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08-04-2013, 06:38 PM   #1
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Vivitar Teleconverter: Fair Test?

Recently bought the Vivitar 1.4 TC to use with my Tamron 70-200/2.8, thinking I could get some extra reach from it. Did some initial tests and thought the IQ wasn't too bad.

Here's one at min focus distance (around 1m)

and a 100% crop of that one:

I got to thinking that my Tamron is almost always super sharp, and generally I love it, so I decided to A-B test the TC vs the lens with no TC. Comparing the TC at 140 to the lens alone at 200, the lens was clearly superior. The TC added softness and some CA that's obvious when pixel peeping.

So the final test: The TC at 200mm, and the lens alone at 200mm, with a crop to get the same FOV. The lens with no TC still won in my book. I think it's sharper, better contrast, less CA, and has more detail.

and similar size crops from both:

So here's where my question comes in, Did I A-B test these two fairly? Here's what I did. I used TAv, and set the shutter speed to 320 for the TC shots, and 200 for the no TC shot. I also set the aperture to an indicated 1 stop smaller (same effective aperture) for the no-TC shots, to get the same DOF. The result is the ISO for the TC shots was about 2/3 of a stop higher due to the shutter speed. My thought is without the TC, I can hand hold at a lower shutter speed.

Now I've read that the SR doesn't know about the TC, so it sets the shutter speed wrong. I'm wondering if that is the reason for the difference in quality, so I'll need to repeat the test on a tripod. I'm not sure I care about that test, though, because I want to shoot hand held, so I may just have to cough up the funds for a longer lens.

As of now, I'm thinking this is going back to B&H. Kind of sad because I waited a month and a half for it to be back ordered. But it doesn't seem sensible to carry around a TC when I can do better cropping in LR.

A final comment, that Flickr page is one I set up just to post tests and things like this. My real Flickr page is here: assuming anyone cares!

08-04-2013, 07:46 PM   #2
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The real test is with lens at 200mm and 1.4 TC--equal to 280mm, vs. lens at 200mm and no TC and then crop the image so it is the same as 280mm. Generally with a good prime lens, and sharp f/stop, the TC will win. With a zoom--maybe/maybe not. Many people believe the lens w/o TC will still be better cropped. I think generally the w/ TC will be better--but only if the lens is good and not wide open.
08-04-2013, 07:47 PM   #3
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My apologies--you did what I suggested.
08-04-2013, 08:00 PM   #4
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By setting the lens 1 stop more closed down w/o the TC you make the comparison complicated--as you may have simply put the lens at a sharper f/stop. I suggest you use the same f/stop and a tripod--and use an f/stop where the lens is sharpest--generally (w/o your experience or testing to give a better value) 3 stops closed down.Actually w/o a tripod at such a long FL it is not a fair test. No SR and mirror up if you can. The TC is tough to use well and it causes some problems--but if everything is optimum--does it let you do something you couldn't have done. Anyway w/o paying for and carrying a bigger/more $ lens. Except for closeup where the TC will generally win even over a longer lens.

08-04-2013, 08:05 PM   #5
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A TC is always gonna cost ya something - that's a given. So why would you make a shot with a TC if there was any non-TC solution to get the shot you want? To me, that makes this sort of comparison meaningless since there would be no other image to compare it to.

I don't use a TC as a "magnifier". Cropping suffices for that purpose with similar trade-offs in IQ.

I use it to change perspective/POV when I don't have an equivalent FL available, to provide stand-off room for close-up situations when no other non-TC solution is at hand, or to reduce the size and weight of the kit. In that case, there's no comparable lens FL to confuse the issue so I have made the best possible choice of lenses at that point in time.

If you can't determine when and why a TC would be your best available solution you don't need one -- unless IQ comparisons are the sole point of your photography.

If you have nothing to compare it with, you have an incomparable image regardless of its absolute quality. The only criteria then is -- does it satisfy you?


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