Pentax-DA 1.4x AW AF Rear Converter
Autofocus
On this page, we'll be taking a look at how the 1.4x teleconverter affects autofocus speed and performance. We performed speed tests with the following two lenses mounted on the Pentax K-3:
We found that the autofocus speed in daylight with the SDM lens is the same with and without the rear converter.
With the D FA 100mm screw drive lens, AF was about 20% faster without the converter. Thus, we can conclude that the teleconverter adds a slight amount of friction to the screwdrive mechanism.
The video below contains footage of some of our tests:
Autofocus With Slower Lenses
The rear converter is designed for lenses F4 and faster. That didn't keep us from trying it out with our favorite travel companion, the smc Pentax-DA 18-135mm F4-5.6 zoom on a K-3. What we found was the following:
Viewfinder mode (phase detect autofocus):
- Even in daylight the autofocus function was sluggish from about 70mm to 135mm focal length (as set on the lens). Sometimes the lens would lock on, but more often it would not. The farther away the subject was, the worse the autofocus performance.
Live view mode (contrast detect autofocus):
- Autofocus was accurate and about as fast as with no converter.
These findings are in line with what Pentax recommends for slow zoom lenses.
Conclusion
The rear converter has no significant effect on autofocus speed at normal light levels as long as lenses F4 or faster are used. In dim light the loss of one F-stop of light with the converter will result in slower autofocus, as the camera will require more time to make up for the lost stop of light while making its calculations.
For lenses slower than F4, contrast detect (live view) AF works best while viewfinder AF may be unreliable.
