Noob question: Difference between contrast detect AF & phase detect AF?

Started Aug 24, 2013 | Questions thread
dave gaines
dave gaines Veteran Member • Posts: 9,829
Difference between CDAF & PDAF

That's a good question. Goblin's answer is a good analogy but a little bit scant on details. It's fairly entertaining too. I like his post. I'll see if I can be more succinct in simple terms.

CDAF focuses directly on the sensor. It uses a distinct lline in the image by moving the lens until that line obtains the highest contrast, which coincides with the sharpest focus. It works best with lenses designed to move with these focus motions. It does not work well with the Olympus E-series DSLR, 4/3 lenses. CDAF has in the past been used primarily on P&S cameras and is now used on micro 4/3 cameras like the OM-D E-M5 and E-P5. It works well on Pen and OM-D cameras when micro4/3 lenses are used but works very slowly with Olympus's 4/3 lenses built for DSLRs.

PDAF uses a sensor placed in the light path created by the mirror. It works basically the same way with any DSLR, or single lens reflex camera, like the E-5. It uses two sensor points to detect the phase difference in the light path and find the exact distance. Then it moves the lens once to the correct focus point. Some of the 4/3 lenses tend to pass this point and hunt for focus several times before locking focus. The 50 mm f/2 macro is an example of this. Macro lenses of all types and brands tend to struggle to find focus. The Olympus 14-35 mm f/2 lens is another lens that tends to cycle back and forth around the focus point before it locks on. Several of the SHG lenses do this. PDAF is currently the fastest method of focusing and is found on the top DSLRs from all brands.

The current rage of discussion here is about PDAF occuring on the sensor without a mirror. Several brands have tried to make it work with varying levels of success. It's still not as fast as PDAF with a mirror and seperate sensor. With the two current top m4/3 cameras, the OM-D E-M5 and E-P5, focus of 4/3 DSLR lenses is very slow, except with a few lenses that were also designed for CDAF.

There are a few good sources out there on the web for info about PDAF and CDAF. This is just a broad overview of the differences.

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