[John Hales]

I have read much about rear converters and that you usually don't end up with the same IQ as without. And that the trade off is IQ verses very expensive lenses. Since I can't afford serval thousand for the lens I would like to try a converter on my K-5.
#1 is it more important to get the converter to match the camera brand or lens brand or it doesn't matter?

#2 the decision of which converter to get is determined by which lens you want to use it with? I am thinking about the Sigma 50 -500 lens.

[Adam]

The Sigma 50-500mm is already so long that you probably won't benefit from anything but the best teleconverters. You get what you pay for when it comes to TC's, so don't shoot yourself in the foot by pairing the Bigma (which is a pretty good lens) with something cheap off of ebay.

[goubejp]

Converter will be efficient with a fast lens, since there will be a drop of one stop for a 1,4 converter, 2 stops for a 2X converter. Unfortunately sigma 50-500 is not fast enough. A 500 f4.5 sigma would be better, but you are not in the same price ligue.

[dms]

My experience suggest this is an area where you buy inexpensive ones and try them (or costly ones if you can return them). Opinions vary considerably.
For macro/close up use converters (TC) are generally excellent. I have no experience w/ zooms, and TC's can be (in my/others experience) fine w/ high quality prime lens. I use Vivitar and other 3rd party lower cost 2x TC's (on Nikon SLR and Pentax dslr)--I spend $10 to $50 on used ones and try them.
Usually I hear they work less well w/ zooms--although if a zoom has a particular sweet spot it may work well. It is even possible (not likely) for a TC to improve the lens IQ--it magnifies the center section and may cancel some lens defects. In this regard a matched TC should do better--and thus a matched TC is the best--but unless you are talking professional and very expensive glass, my gut feeling is a matched TC is more likely to be marketing ploy.

[pacerr]

It's a given that there will be some undesirable effect from adding a TC to a specific lens -- generally proportional to the quality of the two components.

But that misses the point of using a TC. If that's the ONLY way in which you can get the in-camera composition you want with the gear at hand you will have the best image possible under those circumstances. In many cases the quality issue involves the difference between cropping or using the TC to frame the original shot.

The question becomes whether you prefer to carry/purchase an additional longer lens over a potential slight compromise in quality using a shorter lens plus TC when there's no possibility of direct comparison in the field.

[marcusBMG]

You can check out:

Bob Atkins Photography

PhotograFun (click Tips 'n Tricks - teleconverters)

Teleconverter Comparison: Tamron 01F vs. Vivitar Macro

[KevinR]

My experience with TCs is that a good TC just magnifies the lens defects, so if you have avery good lens the TC has minimal impact on IQ. I find this with the DA*300/4 which works fantastically for me with new DA-1.4x AF TC. Some tests on sharp lenses https://www.pentaxforums.com/forums/10-pentax-slr-lens-discussion/267012-some...nce-tests.html. Even 2.8x TC stack on DA*300 produces very good IQ although I think these is noticable IQ degrade (but this could also be motion, air distortion, etc as the magnification is now extreme - 1260mm on 35 equivalent).

Not sure your 50-500 character, but my 170-500 is soft on the long end and seems not to be crisp on distant subjects. So I would expect poor IQ with TC, and you might also be pushing the AF system sensitivity a bit.

I could try a test on 170-500 with new DA-1.4x AF TC if that would help..??

[ElJamoquio]

I have read much about rear converters and that you usually don't end up with the same IQ as without. And that the trade off is IQ verses very expensive lenses. Since I can't afford serval thousand for the lens I would like to try a converter on my K-5.
#1 is it more important to get the converter to match the camera brand or lens brand or it doesn't matter?

#2 the decision of which converter to get is determined by which lens you want to use it with? I am thinking about the Sigma 50 -500 lens.

I went through this with the 150-500. I tried the Sigma 1.4, the Tamron 1.4, the Pentax 1.7... no TC was better than cropping. The HD Pentax 1.4 wasn't out yet but I doubt you'd have a different result with that T/C.

Don't know if you have a K-3 but I think an upgraded pixel level is your best bet.

T/C's really work the best with the best glass.

[severalsnakes]

I don't have a ton of experience and I don't take a lot of long shots, but experience is experience, right?

I have an old Promaster Spectrum 7 2x teleconverter that a nice old camera shop man threw in with my purchase of the M135/3.5, as a gift, kind of. He said, "You know, these things used to sell for $50!" I gave him a little extra when he rung me up, just for being cool...

I use the teleconverter rarely, when it might make a difference, for birds or other wildlife, and I typically only use it with my longest prime, the M200/4. Compensating for a couple stops lost by the addition of extra glass is not a problem, but the lens' performance suffers noticeably with the addition of the teleconverter. My copy of the M200/4 has great contrast, normally, but the teleconverter takes it down and I always have to boost contrast in post (this is on my K-30). The M200/4 also reportedly, in the experience of others, has significant chromatic aberration and purple fringing. I've never had it be a problem, unless I'm using said teleconverter. The the PF is absolutely obnoxious. For the situations I use the M200/4 and the 2x teleconverter (birds, usually), this is just deadly, since I'm almost always pointing into a tree with a nice bright sky behind. Therefore, unless I decide I really need the reach, it makes more sense for me to crop an image straight out of the M200/4 that has better IQ, than to get a little closer with the teleconverter and sacrifice clarity, contrast, and f-stops.

This is just my experience and, as they say, "your mileage may vary".

[John Hales]

I appreciate all of your insights and experience. It sounds like I should try more cropping. A K-3 is on the wish list which would also help because of the mentioned increase in MPs.
From everyone's comments it doesn't seem to matter whether the TC is a different brand than the lens or the camera.