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07-28-2013, 08:24 AM   #1
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Sigma 1.4x EX APO DG Teleconverter -- Corner Performance

I'd appreciate input from owners of the Sigma 1.4x TC -- the one with the protruding lens elements that only matches certain long focal length lenses -- regarding its corner performance.

I've had the chance to try one and unless I stop down to f/8 the corners don't look that great. At f/6.7 they reach somewhat acceptable level, but at f/5.6 or f/4 (on a Sigma 100-300/4 EX) they look pretty bad to my eyes.

The centre looks excellent, but the quality degrades pretty quickly towards the edges. There is increased CA and pronounced fuzziness. I saw the same when combining the TC with my FA* 200/2.8.

I also have a Pentax 1.7x AF TC that shows better corner performance. In the centre it does not appear to be as good which is what you'd expect from the higher magnification.

Is this normal? Is the Sigma 1.4xTC really meant to be used at f/8?

I'd very much appreciate input from users of this TC.

07-28-2013, 10:34 AM   #2
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I used my copy on the Sigma 500mm f4.5. This is a birding lens where nearly everything gets cropped anyhow so I wasn't that concerned with corner sharpness. I confirmed with more-or-less rigorous tests using a test pattern that the Sigma 500mm plus the 1.4x TC produces an image with slightly higher center resolution than the Sigma 500mm w/o the TC (but with the image cropped to the same FOV as produced with the TC). Any TC that fails that test shouldn't be used at all, right, because you could do just as well by simply cropping. But even though the TC produced slightly sharper images than equivalent cropped images from the naked lens, I hardly ever used it because I would lose a stop to the TC itself and needed to stop down a stop to gain maximal sharpness. So there I was at f/8 when I nearly always needed to be a f/4.5

I don't have any experience with the two lenses you're using, but the general sense of the users of nearly all TCs across the camera brands is that they're sharper stopped down one stop, which means you're generally losing at least two stops to get a really sharp image. My decision after being seduced time and again by the prospect of increasing reach through a good TC is that it is generally better to shoot w/o the TC at lower ISO and crop.

I'll add that using TCs that pass through the electrical info on Pentax bodies creates another issue in that the camera doesn't know it is working with the longer focal length. Thus the shake reduction system is working with a parameter that is badly off. That leaves the best use of the Sigma TCs on Pentax to a tripod or fast shutter speed with shake reduction turned off, IMHO.
07-29-2013, 12:11 AM   #3
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QuoteOriginally posted by oeriies Quote
I used my copy on the Sigma 500mm f4.5. This is a birding lens where nearly everything gets cropped anyhow so I wasn't that concerned with corner sharpness.
First, thanks a lot for your reply.

I guess you didn't check the corner sharpness? I'd love to know whether I just tried a subpar copy or whether this fall-off of image quality is normal unless you stop down considerably.

I realise that stopping down helps with IQ in general and that corners are always weaker than the centre, but with no other lens or converter, I've seen such a quick and high degradation of IQ towards the edges at wider apertures. Maybe it was designed that way to optimise the centre performance in the knowledge that people will crop anyhow?

QuoteOriginally posted by oeriies Quote
I'll add that using TCs that pass through the electrical info on Pentax bodies creates another issue in that the camera doesn't know it is working with the longer focal length.
Yes, but I'd only use the TC on a tripod anyhow (with SR turned off).
07-29-2013, 06:13 AM   #4
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I use both the 1.4x and 2x sigma TCs with my APO-70-200/2.8EX (non GD non Macro)

As oeriies has said, most shots are taken with the intent to even crop in further so edge performance is not critical. Also, considering the very narrow DOF when shooting most subjects I find that corner performance is not really relevant in many shots because the subjects are NOT in the focal plane and therefore just disappear into the bokeh

Unless there is a real need to have corner to corn sharpness I doubt anyone really notices this issue if it exists

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