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09-08-2012, 09:46 PM   #1
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AF assist light

Quick question:

I've never used a Pentax newer than the K20d, I know that the pop up strobe AF assist doesn't really help that much...the SB mode in the AF360, doesn't seem to make a significant improvement either (both when shooting indoors).

On the current generation of Pentax DSLR, that include a dedicated AF assist LED on body...does it make a significant difference in indoors AF?

09-08-2012, 10:26 PM   #2
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absolutely and is much less annoying to others.
I used K10D and K-x with strobe and now the Q ( red light ) and K-5 and K-30 ( both green light)
09-09-2012, 01:55 AM   #3
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I found the SB mode of the flash to work perfectly though.

The problem i've with the assit of the K5 is that it doesnt work with most of my lenses because the lamp is to close to the mount so the lens cast a shadow.

I don't feel that the green light works better then BS.
09-09-2012, 03:20 PM   #4
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Thanks for y'all input.

In my case, the SB light works...but it doesn't seem to make a significant difference in AF performance.

Just curious, what is the useful distance range for the SB light.

09-09-2012, 11:10 PM   #5
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SB works great for me up to 10 meters haven't tried it further.
But i doubt the green light goes further, the SB light is a bit smarter i believe.
09-10-2012, 11:27 AM   #6
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This may not be what was wanted - however I'd like to share my experience.

The common take seems like there is always something lacking in Pentax dSLR AF.

I have used the K-x quite extensively (about shot count 58,000) and regularly in very dark environments -
to where there are parts that are literally below both the metering and AF limits of the K-x -
I am not saying the K-x never hunts - but mostly I can focus even in those very dark conditions.

I am not saying for a moment that my copy of the K-x is some super-boosted AF focusing monster -
it is "bog-standard" and I have not done any modifications to it, not only that I have only the two most humble kit zooms (18-55 and 50-200)

Please see Post #132 and #131 in thread Kx in Use ( 1 2 3 ... Last Page)

It is quite common to have difficulties when trying to focus on the face/eyes in the dark.

So I look for something else close by that may be easier to focus on - it may not be as critically correct - but mostly the depth of field takes care of any differences.

take this example -

the main subject's face was basically in the shadows - and it was not possible to focus on the face/glasses.

However the bell of the trombone had bright highlights which contrasted with the rest in darkness - gave a much easier target to focus on.

How dark was this?
ISO5000, f/3.5, 1/5sec, -0.7 comp; 18mm
metering segment read outs:


This one -



certainly could not focus on any face -
and even if the K-x had a focus-assist light it probably would not have helped at this distance -
focused on a candle flame...

So one can see the K-x is more than capable of focusing in very low light without any focusing aid.
09-10-2012, 05:23 PM   #7
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I would say that the highlights from the guys spectacles or face might be enough...I'm not sure.

Now that you mention it...how does the Pentax AF sensors work (I keep hearing words like cross type, etc.)?

Obviously some form of hard edge in contrast must be present in the AF sensor area in order for it to work (in all AF systems). Something I'm finding out is that the K20d seems to focus much easier on objects than on living things. Things like a watch, a fan, a bed frame, a door hinge (even on warm light), seems to be easy on the AF...on the other side, when shooting humans and plants (I haven't shot animals yet) even outdoors, seems to give more AF errors (accuracy and difficulty to lock).

I'm starting to think that it has to do with the shape of the hard edge of what the AF sensor is seeing...and it seems to favor easier patterns, like lines, regardless of light or contrast levels. Does this make sense?
09-10-2012, 07:03 PM   #8
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QuoteOriginally posted by HSV Quote
I would say that the highlights from the guys spectacles or face might be enough...I'm not sure.
Perhaps seeing the original unprocessed (other than re-sizing) might help to visualize the scene a bit better?
EXIF attached

according to the metering segment it is about -1.3LV at the face
this is about -1.3 stops below the spec'd metering,
and -0.3 stops below the spec'd AF limit of the K-x -
metering is supposed to be 0LV and AF -1LV, but using a f/1.4 lens -
I am using f/3.5 which is -2 2/3 stop slower/dimmer..

I was there and I really tried -
I only resort to focusing elsewhere when the K-x fails to focus where I would like it to.

QuoteOriginally posted by HSV Quote
I'm starting to think that it has to do with the shape of the hard edge of what the AF sensor is seeing...and it seems to favor easier patterns, like lines, regardless of light or contrast levels. Does this make sense?
......
(I keep hearing words like cross type, etc.)
Yes, kind of - AF on Pentax dSLRs is Phase- detection (or matching)
from: LensRentals.com - How Autofocus (Often) Works
QuoteQuote:
Phase detection uses the principle that when a point is in focus, the light rays coming from it will equally illuminate opposite sides of the lens (it is ‘in phase’). If the lens is focused in front of or behind the point in question, the light rays at the edge of the lens arrive in a different position (out of phase).

There are different ways to determine if the light is in or out of phase, but most current systems use mirrors, lenses, or a prism (beam splitter) to split the rays coming from opposite edges of the lens into two rays and secondary lens systems to refocus these rays on a linear sensor (usually CCD). The autofocus sensor produces a signal showing where the light rays from the opposite edges of the lens strike. If the image is properly focused, the rays from each side strike the sensor a certain distance apart. If the lens is focused in front of or behind the object, light rays from opposite sides will strike too close together or too far apart
So the more contrast and defined a "target" is - like a hard edge
- the easier it is to detect when it is in or out of phase.

read more at:
Autofocus - Wikipedia, the free encyclopedia

Simple (linear) sensors are usually horizontal or vertical -
a cross-type can focus on horizontal or vertical orientation.


Last edited by UnknownVT; 09-10-2012 at 10:10 PM.
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