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03-13-2018, 06:44 AM   #1
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Effects of 2x converter

I have a SMC Pentax M 200mm prime I plan to use with my K-70. I also have a 2x converter. If I pair the two will I effectively have a 400mm lens? Do I lose anything using this setup versus dedicated 400mm lens (prime or zoom)?

03-13-2018, 06:50 AM   #2
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You should expect the lens losing 2 stops of light, the amount of light reaching the camera is reduced so if you are shooting at F:8, you will get the amount of light you would have expected at F:16.
You will probably notice a marked reduction in image quality too when comparing results to those achieved without the tc.
However, you do have effectively a 400mm FL lens with the tc.
03-13-2018, 06:57 AM   #3
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Better to listen too someone who has a similar combo, and you didn't say which 2x converter. From what I can tell, people's satisfaction with their teleconverters depends a lot on what brand they are. Off brands can be really crappy.

The lens looks sharp enough...
SMC Pentax-M 200mm F4 Reviews - M Prime Lenses - Pentax Lens Reviews & Lens Database

The ideal would be the F 1.7x AF adapter... that would give you a little more light to focus with and limited AF.

Last edited by normhead; 03-13-2018 at 08:41 AM.
03-13-2018, 07:00 AM   #4
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2x converters aren't typically that good optically compared to 1,4x. Some dedicated models exist that are quite good but most aren't. So that's problem 1. Also they will double the f stop of the lens, but if your 200 is a fast lens like an f/2.8, that's not a disadvantage since most prime 400's are around f/5.6. If you have a slower 200 then there's problem 2.

Despite all of that there's not really much harm in trying it. Just do comparison shots with and without and crop the ones without to similar image and see which is better.

If you had a faster 200 like the K 200 f2.5 then the F 1.7x autofocus converter would be a great option, but as I assume you have the M 200 f/4 that's not as good of an option due to the slower lens. However the 1.7x is quite good with a sharp lens, so your results could be good although the slower aperture may be an issue.

03-13-2018, 07:06 AM   #5
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QuoteOriginally posted by UncleVanya Quote
2x converters aren't typically that good optically compared to 1,4x. Some dedicated models exist that are quite good but most aren't. So that's problem 1. Also they will double the f stop of the lens, but if your 200 is a fast lens like an f/2.8, that's not a disadvantage since most prime 400's are around f/5.6. If you have a slower 200 then there's problem 2.

Despite all of that there's not really much harm in trying it. Just do comparison shots with and without and crop the ones without to similar image and see which is better.

If you had a faster 200 like the K 200 f2.5 then the F 1.7x autofocus converter would be a great option, but as I assume you have the M 200 f/4 that's not as good of an option due to the slower lens. However the 1.7x is quite good with a sharp lens, so your results could be good although the slower aperture may be an issue.
With the F 1.7x checking in at 6.3 on an 4 lens that's probably not an issue. When I put the 1.7 on an 5.6 lens, I definitely have issues, so 6.3 is close to the line but not over the line.
03-13-2018, 07:09 AM   #6
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Bob Atkins has a couple of good pages thoroughly discussing tc's:

Teleconverters (Multipliers) - how well do they work?
03-13-2018, 07:11 AM   #7
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QuoteOriginally posted by normhead Quote
With the F 1.7x checking in at 6.3 on an 4 lens that's probably not an issue. When I put the 1.7 on an 5.6 lens, I definitely have issues, so 6.3 is close to the line but not over the line.
I defer to you on this. I use mine infrequently except with faster lenses and my FA* 300 for moon shots and have little practical experience at that f/stop in general use with the adapter. I had a 70-210 f4 lens but it suffered internal failure so I no longer have it to test with.
03-13-2018, 07:20 AM   #8
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If the light is good my K5 autofocusses at f8+ (DAL 55-300mm f3.5-5.8 + 1.4x tc), also depends somewhat on subject.

03-13-2018, 07:36 AM   #9
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QuoteOriginally posted by marcusBMG Quote
If the light is good my K5 autofocusses at f8+ (DAL 55-300mm f3.5-5.8 + 1.4x tc), also depends somewhat on subject.
MY A-400 works with the 1.4 TC not with the 1.7 at distance in direct sun. With the 1.7x I turn off focus confirmation and auto-focus and just focus manually, and get better results.

But the 1.7x makes a purple fringing lens even worse. It's designed for * quality 2.8 lenses. That would be my big concern with putting a TC on older glass. If made before Low Dispersion glass and other corrective elements, like aspherical optical plastics were common, those lenses are likely to purple fringe badly on digital sensors. A TC makes it worse.
03-13-2018, 08:25 AM   #10
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Generally with the old non-af tc's, the number of elements can indicate which is a better tc, 7 being probably better than 5.
If you intend to hand hold, use a suitable shutter speed. e.g. 1/600ths of a sec or faster for a Pentax-M 200mm.
As has been mentioned, the tc will multiply the lens's inherent problems.
This is an image taken with a Vivitar 300mm F5.6 and a Vivitar 1.5x tc. Colour fringing is evident in the high contrast areas above the squirrel.
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03-13-2018, 12:16 PM   #11
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And do not forget that using TC makes DOF noticably more shallow so this is the reason to to use smaller aperture eg 11 instead of 8 and this may lead to IQ degradation due to diffraction.
03-13-2018, 12:53 PM   #12
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I've got a Takumar-A 2x teleconverter that I fortunately didn't pay a whole lot for. The only lens I use it with is my Pentax-A 50mm f2.0 lens, which makes a mediocre combination at best. I'm usually better off with a 28-105mm kit zoom.
03-13-2018, 01:08 PM   #13
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QuoteOriginally posted by E-man Quote
I've got a Takumar-A 2x teleconverter that I fortunately didn't pay a whole lot for. The only lens I use it with is my Pentax-A 50mm f2.0 lens, which makes a mediocre combination at best. I'm usually better off with a 28-105mm kit zoom.
I would agree with that. I have a Pentax A 2x teleconverter from the Film days but I find better results with taking a shot and using Lightroom to zoom in.

A couple of years ago I photographed the moon and performed an experiment using all my lenses. I used every lens I had - see the "my stuff" in my signature block - with and without my 1.4x and my 2x teleconveters but my old manual Vivitar 75-205 F3.8 gave me the best results in that instance.
03-13-2018, 01:10 PM   #14
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QuoteOriginally posted by nobody Quote
And do not forget that using TC makes DOF noticably more shallow so this is the reason to to use smaller aperture eg 11 instead of 8 and this may lead to IQ degradation due to diffraction.
Putting a 1.4 TC on the 100 2.8 makes it a 140 ƒ4...

Pentax K-30
According to DoF Master
Online Depth of Field Calculator

100mm , ƒ2.8, @ 10 feet, total DoF .17 feet.
100mm, ƒ4 ---,@ 10 feet, total DoF .47 feet
140mm , ƒ4... , @ 10 feet, total DoF .24 feet

So wide open, more DoF wide open on the 140 ƒ4. From the same position you actually get more DoF wide open with the TC.
Diffraction tends to become an issue at ƒ8 and goes downhill from there.

100mm 15 feet DoF = total DoG 2.16 feet
Back up to 21 feet to keep the framing roughly the same

140mm, 21 feet, ƒ8 - total DoF = 4.28 feet.

You can actually double your DoF without changing your framing by using the TC and backing up, at the same ƒ-stop.
These kinds of things are usually a lot more complex than most people expect.
03-13-2018, 01:54 PM   #15
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QuoteOriginally posted by E-man Quote
I've got a Takumar-A 2x teleconverter that I fortunately didn't pay a whole lot for. The only lens I use it with is my Pentax-A 50mm f2.0 lens, which makes a mediocre combination at best. I'm usually better off with a 28-105mm kit zoom.
The best use for the Takumar A 2x is to deglass it and give yourself an Aperture coupled A series capable extension tube. LOL. At least that is what I did with mine.
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