What am I doing wrong with BBF?

[Jedicolin]

I can not seem to get the front of the train (see below) in focus using ‘back button focus’.

This is what I am doing:

When the train is in view I put the centre focus button on the front of the train, using the back button I hold it down and recompose, then when the train is near the right place I take a number of shots.

But as you can see the front of the train is not in focus. Any help would be much appreciated thank you.

5d mk3
70-300 F4 – 5.6 L IS USM @ 116mm
1/1000
F10
ISO640

IMG_3835 by colin wallace, on Flickr

[AndyMulhearn]

Re: What am I doing wrong with BBF?

Does the mk III have 3D tracking? If it does you could enable all points and set the starting AF point to be the centre. Then using BBF, lock on the front of the train and allow the camera to change the AF point as the train moves through the frame. Keep the AF Start button held down at all times.

If the camera doesn't have 3D tracking, select an AF point in the frame where you want the front of the train to be when you take the shot and then use that point all the way through the framing. You will need to keep the AF On/Start button held down and the selected AF point over the front of the train to ensure it focuses correctly.

Focus and recompose works OK for stationary subjects but for this I think you're not going to be able to achieve what you want unless the camera can shift the AF point automatically to keep a different AF point over the initial target. The 7D2 and 1DX can do this with varying degrees of success, I'm not sure about the 5DIII.

[Tigger]

Re: What am I doing wrong with BBF?

not unless I'm missing something put the camera into AF hold the back button down and it will track it, thats all I've done, although I have got limited experience with moving subjects

[Parsen66]

Re: What am I doing wrong with BBF?

not unless I'm missing something put the camera into AF hold the back button down and it will track it, thats all I've done, although I have got limited experience with moving subjects

Yup that's how I use it too

[Jedicolin]

Re: What am I doing wrong with BBF?

Yup that's how I use it too

Which AF Area Selection Mode would you suggest I use for moving steam trains?

[El Sid]

Re: What am I doing wrong with BBF?

It may be that i'm reading the post wrong but you appear to be trying some kind of pre-focus?

The way I would normally do this is to use normal one-shot focus to focus on the track at point where I want the engine to be in the composition, hold the focus button down and recompose then wait and release the shutter fractionally before the engine reaches the right point - I'd also be chicken and use the motor drive to take more than one shot to improve the chances of being dead on.

The reason I say fractionally before the point is that there is a some shutter lag between pressing the shutter and the exposure being taken as the mirror has to get out of the way.

One thought regarding what you currently do is that you might be subconsciously releasing the back button as or just before you trip the shutter - with a moving object that may be enough to catch the AF out.

[H2OJunkie]

Re: What am I doing wrong with BBF?

Since you are using a Canon I think you may be making it too complicated. Your EXIF date shows you used Auto Exposure mode. That could mean many things but gives a hint as to what happened. What I would recommend is two things. First, go aperture priority set at the f/10 you are aiming for. Then set AI Servo mode, high speed and single point auto focus. In AI servo mode the camera will maintain focus on your chosen point AS LONG AS THE SHUTTER BUTTON IS HELD HALF WAY DOWN! Your Mk III isn't a screaming fast camera but you should be able to fire off enough shots to do what you want. You will get some speed blur depending on how fast the train is moving at f/10 depending on your ISO. I think your #1 problem was probably your choice of modes. You may have used multiple points of focus which averages the focus to cover the entire image, many times compromising what you consider to be the important part. By going single point AI Servo you are telling the camera that what you are pointing at is the most important part of the image and it will concentrate on that exclusively...