Autofocus speed

Sansovino

Sansovino

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I am a bit confused about how autofocus works - I have a Canon 400d, which may or may not be relevant.

I am missing alot of wildlife shots because my lens can't pick out the subject (it's a Canon 75-300mm IS) and by the time it has focused, the subject has gone - especially true when trying to photograph a flying bird. The lens often tries to focus closer first, then further away, even if the subject is in the distance. If I bought a better lens, say the Canon 70-200 f/4 L (non-IS), would it be quicker and more accurate at picking out a subject, or is it the camera itself that is at fault?

Any help would be appreciated. What focusing points do people use when photographing wildlife?
 
AIming at moving animals takes a lot of practice. However, I too have that same lens and it's not the best for birds in flight. It works great for motosport, but I've never managed to catch a bird on the wing myself either...

Whether this is down to technique or the equipment I'm not sure - I do know that, fully extended, it needs a lot of light to work well.

I've been thinking about hiring the 70-200 f2.8 to see if it is any better but will wait until I go somewhere special to use it.

Steve
 
I really wouldn't be able to construct a post to explain how it works but it comes down to the lens not the body....so yes the 70-200 f4 would focus quicker....hopefully someone will be along soon to tell you why..

Things I do know though....

USM/HSM motors normally quicker I believe....

The bigger the aperture the more light is let into the lens and therefore the easier it is to focus..
 
try just using the centre spot for focus
I don't about that particular lens, but some Canon zooms allow you to select a minimum focus distance, go for the longer setting
 
A quick potted tome on the workings of AF on Canon EOS setups.....

The image is gathered through the lens and passes through the mirror (yes, through the mirror) and onto the sub mirror behind. It then gets reflected onto a prism and through a focussing lans onto the AF sensor.

This sensor has several areas (focus points as seen in the viewfinder) that will measure the contrast (intensity) difference between adjacent pixels. The body then calculates where to position the lens based on the percieved overlap of the out of focus image. The command sent to the lens is a digital byte or bytes and the lens will move to this position.

Things that will help you here.....
Select centre point AF only and position the centre point over a high contrast differential boundary.....ie, the edge of the bird against the sky.
Use AI servo mode to keep the focus tracking active.
Make sure that the main mirror is clean....they have a habit of developing a bluish tint to them and this impacts AF speed and accuracy.
Use a lens with a wider aperture...the more light that the AF sensor receives then the bigger potential contrast differential it has to work with.

Those details are just the wooley basics to give you something to digest.

HTH

Bob
 
The best technique I've found for tracking birds in flight is to zoom out a bit while the camera finds focus (using the central focus point since it tends to be the most sensitive and accurate) then zooming in while keeping the bird in the focus bracket. Takes a fair amount of practise but it works for me!
 
Nod said:
The best technique I've found for tracking birds in flight is to zoom out a bit while the camera finds focus (using the central focus point since it tends to be the most sensitive and accurate) then zooming in while keeping the bird in the focus bracket. Takes a fair amount of practise but it works for me!
Click to expand...

This success of this depends heavily on whether you are using a parfocal zoom or not.

Bob

Edited due to a bum steer I gave....oooops!
 
75-300mm is not known for being super fast focusing. It's quite a small aperture lens and even with the USM it can struggle in the gloom. If the lens is causing you to miss shots you should change the lens. the 70-200 F4L is a good one but not cheap, Sigma do some decent lenses in this range and should be better than the Canon. Alternatively seek out a Canon 70-210 like mine, smaller reach and no focusing problems whatsoever, pretty cheap too. You've got a decent body so a bit of a crop won't be a problem.
 
I always try to use the centre focus point, and with servo mode set. However, I think that trying to focus on the edge of the bird with that focus point is currently beyond my skill. Quite often, my lens goes so far out of focus that I can't even see the bird through the viewfinder because the lens is focusing 3 feet away! :bang: I then have to focus on a tree or house and try again, by which time the bird is in the next timezone!

I'm hoping that a faster focusing lens will partly solve these problems, but, Canon Bob, your comments imply that the camera body has a lot to answer for too, and that I may need to upgrade to a more serious camera (when budget allows). In the meantime, I will try and improve my technique and look at alternative lenses.
 
dolomitefan said:
Alternatively seek out a Canon 70-210 like mine, smaller reach and no focusing problems whatsoever, pretty cheap too. You've got a decent body so a bit of a crop won't be a problem.
Click to expand...

Now there's a statement.....

I have the old 70-210 (1989 I think) and it's a cracking lens BUT it definitely focuses slower (due to the older technology I guess). I bought the 70-300 IS because I wasn't getting the shots on the other lens.

And yes, as mentioned before, if you zoom out to find the birds, focus, then zoom in, you'll lose your focus. You need to have a reasonably well focused lens to start with, then zoom in, focus again, then shoot.

You may find that some of it may be down to camera shake too. Up the ISO and shoot at around 1/500 if you can.

Steve
 
Sansovino said:
....I'm hoping that a faster focusing lens will partly solve these problems, but, Canon Bob, your comments imply that the camera body has a lot to answer for too, and that I may need to upgrade to a more serious camera (when budget allows). In the meantime, I will try and improve my technique and look at alternative lenses....
Click to expand...
Your 400D will do the job just fine when mated with the right lens.....spend your money on a better lens and don't worry about the body for now.
It's worth popping the lens off and looking at the mirror....does it look silver or has it got a bluish tint to it?

Bob
 
My 70-210 is the f3.5-4.5 USM version. They also did a non USM f4 version which was one of the first L lenses. I must admit selection of the focus point is pretty critical as the camera will always default to the nearest object, centre point is normally pretty good for most things.
 
Canon Bob said:
This success of this depends heavily on whether you are using a parfocal zoom or not.

Bob

Edited due to a bum steer I gave....oooops!
Click to expand...



Oops! Forgot to say that I keep the AF mode set to continuous and the shutter release 1/2 pressed, so the camera keeps the focus as the focal length changes. (Works using a Nikon D200 and either a Nikkor 70-300 VR or a Sigma 170-500)
 
The advantage of parfocal lenses is that the focus doesn't change as they are zoomed. This means that AI servo is tracking the subject and making small corrections as the distance changes. Non-parfocal zooms lead to far bigger changes in the required focus position and only slowly extending the zoom will allowing accurate tracking.
Constant aperture zooms also have the advantage that the contrast differential will remain the same through the zooming process whilst the variable aperture zooms will be adding in a reducing differential as they extend.

Bob
 
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