How Come My Camera Keeps Doing This?

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my guess its changing the F number because the lens is a variable lens, its F2.8 starting out but as you up the mm it will go to F 4.5, F5.6 etc

tekkie is right

Like 18-55mm f/3.5-5.6

So your aperture will vary from 3.5 to 5.6 depending on the focal length of your zoom.

tekkie wrote in post #4292220
my guess its changing the F number because the lens is a variable lens, its F2.8 starting out but as you up the mm it will go to F 4.5, F5.6 etc

Exactly. For instance, the 28-135mm f/3.5-5.6 will change the f/ stop as you zoom out. It will remain at f/3.5 at 28mm and will be f/5.6 at 135mm. If you have a lens, such as the 70-200 f/2.8 that has a fixed f/stop, it will remain at f/2.8 throughout it's range of zoom.

you can get fixed 2.8 lenses.. a fixed 2.8 lenses cost a lot.. I don't know why they made it like that.. my guess is for people who don't want to spend a fortune on lenses..

Taken from the Canon brochure for the A710 IS

"Maximum apertures vary from f/2.8 to f/4.8 across the zoom range"

There's your answer in a nut shell.

It would be better, but it cost much more in design and glass to be able to make it a constant f/2.8 aperture.

The reason for this is, that the longer the focal length, the more glass you need to take in enough light for f/2.8.

Variable aperture zoom lens:
A zoom lens which has different maximum apertures at different focal lengths.

Constant aperture zoom lens:
Also “fixed aperture zoom lens.” A zoom lens which has the same maximum aperture at all focal lengths.
Such lenses tend to be expensive professional lenses because of the optical complexity in designing such a lens. Consumer lenses tend to be variable aperture lenses - in other words, their maximum apertures vary with focal length - typically decreasing as the focal length increases.
For example, a professional lens might maintain a constant maximum aperture of f/2.8 as the lens zooms from 70mm to 200mm. A consumer lens might have a maximum aperture of f/3.5 at 70mm, f/4.5 at 125mm and f/5.6 at 200mm.

Yep, it's nice to have a constant aperture but the downside is that it's expensive. Canon does make constant aperture lenses for those willing to pay for them and for those wanting to save some bucks they make variable aperture lenses.

The salesperson didn't explain that to you I guess...

It's not just cost, it's size. Johnny's lens would need to be a lot bigger to be f/2.8 at the long end.

The aperture of a lens is expressed as a ratio of the lens pupil to the focal length. So long lenses that are fast also need to be big.

So Johnny.....are those white telescopes you used to rant about starting to make more sense now?

DVS_WiNdz wrote in post #4292273
you can get fixed 2.8 lenses.. a fixed 2.8 lenses cost a lot.. I don't know why they made it like that.. my guess is for people who don't want to spend a fortune on lenses..

The Tamron 28-75 is a fixed 2.8 throughout. And it is certainly not expensive.

JohnnyGalaga wrote in post #4292367
Wouldn't it be a lot better if you could just keep it at 2.8 no matter what?

Isn't that like asking "wouldn't it be a lot better if you could have a lens that could zoom to infinity?" ?

Well you figured it out, it's all a scam...Canon stock will crash in the morning!