What makes a lense a 'fast' lense?

[Spinners]

I keep reading posts about the need for faster lenses? what makes a lense fast?

is it based on the lowest fstop? or maybe focus speed? i just seem to be missing something here? i mean what good does an fstop of 1.whatever do for anyone?

just curious.

[Belmondo]

Generally, fast lenses are those with the lowest focal ratio (f-stop number). They have larger openings to let in more light, and allow faster shutter speeds (hence the term fast).

An f/2.8 lens is faster than an f/4.0.

Generally, faster also means 'more expensive.'

[robertwgross]

In general, a "fast lens" is one with a very low number for wide open aperture. In some short lenses, this may be f/2 or better. In some long lenses, this may be f2.8 or so. Although there may be some differences in autofocus speed associated with the lens, this is not much of a factor.

Where it gets that term "fast" is that it allows the operator to shoot with faster shutter speeds. For certain low-light and sports subjects, this is important.

Having that very low number for wide open aperture allows the operator to have very fine control over depth of field, in case that is important for some subject. Note that often times a beginner is not so stressed over controlling depth of field so precisely.

---Bob Gross---

[Spinners]

ok, that's what i thought, but who's gonna get any depth of field at F2.0. i mean if you shoot sports, wouldnt you want a little more depth of field? or is that fact that your so far away not effect DOF.

[Mark Kemp]

generally a lens goes faster the harder you throw it

[CyberDyneSystems]

It is true that DOF decreases as your aperture increases (fstop lower)

But;
1. It is better to at least have the ability to get the desired shutter speed to stop the action than not have the ability at all with a lens that has to small an aperture

2. Shallow depth of feild can vastly inprove an action image by blurring the background and isolating the action from the crowd or other distraction.

3. As you surmise,.. by increasing the distance the actual DOF in feet or inches does increase (although I believe the ratio remains constant )

[minicooper]

When shooting sport, a very shallow depth of field doesn't matter because the focus of the action is generally going to be happening in one place.

[Spinners]

so then in conlcusion, fast lenses are most likely to be used by those shooting sports, and possible wildlife. someone taking portraits would have no use for them eh?

[minicooper]

I use my 2.8 70-200mm alot for portraits- it's very flattering and using a shallow depth of field throws the background right out of focus bringing alot more attention to the face and adding impact to the photo.

[robertwgross]

spinners wrote:
so then in conlcusion, fast lenses are most likely to be used by those shooting sports, and possible wildlife. someone taking portraits would have no use for them eh?

Basically, this is not correct.

As I indicated earlier, fast lenses can be very useful for a limited light subject. Suppose you want to shoot a portrait with a very carefully-controlled dim light with interesting shadows. The fast lens is going to allow you the light-gathering ability to get there. How much of this you need depends somewhat on the local length of your portrait lens.

---Bob Gross---

[CyberDyneSystems]

Another thing that makes a lens "faster" is its autofocus.

This is not what people are referring to when they say "it's a fast lens" but it is related.

Although many aspects of a lnese design can contribute... since a lens will allways autofocus at max open aperture.. this USUALLY means a larger aperture lens can tranlate into a faster focusing lens. (more light faster foucs) especially in darker settings.

So even if you are shooting at f/8 with your 300mm f/2.8,.. the lens will have that big f/2.8 opening and lots of light to do it's focusing.

[Jesper]

A fast lens makes you poor faster..... :D

[iwatkins]

.... Amen :)

[Vince_Z]

...except for the 50mm 1.8.