Visit MPB Visit MPB Visit MPB

K5II - centre focus point - AF at f2.8 - What?

JohnX
Posted 25/04/2013 - 09:34 Link
I saw a post elsewhere that commented on how the centre focus point of the K5II will work at f2.8, (in the horizontal plane), and that this was an improvement on the K5 et al.

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?
Edited by JohnX: 25/04/2013 - 09:35
johnriley
Posted 25/04/2013 - 09:38 Link
It sounds nonsense to be honest.

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?
Best regards, John
johnriley
Posted 25/04/2013 - 09:42 Link
Maybe what this was referring to was using the 1.7x AF converter. The original instructions say that to ahieve reliable AF the manual focus lens used should be at least f/2.8. As the converter loses light, then at the time it was issued no doubt this was right.

Whether even that still holds true I don't know, perhaps someone with a 1.7x could try it and see?
mille19
Posted 25/04/2013 - 10:18 Link
I use the 1.7AF converter with the DA* 300mm F4 and it works OK
JohnX
Posted 25/04/2013 - 13:38 Link
Thanks guys, but no mention of using a converter, although I can understand how/why this would be better with improved AF.
Mike-P
Posted 25/04/2013 - 17:44 Link
JohnX wrote:

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
JohnX
Posted 25/04/2013 - 19:02 Link
Mike-P wrote:
JohnX wrote:

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?
Edited by JohnX: 25/04/2013 - 19:03
sbrads
Posted 26/04/2013 - 15:24 Link
JohnX wrote:


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.
Edited by sbrads: 26/04/2013 - 15:25
JohnX
Posted 26/04/2013 - 15:59 Link
I appreciate all your efforts, but I still am not getting it.

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.
johnriley
Posted 26/04/2013 - 16:03 Link
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.
Best regards, John
Mannesty
Posted 26/04/2013 - 16:24 Link
johnriley wrote:
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.
Peter E Smith - flickr Photostream
JohnX
Posted 26/04/2013 - 17:19 Link
This is a link to the source of my confusion. link

Apologies; it's on another forum
Edited by JohnX: 26/04/2013 - 17:36
johnha
Posted 26/04/2013 - 20:50 Link
I think it's a DoF issue (the AF systems ability to accurately focus wide aperture lenses). IIRC in a lot of the pro Canikon bodies, many of the 'outer' AF areas are switched off when fitting some fast lenses as they're not sensitive enough to 'see through' the DoF of the wide-open lens.

Otherwise shots might be out of focus and photographers would be complaining their new camera doesn't work.
JAK
Posted 27/04/2013 - 01:12 Link
JohnX, I'm as confused as you are!

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?
John K
Edited by JAK: 27/04/2013 - 01:16
sbrads
Posted 27/04/2013 - 08:54 Link
I'm confused as to what the question is, but this info may help. The AF sensor's f-stop rating has much to do with how far apart the 2 halves of it are spaced. A dim (i.e. small) lens will throw a narrow beam straight between these and miss them completely. A bright lens has a wide enough beam to hit them and function.

Add Comment

To leave a comment - Log in to Pentax User or create a new account.