streetmarty Loc: Brockton, Ma

Hi, I have been using BBF for a little while but still have this question. Steve Perry says as long as your subject stays on the same focal plane you are locked in. Steve shows in his video a cartoon of a deer moving laterally and staying in focus because the focal plane hasn't changed. My question is what type of range will stay in focus. If the deer moves 10 feet laterally am I still in focus? There has to be a limit here. What about if the deer moves 2 feet closer, or two feet back, the focal plane has now changed correct? I guess am asking what kind of parameters does back button hold? Thank you.

imagemeister Loc: mid east Florida

It sounds to me like this is a theoretical tutorial scenerio ........As a practical matter - in the real world - you need to always be focusing on a moving target - no matter which way you may THINK it is moving ! !

jpendasulo Loc: TN

It all depends on the focal length of the lens and f stop set. Google depth of field tables for the parameters you are using.

I agree with keeping the BBF button pressed for a moving subject.

oldtool2 Loc: South Jersey

Draw a circle, put an X in the middle. Any thing on that circle line will be in focus, + or - your DOF.

Is this what you are asking?

Jim D

streetmarty Loc: Brockton, Ma

Thank you.

streetmarty Loc: Brockton, Ma

Thanks

streetmarty Loc: Brockton, Ma

I can draw a pretty big circle. Are you referring to what I see through the view finder as my drawn circle? Pretend I'm 6 years old instead of 60. LOL Thanks Jim

amfoto1 Loc: San Jose, Calif. USA

Just watched that video (https://www.youtube.com/watch?v=FzqQskGoURE) and, no, this is not what Steve is showing or saying.

He does show how when the subject stops moving... and especially when you are using a single active AF point (which is generally the most accurate method) and want to focus and recompose... you can stop/lock AF at a point simply by lifting pressure off the rear button of the camera. He goes on to mention how you can instantly start/stop continuous focus using BBF and how it gives you more control over the camera. But I don't see him encouraging you to look for subjects moving parallel to the plane of focus and lift pressure off the focusing button when you find them!

Most of the time, you will simply want to apply continuous pressure and keep AF running. Subjects start and stop moving or move erratically or you might misjudge whether they are staying in the same plane of focus or not.... So why not just keep AF running? It's a better bet that you'll get an in-focus shot by keeping AF running and updating, than if you are trying to anticipate it's movement and start and stop AF.

I agree... the "plane of focus" varies considerably depending upon focal length, aperture, and working distance. Using my 500mm lens wide open at f4 near it's closest focus distance, sharp depth of field is only a few inches. Working with a subject at more of a distance, or with the lens stopped down a bit, or if using a lens with shorter focal length, depth of field increases (and might cover some focus error)... But why risk it, if it serves no purpose? Just keep focusing. The last thing I'm going to do out in the field shooting moving subjects is get out my depth of field calculator and start doing the math. Might do so for a more sedate scenic shot.... but not with active subjects.

One place you can run into concerns with the plane of focus/depth of field is if shooting a group of people, posed in two or three rows. Particularly if you are shooting with a large aperture and relatively close with a short tele lens, focusing and recomposing can be tricky. You have to be aware that the rotating the camera. Your movement is more of an arc shape as you rotate, so your original focus point on the plane of focus might no longer be in focus, after you recompose. If you search for "focus and recompose problem(s)", you'll find some illustrations of this that might explain it better than me.

Overall, Steve's video about using BBF is quite good and succinct (and very similar for either Nikon or Canon shooters, though he's giving Nikon examples and the terms and setup methods are a little different for the Canon folks.... I have no idea about Oly, Sony or Pentax).

One thing that irks me a little in Steve's video is in a couple scenes he appears to be shooting with his lens hood reversed in the storage position. I see people doing that at times and it just seems dumb to me... Why bother carrying around the lens hood at all, if you aren't going to use it? Personally I almost always use a lens hood (only reversed for storage when the lens is in storage... when I take the lens cap off, the hood immediately gets installed in the shooting position). A hood protects the lens physically better than some thin glass filter ever could. Plus a hood cannot possibly degrade an image, actually might improve it as well as help focus speed and accuracy by keeping oblique, "scattered" light off the lens. I don't like to see someone who's calling themselves a pro encouraging poor technique like this, even if it's just an oversight.

But that's just a minor complaint. Overall, the video does a very good job explaining the why and how of BBF.

Basically, IMO, BBF allows you to use continuous AF mode as your default, which, as Steve says, gives you the best of both worlds. Without BBF, you really can't leave the camera in continuous AF mode all the time and may find yourself switching back and forth between the modes, spending more time setting up your camera than shooting with it! I first tried BBF with my Canon cameras about 10 or 15 years ago... got used to using it in a few days and have never looked back. Since then I've been a strong proponent for the technique and now I find it far more "alien" to switch BBF off and try to go back to the "old way" of shooting (I really only rarely switch it off, such as when loaning a camera to someone who isn't familiar with BBF... But only if I can't convince them to give it a try.)

Rich1939 Loc: Pike County Penna.

One more thing. I don't recall if Steve mentioned this in his video but maybe this will help you. While you are tracking your subject hold the back button in and continue to hold it while you press (release) the shutter. Shutter actuation will override the focus at the instant of actually recording the image, hopefully giving you the most accurate focus. As a side benefit it will have you in position for a follow up shot.

streetmarty Loc: Brockton, Ma

Awesome answer, thank you very much.

BHC Loc: Strawberry Valley, JF, USA

Assume a 3-4-5 (a-b-c) right triangle. If you are focusing at a subject 100 yards away and it moves laterally (at a right angle) 75 yards, it is now 125 yards away. Depending on your DoF, is it still in focus? If you were in continuous focus, yes; if not, maybe not.

Short answer: it depends......

Psergel Loc: New Mexico

Unless you are in AI Servo mode and keep the focus button depressed, the deer will be out of focus as soon as the distance from you changes. How much out of focus would depend on DOF which in turn depends on focal length, distance to subject and aperture.
If you are not in AI Servo mode (focus tracking), the only way the deer would stay in perfect focus is if it were running around you in a perfect circle with you in the center while you kept turning to face it.

Gene51 Loc: Yonkers, NY, now in LSD (LowerSlowerDelaware)

There is no exposure "locking" unless you keep the BB pressed in - in which case the camera tracks the subject. If it moves and the subject to camera distance does not change, you won't have to reacquire focus. so your other assumptions are 100% correct.

jerryc41 Loc: Catskill Mts of NY

A smaller aperture will make a larger area appear to be in sharp focus, but since you'd be swinging in an arc to capture the moving deer, the distance would increase from straight ahead. As others have said, continuous focus would be good in this situation.

streetmarty Loc: Brockton, Ma

Thank you.