If telephoto is f4.9 and aperture is f8,

On my Canon s50, the zoom range is 7.1mm-21.3mm; f2.8-4.9. I can set the aperture up to f8. I know that digital f-stops are different than film, but I wondered what the net effect was on the sensor. Is it f4.9 + f8 = f12.9? That would explain a lot of underexposed images! Thanks.

Hi Skeeter,

The f2.8 is the MINIMUM f-stop at 7.1mm

The f4.9 is the MINIMUM f-stop at 21.3mm

From there, you can bump up the f-stop, to say f8, but when you get there, you don't add it together, you are at f8.

Digital f-stops are NO different than film. An f-stop is an f-stop

HTH

James

James,

That's a little confusing. My Oly says the same thing in the specs however when you've got it fully zoomed it allows you to choose a f2.8 all the way up to f8. So does it mean that anything I choose below f4.9 is irrelevant?

I believe you mean when the camera is fully zoomed OUT, you can choose anything between F2.8 to F8.0.

The indication on your camera (F2.8 - F4.9) means that when you're fully zoomed out (widest angle of view) you can choose the full range of apertures offered by your camera (in your case F2.8 to F8.0), but when you're fully zoomed in (smallest angle of view), you have a smaller range of apertures to choose from (in your case, between F4.9 to F8.0).

This could explain the underexposed shots mentioned by Skeeter: if they were taken at the highest zoom, the maximum aperture possible is F4.9, which may not be sufficient to get a properly exposed shot with a decent shutter speed...

Matt or James,

Does the P&S camera actually restrict the aperture down to f4.9 when zoomed in? Or is it simply a function of the amount of light available when the lens is extended?

F-stop is a physical property of the light coming into the camera. It's a function of the size of the hole (aperture) and the lens's focal length.

At longer focal length, with the same size "hole" the f-stop goes up.

Cheers

James

I just had a flash back to high school math class after asking a dumb question. The self-imposed humiliation is too much to take....haha. Thanks,

digital F-stops and film f-stops are the same. this is a mechanical part of a camera.

a lens that has a specs of 7.1mm-21.3mm f2.8-4.9 means at 7.1mm your maximum aperture (wide open) is f2.8 and when you fully zoom out at 21.3mm your maximum aperture at that focal length is f4.9. meaning if you are using the widest angle of your zoom lens 7.1mm, and choose to use the bigest aperture you get the limit of 2.8 and the smallest limit of f8, while if you zoom it all the way to 21.3mm your biggest aperture will only be f4.9 and not 2.8... but of course you still can get the smallest limit of f8. take note: the smaller the number the bigger the aperture, the bigger the number the smaller the aperture. which means everytime you use a bigger number you are only allowing a small amount of light to your sensor or film for that matter.

hope this helps you clear up the confusion

edwin

For those looking for a nerdy answer (helloooo, anybody out there.... <silence>) I'll complement James earlier post.

1: Your max aperture at the widest angle of view of 7.1mm is F2.8.

2: Your longest focal length is 21.3mm, which is exactly 3 x 7.1mm (i.e. 3x zoom).

3: The F-stop is proportional to the area of the opening (as compared to the focal length).

4: The area of the opening is related to the square of the diameter.

5: If you use a focal length length that is 3x longer, in proportion, the max. opening will be equal to the original opening divided by square_root_of_3 (the actual size of the opening is the same, it's just smaller WHEN COMPARED TO the new focal length).

6: Point 5 above results in 2.8 x SQRT(3) = 4.9. Your max aperture at the longest focal length is F4.9 (tada!!!)

Boy, that was relieving

Where does pie fit into that?

that hard to understand formula (for me) must be the reason why those zoom lenses with fixed aperture at f2.8 are so expensive because they have a lot of mathematics in there hahaha

When f./stop is written properly you can understand that the actual hole or aperture in the lens is f (focal length of the lens) divided divided by a number. Therefore f./4 is a hole of a diameter equal to a quarter of the focal length (or the lens-to-imageplane distance when focussed on infinity). f./8 is on e eighth. f./16 is a sixteenth and so on.

From this you will see that the actual size of the aperture or hole increases with focal length but the f./number remains constant.

So any given f./number always gives the same exposure on any fixed lens.

With variable focal length (zoom) lenses, if the hole remains the same size, the f./number varies as you change the focal length (zoom).

So the maximum size hole may be f./4.9 at 21.3mm focal length (zoomed out) but will be f./2.8 when zoomed out to the shortest focal length.

In simple terms, if your maximum focal length was 20mm and the maximum lens aperture was f./8, at a minimum focal length of say 5mm, the same aperture would be f./2. The diameter of the hole would be 2.5mm in both cases.

The f./stop is a fraction. That is why the smaller numbers represent wider apertures (or bigger holes).

I should have said that zoom lenses havea mechanism that automatically widens the hole as you increase the focal length so that the f./stop remains the same. Of course the maximum is exactly that - so your lens will be one maximum f./stop at the widest angle-of-view and another at the narrowest.

Well, John, I'm not sure I follow *everything* you stated above, but I don't need any help to recognize that your gallery is outstanding! Please post a few comments and pics from Sudan when you return. I was invited to go last year to Port Sudan on a liveaboard science expedition but I had to work instead - been frustrated ever since!