Is there a trick for zooming out and focusing?

[hank1105]

As some of you already know I picked up the 70-200mm f/4 last week. One item that I seem to be screwing up on is zooming out of the subject and still keeping focus. Like a dog running towards me. I had a few photos that were focusing on the back of portion of the subject, but not on the subject itself. Just wondering if there are any tricks to get nice clear captures of subjects that are moving towards you? I will continue to practice. Thanks for any replies.

BTW: I am using AF not MF.

Hank

[mdude85]

I suppose you could figure out exactly where you want the dog to be positioned when you shoot the picture, and then keep the lens at a fixed length until the dog reaches that point. Then shoot the picture in focus at that length.

[Scottes]

Are you using One Shot, rather than AI Servo?

In One Shot, the dog is running towards you and you depress the shutter button which AFs on the dog and then the shutter releases. Well in that time between the AF and the shutter release the dog has moved closer, so now it's closer than the AF point, and you get an image with the dog's butt in focus but his head isn't in focus.

AI Servo is supposed to predict where the dog will be when the shutter is released so the above shouldn't happen. Note that it will often blow the shot because it's not perfect, but AI Servo will increase the number of focused shots.

In the old days, before auto-focus and AI servo, photographers would guess at where the dog was going to be, manually focus on that point in front of the dog, and then click the shutter.

[hank1105]

I am not using AI servo. Is that Canon's algorithms at work? Well at least I am glad that it isn't me so much, basically everyone experiences this issue. I think, not positive, that the 310d hack allows for AI servo, I could be mistaken. Have to research that.

Thanks.

Hank

[ejwebb]

It also helps to use a smaller aperture which provides a wider depth of field. While this won't help you get an exact focus on the head it will give the head better or more acceptable focus than you would get with a larger aperture and the subject move away from the focus point.

It's easier to get acceptable shots of moving ojects if you shoot them moving accross the field of view rather than directly toward you. Panning the camera with the subject movement in such shots helps as well.

The hack enables a choise of focus methods but AI-Servo does not work completely. If you search you will find that there is a work around to use it but I'm not sure how effective it is.

[hank1105]

Good advice. Please excuse my noobness, but does lower apeture equal better depth of field? I will definitely give what you said a try.

I did notice that the AI Servo is not fully hacked as of yet.

Hank

[ejwebb]

Yes, smaller aperture (meaning larger f/ stop number - f/11 is smaller than f/4) provides larger depth of field. I forgot to mention that the length of your lens also impact the depth of field - longer lens = narrower depth of field and shorter lens = wider depth of field. so when you use maximum zoom you are also decreasing the depth of field.

You should experiment with some stationary objects at different apertures and zoom points to get a feel for how it all comes together.

[hank1105]

Thanks again ejwebb for the info. I definitely have to experiment more, time is hard these days.

Hank