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04-03-2011, 01:53 PM   #1
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How much does a teleconverter change the IQ?

I have a Lentar 2x teleconverter. If I put that on a 200 f4 lens, does that mean I am at 400mm or with cropped body 600mm? Does the teleconverter degrade the image significantly?

04-03-2011, 02:56 PM   #2
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Unless you shoot film or are trying to converse with someone who does, crop factor has no relevance, whether talking about TC's or not. A 2X TC doubles the focal length, just as the name implies. So a 200mm lens becomes a 400mm lens. Whether that 400mm happens to have the same field of view as a 600mm lens you don't own on a film camera you don't own doesn't seem very relevant to me.

Basic rule of thumb as for IQ: a TC that costs $100 hurts IQ about exactly as much as subtracting $100 from the value of your lens. The cheaper the TC, the more value it subtracts from the lens. The more expensive the TC, the less value it subtracts. Unless that TC was very expensive, and the 200/4 you are attaching it to equally so, then I expect the results to be noticeably worse than simply cropping - and cropping doesn't steal 2 stops of light.
04-03-2011, 04:09 PM   #3
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aside from loss of depth of field narrowness, any weakness that the lens possessed will be magnified as well. it becomes less sharp and less contrast. the rule of thumb is, only use a TC if the lens is an excellent one.
04-03-2011, 06:28 PM   #4
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QuoteOriginally posted by Pentaxor Quote
the rule of thumb is, only use a TC if the lens is an excellent one.
I emphatically agree .

04-04-2011, 01:49 AM   #5
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QuoteOriginally posted by justtakingpics Quote
Does the teleconverter degrade the image significantly?
I've been wondering that, too, so I did a little test. I'll start with the disclaimers:

This was a quick, completely unscientific endeavor. There's variation in the exposures, but I was careful to get the best focus possible on all images. The lenses and TCs used were (mostly) a motley bunch. Not exactly the best and the brightest. That said...

camera: K10D, jpegs, no PP except for WB on the Tokina
lenses: Tokina SD 400 f/5.6, Vivitar 200 f/3.5, Prinz 135 f/2.8
teleconverters: Kiron MC7 2x (7 element), Focal 2x (4 element?), Vivitar 3x (?? element)

All shots from the same spot, on a tripod, 3 second remote control timer, all @ f/8.

The Tokina 400 was the baseline. I made a moderate crop of that picture and cropped the rest to match the FOV of the (cropped) Tokina shot. I shot the Vivitar 200 alone and then with each 2x TC. I shot the Prinz 135 alone and then with the 3x TC. For reference, the first picture is the original, uncropped Tokina 400.
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PENTAX K10D  Photo       

Last edited by .a.t.; 04-04-2011 at 02:29 AM.
04-04-2011, 01:52 AM   #6
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Cool comparison. Something's gotta be off with the focusing though because the Prinz crop with TC actually looks sharper (though less contrasty) than the other one. Also, 3x TC? I didn't even know those existed!
04-04-2011, 02:20 AM   #7
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Bottom line? My teleconverters were cheap and are fun to play with, but there's no free lunch. As you can see, there's very little difference between cropping and using a TC - with the 200mm, anyway. There may be a slight IQ benefit to using the 3x on the 135. But, considering the three stop light penalty, someone wanting length would be better off looking for an affordable longer lens instead.

Tim

edit: Hi Erik,

I can't even remember where I got my 3x TC, but I know I didn't pay much for it. It did better than I thought it would. As for the focusing, it was difficult, but I focus bracketed and chose the best one each time.

Last edited by .a.t.; 04-05-2011 at 10:33 PM.
04-04-2011, 04:46 AM   #8
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I've been using a Tamron SP 60-300 and Tamron SP 2X TC (01F) for wildlife shots & moving birds. I've gotten some good results with the combo, but this has me wondering if I wouldn't be better off shooting at 300mm & cropping... I may have to test that out. I always figured I'd lose to many pixels to cropping at that level.. (using a 6MP body.)

One thing's certain, it's a lot easier to focus & shoot at 300mm/f:3.8 than it is at 600mm/f:8...

04-04-2011, 04:54 AM   #9
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QuoteOriginally posted by GibbyTheMole Quote
I've been using a Tamron SP 60-300 and Tamron SP 2X TC (01F) for wildlife shots & moving birds. I've gotten some good results with the combo, but this has me wondering if I wouldn't be better off shooting at 300mm & cropping... I may have to test that out. I always figured I'd lose to many pixels to cropping at that level.. (using a 6MP body.)

One thing's certain, it's a lot easier to focus & shoot at 300mm/f:3.8 than it is at 600mm/f:8...
I also use this lens, but with the Pentax AF-1.7x TC.
Haven't tried it with the K-5 yet, but I've meaning to see how it works with contrast AF in liveview.

PS. at f/8, this combo matches or exceeds all of my long primes(200-400mm).
04-04-2011, 06:21 AM   #10
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QuoteOriginally posted by justtakingpics Quote
I have a Lentar 2x teleconverter. If I put that on a 200 f4 lens, does that mean I am at 400mm or with cropped body 600mm? Does the teleconverter degrade the image significantly?
"does that mean I am at 400mm?"
No.
It means that your 200mm is now functioning at an apparent FL of 400mm. A TC cannot change the native FL of the objectives.

What a TC does do is add a negative focus group to the back end of the Tele. This makes the light cone of the native system less acute thus the image formed at the sensor appears to be at 400mm.

BTW all true Telephotos already have, essentially, a TC (a negative focus group) already built into the back end of the lens and it's called a "telephoto group". This is done to shorten the length of the telephoto and make it more compact than it would be if the lens was at true prime focus. This is the best sort of "TC" because it can be precisely matched to the lens' optical system.

But when you add a TC to a Telephoto you are essentially adding a well matched TC to a, more often than not, poorly matched TC thus degrading the image quality. Even it the TC was the best possible match to your lens the added elements always introduce added aberrations of their own to the light path that are obvious to my eyes at least.

"Does the teleconverter degrade the image significantly?"

Compared to what?

All else being equal If you compare a 200mm tele with a 2x TC to a true 400mm tele with no TC it's been my experience that the loss in quality between the two is obvious to the naked eye. So, yes, to me a TC does degrade the image "significantly".

But at the end of the day sometimes you just need the added reach a TC gives more than you need absolute drop dead optical quality and you have to live with that trade off.
04-04-2011, 06:26 AM   #11
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JohnBee:

So what do you think? Use the TC or crop? I'll probably do some test shots today to compare, but was just wondering if you had ever compared the TC to a crop with your Pentax 1.7x. What primes are you comparing the Tamron/TC combo to, if I may ask?

Yeah, I love my Tamron. I find myself using it probably 70% of the time. It's very sharp with great colors. The macro mode's very useful, as well... It's a versatile lens. Love the build quality, too.

Not too crazy about the PF (which is worse with the TC) , but I can remove that easy enough with Photoshop.

I've found that the numbers of good long tele options over 300mm are few and far between... and usually expensive.
04-04-2011, 06:43 AM   #12
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QuoteOriginally posted by GibbyTheMole Quote
I've found that the numbers of good long tele options over 300mm are few and far between... and usually expensive.
"usually"? How about "always" (although I guess you could get really lucky at a yard sale ;~)
04-04-2011, 06:56 AM   #13
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As a result of my shotgun approach to lens buying -- buy a cheap lot, keep what I like, try to sell the rest, sometimes get stuck with stuff no worth selling -- I've accumulated a pile of TCs in PK and M42 that I just don't use. I tell myself that someday I'll put those two 2xs and two 3xs onto my M42 Rubinar 1000/10 to make a 10000mm f/100 monster. Yeah, that could happen.

Actually, I do use a couple PK TCs quite a bit. These have A-type contacts. They weren't too expensive. (Focal, US$23; and Promaster, US$9.) To make them usable for my purposes, I merely removed the glass. So now they're A-type macro tubes, costing less than items marketed as such. Yeah, just lose the glass, to keep the IQ good. Heh heh.

Here is my take on intact TCs: They can be useful in situations where content is more important than image quality. Surveillance, blackmail, crime planning, that sort of stuff. Or if I *want* that diminished IQ effect and am too lazy to add it in PP. There's more to photography than just photorealism. Think of it as art. Right.

Last edited by RioRico; 04-04-2011 at 07:03 AM.
04-04-2011, 07:00 AM   #14
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QuoteOriginally posted by GibbyTheMole Quote
JohnBee:

So what do you think? Use the TC or crop? I'll probably do some test shots today to compare, but was just wondering if you had ever compared the TC to a crop with your Pentax 1.7x. What primes are you comparing the Tamron/TC combo to, if I may ask?
I did in fact run some w/without TC tests using the SP 60-300, I'll try and dig those up today for you.

However I did not do any controlled tests using the SP against other primes. So my statement is based entirely on individual observations.

As for the primes, I've compared it to a couple of my older lenses:

67 160mm w/PK adapter
Tamron 200mm f3.5
Telear 200mm f3.5
Jupiter 21A f4
Pentax-M 200mm f4
Pentax-K 300mm f4
Tamron SP 300mm f5.6
Komura 300mm f5

And perhaps a few Takumars but I can't quite remember what models they were atm.

The only lens that the SP 60-300 can't compete with is a Nikkor ED* 180mm 2.8 that was retrofitted for K mount(better than Pentax M * equivalent).

Hope this helps
04-04-2011, 07:48 AM   #15
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The IQ with tele converters is kind of a crap shoot. Some converter/lens combinations work very well and others don't. Some TC's are designed to work with certain lenses and you will probably have good results with Sigma's TC's and staying with the recommended list of their lenses. Their TC and the 70-200/2.8 are a matched combination and there are a lot of great shots all over the forum with this set up. My old Vivitar M42 2x TC on my 200/4 Tak does pretty good much of the time but my results have been mixed. I have had a few very sharp shots but most are a little soft. I really haven't taken the time with the combo to figure out if it's me or the TC. With my 55/2 Tak, it works great. On the 200, it's a heavy beast and difficult to hold steady.
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