What is the cross type AF in Canon 20D? Better AF in future?

[f_k2]

In the 20D manual it talks about the centre crossed type AF sensor, and how

it's supposed to be better with f2.8 or faster lens. What is it supposed to be

and how does it work? By the way, I think the next upgrade for the 30D should

have a bigger viewfinder and faster AF since many entry DSLRs now have better

AF systems- 400D, Sony alpha, Pentax K100, D80, etc. I think it needs more AF

points and better low light conditions. Thanks

[raphael_emond1]

"entry DSLRs now have better AF systems- 400D"

 

the AF system in the 400D is the same as the 30D, and one of

the best on this range of price.

 

f/2.8 and faster lens have better accuracy than on others models

the 20D/30D/400D use this type of focusing

 

Normally, if the AF system cannot focus because of lowlight, it's too

dark to shoot anyway. If you need flash, it provide the AF assist beam.

[leya216]

Is this with only Canon 2.8 lenses or with third party lenses as well?

[mormegil]

SLRs can get better AF with a narrower depth of field, same way you do manually.

 

F/2.8 lenses have a narrower depth of field, but in addition the "High Precision Cross Type Sensor" goes into triple accuracy with these lenses.

 

With slower lenses, I think the cross type is already more accurate. But with 2.8 it seems to shine. This is why I'm saving for the 70-200 2.8L IS instead of getting the 4L IS.

 

 

Also, the "cross type" means that it autofocuses on contrast both horizontally and vertically. You'll notice that most of the sensors regions autofocus in one direction only. Try focusing on a surface with black on one side and white on the other separated by a single straight line (think picture frame edge). The outer focus points only focus when the line is perpendicular to the AF sensor strip. The cross type goes in both directions, so either way works, and it's more precise.

[jim_larson1]

The cross type is "more accurate" because of the ability to detect contrast both vertically and horizontally.

 

But a horizontal sensor seeing the same horizontal contrast as a cross sensor (at F4->F22) will be just as accurate.

 

The "high precison mode" is stated to be accurate to 1/3 DOF. The "standard precision mode" is stated to be accurate within DOF. But that information dates back to the EOS-1V days.

 

The prosumer (20D/30D/400D) AF systems use different sensors than the 1 series camera. Canon has never published the capabilities of the prosumer sensors.

 

The most obvious difference is that the AF area for each sensor is HUGE compared to the 1 series cameras. (about 2.5 times the area shown in the Viewfinder) Even assuming the same 1/3 DOF accuracy, there is no telling where the camera will choose to achieve focus. Usually, it will be on your subject. Sometimes, it will be on a tree behind your subject.

 

You should most noticably see the difference when shooting in low light. . .where the "standard precision" AF may struggle while the "High precision" locks in.

 

For critical applications (ie, when using something like a 85/1.2 where the DOF is 1.3 microschnecks), the prosumer center sensor may still give you trouble even in high precision mode.

 

***

 

The 70-200/2.8L-IS has many merits over the new 70-200/4L-IS. Considering the relatively close prices of the two lenses, the 70-200/2.8L-IS is a good choice. (but the 70-200/4L (non-IS) is still quite a bargain)

[lovcom_photo]

Ralph wrote "...Normally, if the AF system cannot focus because of lowlight, it's too dark to shoot anyway. If you need flash, it provide the AF assist beam...".

 

Not necessarily true....just because a shot is too dark for the AF to lock does not mean the shot is too dark anyways. I made many picutures inside very dark Italian churches that were too dark for AF but not too dark to take the shot. Those pictures came out really good.

 

The real problem is that Canon chose to give their 20D/30D just one cross sensor...that is the real problem

[awindsor]

Unless it is perfectly dark you can always make an exposure with a long enough shutter speed. Of course you need a tripod, or other support, to hold the camera steady for such a long exposure.

 

My 20D merrily autofocuses using an f2.8 lens in light conditions where I am shooting 1/40 f2.8 ISO 3200 . Actually this probably underexposes but I was already pushing my ability to handhold the 90mm lens.