I dont get this TC loss of auto focus issue

rodderstrotter

rodderstrotter

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Looking to get a 1.4x TC for Pentax 55~300DA, reading reports I understand that I will use 1 stop, can live with that. What puzzles me are the reports that say the lens AF will be adversly affected..... why, when my lens will easily AF when stopped down to f22!

Can someone explain what I am missing.:help:

Many thanks.
 
Auto focus is always done wide open.

The lens doesn't stop down until you take the picture.
 
Your lens is not at f22 when the camera is focusing.
The camera will focus with the lens wide open and then stop down to f22 just at the photo is about to be taken.
 
The camera will focus and meter with the aperture wide open and will only stop down when you take a photo. So if you're shooting say a f4 lens at f22 the camera will focus using the full f4 then only stopdown briefly for the exposure then open back up again.

As the max aperture gets smaller the camera will find it harder to AF, so by adding a TC you make focusing worse.
 
rodderstrotter said:
Looking to get a 1.4x TC for Pentax 55~300DA, reading reports I understand that I will use 1 stop, can live with that. What puzzles me are the reports that say the lens AF will be adversly affected..... why, when my lens will easily AF when stopped down to f22!

Can someone explain what I am missing.:help:

Many thanks.
Click to expand...

:bonk: Durrr, showing my ignorance! Thanks to Squishy and Sunraygp for putting me straight, now I get it.

How does one know at what f nbr a particular lens will start to have a problem with AF?
 
I think it varies with make / model of camera (IE the specific AF sensor used), but typically f/5.6 will be OK and f/8 will probably fail.
 
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