AF point arrangement

[swifty]

Anyone know definitively why PD AF points on full frame cameras are all bunched in the middle? It's one feature that I haven't really seen improved generation after generation of DSLRs.
I suspect it's got something to do with the sub-mirror size limitations that directs light to the AF system but can't really find any confirmation.
But even Sony's SLT's don't seem to have a wider spread of AF points although on the A99 there appears to be some non-selectable on-sensor PD-AF points at the periphery.
Perhaps PDAF points near the peripheral aren't accurate enough or light fall off is too great at the edge for reliable AF? :confused013
Maybe there are FF cameras with wide AF points.. If so can someone direct me to it.

[ricktas]

I actually wondered if it was done due to lens physics, in that lenses are always sharper in the centre than at the edge, so therefore it made sense to have to focus points where lenses were the sharpest. Could be wrong, as this is just my personal thoughts

[arthurking83]

Yeah! ... never made sense to me either.

I love the way the D300 is almost fully covered by AF points, and then when you look through an FX finder, they're all in the middle.
I'm always using the outermost/periphery AF points when using wide aperture values .... it's far more accurate than focus and recompose!

I suppose we just have to live with it until they decide it's a feature they think we want! :rolleyes:

As for why(they don't or haven't done so yet) .. who knows? Could be as Rick said, lens physics.. not being so sharp and hence doubtful AF accuracy.
Maybe they've simply decided that CDAF is the way of the future and that PDAF will eventually be phased out!(pun intended).

Personally I'd have preferred to see the af points not so bunched up(ie. just not so close together) as I've never and still don't find any value in flicking from one af point to the one next to it, in any direction!
I'm always flicking a couple of spots at least .. and that's on Dx not Fx where they're more bunched up.

[swifty]

Looks like it remains a mystery.
But I can't imagine there wouldn't be more requests for this feature if it wasn't a technically difficult thing to implement.
Looks like we'll have to wait til PD-AF points make it onto the sensor in DSLRs b4 we get this feature. Good thing Nikon's already doing it in their V1/J1 CX cameras.

I hardly ever use all the 51 AF point selection on my D700, just too many button press to get to the point u want. I prefer the 21 point AF arrangement and by extension, a 7X5 grid (35 points) spaced evenly to the periphery would be pretty ideal.

[Ian Brewster]

Hi Swifty,

your query took me off on a tangent from my planned afternoon reading. Here are a couple of sites that add to the knowledge, although not completely definitive. From the second one, though it appears, if you can afford the camera, AF points could be everywhere.

http://www.cambridgeincolour.com/tutorials/camera-autofocus.htm

programming4.us/multimedia/5860.aspx

Ian

[swifty]

Thanks Ian for the links although in the second link, the guy kinda gave a non-answer lol.
Anyways I've had a chance to do some brief research and it appears Rick and AK's inklings are correct and the submirror doesn't appear to be the issue (probably cos no one has overcome the engineering challenges of putting PDAF points at the periphery so they never had to try).
Anyways it appears the issue is to do with the way PDAF works in relation to the physics of light rays at the periphery being away from the lens axis (middle). The further away the less reliable as light passes through the lens diaphragm and all sorts of things happen to it (I won't go into it but its similar to why corner lens performance is poorer than center including sharpness, vignetting, CA etc.).
And by extension this is related to the image circle that's projected by the lens. Most lenses only project an image circle just big enough for the maximum sensor size of that mount. If the image circle projected was much bigger (eg. In tilt-shift lenses) then you'd overcome much of the corner problems within the sensor area but manufacturers just don't do that, for weight, bulk, cost reasons.
So even on DX where Nikon's AF sensor are closer to the edge, there would probably be a limit of how close to the edge it can be when used with DX lenses which probably has an image circle just larger than an APS-C sensor in most cases.
I wonder whether anyone has noticed a difference in edge AF point performance comparing a DX lens and an FX lens. I must admit I haven't and my only current DX lens is a 35 1.8G which has an image circle covering FX so it wouldn't be the best test lens.
But anyways that appears to be the reason. If I find further info contrary or further supporting it I'll post it.

[William]

Got this bit in relation to why the AF points are close to the centre on a full frame , This is from a 6D review

Quote from DP review : However, like other full frame SLRs, one disadvantage is that the autofocus points are rather clustered towards the centre of the frame (increasing the coverage would require a larger body to accommodate the larger relay optics needed).

I dont understand it , But you guys may - Bill :)

[Bennymiata]

The Canon 5D3 and 1Dx have focus points that are getting close to the edge on both sides, so perhaps with newer cameras, they will start to get closer and closer to the edge.

The funny thing is that according to the rules of good composition, if your subject should be one third in from the edge of the frame, why don't most cameras have auto-focus points there?
My 5D3 does allow this, but my 60D doesn't.

[swifty]

increasing the coverage would require a larger body to accommodate the larger relay optics needed

Not 100% sure what that means either.
Relay optics may indeed refer to the sub-mirror that's under the main mirror which 'relays' light to the AF module such that AF operations can be performed.

- - - Updated - - -


The Canon 5D3 and 1Dx have focus points that are getting close to the edge on both sides, so perhaps with newer cameras, they will start to get closer and closer to the edge.


Not very familiar with the 1DX AF but a quick google search shows that its probably similar in coverage compared with Nikon's FX cameras.
http://learn.usa.canon.com/resources/articles/2012/1dx_guidebook.shtml

Now using that same picture of the 1DX's AF point array, wouldn't it be great if the overlayed 5x3 grid had an AF point at each intersecting lines.