what does a fast lens actually mean

[mart_d50]

Hi, I wonder if you can help me understand something I'm stuck on - I get the impression especially where big zooms or prime lenses are concerned, that a fast lens, in other words having a bigger aperture/ower f-number i.e f2.8 are considered to be the best and most expensive lenses

1) what does the 'fast' bit actually mean - what is 'fast' referring to ?
I know it's not focus speed, but why is a lower f-number 'fast' ? is it the speed of the light going into the lens ? (although speed of light is a constant isn't it ?)

2) why are f2.8 lenses sought after (in this case birding/wildlife big zooms) - wouldn't taking a photo at f2.8 only render focus on a small part of the bird/animal, from what I've seen it's much better taking shots at f5.6-f11 to get sufficient focus detail all over the subject ?

3) assuming build quality is the same (and not taking into account quality factors such as vignetting/CA/etc at different apertures) assume you have two lenses taking a pic of the same subject at 300mm focal length, one is an f2.8 lens minimum, and the other is f5.6 minimum, they are both set to f8 and a photo taken
will the f2.8 lens have a better quality picture ?

thanks in advance and hope I didn't confuse anybody !
Martin

[matt_xyz]

a 'fast' lens refers to the fact that a large aperture allows a faster shutter speed to be used. This is obviously desirable when using long lenses. As you point out, though, a large aperture will give you limited DOF so is not always desirable. However, in poor light a large aperture may be the only way to get a shutter speed fast enough to avoid camera shake (or blurring of a moving subject matter).

Matt

[Kev Lewis]

The other significant areas that people forget is that a fast lens lets more light through it and the view through the viewfinder is considerably brighter. This is useful in low light conditions

[Andy Warne]

Originally Posted by Kev Lewis

The other significant areas that people forget is that a fast lens lets more light through it and the view through the viewfinder is considerably brighter. This is useful in low light conditions

Yep. Brighter image is better for focusing and composing and allows the use of teleconverters.

[eeyore]

and generally the fast lenses are the pro ones so they are better built and have better quality glass whichv leads to better pics - assuming photographic talent as a given of course, no lens in the world will help if you cant take a decent pic to start with.

[mart_d50]

many thanks for your advice !

so to summarise:

1) 'fast' refers to the lenses ability to allow a higher shutter speed

2) f2.8 are useful in low light situations allowing a higher shutter speed and a brighter viewfinder but limited DOF; even in bright conditions f2.8 is still going to yield limited DOF so a shot taken in any conditions at f2.8 is still just a compromise ?

which brings me to the last point I'm not sure about...

3) assume you have two different lenses taking a pic of the same subject at the same focal length ie.
sigma 50-500 f4-6.3 set to 300mm f8
sigma 120-300 f2.8 set to 300mm f8

ignoring the individual characteristics and quality differences of each lens will the picture quality theoretically be roughly the same at the same settings ?

thanks
Martin

[paulthomas]

Originally Posted by mart_d50

ignoring the individual characteristics and quality differences of each lens will the picture quality theoretically be roughly the same at the same settings ?

I'm not quite sure what you mean by quality but they'll both have the same DOF.

[RichBrew]

f/8 is f/8 on any lens. It still transmits the same amount of light.

[mart_d50]

the same DOF and the same amount of light - excellent

so essentially an f2.8 lens will not give extra quality at other aperture settings compared to other lenses 'just because it's an f2.8 lens', it merely allows you to take a shot at f2.8 albeit less DOF

fantastic !

thanks it just confirms in my mind I've no need to spend loads more on heavier more expensive f2.8/f4 lenses when I really want to be taking shots at at least f5.6 to get sufficient DOF on say a bird or animal; an f2.8 lens at f5.6 costing thousands isn't going to be letting in any more light than say a bigma at f5.6, assuming both the quality of the glass is the same it should result in the same quality/sharpness of picture at the same aperture and focal length

thanks

[Kev Lewis]

the f2.8 lens will let more light in mart and allow easier and faster focussing. The aperture doesn't stop down to the selected aperture until the moment you press the shutter.

Most of the fast lens will also give you a sharp image when they are fully open, with reduced DOF. This means you can use a higher shutter speed rather than crank the iso up