Is a "faster" lens faster only when wide open?

[Janmc]

this may be a silly question - but supposing I have a zoom that's 2.8 through-out, and then a variable zoom, or a constant f4 - will the faster zoom focus faster only when it is set to 2.8 (or any value wider than the variable zoom (of f4 zoom) equivalent)? Or would it focus faster regardless of the f stop?

 

[Overread17 5]

Focusing on any normal lens happens when the aperture blades are wide open, so it is always done at whatever the maximum aperture of the lens is capable of. In a variable zoom lens this means that the AF speed and accuracy can degrade as you increase the focal length and the aperture reduces (although in decent lighting you likely won't notice the difference - in dimmer conditions you will).

With a lens that has a constant aperture through its zoom range the AF speed should remain the same through the whole range of focal lengths as the amount of light getting into the camera remains the same.

Note that fixed aperture lenses can't change their aperture at all (ergo why its called fixed).

Also note that during taking a series of photos in burst mode the aperture blades flick open and shut, and the mirror inside the camera flicks up and down so that light can reach the AF sensors to keep the AF functioning during the process.


Ps the term "fast glass" generally refers not to the AF speed, but to the possible range of shutter speeds with the light present without changing the ISO. If you have a lens with a wider maximum aperture then it will be capable of giving you "faster" shutter speeds in the same lighting conditions than a lens with a smaller maximum aperture (provided of course, that you're comparing both at maximum apertures).

 

[dutcheagle]

The word "Fast" in a f/2.8 lens does mean that you can keep a fast shutter-speed for a longer time then with a slow lens like a f/4 it has nothing to do with focus speed, that depends on the camera and the amount of light that is available.

Lens speed refers to the maximum aperture diameter, or minimum f-number, of a photographic lens. A lens with a larger maximum aperture (that is, a smaller minimum f-number) is called a "fast lens" because it delivers more light intensity (illuminance) to the focal plane, achieving the same exposure with a faster shutter speed. A smaller maximum aperture (larger minimum f-number) is "slow" because it delivers less light intensity and requires a slower shutter speed.

 

[Judobreaker]

The word "Fast" in a f/2.8 lens does mean that you can keep a fast shutter-speed for a longer time then with a slow lens like a f/4 it has nothing to do with focus speed, that depends on the camera and the amount of light that is available.

For you maybe...

 

[dutcheagle]

The word "Fast" in a f/2.8 lens does mean that you can keep a fast shutter-speed for a longer time then with a slow lens like a f/4 it has nothing to do with focus speed, that depends on the camera and the amount of light that is available.

For you maybe...

Please explain, I don't know what you try to say.

 

[480sparky41 40]

The word "Fast" in a f/2.8 lens does mean that you can keep a fast shutter-speed for a longer time then with a slow lens like a f/4 it has nothing to do with focus speed, that depends on the camera and the amount of light that is available.

For you maybe...

Please explain, I don't know what you try to say.

How do you 'keep a faster shutter speed for a longer time'?

 

[dutcheagle]

How do you 'keep a faster shutter speed for a longer time'?

Sorry bad explanation from my part .
For example if you are outside and it is getting dark. When I meter the light for a shutter-speed of 1/125 of a second I still can take pictures with a f/4 lens and a f/2.8 lens. When it is getting darker I can open up a f/2.8 lens and still have a shutter-speed of 1/125. This is not possible with a f/4 lens when you are at the maximum aperture. (I know you could change the ISO but that is another discussion) This is why a f/2.8 (or f/1.4 or f/1.8 etc) is a fast lens and a f/4 is a slow lens. Keep in mind you pay a lot more for a fast lens.

 

[The_Traveler]

ANALOGY:

Getting light into a lens is like pouring water into a bottle through the neck. To get a good exposure you must fill the bottle.

If the neck is large [large opening f2.8), water (light)] can potentially get be poured in very quickly (thus a 'fast' lens).
Conversely an smaller opening (f4) means that the speed of filling the bottle is slower.

Since the glass elements in 'fast' lenses must be larger to capture the light, these fast lenses generally must be made with more stringent requirements to get a good sharp image.
Thus they are larger, heavier and generally more expensive.

That doesn't mean you always have to use them at f2.8 but they often/usually are of higher quality overall.

 

[Janmc]

well, that turned on some light bulbs on in my own head - and I actually didn't know that a lens always focuses wide open. But everything said makes sense now.
thanks for all the clarification guys!

 

[dutcheagle]

well, that turned on some light bulbs on in my own head - and I actually didn't know that a lens always focuses wide open. But everything said makes sense now.
thanks for all the clarification guys!

That's why we think there are no silly questions.
Be aware that focusing is a whole different story. In various light situations even a fast lens may have trouble focusing they say the lens is "hunting" for focus.