3D image tracking - the size of the bird in your viewfinder matters

Black Tern, Red Deer, Alberta - the difference is how far the subject was away from the camera

3D image tracking is very handy when trying to photograph flying birds, although there are a lot of issues with it working well all the time.  Some of them I have already discussed in a previous blog.  While tracking black terns in Red Deer, I noticed a relationship between proximity to the camera, ability to get a focus lock, and the usability of any resulting image.  

The further away your subject is, the easier it is to lock focus and track it.  The exception is when there is a lot of background clutter.  The greater distance makes the bird appear to travel slower, the same way that a plane can travel at 400 km/hr but seem to only crawl across the sky.  As the bird gets closer, it can dart across the field of view in no time at all.  Your motion of following the subject has to be that much faster so as to keep the subject somewhat centered.  So, it turns out there is a relationship between the size of the subject in your view screen and the ability to get a lock.

It doesn't end there, though.  If the bird is flying straight and level, keeping it in the viewfinder is simple.  If it is moving erratically, tracking is successful indirectly to its image size.  Small and distant is easy, as there is more room for the bird to move within the field of view.  As the bird gets closer, it can move out of the field of view quickly, and your motions have to be that much faster to keep the creature in it.

Then there is the issue of capturing an image with some detail in it.  Lower resolution sensors mean less detail.  Less magnification from the lens means less detail. Farther away means less detail.  Essentially, there is a sweet spot with whatever equipment you are using where the bird can be tracked successfully by the user, and the resulting image has enough resolution to meet your needs.

The trick is to take lots of photos, checking exposure (backlighting means +2 or more stops for exposure) as you go, and to try tracking when the bird is closer to your position.  If you use a zoom lens, there are some adjustments you can make to keep the bird's image an acceptable size.  But the issue remains:  A smaller subject is easier to track but will produce a poorer image with reduced resolution.  More magnification means harder tracking but improved images.  One way around this is to take lots of photos.  Out of 100 shots, maybe you get 10 that have some value, and one or two that are exceptional.  You may only have to take ten photos when zoomed out or if the bird is at a distance to get a nice capture, but it may not have the resolution you want.

In the end, if you can't use the image, what good is it?  Find that sweet spot between your equipment, your ability to track, and the bird's location, and you will end up getting better and more usable results.

Thanks for reading.

Eric Svendsen     www.ericspix.com


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