autofocus - How can I effectively use the focus points (of Canon DSLR), to get accurate focus on a small subject?

I'm shooting birds with my Canon 1200D and 55-250mm lens.

I know this is not ideal lens and focal length for bird photography. But I am new to this field and before investing on expensive equipment, I would like be more thorough with the craft.

So to the scenario now. Often when I'm shooting birds with my 55-250mm, they are so far away that they appear as tiny dots in my viewfinder. Often little larger than the focus points.

Now my question is, in my frame if the bird is only slightly bigger than one of the focus points (and the rest of frame is say trees and forest), can I use that focus point to accurately focus on the bird and get a reasonably sharp image of the bird?

I have obviously faced this in the field, and the results are not so impressive. So that led me to wonder - if these focus points are designed to focus on that area of the frame - then technically I should be able to get a sharp focus on that object.

But I don't, is it because I need lot more practice, or is it fundamentally not possible?

Answer

You're going to have a very tough time shooting far-away birds with that setup. Capturing birds-in-flight is one of the most challenging forms of photography there is in terms of gear limitations and photographer skill. In addition to photographic skill the photographer must also practice excellent fieldcraft to get as close as possible to the subjects.

You might be better served to find other subject matter that will be more within the capability of the gear you have and learn how to use it on subjects that will allow you to learn progressively and see more of the differences in your capabilities as you improve. Depending on where you live, go to a park and shoot the animals that will allow you to get closer to them. Hang out near an airport and photograph the planes taking off and landing. Shoot your friends' and family's pets and children at get togethers.

With pretty much any modern AF system the areas of actual sensitivity are larger than the little markers for each AF point that you see in your viewfinder. The good news is that each one covers a larger area than you think. The bad news is that each one covers a larger area than you think. If your target is very small but there is an area of even greater contrast within the area of sensitivity, the camera will almost certainly focus on the area of greater contrast. For a look at how this works out practically when shooting, see this entry from Andre's Blog. For a look at how AF accuracy can vary from shot to shot, see this entry from Roger Cicala's blog at lensrentals.com.

The EOS Rebel T5/1200D has a very basic AF system. For action you're probably going to be limited to the center cross-type point only as it will perform faster and more accurately than the others. The EF-S 55-250mm f/4-5.6 in it's various versions is a fairly slow lens - both in terms of maximum aperture that limits what shutter speeds you can use in less than optimal light and in terms of AF speed.

The AF systems of cameras more optimized for action shooting are so much more sophisticated and configurable that moving from a basic AF system like the 1200D's to one like the 80D's or the 7D Mark II's is going to be a little like learning to drive in a Hyundai Accent with an automatic transmission and then moving to a Corvette with 6 speed manual. Yes, the skills you learn on the 1200D will be needed with the advanced camera. But a lot of other knowledge and skill that the 1200D does not require you to have (or allow you to use if you already have it) will also be needed to control the more advanced tool set.