Back button focusing

[Lmphotos]

I need some precise answers to back button focusing on a canon. When you back button focus as long as I don't zoom it will stay in focus right? But what if I move or the subject moves? Also of I put it in al servo mode can you focus and recompose with BBF?

 

[jbkm1994]

Just keep your finger on the button. You can focus and recompose but remember it might result in back focusing. It really depends on the situation that you are shooting. And you thought the Pythagorean theorem wouldn't come in handy...LOL.

 

[jbkm1994]

Check out this post: http://digital-photography-school.com/the-problem-with-the-focus-recompose-method

 

[TCampbell]

When you use back-button focus in "One Shot" mode the camera will focus and stop once focus is locked. When you use it in "AI Servo" mode it will continuously focus as long as you HOLD the back-button down.

Depending on camera model, you can pick which button you want to use. Usually you can pick between the "AF-ON" or the "*" button.

 

[Lmphotos]

When you use back-button focus in "One Shot" mode the camera will focus and stop once focus is locked. When you use it in "AI Servo" mode it will continuously focus as long as you HOLD the back-button down.

Depending on camera model, you can pick which button you want to use. Usually you can pick between the "AF-ON" or the "*" button.

So in all servo I could focus and recompose as long as I continuously hold it down? While if I'm in one shot if anything changes i must refocus?

 

[jbkm1994]

There is a 3 part video that you should watch. I learned a lot and I believe it will answer your questions.

 

[TCampbell]

So in all servo I could focus and recompose as long as I continuously hold it down? While if I'm in one shot if anything changes i must refocus?

If you are in AI Servo mode and want the camera to continuously focus (something you should only do when you know you the focus distance to your subject is continuously changing.. e.g. action shots) then you need to keep the active focus sensor on your target. If you "focus and recompose" then it'll just switch focus to whatever target works best for your new composition... possibly not the subject you had in mind.

Some camera bodies are very good about following focus and transitioning a focus target to a new focus point if the target is moving around in the viewfinder... but even then there's a bias in the computer that says "how long should I let the target go missing from the focus point before I give up and assume the target is never coming back?" At some point, the camera WILL eventually give up looking for the old focus target and start looking for a new focus target. The major point of back-button focus is that it lets YOU decide when the camera should start or stop focusing. If you've locked focus on a subject and you notice that at the edge of your field of view, someone else is about to get into the way, you can let go of the focus button while continuing to shoot. Since the last thing the camera locked focus on was your subject, they'll still be in focus as long as they haven't moved too much. When the obstacle moves out of the way, you can resume pressing the back-button again and it'll re-tweak the focus if needed.

Watch the 3 part Canon videos. They are VERY good. Each of the three parts is about 30 minutes, but WELL worth it. It will hopefully lift the veil of mystery regarding how and why the camera decides what to do when autofocusing. Once you know how the camera's focus system "thinks", you'll know what it'll plan to do in any situation and you'll know what YOU need to do to get the results you actually want.

Lastly... just in case anyone else is reading this thread, if you own the new EOS 1D X or 5D III then some of this information has changed. The focus system on those cameras is extremely advanced and can be tuned or customized. For those cameras, Canon has a whole 47 page guide *just* for the focus system alone and it's a "must read" guide if you have one of those cameras. That link is here: Canon DLC: Article: Master the EOS-1D X's AF System (PDF is near the bottom of this page). But this ONLY applies to the 1D X and 5D III. If you own any other model, this information doesn't apply to your camera.

 

[TCampbell]

BTW - since this thread is fairly Canon-specific, I'm thinking maybe it should be moved to the Canon section.

 

[ChrisHeathcote]

Simply put Back Button Focus replaces the half shutter button press. So to put this into practical benefits:

Camera set to One Shot - Press the button, focus locks on the relevant focus point. Release the button and recompose. Good for if you want to prefocus on a particular area. ie. motorsports. Focus on the point and then press shutter when the subject enters it.

AI Servo- Focus locks onto focus point and then tracks whilst the button is held down. Usefull for moving targets ie birds in flight, sports etc.

 

[chuckdee]

I need some precise answers to back button focusing on a canon. When you back button focus as long as I don't zoom it will stay in focus right? But what if I move or the subject moves? Also of I put it in al servo mode can you focus and recompose with BBF?

If you're in focus, you can zoom in and out and if your subject has not moved, you will be in focus.
Keep your finger on the button in servo and your lens will adjust accordingly.


____________________
Chuck Dee - AKA Chris
"My job as a portrait photographer is to seduce, amuse and entertain." - Helmut Newton

 

[TCampbell]

I need some precise answers to back button focusing on a canon. When you back button focus as long as I don't zoom it will stay in focus right? But what if I move or the subject moves? Also of I put it in al servo mode can you focus and recompose with BBF?

If you're in focus, you can zoom in and out and if your subject has not moved, you will be in focus.
Keep your finger on the button in servo and your lens will adjust accordingly.

This only works on zoom lenses which are "parfocal"... there aren't very many zooms that technically qualify. Most zooms have to be re-focused is you change the focal length after focusing.

Here's a list (it's not very long): LensRentals.com - Photo Lenses for Video

 

[BrianV]

Depth of Field is reduced as you "zoom-in" on the longer focal length of the zoom. So- if you focus at the shortest focal length of the zoom, then "Zoom-in", the focus becomes more critical because you lose depth of field. With a manual focus zoom lens, it was always best to focus at the longest focal length. "In the Day", a Zoom lens implied that the lens stayed in focus throughout the zoom range, using "Optical" or "Mechanical" compensation. "Vari-Focal" lenses required the lens to be refocused as you changed focal lengths. There were not many made, Vivitar had one in their Series I line-up.