500mm vs 600mm?

Steve Balcombe wrote:

BirdShooter7 wrote:

If you are suggesting that there is no advantage of having an f/2.8 lens vs f/4 lens vs f/5.6 lens... when it comes to AF in a given lighting condition (particularly low light) then you don't have a clue what you are talking about. That seemed to be what you were suggesting in your original post.

Yes giving more light to a lens with a max aperture of f/8 will not help if AF when the sensors are only good to f/5.6. I don't know of anyone personally who believes such a thing. It seems like you are just making this statement to make yourself feel big.

WilbaW could work on his bedside manner, but on this occasion he's right. It's easiest to explain with a diagram:

Schematic!!

Phase detect sensors work by looking at a small strip of light from one side of the lens and another from the opposite side, and comparing them. In the diagram above, the blue discs represent the aperture and are drawn in correct proportion, e.g. f/5.6 is half the diameter of f/2.8. The black rectangles represent the two parts of the aperture that an AF sensor looks at, and the difference between the f/2.8 sensor and the f/5.6 sensor is that the f/2.8 rectangles are further apart. The phase detection process is not triangulation as such, but it will do no harm to think of it that way - and by being further apart the f/2.8 sensor can be more precise.

Starting with the f/2.8 sensor on the right, you can easily see why it only works with an f/2.8 lens. With an f/4 or smaller lens, the sensor is trying to look at an area outside the aperture of the lens.

The f/5.6 sensor, on the other hand, can use an f/2.8 lens right down to an f/5.6 lens. (I'll come back to the f/8 one.)

Now we come to the key point for this discussion. We say an f/2.8 lens is two stops 'brighter' than an f/5.6 lens, but the light arriving at the aperture is not brighter, it is the same brightness covering a bigger area. It's only when focused into an image that the end result is increased brightness. The AF points used a small fixed area of the aperture only, so provided those black rectangles fall within the blue disc, any additional area (i.e. a faster lens) makes no difference. The f/5.6 sensor will perform exactly the same with an f/4 or f/2.8 lens. The f/2.8 sensor won't work at all with an f/4 lens and a brighter subject would make no difference to that.

The f/8 case is interesting. As I've drawn it, the AF sensor is roughly half in and half out of the aperture. This gives an insight into why an f/8 lens+TC combination might sometimes work with an f/5.6 sensor, but might give unsatisfactory performance. It also explains why f/6.3 lenses like the Sigma and Tamron tele zooms works pretty well - I haven't illustrated this but I'm sure you can visualise how the AF sensor falls mostly within the aperture. In reality, depending on the precise specifications of both lens and sensor, it might work just as well as an f/5.6 lens.

Also not illustrated, but easy to visualise, is the case of an f/2 or f/1.4 lens - the extra area has no effect at all on AF performance, because all that extra aperture falls outside those black rectangles.

Hope that helps.

Good explanation.  Thanks.  I'm curious whether IS comes into play or is independent.

even if f/2 or f/1.4 does not help with AF, I imagine there are exposure advantages.