E-mount reverse engineering

Started Jul 12, 2015 | Discussions thread
OP Entropy512 Veteran Member • Posts: 5,805
Plots with additional data (Another Techart III issue)

OK, I've plotted a few additional things:

1) Focus position from the AF cycle start command (packet ID 0x27) - bytes 26/27

2) Focus position from the 0x34 (focus complete?) lens status packet - bytes 11/12

3) Focus position from the 0x32 (focus motor changing speed/direction?) lens status packet - bytes 9/10 and 26/27

4) Focus motor speed (steps/second) where available (e.g. when I received two consecutive 0x30 status packets, so no data is shown when a 0x32 or 0x34 pops up)

Focus pos1/pos2 from 0x30 are red/blue lines

Focus from AF cycle start command (0x27) are blue triangles

Focus from 0x32 packets are yellow/green triangles

Focus from 0x34 packets are brown triangles

Focus motor speed is a black line

First, the SEL55210 @210mm (this is the exact same data as the previous graph, just plotted differently)

SEL55210 @210mm focusing on wire shelf

Note that the focus motor speed is changing, in approximately powers of two ratios. Fastest rate is approximately 5275 steps/sec, next speed down is approximately 2630 steps/sec, next is 1315 steps/sec, then 900 steps/sec (This one isn't half - closer to 2/3 - although there's some variation here, some of it is only 840 steps/sec), then 420 steps/sec

During the focus cycle, it typically starts at the 1315 rate, then if a reversal is necessary, it jumps to the 2630 rate and changes direction.

Also, the commanded focus position in packet 0x27 is a bit strange. When it commands an initial jump forward, the lens moves to approximately twice the distance commanded then immediately reverses. When the lens initially jumps backwards before moving forwards, the commanded focus position is extremely high (the blue triangles near the top of the graph)

Zoomed in version for easier viewing of three AF cycles:

SEL55210 @210mm focusing on wire shelf - zoomed in version of above chart

Now for the Techart III + EF50/1.8 STM combo - this is the exact same data as in my previous plots, just plotted differently:

EF50/1.8 STM @f/1.8 + Techart III in Fn mode focusing on computer monitor - varying subject distance

Look at the motor speed - it's hitting +/-7000 steps/sec, and the only cases where the graph shows lower speeds is where the focus movement is so short the motor doesn't have time to accelerate to full speed. It's clear Techart is not implementing the E-mount motor speed controls here.

It's clear that the body is having trouble keeping up with the lens, since the body updates at 60 Hz. The end result is overshooting/hunting, so the faster motor is actually leading to slower AF in terms of time-to-lock.

Now for the Techart III combined with the EF85/1.8 USM, which is notorious for hunting severely and not locking focus when used with the Techart III:

EF85/1.8 USM @f/1.8 + Techart III in Fn mode, focusing on monitor. Varying subject distance

Yes, that's right, +/- 30000 steps/second - this lens has an insanely fast motor and the body simply can't keep up with it. Also, it's hard to see in the graph, but there appear to be spurious 0x32/0x34 packets coming from the lens mid-cycle - I need to look through the raw data to see if there are any command packets triggering these, but they definitely don't look right on the graph. They may be the result of the body attempting (and failing) to command a new focus motor speed.

So it's clear the Techart III does not properly implement motor speed control. This is supposedly a capability of the EF mount via command 0x50 - https://pickandplace.wordpress.com/2011/10/05/canon-ef-s-protocol-and-electronic-follow-focus/#comment-59 but it's obviously not being implemented or not being used correctly in Fn mode.

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