Why does Aperture decrease when Zoom Increases

[SpiralOcean]

For point and shoot cameras I've noticed the aperture stops down the more a person zooms in. This seems strange to me and I haven't found any answers out there that specifically say why this is.

The answer I've put together from various sources is:

The smaller the aperture, the more sharp or in focus a picture is. Also, the more depth of field a picture has. Camera manufacturer's stop down on the aperture in order to keep the picture in focus.

Is this a correct assumption?

[rc53]

Apertures are described in f stops, the relationship between the focal length of the lens and its diameter: if the focal length is say 50 mms, and the diameter of the aperture is 25mm, then the f value is written as f:2.

If you then zoom the lens so that the focal length is now 100 mms, but the aperture remains the same, then the lens is f:4.

P&S cameras will have a maximum aperture value at their widest zoom; zooming to telephoto won't change the size of the aperture, so the f number will increase, as above.

simple optics, nothing to do with smaller apertures [larger f numbers] giving sharper pix.

[SpiralOcean]

Thank you for the help rc53. The f stop ratio finally has sunk in! Many thanks to this website and rc53.

I am coming from a video camera perspective.

With a video camera, you set the aperture to an f stop, then you can zoom in and out. The aperture doesn't change on the video camera although the focal distance is changing. Maybe the change in the focal distance is not enough to change the aperture until the focal distance reaches a point where the aperture must change?

There is a maximum aperture for the telephoto and for the widest zoom.

Maybe the question is better described as, why is the maximum aperture at the telephoto different than the aperture at the widest zoom?

The answer you posted makes a great deal of sense to me. The only confusing thing about putting that answer into my experience is, shouldn't the f stop be always changing when I am zooming in and out? Changing the focal distance?

[rc53]

The answer you posted makes a great deal of sense to me. The only confusing thing about putting that answer into my experience is, shouldn't the f stop be always changing when I am zooming in and out? Changing the focal distance?

Yes it will -- it is a direct effect of the optics. But to keep the exposure correct, the camera's auto exposure will alter the aperture appropriately. Two things are happening; the bigger or smaller aperture from the change of zoom, and the corresponding auto-exposure change of the aperture to compensate.

[NB some zooms for still cameras do have a constant f number throughout the zoom range]

[SpiralOcean]

We've got two threads going with relatively the same information. My mistake. I don't mind going to both.

I'm beginning to think that the f stop naming convention was began before there were lenses that could be different focal lengths. (zooming).

There isn't any physical reason for the aperture to change when the focal length changes. The ratio will be wrong, but that's just a naming convention.

If the aperture changes during a zoom, then light level hitting the CCU is going to change. If the aperture doesn't change during a zoom, then light level hitting the CCU is not going to change.

[rc53]

I'm beginning to think that the f stop naming convention was began before there were lenses that could be different focal lengths. (zooming).

Correct

There isn't any physical reason for the aperture to change when the focal length changes. The ratio will be wrong, but that's just a naming convention.

The physical size doesn't change, but the ratio between the diameter of the aperture and the focal length does, hence the change in f number.

If the aperture changes during a zoom, then light level hitting the CCU is going to change. If the aperture doesn't change during a zoom, then light level hitting the CCU is not going to change.

Correct

[SpiralOcean]

This has been tremendously helpful in my understanding of aperture and lenses. Thanks for helping me out rc53.

[margaret tan]

During a zoom, the aperture size remains the same though the f-stop increases.
Then, under what situatiom or lighting condition does the aperture change its diameter size

[rc53]

During a zoom, the aperture size remains the same though the f-stop increases.
Then, under what situatiom or lighting condition does the aperture change its diameter size

The OP's question seemed to relate to video cameras, not to still cameras.

The f value is a measure of the focal length and diameter of the aperture. In a video camera during zooming the aperture size may have to change to keep the exposure constant -- depends a bit on the scene and its contents.

Tha aperture will change if you change it yourself in manual M or Av modes, or if you have selected Tv, and then change this value. In full auto or green box mode, the aperture [Av] and shutter speed [Tv] are determined by the program, and both vary with the lighting.