Nighttime Aerial 5DII focus

[frankcarleton]

I have recently been shooting night aerials (from an airplane) with my 5DII and (mostly) my 70-200L.  I have an issue with focus.  It isn't terrible, but I'm looking for improvement.  I think I have eliminated motion blur as the culprit - I use a gyro and am able to get enough light so that the shutter speed is at least twice as fast as the focal length.  Also the lens has IS which Canon claims buys me four stops (color me skeptical).  Some images are absolutely tack sharp, but at least half are a little soft.  I usually shoot at either f2.8 or f4 as I am struggling for light, with ISO of either 1600 or 3200.  

I am shooting center point, one-shot AF.  Because I'm in a plane - everything is at 'infinity'.

Any thoughts as to how to improve my percentage of sharp images would be appreciated.

[RobertTheFat]

Er ... I guess I'm missing something here. If everything is at "infinity", how can focus be the problem? You've presumably set the lens to manual focus at infinity.

Or maybe not. Many aotofocus lenses are capable of focussing beyond infinity; otherwise you'd be limited in your ability to calibrate them with autofocus microadjustment. So maybe you are using autofocus. But lenses sometimes have difficulty achieving infinity focus under very flat lighting conditions. I've had it happen to me at the beach, especially if it's slightly hazy. Maybe something similar could be happening to you if your lens is trying to achieve infinity focus against a night sky. I guess the cure would be to make sure that the active autofocus point always has something contrasty to look at. Or figure out where the infinity focus point on your lens is and set it manually.

Another possibility would be to increase your odds by telling the camera to use all of its autofocus points, rather than only one. If everything is at infinity, it doesn't matter which point achieves focus.

Hope this helps, but it's obviously a wild stab in the dark(!).

[frankcarleton]

Thanks Bob.  Since I use a zoom lens, the exact point of focus varies with focal length.  My kingdom for a zoom lens that is permanently focussed at infinity.

You are likely on to something with the 'flat spot' problem.  At 80 knots, I have very little time to compose the image, so I can't easily center the camera on a contrasty area for focus, then try to compose the image.  I need the camera to do all the dirty work.  Just wondering if there are any tricks I haven't thought of, besides buying a Mk III.

[cicopo]

I'll start by asking if the camera is set to AI Servo & if prior to taking the photo the AF has been started so the camera can work on predicting subject distance as you move.This assumes you are actually using AF which you do say you are.

The 5D2 isn't know as a good body to use for low light photography when it comes to it's AF. Now you've put that body in motion which has to amplify any AF issues from lack of contrast etc. I strongly suspect the keeper rate is close to "as good as it can do under the circumstances" but I'm not so sure about the lens being set to infinity and still having the AF set to on. If the AF is active it should be in charge, not the infinity setting you dialed in. What altitude do you normally shoot from? Is the AF in charge or is the lens set to infinity AND shooting at that setting? If the AF in in charge I'd do as suggested and turn on all the AF sensor points to see if it makes any difference plus try any other AF settings available.

[frankcarleton]

The camera is absolutely in charge of focus.  I have always used One Shot AF - I may try servo, but am skeptical.  The scene is moving, but the required focus distance is constant at infinity.  Basically, I agree that the camera is probably doing the best it can, but always looking to improve.

[cicopo]

If the camera & lens are set to AF then setting the lens to infinity has no bearing on your results. The moment the AF is activated the system will start the process of re focusing the lens. If you have the camera set to ONE SHOT the AF assumes your target is stationary & will lock onto whatever it saw at the moment the shutter was pressed but your aircraft will have moved before the shutter opens & closes.

[cicopo]

Here's another thought re the lens / IS. Are you giving the IS time to spin up before taking the shot & what mode are you using? Normally we are shooting upwards while you're shooting downwards which may influence just how the IS helps or hinders, especially if you don't give it time to spin up & settle.

[ebiggs1]

 I did this just one time.   It was enough!   It was in the film days and I was in a Cessna 150 with the tandem seats.

If I were to do it again I would use manual everything as I did in the film days.  Most lenses focus past infinity, not because they are zooms, but because of temperture.  Even our old FD prime teles did that.

Turn everything off and go manual.  

(Take a lot of shots, too!)