theSuede wrote:
Pulse counter in parallel over the input from the encoder, right at the logic chip on the lens mainboard. One full rack > 6120 pulses. This was the 70-200/2.8VRmk1, not the newest version. Since you can often get at the mainboard just by disassembling the rear outer barrel and mount of the lens, this is the easiest point of access. But some lenses have "access windows" over the MR head in the lens barrel.
Actually I'm a bit confused about how to interpret the result - since the polarity (direction of the motion) is derived from having two heads with a half-pulse offset the actual resolution is half of 1/6120, or 1/12200. Close to 16-bit resolution.
Nikon has never published their actual specification, so most of it is stuff I can't write about here. Reverse engineering isn't illegal per se, but you can get in trouble for publishing your results....Show more →
Does the lens servo actually receive a unique pulse for each step it must travel across its range for a full-rack focus, ie the pulses are strictly linear from the lens firmware? I've never seen a data sheet for a lens motor but I always assumed there was a non-linear acceleration/deceleration factor even at the lowest levels of the motor's register interface.
snapsy wrote:
Does the lens servo actually receive a unique pulse for each step it must travel across its range for a full-rack focus, ie the pulses are strictly linear from the lens firmware? I've never seen a data sheet for a lens motor but I always assumed there was a non-linear acceleration/deceleration factor even at the lowest levels of the motor's register interface.
That would make sense *to me* if he were reading off of some motor control output, but he was reading off of the encoder right? I have never heard of a non-linear encoder.
(you guys are way over my head at this point, but I am loving the discussion)
theSuede wrote:
Pulse counter in parallel over the input from the encoder, right at the logic chip on the lens mainboard. One full rack > 6120 pulses. This was the 70-200/2.8VRmk1, not the newest version. Since you can often get at the mainboard just by disassembling the rear outer barrel and mount of the lens, this is the easiest point of access. But some lenses have "access windows" over the MR head in the lens barrel.
Actually I'm a bit confused about how to interpret the result - since the polarity (direction of the motion) is derived from having two heads with a half-pulse offset the actual resolution is half of 1/6120, or 1/12200. Close to 16-bit resolution.
Nikon has never published their actual specification, so most of it is stuff I can't write about here. Reverse engineering isn't illegal per se, but you can get in trouble for publishing your results....Show more →
Have you ever tried doing this on a sigma 50mm or sigma 85mm? I would be VERY interested in the results.
The encoder strip is a linear type, i.e you have the same linear resolution at one end as at the other.
The motor in itself usually has three driving speeds, which is governed by the frequency modulator controlling the HSM ring motor. This motor is purely frequency controlled - as long as there's enough voltage to drive the piezo elements into full actuation (about 5V I think) the frequency dictates the "tick/tock" of a piezo motor. Each tick is roughly the same travel distance.
You can modulate the driving speed or tick travel distance by lowering the voltage, but only by a little bit - then the motor stalls completely. That point is when the square "cogs" of the actuator no longer connect/disconnect as they should, and usually that seems to be at about -10 or -15% rotation speed.
Acceleration and deceleration is handled by two things in most setups - each lens has a band-gap compensation in firmware (programmable) that controls the amount of overshoot/undershoot. And then you also almost always set speed to the second lowest actuator speed just before the stopping point, to lower this error margin. Remember that the motor in itself is a secondary part of the control chain, it only does what the position control strip tells the motor control to do.
...........
On Topic:
Can't wait to try the 35 out for real. Both according to the 24MP images I have and the calculated MTF it should be better than the Nikkor wide open, and the Nikkor is better than the Canon. The question is how good it gets at F2.0, since that's where I need the performance. The Nikkor sharpens up beautifully at just a few thirds above wide open.
Oops, no, Allen's camera in Philly. While on the phone, they had just rec'd 10 of the new 70-200 f4 lenses and all were sold that day. Great place to buy, and he's always on the money about when stock comes in.
I received the lens Friday. and gave it a test run. Very cold outside, so decided to take my toddler to an indoor playground at the mall around 9 pm. Here are some quick and dirty results.
-All shot using D800E
-No fill flash applied
-No sharpening or noise Reduction applied in post
-Some minor cropping and some bringing out of shadows in post only (shot indoors and no flash). I added minor saturation in a couple as well.
-All shot at 1.4
In terms of sharpness I dont have any complaints. If I missed the focus, it was my fault (thin DOF, slow SS of 1/160 with a toddler). Again, I have applied NO sharpening to any of these in post.
Also, keep in mind that I am an amateur so if things look bad, most of the time it is going to be my fault, not the gear's.
