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10-28-2015, 03:33 AM   #1
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suitable 1.4x tele-extender for Takumar 200 f/4 astronomy?

I'm trying to extend my focal length cheaply right now because I cannot afford a higher focal length lens. I currently own a SMC Takumar M42 200mm f/4 lens that I use with my KX for astronomy. I'm wondering if it would be possible to extend the focal length to 360mm with a 1.4x tele-extender and how this impacts the lens when used for astronomy? What tele-extenders would be recommended?

Thank you!

10-28-2015, 05:19 AM   #2
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I would expect a good quality 1.4x would work. It could be true that some are better for your purpose than others. Are you specifically looking for one that doesn't increase coma or ? What type of Astro shots do you plan to make?

Generically I would expect the Pentax branded 1.4x-L to be a good match assuming the rear element is recessed on your lens. However this adapter can be pricey.

Is there any reason you are limiting this to a 1.4x? I realize these create less optical compromise and take fewer f-stops, but some are reasonably good quality.
10-28-2015, 06:48 AM   #3
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The Takumar lens does have some CA near the edges, but very little, if any, coma. I would hope that the extender would not increase this much, but I would be concentrating toward the middle as with most astro images. I plan on deep space object shots. I was looking at 1.4 instead of 2 because I understand that the 2x also reduces by 2 stops, not desired for astrophotography, where the 1.4x by 1 stop which would be acceptable. I'll be using a tracking mount, but even that will have limits so prefer as much incoming light as possible.
10-28-2015, 11:07 AM   #4
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My experience with converters is that if good quality, they tend only to magnify the lens defects with very little degrading from the TC itself. If the lens is sharp with low CA, you should be good. The Pentax A 1.4xS should be a good match at that FL. I tested one on the DA*300 prior to purchasing the DA-1.4xHD model and it produced good results. The Pentax A 1.4xL is a better design optimised for longer FL but has a protruding front which will likely not be compatible with the Tak 200/4.

I am also lucky enough to have a random Saitex A-2xTC which works very well, and I have test stacked with DA-1.4x on DA*300. The results were very good and much much better than cropping.

Nothing beats optical zoom imho. Just need good version of TC.

10-28-2015, 11:08 AM   #5
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i would think a quality 1.4 teleconverter would cost as much as most 2nd hand lenses
10-28-2015, 11:28 AM   #6
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You might also consider either a Tamron or Sigma APO converter. Look on EBAY for bargain prices. There are a lot of converters up for sale.**


**Why? Teleconverters were really intended to fill the frame when using chrome (slide) film, because cropping post-exposure was really difficult. With digital, cropping color images post exposure is a cinch, so converters are used by fewer photographers. I remain skeptical of the merits of using using a 1.4X teleconverter on a lens such as the Pentax 300 f4, as opposed to simply cropping in PP. To be worthwhile, the 1.4X image must have no loss of sharpness and no increase in aberrations compared to cropped/enlarged image from the same lens without using a teleconverter, and I'm not at all sure that is possible with even the very best converters.
10-28-2015, 11:51 AM   #7
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I'm of a similar mind - the fact is however in A/B tests I have seen and made the 1.4x HD TC has beaten cropped photos. I don't however have any handy to share. When I get my FA* 300 back from a recent CLA by DCShooter I will try to post some. I have found the 1.4x-S Rear Converter A to be of similar quality to the HD but not quite as good and obviously not AutoFocus. For moon shots (infinity focus) that's less of an issue and the same goes for a lens that is already manual focus.

The great that about a TC is that it is an investment that you can leverage across a number of lenses if you have the need - so if you have gaps in the lenses you carry and find that cropping isn't sufficient IQ then carrying a TC can be cheaper than adding a few lenses. But I agree that using a 1.4x TC on a single lens may be almost as costly as buying a bigger/better lens. As an example, if you carried a 35, a 77, and a 200 adding the 1.4x TC would give you the option of a 49mm, a 108mm, and a 280mm to fill in gaps and extend range.

BTW - Full disclosure. I have a 1.4x-S Rear Converter A and a 2x-6 non A series both for sale at the moment. I also own the HD TC and find that I like it but use it less often than I wish for the cost. If anyone is interested in my sale items PM me if you can't find them in the marketplace.
10-28-2015, 01:54 PM   #8
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I posted a fairly extensive testing of TCs with DA*300/4 and also FA*600/4 here: https://www.pentaxforums.com/forums/10-pentax-slr-lens-discussion/267012-some...nce-tests.html

Hopefully this shows the TC benefit over crop. BTW, the Saitex Auto 2xTC cost me about $15 so there are some obscure gems out there.

10-28-2015, 04:32 PM   #9
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I found a guy selling some very old M42 Vivitar TCs on Ebay for $20. I bought a 2x. It was brand new in the box although it had probably sat in a warehouse somewhere for decades. It does a good job although I rarely use it. Images are still sharp but it makes the CA worse.
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