or a lens) is it physically (like in science of physics) possible to attach glass in front of it or after it to actually increase its speed? I doubt it.

[MrEspie]

Assuming it were optically possible, we'd effectively be converging the 35mm image circle into the smaller APS-C circle, increasing the density/intensity of the light incident on the sensor. More photons per unit area = faster exposure. I've ben thinking more on this and, while a rear converter to this effect for an SLR would be unwieldy, it might be possible to make for an EVIL with a shorter registration distance. An adapter would be needed to hold the K-mount lens out at its usual registration distance anyways - why not add a simple afocal converging pair in that added space?

[Boris]

It suddenly occurs to me that A 50/1.2 with such a converter (should it be possible for K mount) would be a contradiction of terms as it seems that f1.2 is the maximal lens aperture under K mount...

[Entropy]

Given a complete optical formula (or a lens) is it physically (like in science of physics) possible to attach glass in front of it or after it to actually increase its speed? I doubt it.

Yes. It's done pretty often with telescopes (see the above post regarding a Celestron wide angle converter).

The physical dimensions of the lens remain the same, but the focal length is shorter, so the f/d ratio drops. A way to think about it is that a certain amount of light illuminates the image circle - If you take that light and concentrate it into a smaller image circle, the illumination will be brighter.

The key issue is, however, that for many lenses (especially wider angle ones), it's simply not possible to do this without making it impossible to focus on the sensor any more. It tends to be a lot easier for long telephotos. In the above example of Celestron telescopes (and most telescope designs), they are capable of focusing WAY beyond infinity, and also capable of projecting an image on a focal plane well behind the normal mounting point of the scope. The wide angle correctors will shorten this maximum distance significantly, but not to the point where it becomes impossible to focus.

You might be able to do this with some of the mirror lenses out there, like converting the Korean 500/6.3s into 300/3.8s