Advice on focusing subjects please

DanTheManc

DanTheManc

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I've only had my DSLR a few months and I'm still learning so go easy on me :D

I have a Canon 550d and I took a few shots at Chester zoo this weekend using my Canon 70-200 F4 non IS lens. In many of my shots, not all of my subject was in focus. Here's an example. I wanted the two mongoose and the rock sharp and the background blurred. In both of the photo's, only one of the Mongooses are sharp. In the 2nd photo the rock also isn't sharp.


Mongoose2 Chester Zoo July 11 108 by DanTheManc, on Flickr


Mongoose Chester Zoo July 11 107 by DanTheManc, on Flickr

Why is this? Is it because my aperture is set incorrectly or is it my focus settings? I always have my focus set to one shot. Is this correct for stationary subjects? What scenarios should I be changing my focus settings?

Dan
 
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Wrong aperture. They are both at f/4. f/8 or higher would have been better as the second mongoose is further away than the first, so the DOF didnt reach as far as the second.
 
Thanks.

So is aperture dependent on how far apart the subjects are from one another? These were fairly close together. Only about 10-15cms apart.
 
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I've never got on with AI focus, it's a hybrid mode that uses single point focus and switches to AI servo if the subject moves.

If my subject is moving or is likely to move then I use AI servo.
 
Hi Dan, what you want to achieve is dependant on subject distance from camera,aperture, and focal length used at the time. Take a look here and you can put in your own settings to see what DOF you have to play with http://www.dofmaster.com/dofjs.html You will also have to take into account that to capture subjects like these you really need to get them when they have paused for a brief moment,having two to contend with makes it even more of a problem as chances are one of them will be moving about.
 
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DanTheManc said:
Thanks guys.

u8myufo, I'll take a look at that link. Thank you.
Click to expand...

Yes, check your depth of field on the link. You must have enough of that to cover the full depth of the subject front-to-back to render it all sharp. DoF changes with f/number, distance, and lens focal length - basically the bigger the subject is in the frame (longer lens, closer distance) the less DoF you have. And low f/numbers give shallow DoF, higher f/numbers more DoF.

You also need to check your focus technique. Use the centre point only, lock it on to the subject and hold it there with half-press on the shutter release, then recompose the shot as you want it. If depth of field is critical, you need to optimise it by focusing about one third in to the zone you want sharp.

With subjects that are changing distance rapidly, use Servo and enable the focus point closest to the most important part of subject. It's not 100% reliable though, and unless your technqiue is really good it's easy to let the AF point slip on to the background where it will immediately refocus and it'll take you a moment to get it back on target.

The trick is to use knowledge and experience to make the best compromise of all the various settings options, then apply sound technique. It takes a bit of time and practise, but understanding how the AF works and the DoF parameters is key.
 
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