Using Pentax converters

Firstly I admit I am posting this partly because I am currently selling one of my Pentax converters (a rear converter A 1.4xS - I have 2 of these, and am keeping one) - but it's mainly to demonstrate a piece of advice that I have given to forum members whenever I have sold a converter or discussed using converters.

I always advise to stop down at least half a stop (and preferably more) to avoid disappointing results, and this advice is even more important when using fast lenses. Converters magnify the image coming through a lens (obviously) but they also magnify the flaws and weaknesses of a lens and this can be very noticeable under some conditions - stopping down can eliminate this, at the cost of a loss of light of course.

The two images below are both taken on a K1 MkII with the SMC A Pentax A*85mm f1.4 and the rear converter A-1.4xS - there are no PP adjustments apart from exposure/shadow/highlight.

The first image is taken at maximum aperture (f1.4) and shows significant purple fringing and of course a slight softness in the "in focus" areas. (click for a bigger image)




The second image is taken at f2.8 and the flaws so obvious in the f1.4 image are virtually non-existent.




Hope this little comparison helps somebody - If I'd have understood this properly a few years ago I might not have returned a 50mm f1.2 lens that I had just bought and assumed was a dud because I judged it by the maximum aperture shots taken with the SMC Pentax-F 1.7xAF Adapter

p.s. The advice still holds even for the most recent Pentax 1.4x DA converter, although it is the best quality converter they have produced and the flaws aren't quite as notable. It's a shame that Pentax have never released a FF compatible version, surely it wouldn't be that difficult to produce

Interesting. I'm not sure whether I've ever used the 1.4x DA with a wide open lens, getting enough DOF is usually a consideration.

For which lens(es) do you think a FF version should be optimised? Presumably it isn't possible for a TC to be equally good for all lenses. And a FF TC would undoubtedly be pixel-peeped by devotees of the art.

Steve

I'll have yo try and remember that the next time I use mine, Unfortunately it is very rare that I use it.

As always, helpful and insightful, John. Thank you.

The Pentax design philosophy back in the A series days was to offer two converters, the S version was optimised for shorter focal lengths (up to and including 300mm) and the L version for selected longer focal lengths. If you read the PF review of the DA 1.4x rear converter you can see that that approach was probably correct as they found that the A-1.4x-L performs better than the modern DA version in terms of IQ with longer lenses. In the same review the A-1.4x-S converter was noted as its resolution being near identical to the DA version!

I suspect most users would want a Full-Frame converter for longer lenses and fast telephoto zooms, and having the advantage of carrying focal length info (with the multiplication factor applied) coupled with AF make a new FF converter attractive. I think you're right about most people wanting decent depth of field and so would stop down as a matter of course - but knowing the fastest usable aperture can be useful particularly in low light or for thin depth of field situations.

LennyBloke writes:

"I suspect most users would want a Full-Frame converter for longer lenses and fast telephoto zooms, and having the advantage of carrying focal length info (with the multiplication factor applied) coupled with AF make a new FF converter attractive. "

I concur... we've loads of mid-range lenses that transmit info back... but to be able to add a connected 1.7x or 2.0x converter to my 150>450 zoom for bird photography and other wildlife shots at the top end of focal length would be marvellous.
I have the 400>600 mirror zoom, but that was bought mainly for the mirror lens bokeh... the results when focused are what I would describe as "35mm-soft"

I've never really got on with my TCs, whether Sigma, Pentax or Nikon. I did have the 1.4DA x which worked reasonably well with my DA60-250/4 but that was it....

When I was looking for a long focal length lens for my Nikon system, I did so in the hope I could use my 1.4x and 1.7x TC... one of the lenses I was looking at was the 200-400/4 and advice was don't use TC with this lens, actually not very good with the primes either....so my 500/4E FL is used without TC, if I want to 'extend' I just use it on the D500.

I take your note about stopping down Lenny. When I had my Nikkor 85mm 1.8D I could not get it calibrated using Reikan Focal, even with auto-tune in the cameras it was hit and miss - the software calibrates at f1.8 and was indicating a massive change (-20) but as soon as you go over 2.8 it was fine. No such issues with my FA*85/1.4 though (don't need a tc on that as I already have the FA135/2.8.

Tim. I too have the 400-600 mirror lens but I find mine amazingly sharp (I really must take it out again!).


This was hand held on a K3ii and reported to be 420mm. Putting any sort of convertor on it would be an invitation to a camera shaking hiding I would have thought? On a tripod you would need to use the electronic shutter!