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Cameras, Lenses and Accessories: Discuss Can someone explain focal length and the f numbers?...Hi guys, another question as i am new to all this. Can someone give me a explanation of focal length, ...
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Old 03-03-2006, 20:58   #1 (permalink)
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Can someone explain focal length and the f numbers?

Hi guys, another question as i am new to all this.

Can someone give me a explanation of focal length, i understand the smaller the number the wider the angle but do not get how this equates to usage in the real world and how much zoom/distance i would get from a certain focal length.

Also what does the f number relate to like f2.8 or f3.6? I have seen some lenses that are 28-300mm f3.5-6.3 and others that are similar but have lower f numbers, are these better?

Would a lens with 28-300mm f3.5-6.3 be ok for general photography and motorsport for someone on a budget (i.e me).

Thanks, sorry it's a lot to ask but with your help i am getting there slowly.
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Old 03-03-2006, 21:06   #2 (permalink)
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From the info i have picked up over the short period i have been doing this is.

The lower the F number the faster the lens is, it can also work better in low light conditions. The lower the F number is normally a lot more expensive than one with a higher number.
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Old 03-03-2006, 21:09   #3 (permalink)
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faster it is to focus i assume? is that not dependant on the camera or is it something inside the lens? would that be a problem if i was tracking something?
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Old 03-03-2006, 21:19   #4 (permalink)
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There is a tutorials and guides section to the site. You will find lots of useful info there like this one -scroll down to F

And focus speed is more down to the design of the lens or lens/camera combination...varies for different makes.
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Old 03-03-2006, 21:27   #5 (permalink)
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thanks
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Old 05-03-2006, 18:18   #6 (permalink)
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Quote:
Originally Posted by ppp
faster it is to focus i assume? is that not dependant on the camera or is it something inside the lens? would that be a problem if i was tracking something?
When people refer to a lens as being 'fast' they are referring to the aperture, i.e. An aperture of F2.8 is faster than a lens with a max aperture F5.6.

It makes the lens fast because you can use a faster shutter speed at F2.8 than you can at F5.6
  • The lower the Aperture number the more light the lens lets through to the camera sensor
  • The lower the aperture number the less distance you get in focus.

Having a 'fast' F2.8 lens is better and generally speaking they cost a lot more than lenses from F3.5 to F5.6

There is a 'rule' in photography that says you need to at least match your shutter to your focal length to avoid blur from camera shake. So when using a 28-300mm at 300mm you should be using a shutter speed of at least 1/320, with an aperture of F6.3 you will need to use a high ISO setting to achieve a balanced exposure.

The downside of using high ISO (800+) is your photo will have more noise (grain) than a low ISO (200)
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Last edited by SDK^; 05-03-2006 at 18:30.
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Old 05-03-2006, 23:35   #7 (permalink)
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many thanks
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