Tip & How-To about Canon PowerShot A520 Digital Camera

Explaining Camera Focus

Camera: Focus We've seen that a real image is formed by light moving through a convex lens. The nature of this real image varies depending on how the light travels through the lens. This light path depends on two major factors:

  • The angle of the light beam's entry into the lens
  • The structure of the lens
The angle of light entry changes when you move the object closer or farther away from the lens. You can see this in the diagram below. The light beams from the pencil point enter the lens at a sharper angle when the pencil is closer to the lens and a more obtuse angle when the pencil is farther away. But overall, the lens only bends the light beam to a certain total degree, no matter how it enters. Consequently, light beams that enter at a sharper angle will exit at a more obtuse angle, and vice versa. The total "bending angle" at any particular point on the lens remains constant.
As you can see, light beams from a closer point converge farther away from the lens than light beams from a point that's farther away. In other words, the real image of a closer object forms farther away from the lens than the real image from a more distant object. You can observe this phenomenon with a simple experiment. Light a candle in the dark, and hold a magnifying glass between it and the wall. You will see an upside down image of the candle on the wall. If the real image of the candle does not fall directly on the wall, it will appear somewhat blurry. The light beams from a particular point don't quite converge at this point. To focus the image, move the magnifying glass closer or farther away from the candle.
This is what you're doing when you turn the lens of a camera to focus it -- you're moving it closer or farther away from the film surface. As you move the lens, you can line up the focused real image of an object so it falls directly on the film surface. You now know that at any one point, a lens bends light beams to a certain total degree, no matter the light beam's angle of entry. This total "bending angle" is determined by the structure of the lens.




courtesy of HowStuffWorks.com

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How do I use my old manual lense with a K200D?


Move the automatic/manual focus switch (on the lower right of the lens mount as you look into the lens) to manual. Focus manually by turning the focus adjustment on the lens itself - align the split image as you would with your old camera and take the picture.
(Use a handheld light meter and set the f/stop and shutter speed appropriately - the K200D doesn't have a visible meter. I would recommend bracketing your exposures -up a stop and down a stop from the 'appropriate' stop.)

Aug 11, 2013 | Pentax K200D Digital Camera with 18-55mm...

1 Answer

Can you use the twist-barrell zoom lens on a Sony DSLR camera (or any DSLR) when the camera is set to auto-focus? I've been told that this damages the gears in the lens if you do. Thanks for your help.


You can zoom completely independent of the focus.

Some lenses immediately override the autofocus when it senses you turning the manual focus ring. Other lenses don't, and you can damage the lens by forcing it. It depends on the lens, so you'll have to consult the documentation for the specific lens.

Jul 06, 2011 | Sony Alpha DSLR-A200K Digital Camera

1 Answer

my sony DSC H4 lens is stuck outside and it doesn go in .. the lens moves back and forth and is stuck ... the lens is also unable to focus.


Hi, Have you dropped your camera on the lens since I've seen that cause it before. If not then it's possible that the motor in the lens has gone. Either way it would be a costly repair which there's a 50/50 chance it would actually work again. Your best bet would be to get a new camera.

Jun 18, 2011 | Cameras

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