EM5 focus slow with Zuiko 12-60

I bought the MMF3 adapter for my EM5 so I can use my E5 lenses.

I've experimented mostly with the Zuiko 12-60 SWD F/2.8. I'm sitting in my living room (daytime), pointing at random things, and it's about 2x slower. Put the same lens on the E5 and it's much faster. The EM5 with the 12-50 kit lens is very fast.

I have the latest firmware installed on everything.

Suggestions?

Rick

Sadly all the 4/3rd's glass is slow at best when it comes to AF

Great optics, but the added size/weight and slow AF make most of them not really worthwhile compared to the m4/3 counterparts

50-200 would be a heck of a nice tele if it wasn't so slow at focusing

Yep, have the 50-200 as well. It's a great lens.

I'm an optical engineer, and I'd really like to know WHY the AF is slower with that lens/body combination. Sure I know that contrast focus is slower, but the EM5 with the kit lens is very fast, so there's more to it than that.

Maybe the EM5 simply doesn't have enough power to run the motors in the bigger lenses at full speed?

No I don't believe it's power related, and contrast based autofocus isn't "really" slower than PDAF. Marginally at most(my G3 focuses as fast or faster with a quick lens than almost any DSLR I've used for most circumstances).

The issue, from my understanding, has to do with the relative designs for motors of PDAF lenses and CDAF ones.

CDAF works by making tons of microadjustments for precises focus. It needs a motor than can make very fast, small changes.

PDAF is more about calculating focus and just getting the lens elements where they need to be. More linear in that sense, less of this quick "micro-hunting", so motors in them aren't designed for the microadjustments of CDAF. I've heard some even say that it's damaging for the motors in those lenses to work with CDAF.

Personally, after trying a 14-35 f2 and 12-60 f2.8-f4 on my G3, I'd rather just use the lenses in manual focus.

luomoalto said:

Yep, have the 50-200 as well. It's a great lens.

I'm an optical engineer, and I'd really like to know WHY the AF is slower with that lens/body combination. Sure I know that contrast focus is slower, but the EM5 with the kit lens is very fast, so there's more to it than that.

Click to expand...

Because the 12-60 and 50-200 Zuikos are designed for use on the phase-detection AF DSLR's and these micro bodies utilize contrast detection AF. Already sold both my 12-60 and 50-200 SWD's because they were frustrating (especially the 50-200) to use with the micro bodies and replaced them with micro optics, and I personally think the AF in the micro lenses with internal focus (MSC) system focus faster on my E-P3 and E-M5 bodies than either SWD Zuiko focussed on my E5 DSLR, which is also now gone.

luomoalto said:

Maybe the EM5 simply doesn't have enough power to run the motors in the bigger lenses at full speed?

Click to expand...

Nope, power is not the issue at all. Those SWD Zuikos focused equally as fast on the E620 DSLR I owned prior to the E5, and the E620 uses the exact same battery as the Pen bodies like my E-P3. Exact same power source, much different AF performance. It has everything to do with them being made for use on phase-detection AF bodies instead of contrast detection.

--
"There's shadows in life, baby.." Jack Horner- Boogie Nights

I've designed various types of autofocus systems for machine vision systems and microscopes. Contrast focus is inherently slower because you have to scan the lens through focus, and read out the sensor many times. Theoretically you can change the focus in discrete steps (step focus, read sensor, step focus, read sensor, etc) or scan the motor continuously while reading out the sensor (in that case the two have to be well synchronized (usually with an encoder) so you know which frame corresponds to which focus position). I'm pretty sure Oly uses the stepping method simply because you can hear it. Either way, once you have the data (multiple frames vs focus), you simply look for where the edges are sharpest (mathematically, you calculate a derivative). Then you tell the lens to go to that focus position. Often you will have a two step process - scan a large range to roughly find focus, then scan a smaller range to get it more precisely. If the light level is low, then the noise level is high, and you may have to try again - which is when the "hunting" begins.

A phase system has separate, discrete, sensors to measure where the focus is. It's faster for two reasons: 1) The lens doesn't have to scan through focus because the different sensors are themselves focused so that the combined signals from them tell you where best focus is. That's the most common way to set up a phase system - details vary, and there are many tradeoffs to be made when designing such a system. 2) These discrete sensors respond MUCH faster than the time it takes to read a bunch of frames from the CCD.

Contrast systems have a hard time getting below 1 sec. Phase systems can be 5 to 10x faster.

The battery isn't the limiting factor. Even an AA could supply enough power to move these motors quickly - just not for very long. But the circuitry on the EM5 may not be designed to supply enough power to move the big 4/3 lenses quickly. Maybe the problem is something in the MMF adapter?

Anyway, I can't see any good engineering reason for the problem. Whatever the problem is, I think Olympus missed it, and it's really a shame. I'm hoping that it's NOT a motor power issue, because then there's a chance it can be fixed in firmware.

napilopez,

Yes, maybe the motor design has something to do with it. Do you know if they are steppers or servos? Steppers, as you might expect, are better at taking small steps. But they can move continuously as well, you just need a good motor controller.

It may be hard to get a servo to make the discrete steps needed for the contrast focus. But if you put an encoder on the lens, then you don't need to worry about that, you just move the lens continuously and sync it with the sensor readout as I mentioned in the other post... Maybe that solution is too expensive for these lenses/cameras but it's certainly possible to do it that way...

The 12-60 is my bread-and-butter lens. I can't do without it, and S-AF with my E-620 is just so-so (in poor light) and nearly unusuable in C-AF.
--
http://www.flickr.com/photos/photography-by-thomas/

luomoalto said:

Yep, have the 50-200 as well. It's a great lens.

I'm an optical engineer, and I'd really like to know WHY the AF is slower with that lens/body combination. Sure I know that contrast focus is slower, but the EM5 with the kit lens is very fast, so there's more to it than that.

Maybe the EM5 simply doesn't have enough power to run the motors in the bigger lenses at full speed?

Click to expand...

It's a question of what kind of motor is ideal for what kind of adjustment.

Phase detection AF has been around in SLR systems since the 1980s, and only very recently have any DSLR lenses been designed to work well on CDAF (the first Olympus DSLR that even had CDAF was released in 2008).

Phase detection AF is able to predict how far and in what direction it needs to move the lens to achieve focus. This means it can use a motor that can move very fast, which will usually be a low-torque motor that can't stop very precisely. It takes a long time and distance to start up and stop and reverse the lens motion, which is what causes slow focus speeds with CDAF cameras.

Contrast detection AF has no predictive properties (to date). It's dependent on constant sampling from the image sensor to determine if the image is at peak contrast. This means the motor needs to be able to stop and reverse direction very quickly and precisely. This requires a higher torque motor, which tends to be slower.

CDAF motors work without fault on PDAF systems. But PDAF motors are less effective on CDAF systems. Each respective AF method can be just as fast as the other, but the high speed/low torque motors that most DSLR lenses use (especially in the case of supersonic drive lenses like the 12-60, and lenses that were designed before CDAF became a consideration in DSLRs) aren't very effective on CDAF systems, but can be the fastest lenses for PDAF systems.

There still aren't many CDAF-compatible lenses for DSLR systems. There's one from Pentax, one from Canon, a handful from Olympus. DSLRs just don't have the same priorities as the rest of the market.
--
http://www.photoklarno.com