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Rear Converter-A 645 1.4X

Reviews Views Date of last review
3 18,131 Tue May 22, 2018
Recommended By Average Price Average User Rating
100% of reviewers $145.00 9.00
Rear Converter-A 645 1.4X

Rear Converter-A 645 1.4X
Rear Converter-A 645 1.4X


The Pentax Rear Converter-A 645 1.4X multiplies the focal length by 1.4 at the expense of one stop of light. It supports all exposure modes of the attached lens. The converter can be used with manual focus as well as autofocus lenses. Even with an autofocus lens attached, the resulting lens/rear converter combination will focus manually only.

Refer the notes in the table as regards lens compatibility.

Rear Converter-A 645 1.4x
©, sharable with attribution
Image Format
645 film
Lens Mount
Pentax 645
Light Loss
1 stop
5 elements, 4 groups
Mount Variant (Rear)
645 A
Mount Variant (Front)
645 A
Weather Sealing
Diam x Length
77 x 31 mm
265 g
Production Years
(in production)
Engraved name: REAR CONVERTER-A 645 1.4x FOR 1:4 300 mm
User reviews
Dedicated to the 645 A* 300mm lens. Also suitable for A and FA 120mm, FA 150mm, A and FA 200mm, FA* 300mm, FA 400mm and A* 600mm lenses.
The converter supports all exposure modes of the attached lens.
Autofocus lenses can be focused only manually with this converter.
Automatic ApertureMedium-Format Support
Price History:

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Site Supporter

Registered: February, 2018
Location: NoVA
Posts: 384
Lens Review Date: May 22, 2018 I can recommend this lens: Yes | Price: $90.00 | Rating: 9 

Pros: Does what it's supposed to do with very little case space invasion
Cons: It's effects are visible at high magnification, but maybe not important most of the time.

The real question is: Should one use a teleconverter, or merely crop the photo from the shorter lens?

Here is a 1:1 center crop from an image made with a 645 A* 300/4, at f/11:

And here is a crop from an image made with the 1.4 converter, downsampled to the same resolution as the image above:

No question: Downsampling the longer lens combination is not better than using the shorter lens. But considering these crops (at least the first one) is part of a 7-foot-wide print viewed at 100 pixels/inch, they are both really good. I'd use the 1.4 when I needed all fifty million of my pixels, but otherwise the A* 300 just has nowhere to go but down. It didn't go very far down here, but the converter still degraded the image more than enlargment.

Another question is whether a shorter lens plus a converter is better than carrying both the shorter and the longer lens.

Here is a 1:1 crop made using the 400/5.6 ED(IF), at f/11:

And here is a crop at 1:1 (without downsampling) from the 300+1.4 at f/11:

Not much difference here, but I would say that the 400 looks a bit better. Clearly, the A* 300 is a (slightly) better lens, but the 400 is worth bringing if one needs to optimize for that focal length.

So, the advice is: If you use the 1.4, start with a great lens. Stop it down a bit, and then don't worry about it. The converter has visible effects at high magnification, but it is still a very capable teleconverter.

Rick "always room for a converter" Denney
Site Supporter

Registered: November, 2011
Location: pontiac mi.
Posts: 371
Lens Review Date: June 28, 2015 I can recommend this lens: Yes | Price: N/A | Rating: 9 

Pros: light, easy hookup,
Cons: manual focus only.

so sharp you can see individual hairs on the arm, no loss of IQ that I can see. the fact that Pentax sells it says that it has to be good.
Site Supporter

Registered: November, 2008
Location: Washington DC, USA
Posts: 564
Lens Review Date: November 6, 2010 I can recommend this lens: Yes | Price: $200.00 | Rating: 9 

Pros: Light
Cons: No AF

This is a nice little converter. It works with more than just the A 300mm f/4. I have used this with my Pentax FA 400mm f/5.6 and the FA 120mm f/4 macro.
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