The SMC Pentax-F 1.7x AF Adapter

Many of us have a converter or two as a "cheap" option to extend the range of some of our fine lenses. Much discussion has taken place regarding IQ degradation, AF lock, and the like and we have reached our own conclusions over which converters are worth the compromises - but one converter stands apart from the rest for several reasons - the SMC Pentax-F 1.7x AF Adapter.

First produced in 1987 it was remarkable for the fact that it came in alongside the first true range of Autofocus lenses (the F series) but was designed primarily for use with manual Focus lenses. It was a demonstration of the commitment to owners of Pentax equipment that the line would be continuous and there would be no need to dump all existing equipment (as some other manufacturers did around this time). Another key feature of this converter (or "adapter" as some insist on it being called) was the Image Quality - if you've tried a variety of converters from this period you'll have experienced softness or lack of contrast from many, but the 1.7xAF was one of the very best - and still is (IMO)!

It does have its limitations of course - the need to "pre-focus" the lens into the approximate AF range, the effectiveness with slow lenses - the Pentax literature states it works with lenses of F2.8 or faster (there are some excluded lenses) although it does work with slower lenses in good lighting conditions in my experience. The loss of 1.5 stops of light, and the fact that it doesn't transmit focal length or MTF date to the camera body. I've found that using it with a lens set at maximum aperture can produce quite poor results (in some situations), but stopped down slightly it can deliver top results.

Here's a shot with the adapter and the A*85/1.4 on the K1 mkII yesterday, it's not a great photo but shows the IQ quite well......


Better viewed big.

I just realised that I've used an old 2x TC on the A135/1.8 but not the 1.7x

The 2x TC despite being an MC7 version was pretty glowy with poor sharpness so it would be interesting to see how the F series one fares.

EDIT: The maximum aperture shown is f/2.8 for some reason but the IQ is very very decent, just like your example. The SR had to be set to 250mm also.

Super IQ - would we have known the converter was deployed ??

Honestly I wouldn't

Just playing with another image taken with the same combination yesterday and the IQ on a fair sized crop is still impressive - as you say Daronl, it doesn't look like a TC is involved

Original:



Crop:



I'm not saying that there aren't other converters that perform well, but in my experience this is still one of the best

Completely agree. From the two copies I have, the not so good one performs like most other TCs while the good one has blown my mind when matched to a proper lens. I didn't realise how good it actually was!

The f/2.8 limit is likely related to the original SAFOX as present in the SFX & SF7 bodies (which probably had an AF minimum of f/5.6).

Modern DSLRs are likely to have much more sensitive AF (and much faster/powerful screwdrive motors).

Thank you for this thread, I had been thinking of getting an 85mm lens for a particular situation I have in mind but I have a couple of 50mm lens (M & A) and a 1.7 adapter so I can give them a try, see what quality I get and whether this is indeed a prime I should buy.
A quick test has been promising.

If you have an AF lens attached to this converter, does it pass the focal length back to the camera of can you set it manually like a manual focus lens?

The FL is set manually, I believe you have to factor in the 1.7x increase yourself. It makes picking certain lengths a little bit awkward. As an example, a 135mm lens would require a FL setting of 200/250mm. I'm not sure which is better.

Also a thought that I forgot to post earlier:
It's also very good from wide open when paired with the DA*300. With this combination you get yourself a 500mm f/6.7 lens with awesome IQ.