CP+ Rumor Roundup.....

Started Feb 4, 2014 | Questions thread
MightyMike Forum Pro • Posts: 37,540
Re: My number one hope is for the Sigma 18-35mm to be announced available for sale in k-mount.

paulkienitz wrote:

MightyMike wrote:

I think you're making a big stink out of nothing, what is not to work? The lens provides the camera with its FL, one assumes the TC alters the FL and therefore SR will be on par with the FL. Honestly with my testing its better to have the SR a little weaker than par with the focal length if you want more SR stops at long FLs. Its not likely AF will change, 90% of third party lenses are still screw drive, screw drive TC's still work fine, one would assume Pentax did the SDM correctly too so nothing to fail there. about the only area where things might fail is the in lens corrections but they don't even exist for third party lenses. so correct ID or not what what will go wrong that isn't already wrong before the TC is available?

The main problem with an unchipped TC isn't antishake or distortion correction, it's accurate autofocus. Phase detect autofocus works by measuring a focus error and precalculating how far to move the lens to correct it. Then, if necessary, it can do a second finer correction once that movement is complete. But the main large correction is handled not by a process of moving it until it gets sharp (like contrast-detect), but by judging in advance how far it has to be moved. With a TC on, this estimation is thrown way off. This is why autofocusing with a TC often involves a lot of hunting, and/or prefocusing manually to get it pretty close, before it'll work.

Presumably the new one will clear this problem right up with DA* lenses, but with third-party lenses it won't be able to, so with them, the autofocus will still hunt a lot.

-- hide signature --

"A good photograph is knowing where to stand." -- Ansel

You're right and wrong... much of the hunting is due to lost contrast and altered spherical aberrations... TC's are essentially supposed to be focally corrected to infinity, if not you couldn't achieve infinity focus of proper close focus. TC's are therefore focus neutral... if your assertion was correct then all TC's for Pentax would fail, clearly there are plenty of us on the forum that have demonstrated over and over that is not the case. I've never had hunting unless the light level was low, the contrast was low on the subject or the combination of TC and lens didn't work well. Slow focusing comes with screw driven TC's as there is a gearing mechanism in them of 1:2 or 1:2.5 depending on the converter.

So if you're getting hunting and bad focus then its not the TC, nor the camera or the lens, its the combination, TC's don't throw any estimation way off.

-- hide signature --

Mike from Canada
"I am not a great photographer! God is a great creator! All I do is capture His creation with the tools He has provided me."
'I like to think so far outside the box that it would require a telephoto lens just to see the box!' ~ 'My Quote :)'

Post (hide subjects) Posted by
Keyboard shortcuts:
FForum PPrevious NNext WNext unread UUpvote SSubscribe RReply QQuote BBookmark MMy threads
Color scheme? Blue / Yellow