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07-07-2012, 10:07 AM   #1
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1.7 TC and image resolution

Has anyone done tests to see how adding a TC to a lens affects MTF numbers? Any lens, any TC? I'm just curious how bad it can be.

07-07-2012, 12:08 PM   #2
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There are loads of comparissons, even here in Pentaxforums. Just use the search function.

Ben
07-07-2012, 01:32 PM   #3
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I do my own comparisons, I was just wondering, mathematically , how bad is it?
07-07-2012, 01:50 PM   #4
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I know only a few Canon tests:

Canon EF 70-200 mm f/2.8L IS II USM review - Image resolution - Lenstip.com

Teleconverters - Canon 1.4X Extender EF III (Tested) - SLRgear.com!

Teleconverters - Canon 2x Extender EF III (Tested) - SLRgear.com!

07-07-2012, 03:45 PM   #5
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QuoteOriginally posted by normhead Quote
Has anyone done tests to see how adding a TC to a lens affects MTF numbers? Any lens, any TC? I'm just curious how bad it can be.

Photozone has some on one/a few of their 200mm lenses... either 70-200 or 200 f/2.
07-08-2012, 01:17 PM   #6
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QuoteOriginally posted by normhead Quote
I do my own comparisons, I was just wondering, mathematically , how bad is it?
You can only raytrace real lens and tc combinations. There is no general rule applicable. Tcs come in very different varieties, some good, some bad, some rubbish. Some tcs are designed to match particular lenses. In these (rare) cases, the tc designer took into his calculation the design of the lens. These combos are usually quite good. Nevertheless, sometimes a lens and a tc match astonishingly well - just by mere coincidence, I think.

I give you an example: The old T6-2X Pentax 2x tc is mediocre at best. But on the quite as old K 300/4 it was surprisingly good, even better than the more modern Kenko 7-element tc, which worked very visibly better with almost any other lens.

I currently own 8 tcs (the 2 pairs by Pentax - the L's and S's, and the 1.7x AFA, the said Kenko KA modell and the pair of Sigma's matched tcs). Each one works best with certain lenses, but there are also inherent quality differences. For example those tcs which have a very long optical path and a protruding "nose" in general provide much sharper images and negligible loss of IQ - but they can only be used with select lenses, for plain mechanical reasons. So there is a place for the usual shorter tcs, as well, because they will simply fit almost any lens.

Then tcs are very dependent on the lens you use it with. The more parallel the rays coming from the rear lens, the better the tc will work. The misuse of tcs behind wide angle lenses or short fl zooms, will always lead to a regrettable loss of IQ.

But these really very general rules, are not based on maths, but on a mixture of physics and ofcourse simple experience and trials and errors. If you look over the shoulder of a lens designer, you will see, that today he will use software (Zemax, Oslo, whatever) to calculate the lens design. But this is only the first step. With the basic design on the screen, he will use his experience to optimize the design, by changing certain parameters (lens spacing or distribution of different lens curvatures to other elements or changing the glass type), just because "he knows", how these changes will influence the outcome. Lens design is still an empirical science, despite a lot of calculations.

So, yes, do your tests and tell about your findings - but don't get lost in mathematical trials.

Ben
07-08-2012, 07:35 PM   #7
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Look at lenstip.com's assessment of some of the 70-200's - they include MTF charts that include a 1.4 TC. The results are variable but usually not good news for resolution.

Then of course there is the issue of the contribution of teleconvertors to CA, distortion, coma, AF performance etc..
07-08-2012, 07:38 PM   #8
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No matter how good, or bad, they are, a TC only makes sense if it's used on the longest lens available at the moment and you KNOW from experience it's use is better than the cropping option or offers better accommodation through the view finder. Better to think of a TC as a better-than-nothing choice if you don't have a better solution (within your economic means).

However, viewing and zooming with a TC's field of view can sometimes offer an off-setting advantage in getting an un-cropped composition when the circumstances warrant.

The only way to tell a GOOD TC shot from an almost-good shot is to have two equivalent images side-by-side . . . and I haven't figured out how to do that economically yet.

All BS aside, I throw away more shots without a TC than I do with one when it's appropriate to use a TC. It's a crutch when nothing else is better -- up to you to learn which suits your own needs from experience.

The truth is, if you don't take the time to do exact comparisons with your own gear and habits it's pointless to guestion the difference and you're only dealing in other folks' myths.

H2


Last edited by pacerr; 07-08-2012 at 08:49 PM.
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