Newbie with a lens question

[400D_Dave]

Sorry if this has been mentioned before i couldn't find anything!

I'm currently reading 'Basic photography' by Michael Langford.

I'm getting the general jist of things but the following is making me curious.

I have a 400D with just a kit lens at the moment while i'm learning.

Lens as follows

Canon Zoom Lens EF-S 18-55mm 1:3.5-5.6

Am i right in saying the lowest f number at 18mm is 3.5 and the highest at 55mm is 5.6?

Also are you allowed to use higher and lower f numbers outside of those stated on the lens? If so what is the idea of the f numbers on the lens then or is it a recommended?

Also what mode am i best shooting in? i'm currently shooting in M and practicing with the settings diff, shutter speed and diff aperture.

Thanks in advance,
Dave

[stevieboy378]

Am i right in saying the lowest f number at 18mm is 3.5 and the highest at 55mm is 5.6?

The "widest" aperture setting ( i.e. lowest f number ) of the lens varies throughout its focal length, from f3.5 at 18mm to f5.6 at 55mm . . .

[tim]

Keep reading the book. The terms "lowest" and "highest" aren't helpful or commonly used. The kit lens can open up the aperture as wide as F3.5 at 18mm, and as wide as F5.6 at 55mm. You can't open them up further, but you can close them up. Open means let more light in, F1 is very open (large hole/wide aperture), F20 is quite closed (small hole/narrow aperture).

[stevieboy378]

Theres a great virtual SLR web site on the net that helped me to understand aperture, and its relationship with shutter speed :
http://www.photonhead.com/simcam/

This site has a good virtual camera too :

http://www.camerasinteractive.com/index.php

[400D_Dave]

Ahhh i get you now, so you can't open anymore than f3.5 at 18mm but you can close say upto f20.

same when you have focal lenth at 55mm you can't open up more than 5.6 but you can close to say f32 for example.

[400D_Dave]

Thanks Stevieboy i will give those a go

[tim]

Ahhh i get you now, so you can't open anymore than f3.5 at 18mm but you can close say upto f20.

same when you have focal lenth at 55mm you can't open up more than 5.6 but you can close to say f32 for example.

Yes that's all correct :) Wide apertures (F2.0, F3.5) give you a shallow depth of field, so the background is out of focus. F32 will give you almost everything in focus (minor simplification but close enough).

[SkipD]

Dave, most consumer-grade zoom lenses have a variable maximum aperture (minimum f-stop number). The reason is that it is far less expensive to design a zoom lens that way. Many "professional grade" zoom lenses such as Canon's 24-70 f/2.8L zoom (the "L" is not part of the f-stop indication, just an indication that the lens is one of Canon's high-end lenses) have a "constant-aperture" design.

To explain the difference better, you need to understand how an older design zoom lens works. By older design, I mean those lenses that had a ring on the lens that you rotate to set the aperture. For this illustration, assume you are using a fully manual camera.

Let's consider a hypothetical 70-210mm f/4.5-f/6.3 zoom lens for discussion. At any particular aperture setting (remember, this is manually set and not electronically controlled by the camera), the actual aperture value does not stay the same as you change the focal length. The iris size is staying the same as you change the focal length, and the "f-stop" dial setting is not being changed (either manually or automatically) as you adjust the focal length. At shorter focal lengths you get more light to the film, and at longer focal lengths you get less light to the film. With this sort of lens on old manual cameras, you had to compensate by either manually adjusting the aperture or the shutter speed as you made changes to the focal length. This is true at any aperture setting. Changing from minimum focal length to maximum with this hypothetical lens requires a two-stop exposure adjustment by either tweaking the aperture ring setting or shutter speed or both.

When the constant-aperture zoom lenses were first designed, the photographer was finally able to take a meter reading and set the shutter speed and aperture and not worry about what focal length the lens was set to. A constant aperture lens physically changes the iris opening via cams (or some other mechanical means) as the focal length is changing. This happens not only at the widest aperture setting, but at all aperture settings.

With today's automated cameras, there is a huge difference in how the lens' iris is controlled. You "tell" the camera what aperture you want (rather than mechanically setting it at the lens). With consumer-grade lenses, you still have a lens that does not change the aperture with focal length changes. However, except at the wide end (low f-stops), the camera itself controls the iris adjustments by communicating with the lens and making it adjust the iris to control the effective aperture. Now, the only real limitation is what the maximum aperture can be at different focal lengths and you don't have the requirement to compensate exposure settings when you change the focal length (if you are using a mid-range aperture such as f/8 or f/11, for example). The camera does the compensation for you.

Thus, the advantage of constant-aperture zoom lenses is a little less today than it was back when the cameras had no exposure automation. However, at maximum apertures, the constant-aperture lens is still an advantage because you can still change the focal length with no need to change shutter speed to compensate.