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Rear Converter-A 2x-S

Reviews Views Date of last review
10 39,103 Tue July 9, 2019
Recommended By Average Price Average User Rating
90% of reviewers $99.22 7.67
Rear Converter-A 2x-S


Pentax Super Multi Coated 2x teleconverter with KA mount lens information contacts.

Features of this teleconverter:

  • Manual Focus
  • Supports Tv or P exposure mode (in addition to M and Av), if the camera allows
  • The converter does not transmit focal length and MTF information from lens to camera
  • Designed for lenses up to and including 300mm (with some exceptions), but can be used with longer lenses as well. But the "L" model gives better result with long lenses.

This converter is NOT compatible with the following lenses due to their protruding rear element:

  • FA 28-90mm f/3.5-5.6
  • K 15mm F3.5 and A 15mm F3.5
  • M 24-35mm F3.5 zoom

This information is from 2006 and predates the introduction of D FA, DA, DA L and DA* lenses. Information regarding compatibility with those lenses is not readily available.

Rear Converter-A 2X-S
©, sharable with attribution
Image Format
Full-frame / 35mm film
Lens Mount
Pentax K
Light Loss
2 stops
7 elements, 6 groups
Mount Variant (Rear)
Mount Variant (Front)
Weather Sealing
HB-90 or S70-70
Diam x Length
64.5 x 39 mm (2.54 x 1.54 in.)
210 g (7.4 oz.)
Production Years
1984 to 2005
Engraved name: REAR CONVERTER-A 2X-S
User reviews
The converter retains all exposure modes supported by the attaced lens.
The converter does not relay information about focal length, MTF, or lens name to the camera.
The converter is designed for lenses 300mm and shorter, but can be used with longer lenses. It is not compatible with the FA 28-90mm F3.5-5.6, the K and A 15mm F3.5, the M 24-35 F3.5 zoom and various other wide angle lenses due to the protruding rear elements of these lenses.

Initially the converter was supplied with a hard case, later with a soft case.

Automatic ApertureFull-Frame SupportDiscontinued
Price History:

Add Review of Rear Converter-A 2x-S
Sort Reviews by: Date | Author | Rating | Recommendation | Likes (Descending) Showing Reviews 1-10 of 10
New Member

Registered: April, 2017
Location: Wisconsin
Posts: 12

2 users found this helpful
Lens Review Date: July 9, 2019 I can recommend this lens: Yes | Price: $35.00 | Rating: 10 

Pros: It WORKS! Well!
Cons: 2 stops darker (just physics)

I just have to concur with the Mr. Stone review about how well this works with "fast glass". I acquired this (mint condition) telecon from auctionsite for about $35 on a "whim", thinking I might, when needing to travel very light, leave a big lens at home and just double a shorter one. I have a couple very sharp longish lenses - a Rokinon 135mm f2, which is just "astoundingly sharp" on its own (by far, sharpest lens I own, based upon my own testing). And a DFA100 f2.8 macro, which is also very sharp, but not like the Roki. To me the acid test of a teleconverter is "does it resolve more than just zoomed in crop"? So I CAREFULLY (tripod, electronic shutter, pixelshift) shot a couple scenes with the Roki and then the Pentax 100mm macro. For the prime alone, I used a "sharpest possible mid-range aperature (like f8). For the 2X telecon, I opened up to about f4 so as to be "realistic" to field use where shutter speed needs to be kept reasonable. So technically this puts my "with teleconverter" shots at a disadvantage. But in both cases, the level of details resolved with the 2X in place is just "dramatically" improved. See attached image files (zoomed way in to same DISPLAYED SIZE for object in center frame - well beyond 100% to show more clearly). I also did a test with the DA*300mm f4 shooting the first quarter moon. Once again, even with this long (and also superbly sharp) prime lens, the 2X dramatically increased the captured resolution of craters. Wow. Talk about $35 well spent!!!
Rokinon 135 Without:
Rokinon 135 With 2X teleconverter:
DFA100 macro Without:
DFA100 macro With telecon:
DA300 without:

DA300 with:
Loyal Site Supportaxian

Registered: September, 2013
Location: Texas
Posts: 495

1 user found this helpful
Lens Review Date: November 29, 2016 I can recommend this lens: Yes | Price: $90.00 | Rating: 7 

Pros: Well matched to A series. Works with fast lenses.
Cons: May not match well with all lenses. Does not transmit focal length and MTF information from lens to camera

My experience matches the fine review written by Stone G. Please see his review for most of the details you will need to know.

I'll try to add some additional unique points.
- It appears the formula really is specific to some lenses. It does a great job with some, but matches poorly with others. Atrocious on the da 60-250.
- This is the only TC I've ever tried that copes well with very fast lenses. It, along with the Rear Converter-A 1.4x-S are excellent matches at low F and Full frame for the SMC Pentax-A* 135mm F1.8
- It also matches very well with the SMC Pentax-A 50mm F2.8 Macro.

I do lament that it doesn't transmit data from lens to camera. It's such a pleasure to use the SMC Pentax-F 1.7x AF Adapter because not only does it pass the data, but it also automatically converts the values to be correct with the TC applied. I miss that every time I use this. HOWEVER, this one has better image quality, so sometimes that wins the day and I use this instead.

- When compared to the Rear Converter-A 1.4x-S , the 2x has less contrast and is slightly less sharp. That's relative, though. This TC (for lenses which is matches well with) is a strong performer. The difference between this and a generic TC is like night and day.

Registered: March, 2010
Location: Frankfurt am Main
Posts: 1,231
Lens Review Date: March 5, 2014 I can recommend this lens: Yes | Price: $60.00 | Rating: 8 

Pros: Optical Quality
Cons: Nothing yet

I just got a used one in very good condition for GBP 36 from eBay.

It should replace my Kenko 2X KAX Teleplus MC7, which once replaced my Vivitar 2X-22.
The Kenko has already been a very big improvement in resolution and contrast (and offered the "A" contacts).
According to reviews in this forum and other places, the A 2X-S will be even better (and it is shorter by 1.5mm). With 7 elements, just the SMC coating may help.

After some usage, I may write a real review.
Just for now, I think I should mention that the manual lists the following lenses as incompatible:

Reason: coupling not possible
- K 3.5/15
- A 3.5/15
- M 3.5/24-35

Reason: strong vignetting
- Many tele lenses longer than 300mm.

As the manual seems to be from the eighties, there may be more incompatibilities with newer lenses.

Registered: June, 2010
Location: North Zealand, Denmark
Posts: 1,516

4 users found this helpful
Lens Review Date: September 13, 2013 I can recommend this lens: Yes | Price: $121.00 | Rating: 8 

Pros: Optical and mechanical quality
Cons: A bit expensive - otherwise none

There are teleconverters and then there are matching teleconverters. I bought my A 2X-S with the specific hope that it would work on my smc DA* 200mm f/2.8 lens because optically it is an FA-design from 1993 and the Pentax product litterature indicates that this rear converter should work on this type of lens (Pentax telephoto lens up to 300mm F.L.).

I have tested it extensively on my DA* 200 and I have not been dissapointed.

Of course, any teleconverter will be an add-on to a carefully optimized (one should hope) lens design and introduce some limitations such as lowering the fully open aperture f-ratio,and any (quality) teleconverter will magnify aberrations of the lens that it is sitting upon. But that's not the point. The point is: Does it work as a quality converter should work on a quality lens?

And it does. I have a hard time seeing the IQ degradation mentioned by others. What I do find, is that this TC unquestionably does help when a quality lens like the DA* 200 outresolves the sensor (the sensor under-samples the image produced by the lens). Images may sometimes at a casual glance look "sharper" without TC, but I have found that to be a matter of under-sampling and pixelation, as the following 100% comparison crops will hopefully demonstrate:

Downsized (approx. 30%) crops: The softness observed at f/2.8 is no worse that what the lens itself without TC would show.

Test Target: Remote building. I am aiming at the guarding rails in front of the windows.

100% non-resized crops of images with and without Rear Converter A 2X-S. Note the bars of the railings.

Starlings on a powerline. Again 100% non-resized crops. Note the speckles on the birds' chests.

And finally the infinitely remote.100% non-resized crops of single, non-postprocessed images. Look for the tiniest craters resolved in the two images.

Be it tiny craterlets on the Moon, speckle's on the starling's chests, bars in the garding rails on a remotine building: With this TC I can capture detail that lens and sensor alone cannot resolve. Add to that the splendid manufacturing quality and I can only recommend this converter - if used with the right matching lenses and if you know what to expect and not to expect from a quality teleconverter.
New Member

Registered: July, 2012
Posts: 6
Lens Review Date: September 16, 2012 I can recommend this lens: No | Price: $125.00 | Rating: 6 

Pros: Not much vignating
Cons: no transmission of lens data, serious loss of IQ

Purchased a used one thinking from Pentax this will be better than any other manufacturer. Even though used, this looked like new. Too much loss of light (more than 2 stops) and due to lack of any communication with lens, need to adjust fl in camera for each shot.
Can be used only when the target is static and you get time to adjust and focus everything
Could use only for moon shots
Loss of IQ was surprisingly very high unlike what others have reported here. on Tripod with MLU also the IQ was poor than handheld for the same shot
PEG Moderator

Registered: August, 2008
Location: Hielands o' Scootlund
Posts: 49,715
Lens Review Date: July 2, 2010 I can recommend this lens: Yes | Price: N/A | Rating: N/A 

Pros: Good quality
Cons: None, that I've found

Seldom used, but can get you out of a fix when you need extra reach that you hadn't planned for or has been imposed on you, eg sound board instead of pit for example at music venue.

Yes two stop light loss, but when used with fast ish lenses not such a great issue, ideal to be kept in bag for emergencies.
Forum Member

Registered: January, 2010
Location: Murcia, Spain
Posts: 93
Lens Review Date: February 4, 2010 I can recommend this lens: Yes | Price: $100.00 | Rating: 10 

Pros: Excellent results. Works with all lenses. Focus trapping works with K10D. You can introduce effective focal lenght for shake reduction.
Cons: Non.

At least my copy is sharper then the Kenko pz-af 1.5x SHQ. Evidently it is not easy to achieve pefect focus with a focal lenght of 600, and maybe some critical reports in this thread are because of this. Tripod should be used, and multiple shots, varying focus slightly (and cable release, mirror lockupů*up to 1/100s). I hadly find any image degradation on well focussed shots.
Regards - Klaus
Senior Member

Registered: March, 2007
Location: Slovenia
Posts: 140
Lens Review Date: October 16, 2008 I can recommend this lens: Yes | Price: $125.00 | Rating: 8 

Pros: Price, build quality, image quality
Cons: none

I bought this converter to acompany my Takumar 300/4 because I sometimes need that extra reach photographing wildlife. I find it really versatile to have converter in my backpack instead of carrying two big lenses with me (300mm and 600mm resp.) I have used it now for a couple of months and am quite pleased with it. Best image quality is of course achieved using a good, sturdy tripod and remote trigger. Hadheld is also possible, because of quite good SR system in my K100D, but, I prefer to use tripod or monopod.
In a combination with my Takumar 300/4 wide opened, some PF can be seen, but used with other lenses (mainly with DA 18-55 and K 50/1,7), PF is not visible.
Veteran Member

Registered: April, 2007
Location: Idaho,USA
Posts: 1,589
Lens Review Date: October 1, 2008 I can recommend this lens: Yes | Price: $150.00 | Rating: 7 


I just want to confirm this converter works well with the DA* 300/4, manual focus of course.
When I first got it and used it I thought it was very soft but found out it isn't too bad but I HAVE to work with it on a tripod and use MLU for decent results. I have taken some pretty cool pictures of the moon using it with the DA* 300/4.
It's very well made.
Again, you can obtain decent results with this, especially for a 2X converter but be prepared for careful work.
Senior Member

Registered: September, 2008
Location: Norway
Posts: 127
Lens Review Date: September 15, 2008 I can recommend this lens: Yes | Price: $87.00 | Rating: 5 

Pros: original product, probably better than 3rd party
Cons: loss of image quality, manual focus only

Bought this (secondhand) mainly because it had a reasonable price tag, and my LBA needed a quick fix. It looked like it was brand new.

It's rarely, if ever used. I've tried it in different situations on different lenses, but have never really been too happy with the results. I don't think it's bad in any way, but it gives softer images than I would like to have.

As many others will tell you, a 2x teleconverter will steal a lot of light (2 stops) and degrade the image quality quite a bit. I have not tried it side by side with other 2x teleconverters, but I guess this is one of the better you can get hold of. For me this converter seems to be less useful on a modern digital slr (K20D) where you can do a 2x crop instead without loosing any sharpness. When comparing a crop vs teleconverter I prefer the cropped image.

It's now hiding in my closet until one day when I get the hold of a reasonably priced 300mm f2.8 where it might give a better impression. When (or if) I do I'll update this review

BTW. Although it is now a discontinued product, the 2X-S is still listed at the local distributor (in Norway). It's now priced at 4495 NOK including 25% VAT, currently about 780 USD. That is just crazy, there is no other word for it
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