Needed: Education about lenses

[Marc Pavan]

As I have mentioned before I am looking at purchasing a D-SLR, and I am learning more and more about them.

In my research I have come up with questions about lenses. When I read a description of a lense there are 2 numbers in MM 18-55mm for example. These are followed by what look like f-stop numbers.

First question - What do the mm numbers mean to me other than the length of the lense? Does a higher number mean a longer zoom?

Second question - Can I only use this lens at the f-stop positions listed in the description? What happens if I use a lense f2.8-f5.6 at the f8 stop? or the f2? How far can I stretch the use of a lense?

I see the 18-55mm lenses are popular? I want a very versatile lense for a backpacking trip in Europe. Sunny hikes in the alps and portraits of my wife on the Ponte Rialto in the evening will both need to be captured. Travelling with one lense is preferred but I might be able to take/buy 2.

Thank you
Marc

[cyclohexane]

What do the mm numbers mean to me other than the length of the lense? Does a higher number mean a longer zoom?

They refer to the focal length, which means the larger the number, the tighter the view. A 20mm lens is considered "wide", because you can fit a lot of what you see in front of you into the picture. A 300mm lens is considered "long" because you can only see a little bit of you.

This is probably easier illustrated with examples, but I don't have my own computer (out for repair) so I can't dig any up for you real fast right now.

Can I only use this lens at the f-stop positions listed in the description? What happens if I use a lense f2.8-f5.6 at the f8 stop? or the f2? How far can I stretch the use of a lense?

The number on the outside of the lens represents the maximum aperture: the largest opening the lens can achieve, letting the most light in.

You can use the lens at the listed apertures, and anything smaller until the lens can't physically close down any more (most APS and 35mm format lenses close down to f/22 or f/32). For an f/2.8 lens, you could also use it at f/4, f/5.6, f/8, f/11.... and so on. Closing down lets in less light, but allows more parts of the image to be sharp. I'll let someone else explain how this concept works.

Remember, f-stops actually represent fractions, with f being the focal length of the lens divided by the number under it. f/2 is a larger opening than f/4, just like 1/2 is a larger number than 1/4.

When you have a zoom lens with two numbers, say an 18-55mm f/3.5-5.6, the lens has a maximum aperture of f/3.5 at 18mm, but as you zoom in to 55mm, the aperture shrinks down to f/5.6 due to the design of the zoom lens. You wouldn't be able to open up to f/3.5 at the longer end.

-----

Two good books to look at are John Hedgecoe's New Manual of Photography and Ansel Adams's The Camera.

[EOS_JD]

Read this.

Understanding Camera Lenses

[EOS_JD]

And even more in depth read this
Photographic lens - Wikipedia, the free encyclopedia