K5II - centre focus point - AF at f2.8 - What?
Cameras need light to focus like eyes do and the last time I checked Pentax were advocating lenses of f/5.6 or wider, but the whole world has changed since then. No doubt the current figure is around somewhere, and it might be f/8, but I don't think it has much bearing on things these days.
Perhaps the poster, wherever he or she was, was a little confused?
Whether even that still holds true I don't know, perhaps someone with a 1.7x could try it and see?
This morning I saw another post where the OP was looking forward to a firmware upgrade enabling his Canikon to focus at f8.
Guessing he has a 5D MKIII.
At the moment the MKIII will only AF up to f5.6 with a teleconverter so for instance the 70-200mm f4 will AF with a 1.4x but not a 2x. Canon are releasing a new firmware sometime this month to enable AF up to f8 which means the 2x will then be usable.
Good news for the 400mm f5.6 users as they will be able to use a 1.4x to get AF 560mm f8
This morning I saw another post where the OP was looking forward to a firmware upgrade enabling his Canikon to focus at f8.
Guessing he has a 5D MKIII.
That's right.
I'm still not at all clear though why/how this works, especially as Pentax say AF at f2.8 is an improvement (this from the original specs for the K5II). On the basis that AF gets more difficult/impossible at smaller F numbers, surely then the converse is true, so why shout about 'improving' AF points so that they work at f2.8? They do anyway, don't they?
I'm still not at all clear though why/how this works, especially as Pentax say AF at f2.8 is an improvement (this from the original specs for the K5II). On the basis that AF gets more difficult/impossible at smaller F numbers, surely then the converse is true, so why shout about 'improving' AF points so that they work at f2.8? They do anyway, don't they?
Pentax made the AF sensor f2.8 so that it will work in lower light when using a lens that's f2.8 or brighter. That's how the -3EV rating was mainly gained.
I understand f2.8 in relation to lenses, but not an AF system.
Surely it's the lens maximum aperture that is responsible for the amount of light 'seen' by the AF system and that would be the same regardless of the camera/AF system.
AF is done at full aperture, so the brighter a lens the more light is available for AF. An f/2 lens would allow more light through than, say, an f/11 lens so AF would find it easier to focus, just like our own eyes would.
It's for this reason that I've switched from using a DA* 60-250mm 1:4 lens to a Sigma 70-200mm 1:2.8 for night time racing work. The DA* 60-250mm was hopeless at focusing on the K5 in low light, it is much better on the K-5II, but still not as good as the Sigma.
There is no doubt in my mind that the DA* lens, once focused, produces slightly sharper and more contrasty images than the Sigma, but that's no benefit at all if you can't focus it accurately. I'm getting keepers with the K5 + Sigma, I rarely did with the DA* lens.
Otherwise shots might be out of focus and photographers would be complaining their new camera doesn't work.
John K
(Must be an issue only if your name's John )
Edit: I'd have thought more light / wider aperture, the better the AF functions.
Further edit: Could it be too much light overpowers the sensor or whatever it uses to ascertain the focus point?
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2388 posts
15 years
South London
This morning I saw another post where the OP was looking forward to a firmware upgrade enabling his Canikon to focus at f8.
Help me please. What is this all about?