70 200 f2.8 vs f4: which one focuses faster?

Started Dec 16, 2008 | Discussions thread
mmullen Veteran Member • Posts: 4,285
Re: I have canon 70-200 F4 IS. F4 is too slow to indoor basketball

John_A_G wrote:

Mike - iso performance isn't the same as focus speed. You can't'
change the laws of physics - f2.8 lets in twice the light as f4.

Focus speed is not determined by the maximum aperture of a lens. As I have already pointed out, the 70-200 f4 IS focuses faster than the 70-200 f2.8 IS. This is per my tests and I have also seen at least one lens review that confirmed my measurements with different copies of the same lenses.

My 70-200 f4 IS focuses MUCH faster than my 85mm f1.8 and will also focus more reliably as light levels drop. I am a strong believer in basing knowledge on real world empirical evidence as opposed to assumptions or theory. I can not explain why some smaller aperture lenses focus better in low light than their large aperture counterparts except to say that the absolute light levels are not the controlling factor. My unproven theory is that the sharpness of a lens wide open is more important than one additional stop of light. That could explain why all of the super-telephotos are such blazing fast focusers even the ones with maximum apertures of f4.

The only real question in this regard is what is the threshold EV
where focus speed is impacted by the difference between 4.0 and 2.8.

As I have already pointed out, many large aperture lenses have low light focus performance that is surpassed by lenses with smaller maximum apertures. Sometimes the amount of difference is substantial. According to your theory, this is not possible.

But there's no doubt I can get focus quicker with my 85mm 1.8 than
say my 24-105 f4 in low light situations.

One example of a larger aperture lens focusing faster in low light than a smaller aperture lens does not prove that focus speed in low light is dependent upon a lenses maximum aperture, especially when there are plenty of counter examples. It would only take one counter example to show that a lenses maximum aperture is not the controlling factor of focus speed in most low light situations but there are plenty of counter examples.

I don't want to sound like I'm lecturing but it is important to avoid the temptation to jump to conclusions based upon applying theory to your understanding of the factors involved. It makes so much more sense to use real world controlled testing to see what actually works.

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Mike Mullen

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