[starbase218]

The motor has to move a greater distance with the DA*50-135 then with the DA18-135, so the motor might indeed turn (slightly) faster but because of the greater distance the focus mechanisms travels the lens focus slower.
You get it?

The DA*16-50 has a shorter focus throw so it focus faster, if the focus throw is comparable with the DA18-135 those might be a good comparison.

The shifting focus with the DA*50-135 might be because of the f/2.8 making the contrast less clear.

Ah, but why would anyone be interested in the speed of an AF motor, whether it is SDM or DC, if it does not tell me how fast the lens focuses? I mean, if Photozone is only saying something about the motor inside the lens, then that might be relevant to some people who are interested in that stuff, but not to its target audience.

Also, it's not just about motor speed and focus throw, but also about the ratio between the revolutions of the focusing ring and those of the motor. With the DA 35/2.4, for example, the ratio is so high that the shaft does not even make 3 complete turns to cover the entire focus scale. The DA 70/2.4, on the other hand, needs about 6 revolutions, but its focus throw is not nearly twice as long (it is only slightly longer). I know this concerns the in-body motor, but the same principle applies to any motor, really. So even if I know the focus throw and relative motor speed, it still does not tell me how fast the lens focuses.

Anyway, I have the 16-50, and I don't think it focuses as fast as the 18-135 in that Youtube clip.

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Last edited by starbase218; 09-02-2012 at 10:04 AM.

[Anvh]

Also, it's not just about motor speed and focus throw, but also about the ratio between the revolutions of the focusing ring and those of the motor.

uhhh that's what i'm saying....

so the motor might indeed turn (slightly) faster but because of the greater distance the focus mechanisms travels the lens focus slower.

[starbase218]

uhhh that's what i'm saying....

At first you only mentioned focus throw, which is how far you have to turn the focus ring to get from minimum to infinity, so forgive me if I did not interpret "focus mechanism" in your next post as you intended it. Besides, in that post, you also mentioned that "the DA*16-50 has a shorter focus throw so it focus faster", which seems to indicate that was not what you were saying. It may or may not focus faster, but that is not the result of it merely having a shorter focus throw.

Funny though: in the original post by Ray Pulley, he also interpreted the Photozone comment as relating to autofocus lens speed, not just autofocus motor speed. As we have now established those are two seperate things. Still, I'm doubtful if the Photozone comment in the 18-135 review actually referred to the rotational speed of the motor used to drive the autofocus mechanism, because a) it would be difficult to ascertain, and b) it would be of no relevance to photographers.

[RobA_Oz]

At the expense of prolonging the discussion, it isn't just motor speed and focus throw that govern the speed with which a lens mechanism can operate. If I understand the engineering correctly, a piezo motor is like an AC synchronous motor, in that it will try to operate at a single speed, governed by the pulse rate of the drive circuits. An AC synchronous motor does the same thing, governed by the frequency of the power supply. Both therefore will absorb more power in their quest to maintain that speed, as load increases. If the power isn't available, then the speed will fall (slip). The difference is that the slip in a piezo motor results in sliding friction (which may be the source of the SDM motor issues, incidentally). Torque changes dramatically with slip in an AC synchronous motor, because of the magnetic pole misalignment, whereas with the piezo motor, the difference is limited to the fall in friction from static (no slip) to dynamic (slip) friction.

A DC motor (and I assume that the DC motor Pentax uses in the 18-135 is like any other DC electric motor) is a rather different device that has full torque available over a wide range of speeds, and doesn't rely on pulses or frequency to govern speed. Speed in a DC motor is governed almost entirely by load, including losses, rather than a particular driving frequency. Given the short focus throw of the 18-135, I would assume that much of the drive time is spent in acceleration and deceleration, and that the advantage of the DC motor is that it can accelerate faster than a piezo motor and therefore achieve the required movement faster. It also won't suffer from wear due to sliding friction.

That's as I understand it, of course. My background is in general engineering, rather than fundamental motor design, so anyone here with a better knowledge may feel free to correct me.

[Ray Pulley]

Definitely not. The 18-135mm uses a simple, small DC motor coupled to a plastic gear train. You can see it when looking through the back of the lens when it is fully zoomed out. It is a cheap and simple implementation. Also, when manually focusing you are actually back-driving the gears and motor..

Apparently there is a lot of misinformation about the AF drive of this lens, but an eye-witness account of the real motor is good enough for me.

One more reason to NOT pay almost $500 for this lens, although I do not recall seeing reports of motor failures with this design.

Ray

[RobA_Oz]

Apparently there is a lot of misinformation about the AF drive of this lens, but an eye-witness account of the real motor is good enough for me.

One more reason to NOT pay almost $500 for this lens, although I do not recall seeing reports of motor failures with this design.

Ray

A personal decision, obviously, but I have to say that the AF drive in my DA18-135 works quickly, quietly and reliably, DC motor not withstanding. The SDM in my DA*16-50 works not quite as quickly, maybe a touch more quietly, and just as reliably, but only after it's been roused from its slumber. The 18-135 might have a plastic gear train, but so does the SDM in the 16-50, IIRC (I have dismantled mine to fix a stuck SDM drive, but it was a while ago).

As for the other reasons, it clearly depends what you want, but I doubt you'll find better value for money in another K-mount equivalent. Of course, the DA*50-135 has better optics and is faster, but it is less versatile in FL range and is nearly twice the price. Like I said, it depends what you want.

[Fontan]

Oh no. Another Sudden Death Motor design scheme. Oh no. Please. Oh no.

[LamyTax]

Oh no. Another Sudden Death Motor design scheme. Oh no. Please. Oh no.

Yes, yes. And the new AF will be slow like a duck, too. Because, why? Pentax, that's why.
Why do you even bother? :P (yes, sarcasm).

[RobA_Oz]

The Pentax SDM motor is, as far as I can tell, just another piezo-electric motor. Call it an ultrasonic or a variety of other names, but they all work on the same principles. Again, AFAIK, Pentax doesn't manufacture their own motors, and the seeming decline in the number of failures of later SDM drives indicates that they've got a supply or design selection problem under control.

[Anvh]

At first you only mentioned focus throw, which is how far you have to turn the focus ring to get from minimum to infinity, so forgive me if I did not interpret "focus mechanism" in your next post as you intended it. Besides, in that post, you also mentioned that "the DA*16-50 has a shorter focus throw so it focus faster", which seems to indicate that was not what you were saying. It may or may not focus faster, but that is not the result of it merely having a shorter focus throw.

You will see that how far the focus ring turns and how many turns the motor needs to make are connected to eachother.

[Anvh]

The Pentax SDM motor is, as far as I can tell, just another piezo-electric motor. Call it an ultrasonic or a variety of other names, but they all work on the same principles. Again, AFAIK, Pentax doesn't manufacture their own motors, and the seeming decline in the number of failures of later SDM drives indicates that they've got a supply or design selection problem under control.

It are not the motors that fail per se, also the AF mechanism have problems but Pentax is the only one the have lenses with 3 different ways of focusing so very complex.

[starbase218]

You will see that how far the focus ring turns and how many turns the motor needs to make are connected to eachother.

I have already seen this is not the case (see what I wrote about DA 35/2.4 vs DA 70/2.4 and ratios). But maybe it's best to agree that we disagree.

Anyway, I don't care if Pentax puts plastic gears, metal gears, a piezo-electric motor, the band AC/DC or a mouse in a wheel in their lenses. Maybe they use the force to move the AF mechanism (I hope that doesn't result in photos from the dark side though). I don't care if it looks cheap or expensive. All I care about is that it does the job I want it to do, and that it isn't significantly slower, less reliable, more expensive or noisier (though I personally don't care too much about that) than what the competition brings out. If Pentax thinks they need to innovate and develop this kind of technology in order to do that, then they have my blessing.

OTOH the act of registering a patent does not mean they are actually going to use this technology. I remember a patent for a DA* superzoom lens, but it was never developed.

[Kunzite]

Maybe they use the force to move the AF mechanism (I hope that doesn't result in photos from the dark side though).

Bingo.
That explains why it works better for some people (those in which the Force is strong) than for others.
Fortunately, while taking photos from the dark side still happens, it happens exclusively with the part of Pentax which stayed with Hoya (i.e. the medical division).

About such patents, I'd guess those SDM micromotors aren't really suitable for longer lenses.

[Anvh]

I have already seen this is not the case (see what I wrote about DA 35/2.4 vs DA 70/2.4 and ratios). But maybe it's best to agree that we disagree.

Have read over that and i don't have either of those lenses so i've no idea.
From what i know and have seen the lenses with a short focus throw are also the faster lenses to focus.

[starbase218]

Have read over that and i don't have either of those lenses so i've no idea.
From what i know and have seen the lenses with a short focus throw are also the faster lenses to focus.

I only used these lenses as an example because I own them and I actually tried it before posting the results. So the question is if you believe me on my beautiful grey-green eyes. (not offended if you don't; I hardly know you)