What is a "fast lens?"

[dxqcanada]

A 70-200mm f/1.8 would cost too much ... and would probably be too large.
There is not enough market to warrant it.

Better to get some fixed focal length (prime) lenses with large apertures.

http://imaging.nikon.com/products/imaging/lineup/lens/af/telephoto/af_85mmf_14d_if/index.htm
http://imaging.nikon.com/products/imaging/lineup/lens/af/telephoto/af-s_vr_200mmf_2g_if/index.htm

 

[Early]

Okay thanks guys. The lowest I can get my camera down to is f/4 on my 70-210mm

and f/1.8 on my macro lens.

I know the macro is "fast" since its an f/1.8 but im not sure about the f/4 (I know you guys said that f/4 is "normal speed" but is it different on a zoom. In other words, if f/4 "fast" for a zoom lens?

THanks so much guys

If you have a macro that open up to f1.8 that is fast. But if it's merely a 50mm that focuses down to about 1 1/2 ft., that isn't considered fast.

 

[ShutterSpeed]

what is a fast lens?

it's not slow.

 

[Battou]

ohh okay. so it doesnt really affect how clear it autofocuses right?

because i am trying to get clearer action shots of birds. so a fast lens would really help that right? birds in the daytime)

Actually it infact does but not directly and not in what could be reffered to as the more common lens range. You begin to notice AF benifits of a fast lens when you start getting into lowlight situations or long lenses.

To explain my self here the two 50mm lenses I mentioned in my original post in this thread (the 50mm 1.4 VS 50mm 3.5) They will AF just the same under normal lighting conditions because they both have a reasonable field of view allowing in ample light to the viewer reguardless of the maximum aperture.

Now Lets step away from the standard lens and look at the 400mm f/2.8 VS a 400mm f/5.6, common serious Birding Lenes. The 400mm lenses are very long and have a narrow field of view these lenses need much more light at the subject because of this. The less light the darker the viewfinder, that is common knowledge. Since most AF is driven by crontrast, a dark viewfinder will result in more searching difficulty by the AF. That said, one will find that the 400mm 2.8 will have superior AF responce over the 5.6 under normal lighting conditions because it is capable of gathering more light from the same narrow field of view. Sadly in order for one to truly grasp what I am trying to explain here, one really needs to see the difference for them self and have experience with the same focal leingth both fast and slow.

Now, as for lowlight situations...I do not care what anyone says, AF should not be used for true lowlight situations, Period.

 

[Battou]

If you have a macro that open up to f1.8 that is fast. But if it's merely a 50mm that focuses down to about 1 1/2 ft., that isn't considered fast.

...a 50mm macro lens with max of 1.8.....that is ****ing insanely fast for a macro lens and will be damn near impossible to focus wide open due to razor thin DOF.

Of all my macro lenses and macro enabbled zooms, I have nothing faster than 2.8 and that is a 28mm.

So my question is is that actually a macro lens or just a 50mm that has a slightly shorter minimum focusing distance?