December 1, 2008
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AF point spread on full-frame cameras
Okay whenever a new full-frame camera body comes out, I complain about the “AF point spread”. Even the high-end, pro-series flagship models.
And people speculate and wonder about why autofocus points are still so tightly clustered towards the center of the viewfinder…
So I thought I’d blog about it. Actually, Canon recently did an interview surrounding the release of the Canon 5D mk2. I’m too lazy to find it, but I do believe that this point came up, the fact that the 40D / 50D have their focus points so spread out around the viewfinder, while the 5D / 5D mk2 have the same focus point arrangement yet it is considerably more clumped towards the center.
There are two things that we need to understand about this situation.
1.) They’re actually using the same physical SIZE autofocus module in the crop / full-frame cameras. It’s just that the crop sensors are, duh, cropped, and so the autofocus spread APPEARS wider.
So, when we consider the 40D versus the 5D and the D300 versus the D3, in the viewfinders it LOOKS like this:
So, the size difference is definitely significant. And one of the main reasons why I simply LOVE my D300. (And a reason why all my 40D shooting friends LOVE their cameras…)
2.) The REASON that they don’t develop larger autofocus modules for the full-frame appears to be this, based on the Canon interview and response to why the 50D has such good AF point spread versus the 5D mk2:
Basically yeah, it is a technical limitation caused by the angle at which light hits a full-frame sensor. On full-frame at wider angles, light hits the sensor at quite an angle sometimes, and I guess this kills the autofocus accuracy. The crop-sensor lenses however usually receive light at a better (perpendicular) angle, so they can AF correctly even at the apparent edge of the frame, since it’s not nearly the edge of the true full frame…
It may have a little to do with the size of the image circle that most full-frame lenses create, but then again DX lenses create a smaller image circle and yet lenses like the 12-24 DX with an equivalent wide end of 18mm still focus perfectly fine at it’s outermost AF points.
It probably also has a bit to do with light falloff, or vignetting. On full-frame MANY lenses exhibit
Anyway, the hope is that full-frame camera makers eventually find a way to spread out their AF points a little more. Maybe if we get a more complete line of new high quality lenses like the latest 14-24 and 24-70. The older Nikon 70-200 2.8 for example definitely has a limited image circle, and I dunno how it would work on full-frame with more spread-out focus points. Vignetting being the reason, in this case…
Anyway, still waiting on the D3X official release, but I expect it within the next 24 hours…
Take care,
=Matt=[EDIT] D3X is announced now, actually. At midnight on the dot, eastern USA time…
http://www.dpreview.com/news/0812/081201nikond3x.asp
Specs are as predicted, D3 body with 24.5 MP sensor, ISO 100-1600 native with 50-6400 non-native, 5/7 FPS, and no video recording. Aces, Nikon, aces…
Take care again,
=Matt=
Comments (4)
Good to know. I hope they find a way to do it soon!
ah HA! I knew it…sad…but i guess there is something more to give up. I pretty much always use my perimeter AF points, and sparingly using my center…idk if i can change!!!
the nikon coverage is still more than canon so they don’t get off the hook that easy! change your sharp diamond shape canon!
@soundz - I actually would not mind a more “diamond” shape like Canon’s 40D, if the focus points were cross-type and more powerful. Everyone talks about how perfectly placed the 40D / 50D (etc) focus points are at the 1/3 points in the frame. Very perfect for framing creatively.
I mean it’s true that the D300 really takes the cake for AF point coverage, but in reality I don’t use those far-edge focus points that often at wider angles since they’re not cross-type. Accuracy is tough any wider than 20/24mm DX. I can still get amazing shots with my 17-55 2.8 wide open at the far edge focus point and 17mm, but not as consistently as I know is possible with cross-type focus points…
But in reality, I’ll do just fine using either the D300 OR the D700, since my focusing technique and methods are solid enough to guarantee me keeper shots.
But still, just for the sake of progress I’d like to see the D4 / D400 / D800 (you get the idea) …IMPROVE the cross-type focus point spread.
=Matt=