SDM speed (or lack thereof)

Started Oct 22, 2007 | Discussions
BTG308 Contributing Member • Posts: 941
SDM speed (or lack thereof)

This is an oddball LBA post in that there will be no pictures, but a brief video clip for those so inclined.

Last week, I finally took delivery of a slightly used SMC Pentax DA* 50-135mm F2.8 ED [IF] SDM lens. I got it from an eBayer who was switching brands. As I had the 1.20 firmware in my K10D, a video camera and a tripod, I figured I'd tape the lens while focusing back and forth and then measure the focus speed from the video clip.

I setup the video camera looking down on the lens' little focusing window and proceeded to half-pressing the shutter button (via cabled remote) with the lens cap on in the AF-S mode to make the AF system go all the way from infinity to close focus and back again. This way, I'd see the "raw" focusing speed by which the focus motors could drive the lens. I did this ten times for the screw-drive, flashed the firmware and tried it in SDM mode, fully expecting the SDM mode to be slightly faster than the screw-drive.

This was not so.

The SDM mode turns out to be quite a bit slower than screw-drive mode.

The average (there were practically no variations except for those caused by the video frame resolution of 1/25s) was 38 video frames for screw-drive and 49 for SDM. At 25 frames per second, this translates to 1.52s for screw-drive and 1.96 seconds for SDM, a 29% increase in time spent. Also, several frames (11) are used up at the close-focus stop, before the drive goes back to infinity. Correcting for this and only counting actual travel fime, the difference is a staggering 37% slow-down when using SDM.

Furthermore, it has been hypothesized that SDM could accelerate the lens mass faster, but the video clearly shows that the screw-drive get up to speed almost instantly, while SDM not only requires a longer time to accelerate, but also has a slower sustained speed.

The conclusion? Well, either my lens is seriously barfed, my screw-drive is amazingly souped up, OR there's no point whatsoever in getting a Pentax SDM lens for it's focusing speed since it simply does not deliver, compared to the screw-drive.

I have a short (4s, 18MB) video clip of two of the test runs here: http://kaukbacken.homelinux.net/files/50-135_SDM.avi

I also did a few more "real-world" focusing tests to see if the SDM would recoup some of it's speed while fine-adjusting focus, but as I mentioned earlier, its slow starts worked against it here too. I don't have any firm figures since this was more of a hit-and-miss affair and it's difficult to maintain the exact same conditions between test runs, but it looks like it's not faster - just much quieter and possibly slightly slower.

So I'd really like the option to turn SDM off in firmware, when speed counts - and turn it back on, when noise counts.

/ Richie - http://www.p-i-x.net

dotnik Veteran Member • Posts: 5,565
Re: SDM speed (or lack thereof)

I am surprised that it is slower. Theoretically it should at least be equally fast. I would also think that they could have been even faster if Pentax did not have to pay attention to the screw-drive mode. I think the price for mixing those technologies is speed.

Anyway even the DL is fast enough for the events I shoot and so speed would not be the reason for me to get SDM lenses. Silence would be much more valuable to me but as long as the shutter makes more noise than my AF then SDM will be of limited value to me.

In fact the only feature that would make by buy a comming Pentax DSLR and SDM lenses instantly would be silent camera operation. Otherwise the K10D / K100D fully covers my needs.

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(unknown member) Veteran Member • Posts: 6,759
Re: SDM speed (or lack thereof)

Not counting the pause period at either ends, did the SDM took longer to travel from one end to another? If so, sounds like the SDM being chosen was underpowered for some reason. Maybe not enough torque or power supply?
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JensR Forum Pro • Posts: 17,971
Re: SDM speed (or lack thereof)

I am surprised that it is slower. Theoretically it should at least be
equally fast.

Which theory? The theory "a bad ultrasonic drive may be worse than a good conventional drive" seems to be confirmed.

I would also think that they could have been even faster

"could have been even faster"? You mean "could have been fast"?

if Pentax did not have to pay attention to the screw-drive
mode. I think the price for mixing those technologies is speed.

I had thought the price for mixing those technologies is price. Who knows...

I also think that if you should be right, Pentax was ill-advised to focus on good non-SDM performance, instead of focusing on good SDM performance - if they could only realize one of the two.

Jens

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JensR Forum Pro • Posts: 17,971
Re: SDM speed (or lack thereof)

Hi Rickard!

Wow. Wow.

I setup the video camera looking down on the lens' little focusing
window and proceeded to half-pressing the shutter button (via cabled
remote) with the lens cap on in the AF-S mode to make the AF system
go all the way from infinity to close focus and back again. This way,
I'd see the "raw" focusing speed by which the focus motors could
drive the lens.

Do you think that these differences reflect the level of difference for real world shots, too? It cannot be outruled that the lens cap throws one or both of the AF systems into a special mode and thus the difference might be skewed and exaggerated.

I also did a few more "real-world" focusing tests to see if the SDM
would recoup some of it's speed while fine-adjusting focus, but as I
mentioned earlier, its slow starts worked against it here too. I
don't have any firm figures since this was more of a hit-and-miss
affair and it's difficult to maintain the exact same conditions
between test runs, but it looks like it's not faster - just much
quieter and possibly slightly slower.

Oh, I guess that answers most of my prior question.

So I'd really like the option to turn SDM off in firmware, when speed
counts - and turn it back on, when noise counts.

Yes, good idea, this was also suggested in one of the recent FW threads. However, wouldn't be fast SDM even better?

Cheers
Jens

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lol101 Veteran Member • Posts: 5,855
Re: SDM speed (or lack thereof)

IMHO USM/SDM/SSM/SWF lenses are not intrinsequely faster than their "non-motored" counterparts (given a sufficiently powerful screwdrive motor of course).

I recently tried a 50f1.4 USM on a Canon body (30D) against my FA 50 f1.4 on the K10 and I couldn't spot any difference regarding speed.

The only difference was that the Canon one was almost silent (I could still ear a faint scritching noise though, confirmed to be normal for this lens by my canonist friend) and thus somehow "felt" faster.

Speed is in the electronics (sensor), computing power and algorithmics.

The best demonstration for that is the difference in AF speed you'll find with the same USM lense mounted on a 400D or a 1DMkIII.

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dotnik Veteran Member • Posts: 5,565
Re: SDM speed (or lack thereof)

JensR wrote:

I am surprised that it is slower. Theoretically it should at least be
equally fast.

Which theory? The theory "a bad ultrasonic drive may be worse than a
good conventional drive" seems to be confirmed.

The theory that Pentax knows what speed their AF system can handle and so dimensioning the SDM system to perform at least as good as screw-drive mode would have been the goal. Anyway Pentax never said SDM would be faster.

I would also think that they could have been even faster

"could have been even faster"? You mean "could have been fast"?

No. I meant that SDM could have been faster than screw-drive mode but that was not I wrote. With SDM alone optimizing for speed would have been easier and cheaper.

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SFT007 Veteran Member • Posts: 5,872
Hmmm...

I've been saying from day one since I got the DA*16-50 - that it is not as fast as some of my lenses like the DA21, etc. I didn't even do a test though - I can just feel it is simply slower - it simply felt slow and I don't find any other of my lenses to be slow excpet the DFA100 Macro - never tried it without SDM - I've had fw. 1.30 from the beginning. I think Lock did a test with the DA*16-50 doing the same thing as you and he found that with the older firmware and then with SDM that the speed was more or less identical so I simply thought that the DA*16-50 simply just didn't focus as fast as the DA21, DA18-55, etc. whether in screw-drive or SDM mode maybe due to the larger and heavier glass? It is surprising since the focusing throw is very short.

What is more surprising though is that it is slower with your 50-135. Maybe the SDM in this particular copy is not up to par? Try again with a fully charged battery? Who knows. Maybe though SDM is just not a great thing and that Pentax's version isn't as good as the competitors? I really doubt that it would be slower though - well so far we have one test saying that with the 16-50 it focuses the same speed with either (actually SDM was about 0.02 seconds faster though I place that on timing errors, etc.) and now your test saying that SDM is significantly slower with the 50-135.

I certainly hope this is jus a case of "more problems from the DA*'s" and not a design flaw with SDM.
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Brett St Pierre
Brett St Pierre Veteran Member • Posts: 7,025
Re: SDM speed (or lack thereof)

lol101 wrote:

Speed is in the electronics (sensor), computing power and algorithmics.

The best demonstration for that is the difference in AF speed you'll
find with the same USM lense mounted on a 400D or a 1DMkIII.

Which gives hope that the next 'advanced' Pentax body may extract more speed from these DA* lenses.

Personally the AF speed hasn't often been an issue for me for some years now... but the silken smooth and quiet of my DA* is nice and gives the grip equipped K10D a very "professional" feel.

(Must say though, I still like the tactile response and feel of the FA Limiteds etc. though when I am keeping it simple with primes)

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OP BTG308 Contributing Member • Posts: 941
Re: Not Surprising

Sinan Tarlan wrote:

Maybe the SDM in this particular copy is not up to par?

That's quite possible.

Try again with a fully charged battery?

It was. Both of them, actually (I had the grip on).

I certainly hope this is jus a case of "more problems from the DA*'s"
and not a design flaw with SDM.

I suspect it's possible to make both good and bad SDM systems and this one happened to be a bad one. I'd actually be interested in seeing others trying their lenses to see if it's just mine, or if it's normal behaviour for this model.

/ Richie - http://www.p-i-x.net

OP BTG308 Contributing Member • Posts: 941
Re: SDM speed (or lack thereof)

wlachan wrote:

Not counting the pause period at either ends, did the SDM took longer
to travel from one end to another?

Yes, 37% longer time. 1.48 seconds as opposed to 1.08 seconds for the screw-drive.

If so, sounds like the SDM being chosen was underpowered for some reason.

Yes, I agree.

And it's a bit of a shame, because the lens is so nice in every other aspect - IQ, handling and so on.

/ Richie - http://www.p-i-x.net

phenri Regular Member • Posts: 294
Should have been 1.30 firmware, not the older 1.20

BTG308 wrote:

As I had the 1.20 firmware in my K10D, a video
camera and a tripod, I figured I'd tape the lens while focusing back
and forth and then measure the focus speed from the video clip.

/ Richie - http://www.p-i-x.net

It's a pity ... but it's likely you'll have to redo all your testing. If I read correctly you tested with version 1.20 of the firmware while version 1.30 is the current one and has been reported to substantially improve the speed of AF in SDM mode.

http://www.pentax.co.jp/english/support/digital/k10d_s.html

lol101 Veteran Member • Posts: 5,855
Re: Should have been 1.30 firmware, not the older 1.20

Firmware 1.20 was recquired for non-SDM testing. He then flashed his camera with 1.30 for SDM support so his conclusions remain valid.

I tested the 50-135 at the Paris Salon and wasn't blown away by it's speed. It didn't feel slow either but I could feel a difference when I tried the 70-200 2.8 USM on a 40D and I had the impression that my FA135 was faster on the K10 (I have no figures to prove it though).

Anyway, in adequate lighting, they all focus fast enough for me.

It's a very nice lens though when it comes to handling and build quality and I'm eagerly waiting for the 60-250 (I like the range better) to appear.

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Russell Evans Forum Pro • Posts: 12,617
Re: SDM speed (or lack thereof)

BTG308 wrote:

I figured I'd tape the lens while focusing back
and forth and then measure the focus speed from the video clip.

I setup the video camera looking down on the lens' little focusing
window and proceeded to half-pressing the shutter button (via cabled
remote) with the lens cap on in the AF-S mode to make the AF system
go all the way from infinity to close focus and back again.

It would be interesting if you could setup the same experiment with a high contrast center focus at infinity: with the lens starting at the opposite end. Variations with auto and spot focus, starting at the half point, and with and without SR to cover a wider range of usage might give a more detailed idea of when SDM use is on or sub par with screw drive use.

Is it possible to test manual focus with a SDM final focus? In low light, does getting a rough focus in manual and then letting the lens autofocus work better in screw or SDM drive?

Thank you
Russell

dotnik Veteran Member • Posts: 5,565
He did flash the firmware /nt

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var1 Senior Member • Posts: 1,156
Re: SDM speed (or lack thereof)

Hi.

I have mentioned in some general posts that to me AF speed is not everything or necessary for me. Coming from another brand system where speed is always mentioned I was often disappointed.

Accuracy is what I need in an AF system and the K10D certainly delivers in this area and so far I am also happy with the speed and to date have no SDM lenses.

Cheers
Ron

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lol101 Veteran Member • Posts: 5,855
Re: SDM speed (or lack thereof)

Brett St Pierre wrote:

Which gives hope that the next 'advanced' Pentax body may extract
more speed from these DA* lenses.

Hi Brett,

Yes, it's highly probable that optimized algorithmics and AF sensor technology will contribute in getting the blazing speed some would like to see in Pentax cameras from these lenses, except if they have chosen a low torque motor to power these lenses and that they are already operating to their max specs.

We'll see that next year.

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phenri Regular Member • Posts: 294
Sorry, I missed the "flashed the firmware" part (nt)
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Russell Evans Forum Pro • Posts: 12,617
Re: Should have been 1.30 firmware, not the older 1.20

lol101 wrote:

Firmware 1.20 was recquired for non-SDM testing. He then flashed his
camera with 1.30 for SDM support so his conclusions remain valid.

There isn't a choice to use or not to use SDM as the case required in 1.30? I thought that choosing screw drive when speed was required was in his conclusion?

Thank you
Russell

phenri Regular Member • Posts: 294
Mask the SDM contacts ?

I suppose you could tape out the two SDM contacts on the body ?

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