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11-03-2008, 10:28 PM   #1
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auto focus speed

does the speed of a lens affect the autofocus speed of the camera. example a 1.2 vs 2.8

Dave

11-03-2008, 11:25 PM   #2
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That's what I have noticed. The FA 1.4 is much faster than eg. 16-45 f4. One thing is that it does less back and forth seeking.
I am sure someone has done this more scientifically.
11-03-2008, 11:47 PM   #3
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... especially in low light. The K20D has AF range EV 1-18 which also tells that in low light conditions the fast lenses should work better.
But nothing competes my fingers with MF lenses, but there are days when the accuracy is not in highest level
11-03-2008, 11:50 PM   #4
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QuoteOriginally posted by dafiryde Quote
does the speed of a lens affect the autofocus speed of the camera. example a 1.2 vs 2.8
Generally, yes, a wider aperture means more light for the AF system to work with. But it might also mean less contrast, as some wide aperture lenses are known for being especially soft wide open. And wider aperture generally means more glass, so the AF mechanism has to physically work harder to make the lens focus. And then there are factors - like what the throw of the focus ring is, or overall build quality - that have nothing to do with aperture but affect focus speed. So I don't know if you can say with absolute certainly that a faster lens will always focus faster than a slower one.

11-04-2008, 12:05 AM   #5
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Yes

But max. aperture isn't the only bit that is involved in AF speed of a lens; there are mechanical considerations regarding the build of the lens. For the sake of consistency this discussion I'm not going to try and compare screw drive vs SDM and instead just address screw drive AF.

Keep in mind I'm no optical engineer, just an amateur photographer who happens to mentally dissect every machine he runs across...

The AF motor in the body is essentially a high-speed precision servo - it spins at one set RPM. There is some variance in this from lens to lens I'm sure based on the mass of the elements the AF has to move but it's probably negligible.

But each different lens has a different element travel distance to focus and different mechanisms to move the lens. They have different focus ring throws too. The focus ring arc (throw) on the DA18-250 is a mere 45 degrees, the DA12-24 is 110 deg. and the DA50-135 spins about 135 degrees. So, how far the moving elements travel and/or how big of an arc the focus ring travels will affect how quickly the AF motor can get the lens to focus lock.

I did an impromptu test earlier this evening between the DA18-250 and the DA*50-135 to see how they matched up and was a bit surprised that the 18-250 damn near out paced the DA* getting a lock.
11-04-2008, 12:08 AM   #6
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Good point on the sharpness wide open, Marc. And btw, that "Yes" was in answer to the OP, you snuck your answer in ahead of mine.
11-04-2008, 10:33 AM   #7
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So what the K20D firmware lacks is a max aperture setting. i am sure i'd get a better lock @ F/2.0 than 1.4 and let it focus at 1.4 only in the dimmest of lights. The extra contrast should help. And i could likely not even tell that the viewfinder is darker than wide open.
11-04-2008, 11:01 AM   #8
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QuoteOriginally posted by morfic Quote
So what the K20D firmware lacks is a max aperture setting. i am sure i'd get a better lock @ F/2.0 than 1.4 and let it focus at 1.4 only in the dimmest of lights. The extra contrast should help. And i could likely not even tell that the viewfinder is darker than wide open.

are you saying that the K20D cannot open a lens to 1.4

Dave

11-04-2008, 11:07 AM   #9
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QuoteOriginally posted by kurt Quote
... especially in low light. The K20D has AF range EV 1-18 which also tells that in low light conditions the fast lenses should work better.
But nothing competes my fingers with MF lenses, but there are days when the accuracy is not in highest level

the K10D has the same AF EV range of -1 to18
actually they both carry the same autofocusing system according to the main specs.

Dave

Last edited by dafiryde; 11-04-2008 at 11:27 AM.
11-04-2008, 11:25 AM   #10
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basically i was looking at getting a 50 1.4 lens, especially for low light.
thru photozone i have noticed that this lens is poor at 1.4 and 2 and only gets good at 2.8 my interest in this lens is not for really shooting at 1.4 or 2 but for the use at 1.4 for accurate and fast focusing in low light.
but i already have two DA* lenses both rated at 2.8, and really would like to know if there is a benefit from the K10/20 autofocus system

Dave
11-04-2008, 11:30 AM   #11
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QuoteOriginally posted by dafiryde Quote
are you saying that the K20D cannot open a lens to 1.4

Dave
No, that's not the problem at all. The problem is, it ALWAYS focuses with it open all the way at f/1.4, where the FA 50 is a little bit (okay, quite a bit) soft and low-contrast.

Some of us suspect that it would focus better (more accurately, possibly faster) if it were stopped down to (e.g.) f/2.0 or 2.8 just for the focusing operation, because the image coming through the lens is sharper at those apertures. Then for the photo, it would set the aperture to whatever you wanted.

In low light, focusing at f/1.4 is great. Otherwise, not so much.
11-04-2008, 11:51 AM   #12
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[QUOTE=Quicksand;384755].

Some of us suspect that it would focus better (more accurately, possibly faster) if it were stopped down to (e.g.) f/2.0 or 2.8

as far as i know, the lens does not stop down to the value of the set aperture until the shutter button is depressed, which is after focus lock is confirmed.

Dave
11-04-2008, 12:46 PM   #13
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QuoteOriginally posted by dafiryde Quote
are you saying that the K20D cannot open a lens to 1.4

Dave
I was expanding on what someone said with lesser contrast on a wide open lens, so why not focus at f/2.0 instead of 1.4 when the lens would be so much sharper there with much more contrast. (Custom behaviour)

Instead of focusing wide open where the lens is softer with less contrast. (default behaviour)

I hope this clears up what the K20D can do and what i think would make a nice custom function.

Daniel
11-04-2008, 01:22 PM   #14
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QuoteOriginally posted by morfic Quote
I was expanding on what someone said with lesser contrast on a wide open lens, so why not focus at f/2.0 instead of 1.4 when the lens would be so much sharper there with much more contrast. (Custom behaviour)

Instead of focusing wide open where the lens is softer with less contrast. (default behaviour)

I hope this clears up what the K20D can do and what i think would make a nice custom function.

Daniel
That's a pretty good idea, Daniel. Maybe some Pentax higher-ups are reading this.
11-04-2008, 03:13 PM   #15
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QuoteOriginally posted by morfic Quote
I was expanding on what someone said with lesser contrast on a wide open lens, so why not focus at f/2.0 instead of 1.4 when the lens would be so much sharper there with much more contrast. (Custom behaviour)

Instead of focusing wide open where the lens is softer with less contrast. (default behaviour)

I hope this clears up what the K20D can do and what i think would make a nice custom function.

Daniel
oh. i now understand what you said when you said it the first time
but do you think there would be a benfit over the 1.4 vs the 2.8 in low light focusing speed

Dave
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