Fixed Aperture?

[Power620]

I am a Newbie to photography. I own canon powershot A620. I want to know is there any way to keep fixed aperture while zooming and why does it change, any option available for this?

[Jon]

Sorry, the maximum aperture is an optical characteristic of the lens. The aperture number (f-stop) is a ratio of the lens focal length divided by the "entry pupil" of the lens. The entry pupil is basically how big the front opening of the lens is and in order to keep a constant maximum aperture, the front opening would have to change along with the focal length.

However, if you're using something other than the maximum aperture, that setting won't change as you zoom.


Lens design is a very complex process. There are expensive SLR lenses which have "constant aperture"; their lenses are designed so there's essentially a magnifying glass in front of the physical opening that's the aperture; by moving this magnifying glass forward and back the entry pupil changes along with the zoom. But they're complicated, hard to design, bulky, and heavy so you won't see them very often.

[Persephone]

As Jon said, this is simply the lens design.

Superzoom cameras will stop down less slowly than smaller range cameras. For instance, my SX110 stays at f/3.5 up to around probably 200-250mm, whereas the A620 will have stopped down to f/4.1 by around 100mm. Actually...the A620 is pretty darn good. The cheaper A cameras stop down to as far as f/5.5-5.9 and the SD cameras stop down to f/4.9.

[Power620]

Thank u friends for this information. :)