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11-20-2019, 06:20 AM   #1
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Pentax 645 Rear Converter 1.4X and 2X

Hello all,

I own the 2X rear converter. I hope to get the 1.4X converter in this week.

I also own Pentax 645 300mm f/4 and 400mm f/5.6 lenses.

1. Why is 1.4X converter advertised as for the 300mm lens? Is it only optimised for this particular lens or work equally well for the 400mm lens?

2. Will the 1.4X converter work equally well for the 645 600mm f/5.6 lens? Any pros or cons?

3. Why is the 2X converter rated inferior to the 1.4X converter? Why is it so much cheaper? I have used it in combination with both the 300mm and the 400mm lenses. I wasn't happy with the result despite taking test shots on a sturdy Tripod and head in broad daylight.

It would be nice to get more information on these two pieces of glass to refine photography strategy for stationary birds and mammals.

11-20-2019, 06:57 AM   #2
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My 645 2X converter is a dismal failure. I could never take a picture I liked with it and have stopped using it. OTOH, the 1.4X gives very acceptable pictures. The front element is flush with the mount and you must take care that it does not interfere with the back element of the lens you are using. I mounted it on the A 200 mm f/4 with success, so it can fit other lenses than just the A 300 mm. It conserves electronic communication with the camera body and permits automatic exposure but, as you probably already know, you will lose Autofocus with the FA lenses.

Last edited by RICHARD L.; 11-21-2019 at 03:33 PM.
11-20-2019, 07:04 AM   #3
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Here excellent results with both
11-20-2019, 11:17 AM   #4
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QuoteOriginally posted by leonine Quote

3. Why is the 2X converter rated inferior to the 1.4X converter? Why is it so much cheaper? I have used it in combination with both the 300mm and the 400mm lenses. I wasn't happy with the result despite taking test shots on a sturdy Tripod and head in broad daylight.
All excellent questions but I can only answer one of them. From my experience, regardless of brand and format, all 1.4x teleconverters have better IQ than their 2x versions. Part of the advantage is that with 1.4x you only lose one EV or stop, whereas with a 2x you lose two EVs or stops. That loss of light results in either the need for higher ISO, slow shutter speeds, and/or less DOF.

Also the 2x is essentially cropping more of the image circle and the resolution limits of the lens are more apparent than a 1.4x teleconverter. The price/cost of the two has little to do with the cost of manufacturing and everything to do with demand and value.

It would be really interesting to do a test comparing a (a) 600mm telephoto vs. a 300mm with a (b) 2x teleconverter vs. (c) 300mm telephoto cropped to the equivalent of the 600mm field of view. I know "a" would have the best IQ, but would be curious to see the difference between "b" and "c".

11-20-2019, 11:48 AM   #5
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QuoteOriginally posted by Andrea K Quote
Here excellent results with both
What do you mean by here Andrea?
11-21-2019, 07:17 AM   #6
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QuoteOriginally posted by fsge Quote
What do you mean by here Andrea?
I mean "I have".
11-21-2019, 12:52 PM   #7
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QuoteOriginally posted by Alex645 Quote
All excellent questions but I can only answer one of them. From my experience, regardless of brand and format, all 1.4x teleconverters have better IQ than their 2x versions. Part of the advantage is that with 1.4x you only lose one EV or stop, whereas with a 2x you lose two EVs or stops. That loss of light results in either the need for higher ISO, slow shutter speeds, and/or less DOF.

Also the 2x is essentially cropping more of the image circle and the resolution limits of the lens are more apparent than a 1.4x teleconverter. The price/cost of the two has little to do with the cost of manufacturing and everything to do with demand and value.

It would be really interesting to do a test comparing a (a) 600mm telephoto vs. a 300mm with a (b) 2x teleconverter vs. (c) 300mm telephoto cropped to the equivalent of the 600mm field of view. I know "a" would have the best IQ, but would be curious to see the difference between "b" and "c".
Thank you @Alex645
11-21-2019, 01:23 PM   #8
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I'll address this question -- 1. Why is 1.4X converter advertised as for the 300mm lens? Is it only optimised for this particular lens or work equally well for the 400mm lens?

Page 21 of the Pentax 645 lens manual (Note: there were many other lenses released after the publication of this manual) states (regarding the Rear Converter A645 1.4X):

"This Rear Converter can be used with not only the 300mm f/4 as described on page 20, but also with the 200mm f/4 lens."

source:
http://www.pentax-manuals.com/manuals/medformat/a%20lenses_s.pdf


QuoteOriginally posted by leonine Quote
Hello all,

I own the 2X rear converter. I hope to get the 1.4X converter in this week.

I also own Pentax 645 300mm f/4 and 400mm f/5.6 lenses.

1. Why is 1.4X converter advertised as for the 300mm lens? Is it only optimised for this particular lens or work equally well for the 400mm lens?

2. Will the 1.4X converter work equally well for the 645 600mm f/5.6 lens? Any pros or cons?

3. Why is the 2X converter rated inferior to the 1.4X converter? Why is it so much cheaper? I have used it in combination with both the 300mm and the 400mm lenses. I wasn't happy with the result despite taking test shots on a sturdy Tripod and head in broad daylight.

It would be nice to get more information on these two pieces of glass to refine photography strategy for stationary birds and mammals.


---------- Post added 11-21-2019 at 03:31 PM ----------

This member, rdenney, has posted a rich assortment of tests. Here is one set of tests with the 645 1.4X converter: Grand Lens Test 5: 645 A 1.4x Teleconverter - PentaxForums.com

QuoteOriginally posted by leonine Quote
Hello all,

I own the 2X rear converter. I hope to get the 1.4X converter in this week.

I also own Pentax 645 300mm f/4 and 400mm f/5.6 lenses.

1. Why is 1.4X converter advertised as for the 300mm lens? Is it only optimised for this particular lens or work equally well for the 400mm lens?

2. Will the 1.4X converter work equally well for the 645 600mm f/5.6 lens? Any pros or cons?

3. Why is the 2X converter rated inferior to the 1.4X converter? Why is it so much cheaper? I have used it in combination with both the 300mm and the 400mm lenses. I wasn't happy with the result despite taking test shots on a sturdy Tripod and head in broad daylight.

It would be nice to get more information on these two pieces of glass to refine photography strategy for stationary birds and mammals.



Last edited by yurihuta; 11-21-2019 at 01:32 PM.
11-22-2019, 12:57 AM   #9
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Thank you @yurihuta. I will check the links mentioned.
01-14-2020, 07:27 AM   #10
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this is a late reply to this post. my shooting is with a 645D, my lenses are the 200mm and 4oomm fa 645 lenses, and the 300mm 67 lens with an adapter. using the 1.4x tc with all the lenses, I've obtained sharp images. using the 2.0x tc I've obtained a lot of very good images. since not all my images need to be tack sharp, I've obtained a lot of good images stacking the 1.4 and 2.0 tc's together. being a " oh what the hell" kind of guy, I've even stacked a 1.4x tc with 2 2.0x tc's and still managed to get some usable images. with a good tc, the better the main lens, the fewer the faults that show up.
with my sony slt a-77 I mount a 200mm f2.8 apo G lens with their 2.0x tc and get tack sharp images all day long. the lens and tc were made by Minolta.
01-15-2020, 08:06 PM   #11
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Thank you very much for your comments. I will be getting the 1.4x soon. I will try out all possible combinations.
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