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03-14-2014, 10:32 AM   #1
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The 1.4x TC and the magical 'AF speed improvement'

The first review of the new HD AW TC has been posted and it seems people still think teleconverters somehow increase AF speed.

AF speed is determined by the lens and the body only, the sole function of the TC is to increase the focal length of the lens and to relay data and AF to and from the lens. Unless the TC unlocks a hidden software AF speed limit, there is no way for a conventional TC to increase the AF speed. The only impact on AF speed might be negative, due to the lower effective aperture.

03-14-2014, 11:14 AM   #2
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I agree, AF should be unchanged or slower. In low light where the loss of one stop hurts more than in bright light we must expect that it will be slower.
03-14-2014, 12:13 PM   #3
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Is it possible that the users' initial impressions are that AF is more "decisive", because the longer focal length results in better subject isolation, in practice?

Perhaps I shouldn't comment since I'm missing the context, and maybe this has been beaten to death in the other discussions.
03-14-2014, 01:34 PM   #4
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I thought I just read in another thread that it actually slows AF down in order to make it more precise. If this is true (it's certainly physically possible), I suppose locking on more quickly could effectively result in faster auto-focusing.

---------- Post added 03-14-14 at 01:37 PM ----------

QuoteOriginally posted by Giklab Quote
AF speed is determined by the lens and the body only, the sole function of the TC is to increase the focal length of the lens and to relay data and AF to and from the lens.
Don't forget that the TC physically becomes part of the lens, including some electronics and mechanical linkages. It could be designed as merely a "pass through" type of device, but it doesn't have to be.

03-14-2014, 02:47 PM   #5
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I also read somewhere in the past couple days where the screw drive in the TC is geared a little differently to help achieve precise focus. Just glanced over it at the time - now looking for where I read that.......
03-14-2014, 02:51 PM   #6
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QuoteOriginally posted by Giklab Quote
The first review of the new HD AW TC has been posted and it seems people still think teleconverters somehow increase AF speed.

AF speed is determined by the lens and the body only, the sole function of the TC is to increase the focal length of the lens and to relay data and AF to and from the lens. Unless the TC unlocks a hidden software AF speed limit, there is no way for a conventional TC to increase the AF speed. The only impact on AF speed might be negative, due to the lower effective aperture.
I haven't read that anywhere, can we have a reference?
03-15-2014, 08:38 PM   #7
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QuoteOriginally posted by normhead Quote
I haven't read that anywhere, can we have a reference?
Page two of the older thread "The teleconverter shipped"
03-16-2014, 11:05 AM   #8
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Two things could change the perception of speed. First is decisiveness. In some situations the magnified subject filling a focus point or more may give the body more information to focus quicker.

The other is to move the focus distance. The distance to turns of the focus ring ratio changes as the subject approaches; to focus two feet of distance at 40 feet requires a tweak of the focus, that same two feet close in is considerably more. I'm thinking that this would seem slower, but I haven't tried.

I suspect it is the first. A more decisive focus on subjects that without the TC are at the edge of the capabilities of the body.

03-16-2014, 06:07 PM   #9
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Any TC, no matter how great, lower effective contrast thanks to adding more elements between the subject and the sensor and creating more aberrations in the image. Lower contrast will slow down any AF system. Any perception of faster focus is because the subject is bigger in the screen and so the autofocus points have an easier time to lock onto a target.
03-16-2014, 06:28 PM   #10
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QuoteOriginally posted by y0chang Quote
Any TC, no matter how great, lower effective contrast thanks to adding more elements between the subject and the sensor and creating more aberrations in the image. Lower contrast will slow down any AF system. Any perception of faster focus is because the subject is bigger in the screen and so the autofocus points have an easier time to lock onto a target.
In lens design, more elements can actually lead to less aberrations.... and there's also no correlation that I know of that means fewer elements means less contrast, also because fewer aberrations generally lead to better contrast, an argument could be made for more elements increasing contrast.... as for more focusing speed, once your AF is fast, deciding how fast it is is a meaningless exercise. Perhaps these things need more consideration before we start stating them as accepted facts.
03-16-2014, 06:46 PM   #11
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QuoteOriginally posted by normhead Quote
In lens design, more elements can actually lead to less aberrations.... and there's also no correlation that I know of that means fewer elements means less contrast, also because fewer aberrations generally lead to better contrast, an argument could be made for more elements increasing contrast.... as for more focusing speed, once your AF is fast, deciding how fast it is is a meaningless exercise. Perhaps these things need more consideration before we start stating them as accepted facts.
I think you misunderstood what I said. I stated that more elements means more aberrations and less contrast. The element of a teleconverter do create aberrations because a teleconverter like the pentax 1.4x is not designed specifically for every lens, A teleconverter is basically another lens stacked on top of your existing lens that multiplies your image size.
03-16-2014, 06:52 PM   #12
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QuoteOriginally posted by y0chang Quote
I think you misunderstood what I said. I stated that more elements means more aberrations and less contrast. The element of a teleconverter do create aberrations because a teleconverter like the pentax 1.4x is not designed specifically for every lens, A teleconverter is basically another lens stacked on top of your existing lens that multiplies your image size.
I know what a teleconverter is, I own the 1.4, I'm not seeing what you say should be happening. I also don't see a speed difference.

No TC image


TC image, approx. the same distance from the lens.


If there's more aberrations and less contrast, it isn't obvious to the naked eye.

Last edited by normhead; 03-17-2014 at 07:35 AM.
03-16-2014, 07:19 PM   #13
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Those are wonderful images and that is a testament to Pentax's optical design if you can get that kind of results with a teleconverter. I'm not saying that a well engineered teliconverter wont give great images, and in fact if it does give those results its well worth the money. If you have enough contrast with a great lens and that particular day's lighting adding a TC will not affect anything because the camera can see contrast and still focus on it. What I'm trying to state is there is no way a TC can make your camera focus faster.

LensRentals.com - Teleconverters 101
03-17-2014, 05:44 AM   #14
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QuoteOriginally posted by Giklab Quote
The first review of the new HD AW TC has been posted and it seems people still think teleconverters somehow increase AF speed.

AF speed is determined by the lens and the body only, the sole function of the TC is to increase the focal length of the lens and to relay data and AF to and from the lens. Unless the TC unlocks a hidden software AF speed limit, there is no way for a conventional TC to increase the AF speed. The only impact on AF speed might be negative, due to the lower effective aperture.
If a native lens requires 10 turns to move from 5ft focus to 7ft focus then providing there is no reduction in gearing the same lens with a 2 x converter attached wiil require 5 turns to move from 5 ft to 7ft.

ergo the AF will be twice as fast with a converter atached.

Traditionally to make a converter function you add a reuction gear as in the above the camera would under PD move ten turns and miss the target before the next lock check, So you reduce the gearing by 2 so for a traditional converter you still require 10 turns to move from 5ft to 7ft but now take twice as long to go from min to infiinty i.e the AF is 1/2 speed but focal length is doubled so relative speed is constant.

It appears with the SDM focusing rather than reduce the SDM speed (not an easy task) the converter reports true focal length to the camera so stepping remains consistant and the relative speed is increased by 40% (for a 1.4 converter)

So reports of faster AF for SDM lens on the new converter are in all proabablilty true.

It is incredibly easy to establish if what I'm saying is true , take any screw converter and count turns of the key side to turns on the slot side if what I'm saying is true it will never be 1:1

And as none are 1:1 then this is obviously wrong and based on some complete misconception of how Converters maintinian AF compatability....

QuoteOriginally posted by Giklab Quote
AF speed is determined by the lens and the body only, the sole function of the TC is to increase the focal length of the lens and to relay data and AF to and from the lens..
When a true statment would be the fucntion of a TC is to increase the focal length whilst mainitaining consistent AF ratio with the convertered focal length.

---------- Post added 17-03-14 at 01:04 PM ----------

QuoteOriginally posted by y0chang Quote
Those are wonderful images and that is a testament to Pentax's optical design if you can get that kind of results with a teleconverter. I'm not saying that a well engineered teliconverter wont give great images, and in fact if it does give those results its well worth the money. If you have enough contrast with a great lens and that particular day's lighting adding a TC will not affect anything because the camera can see contrast and still focus on it. What I'm trying to state is there is no way a TC can make your camera focus faster.

LensRentals.com - Teleconverters 101
There is nothing in rogers paper to back up your claim "there is no way a TC can make your camera focus faster."

He doesn't mention AF gearing or any other methods to adapt PD to the new focal length.

So let me summerise , It can be easily tested

1 if end to end Focus speed remains constant then relative AF speed is increased by the Focal multiplier
2 if end to end focus speed is reduced by the focal multiplier the relative AF speed remains constant

In the case of all pre HD 1.4 AW converters clause 2 applied
The HD 1.4 AW seems to apply clause 1 to SDM focusing by correcting focal length reported to the camera.

This is also why currently HSM lens are broken on this converter as no corrected focal length is reported to the camera the PD af system overshoots the targets repeatedly leading to the 'hesitant no lock'

Last edited by awaldram; 03-17-2014 at 06:11 AM.
03-17-2014, 07:03 AM   #15
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"If a native lens requires 10 turns to move from 5ft focus to 7ft focus then providing there is no reduction in gearing the same lens with a 2 x converter attached wiil require 5 turns to move from 5 ft to 7ft."

This is where you are incorrect. A 'native' lens will ALWAYS take the same number of turns to focus from 5ft to 7ft, no matter how many teleconverters, filters or lens caps you stack on top. The TC simply extends the focal length, the focus range remains identical (except with the AFA 1.7x, but that's the one and only exception).
7ft away is 7ft away no matter what.
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