HELP! Infinity Focus Question

[RadarContact]

Hi all,

I'm having trouble understanding infinity focus and if I can use it for shots of airplanes flying. I have the Canon EF-S 55-250mm lens. My understanding is that it can focus everything beyond 1.1m. My problem is that some things are wonderfully in focus, but others are blurry. See example below.

Perfectly in focus: http://i.imgur.com/WMhKPkF.jpg
Blurry and out of focus: http://i.imgur.com/YNSxyMm.jpg

What am I doing wrong? I would like all of my shots in focus but as you can see I'm only getting some in focus.

Any help would be GREATLY appreciated.

Thanks!

 

[Derrel]

I would agree. The shot with the larger image of the aircraft is what is called "dead, nuts-on" in-focus. The other shot...has the aircraft's main fuselage just a wee bit out of the perfect focus zone. At long distances, like beyond 50 meters, most AF lenses have very hair-trigger focusing travel, meaning that a tiny, and I mean tiny movement of the focusing ring can shift the focus anywhere from 10 feet to fifty feet. At longer ranges, an error of 10 to 50 feet might not be that big of a deal, since depth of field will usually "cover" smaller focusing errors. Sometimes, a focusing error shows up. As in your second photo...that's juuuuust almost in focus, but not quite perfect.

I saw the same thing with my ex-wife's D40 and 55-200 VR telephoto...it had a pretty simple, limited area AF system, and the thing is, the lens itself is "slow" in terms of aperture; at longer distances, the in-focus and out-of-focus differentiation is not really clear-cut, so the typical phase detection AF system that a d-slr uses can basically declare "focus=achieved!"...and yet the picture will still be a wee bit off on the focus. Slowish consumer zoom lenses like that, are in my experience, often prone to these types of not-quite-right AF issues.

As your shots show, there is dead-on, totally NAILED IT! focus [ http://i.imgur.com/WMhKPkF.jpg] ...and then there's "Hmm,darn it...not quite perfect focus". Maybe try using a group AF focusing approach, more points active. That's one possible way. What AF pattern and mode were you using?

 

[RadarContact]

I would agree. The shot with the larger image of the aircraft is what is called "dead, nuts-on" in-focus. The other shot...has the aircraft's main fuselage just a wee bit out of the perfect focus zone. At long distances, like beyond 50 meters, most AF lenses have very hair-trigger focusing travel, meaning that a tiny, and I mean tiny movement of the focusing ring can shift the focus anywhere from 10 feet to fifty feet. At longer ranges, an error of 10 to 50 feet might not be that big of a deal, since depth of field will usually "cover" smaller focusing errors. Sometimes, a focusing error shows up. As in your second photo...that's juuuuust almost in focus, but not quite perfect.

I saw the same thing with my ex-wife's D40 and 55-200 VR telephoto...it had a pretty simple, limited area AF system, and the thing is, the lens itself is "slow" in terms of aperture; at longer distances, the in-focus and out-of-focus differentiation is not really clear-cut, so the typical phase detection AF system that a d-slr uses can basically declare "focus=achieved!"...and bit a wee bit off. Slowish consumer zoom lenses like that, are in my experience, often prone to these types of not-quite-right AF issues.

As your shots show, there is dead-on, totally NAILED IT! focus [ http://i.imgur.com/WMhKPkF.jpg] ...and then there's "Hmm,darn it...not quite perfect focus".

Thank you so much for explaining it. You are right, the second shot was almost the full 250mm focal distance. Thanks again!