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Old Nov 4, 2008, 12:29 AM   #1
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does the speed of a lens affect the autofocus speed of the camera.
example a 1.2 vs 2.8

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Old Nov 4, 2008, 4:57 AM   #2
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Under low light conditions a slow lens (ie one with a high f number ) will tend to hunt as tries to focus because ther eis not enough diffence in brightness between objects.

The autofocus system has a hard time telling if the transition between objects is sharp or not.

This is why sports photgraphers use zooms with low f numbers such as the Pentax DA* 50-135mm f2.8. The SDM focus motor also helps in faster action.

The others make good 70-200mm f2.8 zooms for similar reasons.



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Old Nov 4, 2008, 9:02 AM   #3
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I'm not sure there would be a huge difference from f2.8 and larger.Most likelyif you have something that's f4.5 or smaller in dim conditions. AF speed also depends on the type of lens - I had a macro lens that was AF - the "throw" on it was quite long so it took longer than normal for it to go from very close focus to infinity, even though it was a relatively fast lens. If I were to get another AF macro lens, I'd want one with an AF limiter.
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Old Nov 4, 2008, 1:12 PM   #4
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There is a trade off in that the wider the aperture, the shallower the depth of field. Many autofocus systems are based on contrast detection, and the lower the light the less the contrast, and the more difficulty there will be in attaining fast or sharp focus. In bright light one or two stops at the wider end should make little difference in the far field, but there could be more opportunity for incorrect focus in the near field (depending on the nature of the subject) because of the reduction in DOF.
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Old Nov 4, 2008, 1:30 PM   #5
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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

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Old Nov 4, 2008, 4:57 PM   #6
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Hi Dave,

There are a lot of factors that contribute to AF speed.

AF in a DSLR (with the exception of those models that feature contrast control AF while in Live View Mode) is done by a separate sensor located at the bottom of the light box. The image is redirected down to the sensor through a half-mirrored section in the mirror and prisms, so the amount of light entering the lightbox through the lens is important -- a faster lens transmits more light, so theoretically should focus faster. . .

. . . but most ultra fast lenses lose some contrast and sharpness when wide open, and that works against the AF system. Remember that with all AF lenses, the camera is always focusing with the lens wide open. The FA 50/1.4 is one of those lenses, but it seems to be an open question as to whether it's good qualities outweigh the detractions.

There have been some who claim that the FA 50/1.4 focuses inconsistently on the digital bodies. I have not found that to be the case, but I'm not that critical a shooter with that lens.

I'm going to assume that the DA*55/1.4 is going to be the successor to the legendary FA*85/1.4 in keeping with what seems to be their thinking of designing their new lenses to replace traditionally popular FLs to reflect the crop factor effect of the smaller APS-C sensors (the 16-50=24-75, 50-135=75-202.5). With the * designation, I expect this to be a stellar lens, and with SDM, should perform very well in all aspects, but it'll probably be expensive at probably 2-2.5 times the price of the FA50/1.4.

I'd say it's worth the risk to try the FA50/1.4. There will always be a market for a relatively inexpensive fast 50mm, so you won't be taking too much of a chance of getting stuck with the lens if it doesn't meet expectations, and it's a very good lens in its class.

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Old Nov 4, 2008, 5:50 PM   #7
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what brought this up, was when i attended my moms 80th birthday and the area where the function was held , developed a low voltage problem. everyone did not mind as this made the lighting a nice warm effect.

my problem was the 50-135 was out of the question as the focus hunted back and forth. the 16-50 hunted way less, but was fustrating waiting for focus to lock. today i am sorry i did not use the onboard flash as this givesoff small bursts of light so as the camera would have something to lock on, the spot beam on the 360 flash and nothing is the same

so i wonder now if the 50 1.4 would have saved the day



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Old Nov 4, 2008, 8:40 PM   #8
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dafiryde wrote:
Quote:
what brought this up, was when i attended my moms 80th birthday and the area where the function was held , developed a low voltage problem. everyone┬*┬* did not mind as this made the lighting a nice warm effect.

my problem was the 50-135 was out of the question as the focus hunted back and forth. the 16-50 hunted way less, but was fustrating waiting for focus to lock. today i am sorry i did not use the onboard flash as this gives┬*off small bursts of light so as the camera would have something to lock on, the spot beam on the 360 flash and nothing is the same┬*

so i wonder now if the 50 1.4 would have saved the day
Hi Dave,

It's hard to say, not knowing the exact conditions, but I think the extra 2 stops would have certainly helped.

There are some other tricks to getting better AF performance.

Shoot with settings as close to manual as possible -- select a specific ISO, single focus point, Tv or Av mode. This minimalization of Auto functions will take some of the load off the camera's processor and subsequently the battery.

Start with a fully charged battery -- this is a bit less important with the Lithium batteries of the K20, but can't hurt.

Realize that with a cross type AF sensor, contrast areas that are parallel either to the sides of the frame (or the top and bottom) are most likely to give you a focus lock. If you have to, tilt the camera to get diagonal lines either vertical or horizontal in reference to the viewfinder frame. Lock focus, return the camera to normal orientation, then take the shot.

Get familiar with the area of the focus sensor points. I choose to use center point only and shoot mostly with the subject in the center of the frame, then compose with cropping. Being familiar with where the borders of the focusing sensor ends can be critical if there is something high in contrast in the background or foreground.

You can test this yourself. In normal room lighting, set your camera to Program Mode, Auto ISO, Auto Multipoint focus. dim the lights until the camera starts to hunt, then set the ISO to a fixed value, Av or Tv mode, Center Focus point, then see if the focus locks more easily. Dim the lights further, and find an area with some contrast -- venetian blinds next to a plain wall works well. First try focusing on the blinds in normal landscape mode, it should lock pretty easily. Now manually unfocus the lens and try to get focus lock with the camera tilted 45 degrees. It should take longer to focus.

To get an idea of the borders of the AF sensor, point to the area of plain wall and try to lock focus, then slowly move the camera, bringing the blinds into the frame, and keep trying to lock focus. At a certain distance from the center of the frame, the focus will lock -- that's approximately where the border of the active area of the AF sensor is located -- then do the same thing from the other side, top, and bottom. Now you know where you have to have some contrast to get a reliable focus lock.

OT, but you can also get an idea of how the different exposure metering modes work. In a dark room, turn on a single light source -- a single LED flashlight with some paper in front of the lens works well. In Av mode, watch the shutter speed, and switching from matrix to center weighted to spot, move the frame around so the light is in different parts of the viewfinder. You'll see that in matrix, there's little difference where the light is located. In center weighted, the shutter speed will increase when the light approaches the center of the frame. Just where this starts to happen shows you where the border of the area where the center weighting begins. With spot, the shutter speed will remain the same until you're just about in the center, then the speed will jump up comparatively quickly, and the value will be higher than it was in center weighted -- you'll also see how small the sensor area that is actually metered in this mode.

I'm probably a little strange, but I like to know just how my camera does things -- more fun and useful for me than shooting focusing and resolution charts. . .:-)

Scott
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Old Nov 4, 2008, 9:27 PM   #9
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will try these, sounds like i will learn something

do you know of any tricks of the trade that would take careof sleepy eyes. i often get that using flash, and everyone comes out looking wasted. or is it the 360 flash causing it.

thanks again



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Old Nov 4, 2008, 10:12 PM   #10
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dafiryde wrote:
Quote:
will try these, sounds like i will learn something

do you know of any tricks of the trade that would take care┬*of sleepy eyes. i often get that using flash, and everyone comes out looking wasted.┬* or is it the 360 flash causing it.
Hi Dave,

"Sleepy eyes" or blinkers are usually caused these days by the preflash in P-TTL digital flash guns.

You can get around this by using the 360's "A" (Auto-thyristor Mode) which doesn't need a preflash. To get into this mode, just press the "mode" button once. An "A" will replace the "P-TTL" on the LCD. You need to set the camera's ISO (basically lower ISO for closer subject distances, and higher ISO for longer distances). There's a distance scale displayed on the LCD that will show you the range of distances for each ISO setting. You can change the value of the ISO on the flash by pressing the "S" button until the ISO value flashes. You then rotate the the wheel to change the value.

On the camera,you have to set the lens aperture and the ISO to the values on the flash. The metering is done on the flash only, and because it isn't through the lens, and the camera isn't communicating with the flash (except to tell it to fire). You might have to make adjustments to the aperture to get it just right.

This is a lot easier to do than to write. . .:-)

The Pentax flashes are more versatile than a lot of people give them credit for. Get to know what it can do, and it will be your friend. A lot of people don't like P-TTL flash because of the preflash. I just see it as a more advanced flash system than I could have ever imagined using a few years ago. The biggest problem is that there's little understandable user information available from Pentax. I pretty much got what I know about flash use from forum posts like this.

The best thing about digital is that experimenting doesn't cost anything but the time that you play around with something and a couple of cents for the electricity to recharge the batteries.

Scott
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