Hi David,

Today I was shooting with my A900 70-300G combo and the new Hoya HRT Circular Polarizer, and when I tried my friend's Nikon D7000 28-300 combo, the first thing I noticed was the Nikon's auto-focus - it has little small squares in the viewfinder, rather than the little red segments that the A900 has. It felt/seemed like the Nikon has better autofocus - the little squares showing up on the viewfinder take up more "space" and areas on the viewfinder, which just makes it feel likes it's auto-focus is "better" - but I don't really know what "better" means here...

Regarding the upcoming A77, could you please provide your thoughts as to whether the A77's auto-focus is "better" or actually more developed than the A900's? I'm really tempted to trade in my A900 to get the A77, because they never gave me the firmware to remove all that awful chroma noise in its jpegs, and I would get 450mm with my 70-300G lens, and the Sigma 8-16 is a great lens, etc. But, is the auto-focus on the A77 improved over the A900? Does it cover more of the viewfinder (like I thought the D7000 did)? Apologies if I don't even know how to ask the correct question correctly here, it's always a learning process for me.

Thanks,
David

The small hashes in the A900's viewfinder, representing the AF points, are lines rather than boxes because they indicate in which direction they are sensitive to details. Line-type AF sensors are sensitive to details contrasted in only one direction. Cross-type AF sensors are sensitive to up-and-down contrasted details as well as side-to-side contrasted details, and the A900 has a cross-type AF sensor at the center of it's frame (actually, it's better than that--it's a double-cross, think "tic-tac-toe board," with an extra large line-type sensor through the middle that's activated when fast lenses are fitted to the camera). In the D7000's viewfinder, 39 AF points are represented, but only nine of them are cross-type, and their representation in the viewfinder doesn't clearly indicate which. It's the difference between stagecraft, and accuracy.

The A900 also had an unusually narrow grouping of AF points in the center of it's frame--suitable rather for an APS-C camera. They're actually just as widely distributed in the image circle as those in the A700, but there's a lot more real estate around them due to the larger sensor.

In the A77, 11 of the AF sensors are cross type, and 8 are line-type, but all 19 are represented with squares in the viewfinder (if you want them represented at all). An A700 user will notice that the standard 11 positions that they're used to are now cross-type, whereas eight more line-type sensors have been wedged in between and around these. The center AF point of the A77 is no longer a double-cross with additional f/2.8 sensor as the A700, A900, and A850 had, but a simple, single-cross-type.

Gee Kevin, you seem like you've had your hands on one of these! Care to spill the beans?

Dusty

The A900 is a waste if you're using JPEGs. It really sings with RAW... JPEGs? not so much.

The recent firmware update to the A900 improved the AF performance. I think it's pretty good now -- almost as good as my Nikon D700. With f/2.8 (or better) lenses, the double-cross main AF sensor on the A900 is fantastic. The A77 doesn't have it, so i doubt it will be as good. AF systems in APS-C bodies like the A77 generally cover a larger percentage of the viewfinder than full-frame AF systems, so in a way, you gain a bit of quantity at the expense of a little accuracy.

Regarding the Sigma 8-16, one should note that the quality control might not be that good from some reports (see the review on Photozone.de), so you may or may not get a great copy of the 8-16.

The A77 AF grouping covers about the same area as the A700 - it doesn't stray too far to the edges. This is good, not bad. The Canon 7D has sensors too close to the frame edge and often gives poor focus as a result, picking foreground points based on 'closest subject wins' algorithms - not much use if the closest subject covered by a sensor is in front of your own feet with a wide angle!

I got the impression from brief use that the A77 is faster and surer to focus in nearly all conditions, than any previous Alpha. This could have been influenced by using the 16-50mm f/2.8 though, which focuses very rapidly and (unlike for example the 24mm f/2 SSM I'm testing now) does not hunt into totally out of focus ranges.

David