If an f6 lens with a 2x doubler won't autofocus why will a lens stopped to f22 still focus?

[frogpoet]

I do not understand ?  If a telephoto zoom lens rated f4-6.3 wont autofocus if connected to a camera with a 2x converter why a lens stopped down to f22 will autofocus?

[RobertTheFat]

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@frogpoet wrote:

I do not understand ?  If a telephoto zoom lens rated f4-6.3 wont autofocus if connected to a camera with a 2x converter why a lens stopped down to f22 will autofocus?

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Because virtually all lenses these days are "preset" lenses. You set (or the camera sets) the aperture, but the lens doesn't actually stop down to that aperture until you press the shutter button. So all the viewing and focusing is done at the lens's maximum aperture.

That, BTW, is why virtually all DSLRs have a button that lets you temporarily stop the lens down to its preset value. The idea is to let you check the depth of field, which depends on the aperture. Of course that works only if the resulting image in the viewfinder is bright enough for you to see such detail.

[frogpoet]

OK, this I understand, however if a 2x converter doubles everything then that puts the f6 rate up to f12-right?

  Are camera AF senser units so precise (dumb/picky) that they wont AF with less light? Sure it will take more time to achieve the focus but shouldnt they sooner or later get it? In the "old days" film camers could get a focus even when you manualy set the aperture down to 24 or tighter. Can the modern wonders not achieve what the "old guys" could do? Or are camera mfgrs just having them set so you have to buy their outragesly priced lenses to achieve the longer lens effect? :>)  

[cicopo]

No it dos not double the f stop number. It adds 2 stops (1.4 adds roughly 1 stop) in light reduction & yes the cameras need lots of light for them to AF.

[frogpoet]

So AF is only valuable for fast changing scenes or snapshooting situations?  Why bother having it if you can focus faster manualy than the in AF.  OH, well, a fella can dream of having his cake and eating it too.  :>)

[ebiggs1]

"Why bother having it if you can focus faster manualy than the in AF"

You are the MAN, if you can beat the AF a of modern DSLR at focusing.  Certainly you can find limits to what they can do but generally they will put the hurt on to you in speed of AF.

Some higher end camera can focus with a f8 lens.  My 1D Mk IV rarely misses focus no matter where I use it.  Of course there is that certain time or two but not often.

Remember a 2x tele converter changes the max f-stop of the lens.  And that is what the camera uses.  Some cameras get a signal from the lens, or converter, that tells them what the f-stop is.  They will not even try if it is less than f5.6 on most and f8 on my 1 series.

You can put tape on the lens contacts to stop this and sometimes the camera will AF with much slower lenses.

[cicopo]

Maybe this might help. It's from today's weekly newsletter.

http://www.bhphotovideo.com/explora/photography/tips-and-solutions/how-focus-works?cm_mmc=EML-_-News...

[RobertTheFat]

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@cicopo wrote:

No it dos not double the f stop number. It adds 2 stops (1.4 adds roughly 1 stop) in light reduction & yes the cameras need lots of light for them to AF.

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Actually, doubling the f-stop number is exactly what adding two stops does. Two stops up from f/5.6 is f/11; two stops up from f/6.3 is f/12.7; etc. Any variance from exact doubling is roundoff error.