Is there "permanent" auto focus

alaios

Hi there,
lets say that you are a wedding photographer and during the ceremony you are standing roughly at the same area as you want to wait to catch some facial expressions of the B&G .

In one of the weddings as I was chasing these expressions I have been using manual focus to keep the focus point fixed. I was wondering if there is any mode where you can use autofocus to get the focus points and keep it there for the time you and your main subject would not be moving. What is the term for this feature?

Regards
Alex

ziggy53

Some Canon camera/lens combinations have a "lens" feature which allows the photographer to temporarily override autofocus by pushing a button on the lens.

Primarily this is on very long lenses and I believe that you primarily use the button so that if you know that a foreground component moves between you and the subject you won't shift focus to that component. When the foreground component, another animal in a herd of animals, for instance, moves out of the scene, you release the lens button to resume autofocus of the primary subject.

I believe that this is called the, "Lens AF Stop Button", and I believe that it is only found in some telephoto Canon lenses. (On professional Canon cameras the button function may also be reassigned to limited other functions.)

More commonly, "most" Canon EF/EF-S series lenses have a switch on the lens which allows either AF or manual AF operation of the lens.

---

In practice, and considering your wedding scenario, I want the camera/lens in One-Shot AF mode during the entirety of the day. (AF Focus selection may vary through the day, however.) The reason is that every once-in-a-while the AF may not perfectly focus on the intended portion of the scene that I want, or I intentionally choose a different subject matter in the same scene.

The reason is that mostly I use larger apertures to limit the DOF to the selected subject matter (or subjects). I may stop down a bit for an oblique angle to the bride and groom, to keep them in the DOF range, but more often I want the bride alone in perfect focus. With large apertures I cannot absolutely depend on the selected focus point always maintaining sharp focus on my intended subject (the bride), so allowing the AF to do its job allows me to make more AF point samples in order to insure that at least of of my choices is accurate.

All of this is possible because larger camera sensors and large aperture lenses allow tremendous control over DOF.

For instance, my Canon 135mm F2L USM, at 15 feet from the prime subject, and at f2.2 on a FF body, has a DOF:

Depth of field
Near limit 14.8 ft
Far limit 15.3 ft
Total 0.49 ft

In front of subject 0.24 ft (49%)
Behind subject 0.25 ft (51%)

(Data from the online DOFMaster caculator)

Since the average human head is less that 8 inches, from the tip of the nose to the back of the head, this allows no margin for error, human error or machine error, so several AF samples must be taken and the best shot chosen in post-production.

r3t1awr3yd

Are you using the A/F selector to select the focal point of your picture or am I misunderstanding what you're trying to do?

cab.in.boston

This is a good case for "back-button" autofocus. Use your thumb on the rear button for AF, then when it's set where you want, just take your thumb off the button and focus won't change when you hit the shutter button. A quick poke at the rear button will reset focus if you or the b/g slightly shift position. Once I started shooting this way, I quickly fell in love with it.

jmphotocraft

^ cab rings in with the right answer. You just have to go into the camera's settings to enable back button AF, AND disable AF from the shutter button.

ziggy53

cab.in.boston wrote: »

This is a good case for "back-button" autofocus. ...

jmphotocraft wrote: »

^ cab rings in with the right answer. You just have to go into the camera's settings to enable back button AF, AND disable AF from the shutter button.

Yes, Cab is correct. (Duh, ziggy.) This is often the first thing that sports shooters change when they get a new Canon body. Good one Cab.

cmason

Don't feel so bad Ziggy, I prefer your method of turning off AF on the lens. Its temporary, and nearly impossible to forget to set back. The AF* I can never get used to

gvf

Best way to get permeant auto-focus is to hit the side of your lens with a hammer over and over

AceCo55

gvf wrote: »

Best way to get permeant auto-focus is to hit the side of your lens with a hammer over and over

^