f/2.8 vs f/4

[Ultimate CC]

I searched but couldn't find an answer to my question...I was wondering if f/2.8 lenses focus faster at f/4 than an f/4 lens at f/4. Reason I am asking is that I shoot a lot of events at night and usually use my 24-70 + 1d mKIII but once in a while I will throw on my 17-40L to get table shots and it has a lot of trouble focusing compared to my 24-70. So I was thinking of upgrading to the 16-35 II but before I spent all that extra money I wanted to make sure that it would indeed focus faster and more accurately even when stopping it down...

[New Hobby]

It depends on the body and the focus point. Some bodies, like the 40D, have a better center focus if the lens is f/2.8 or faster.

[Ultimate CC]

yea i know my 1d mkIII does but my question is do they focus faster even if your not shooting at 2.8?

[New Hobby]

The camera will focus at f/2.8 or better even if you choose a slower f-stop.

[tonylong]

Just to elaborate on what New Hobby is referring to:

With an our EF lenses, the camera focuses with the lens aperture wide open, then stops it down at the point of shooting. So, your 24-70 (and the 16-35) will always focus at f/2.8, giving your Mk III that "f/2.8 advantage".

Differences in AF performance can have other factors as well (such as the AF motor type). For instance, the 85mm f/1.2L is a notoriously slow-focusing lens, terrible for sports/action shots because of this, whereas the 85mm f/1.8 is notably fast and widely used for sports/action. So, it's not just a question of aperture wideness.

[Ultimate CC]

perfect that answered my question...thanks for the help...well i guess i know what to order next...

[dave_p]

Not to threadjack, but does anyone know of the XSi does the more accurate focusing at 2.8 and faster?

[kevindar]

dave, yes it does. the extra sensitive middle focus point has been incorporated in xxxd cameras, starting with xti. I dont know what the answer is for xs, but xsi and 40D have the same focusing.

[nphsbuckeye]

Just to elaborate on what New Hobby is referring to:

With an our EF lenses, the camera focuses with the lens aperture wide open, then stops it down at the point of shooting. So, your 24-70 (and the 16-35) will always focus at f/2.8, giving your Mk III that "f/2.8 advantage".

Differences in AF performance can have other factors as well (such as the AF motor type). For instance, the 85mm f/1.2L is a notoriously slow-focusing lens, terrible for sports/action shots because of this, whereas the 85mm f/1.8 is notably fast and widely used for sports/action. So, it's not just a question of aperture wideness.
Is the same true with my 40D shooting at F4.whatever with my 24-60 that it will focus at 2.8?

[tonylong]

Is the same true with my 40D shooting at F4.whatever with my 24-60 that it will focus at 2.8?

Yes, since the 24-60 has (from your sig) a constant f/2.8 aperture, the AF will focus at f/2.8.

It should be noted though that a variable aperture zoom, even if it ranges from f/2.8 to whatever, will only AF at the aperture active for the current focal length. Of course, most zoom lenses have a more narrow range, such as f/4-f/5.6, so it has less to do with the f/2.8 advantage than it does with, say, putting a 1.4 TC on such a lens, and when fully zoomed finding that your AF doesn't work because you are now shooting with an f/8 aperture.

[nphsbuckeye]

Thanks.

[primoz]

I was wondering if f/2.8 lenses focus faster at f/4 than an f/4 lens at f/4.
Yes it will always focus better with f2.8 then with f4.0 lens. And it will focus even better with f1.0 lens. Reason is pretty simple. More af sensor gets more light, which means more contrast, which means easier work for sensor and electronics behind it.

[E-K]

Yes it will always focus better with f2.8 then with f4.0 lens. And it will focus even better with f1.0 lens. Reason is pretty simple. More af sensor gets more light, which means more contrast, which means easier work for sensor and electronics behind it.

I think you need to add an "everything else being equal" on to that ;). My 50mm f/1.8 cannot even focus in some conditions that my other slower lenses can.

e-k

[primoz]

Not really... it's not lens which is focusing, it's camera ;) Lens can have faster or slower focus motor, but that's about it.
But anyway... normally f2.8 lenses are at least as good as f4.0 lenses, so normally everything else being equal goes for this ;)

[E-K]

True it is the camera doing the focusing. I did some additional testing and my 50mm f/1.8 did as well as my 24-70mm f/2.8 as far as being able to acquire focus (must of been remember incorrectly or I had a giant grease blob on the 50mm when I first tested ;)). That being said, the 18-55mm f/3.5-5.6 did just as well.

So what I'm saying is that the bigger factor is the lens itself versus aperture. For a normal sensitivity AF point, I doubt you would find much difference between an f/2.8 and f/4 lens (assuming they are of equal quality - same focus motor, focus type, etc...). You are not getting that much more light as the prism's for the AF sensor are looking at a specific area of the aperture (i.e. the aditional light misses the AF sensor).

Take a split prism screen for example. The prism stays at basically the same brightness until you start getting around it's maximum usable aperture.

Doug Kerr has a good write up here (http://doug.kerr.home.att.net/pumpkin/Split_Prism.pdf).

e-k