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Old Jan 8, 2005, 11:30 PM   #1
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:?I don't understand why so much talking about the AF-assist light. I think most of the regular SLR cameras don't have AF-assist light. When using the EVF instead of the LCD to take pictures. When you press the picture-taking button half-way, the square symbol inside the EVF turn green color. Is it means the picture is in focus and ready to be taken? Also is the green color square symbol inside the EVF like the AF-assist light outside the view finder on the regualr cameras? Anyway, I notice sometime the camera has problem deciding how to focus the subject, is this because the lighting on subject is too low or is it because the capacity of the CCD is limited?
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Old Jan 8, 2005, 11:33 PM   #2
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If we don't know what camera it is...then there's no way of telling what the green light means.
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Old Jan 9, 2005, 2:29 AM   #3
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I think the AF-assist lamp that you are talking about is something else. I think you are talking about an illuminator of some sort (which is used to light up the display or something). In contrast, the AF-assist lamp is a beam of light that is shot from the camera at the subject, in order to help the camera focus better in low-light situations...

jimmy380 wrote:
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When using the EVF instead of the LCD to take pictures. When you press the picture-taking button half-way, the square symbol inside the EVF turn green color. Is it means the picture is in focus and ready to be taken?
Yep... this depends on the camera but usually it means the camera has determined all its settings (focus, shutter speed/aperature, etc) and is ready to take the picture.

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Also is the green color square symbol inside the EVF like the AF-assist light outside the view finder on the regualr cameras?
I think you are mixing up an illuminator and the AF assist beam...

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Anyway, I notice sometime the camera has problem deciding how to focus the subject, is this because the lighting on subject is too low or is it because the capacity of the CCD is limited?
Hopefully someone more knowledeable like JimC or others can respond... but from what I understand, this depends on many factors. Usually it is due to too low of a light, resulting in too low of a contrast and the inability of the camera to distinguish objects. In other words, the light isn't bright enough to seperate objects--the camera can't "see" the object apart from the background/darkness. However, I am guessing that slow CPUs (processors) and inefficient firmware/software (particularly the image processing algorithms) might also be cause of slow focusing.

The sensor (CCD or others) really don't impact this per se. I guess the more megapixels, the greater the CPU power required but apart from that, the sensor isn't generally the cause of slow focus problems....
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Old Jan 9, 2005, 3:51 AM   #4
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The reason why AF is not working is not only depending by lighting but even how the subject is composed. AF, in digital cameras, is working in a passive way. It means: no beam out just "evaluation" of the subject. AF can't focusing horizontal lines only vertical. If AF doesn't work in medium lighiting too is because can't find a "contrast" to work to.

Check this. Try to focus a borderof a paint or mirrorat mediumand low lighting. Try first thehorizontal and then thevertical border.

Let me know your results.
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Old Jan 10, 2005, 9:34 AM   #5
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Adwulf wrote:
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AF can't focusing horizontal lines only vertical.
Why? What's so special about vertical contrast versus horizontal or diagonal? Wouldn't it be better to focus using diagonal lines (since they sort of capture the effect of horizontal and vertical lines, as well as diagonal lines)?
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Old Jan 10, 2005, 9:43 AM   #6
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Yah...so if it can't focus on horizontal lines, and there happen to be horizontal lines there ... then is it possible to rotate the camera 90 degrees to get a focus lock?
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Old Jan 10, 2005, 11:37 PM   #7
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Some people have reported a little bit of low light focusing success when using the red eye reduction mode on the S1.

Since there is no formal AF assist light, but there is a very fast ultrasonic motor and digic processor, the theory is that when you use red eye reduction, the pre-flash allows the camera an extra fraction of a second to get a better focus. It's not long enough to get a lock, but in my very informal testing it does seem to work better than not having the red eye feature on. I took about half a dozen shots in my low-lit living room with red eye on, and off. The ones with it off were all pretty out of focus, but the ones I took with it on, were less so. They were not what I'd call tack sharp, but they were not extremely blurry either.

Experiment with that and see what happens.


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Old Jan 11, 2005, 11:03 PM   #8
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:?I wrote to Canon about the AF assist beam, how to overcome the low ambient lighting problems. They told me to focus manually using the manual focusing mode. They avoid answeringmy question "why Canonnot equiped this camera withan AF assist beam" since it considered so important to any camera. I will write to them again regarding the matter. Thanks all for all the discussion. I have benefited a lot from you guys.
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Old Jan 12, 2005, 12:49 PM   #9
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Hi Sivaram. If You want to know more about autofocus You can check:

http://electronics.howstuffworks.com/autofocus.htm.

You will get a lot of explanations. I hope it will help.
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Old Jan 12, 2005, 6:34 PM   #10
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:-)Adwulf: Thank you for directus to this great site. Now I understand more about the AF mystery. With or without the AF focus beam is entirely depends on the type of camera you have, it doesn't mean it's good or bad. Thanks again for directed us to this site, so we all can understand the subject better.
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