DPChallenge: A Digital Photography Contest You are not logged in. (log in or register
 

DPChallenge Forums >> Hardware and Software >> Why multiple focus point matter?
Pages:  
Showing posts 1 - 16 of 16, (reverse)
AuthorThread
05/08/2012 04:35:58 AM · #1
Ok i searched the forums but too many results (irrelevant) returned. From the prespective of someone who only shoot with fast primes (90% of my shots are of kids indoor in low light with Sigma 30/1.4 or Canon 50/1.8), i don't get why would i want my camera to decide where to focus. At 1.4 the DOF i so razor thin that if the focus is not on subject's eye, the entire face will blur into bokeh. Why then 7d focusing sytem be of any advantage when all i do is use the center point? Maybe its useful for videographers or shooting bursts at smaller aperture and maybe that's the target audience for it? For a center point user, the new 5dIII then does not offers much improvement over 5dII in auto-focus area?

Message edited by author 2012-05-08 04:37:13.
05/08/2012 05:32:25 AM · #2
I must admit I only ever use spot focusing, think its a control thing, unless your a sports photographer I can't see much benefit in the camera deciding for you, there probably is though but I like the control.
05/08/2012 05:43:36 AM · #3
I generally only use centre spot focussing as well these days. I also only tend to use most fast primes but that can make things tricky shooting wide open and using a focus and re-compose way of shooting so one needs to be careful.
05/08/2012 05:46:37 AM · #4
Originally posted by MEJazz:

...the new 5dIII then does not offers much improvement over 5dII in auto-focus area?


Oh it certainly does. Used my MKII for the first time in a while this morning and even using a single point - sometime the centre one, it hunted. My MKIII would have just snapped into focus instantly.

However, one of the things the MKII is good at, and I haven't found a way to do this with the MKIII, is to hit the top right button that selects all points and the camera averages the focus across a scene. I 'think' (I wish I knew) that it factors aperture into that functionality and will use a wider spread of points and the notion of the hyperfocal distance to set the focus based on the scene and the set aperture - but perhaps I kid myself...

But, I haven't found a way to get the MKIII to automatically pick focus points across a scene from its vast array with a single button push.

Message edited by author 2012-05-08 07:13:29.
05/08/2012 07:09:30 AM · #5
The zones on the 7D are incredibly useful if you're a nature photographer. If the subject is standing still, I'll spot focus, selecting my focus point depending on how I want to compose my subject. I find it much better to change to the appropriate focus point than to use the center point and recompose. Less room for error.

However, a single point focus is almost useless for a moving subject -- unless it's large and slow. :)

When photographing birds in flight, they usually move too fast for a single point focus to catch them. Using the focus zones works so much better. Usually I don't want to center the bird, I want it more to the side so that there's room for the bird to fly into the shot. So I use one of the side focus zones.

I find it one of the most useful things about the 7D!
05/08/2012 07:19:19 AM · #6
Originally posted by MEJazz:

Ok i searched the forums but too many results (irrelevant) returned. From the prespective of someone who only shoot with fast primes (90% of my shots are of kids indoor in low light with Sigma 30/1.4 or Canon 50/1.8), i don't get why would i want my camera to decide where to focus. At 1.4 the DOF i so razor thin that if the focus is not on subject's eye, the entire face will blur into bokeh. Why then 7d focusing sytem be of any advantage when all i do is use the center point? Maybe its useful for videographers or shooting bursts at smaller aperture and maybe that's the target audience for it? For a center point user, the new 5dIII then does not offers much improvement over 5dII in auto-focus area?


its extremely useful if you shoot primes, especially wide open. if you use center point and recompose that extremely small DOF wont be in right spot.

when you focus the lens is focused at a specific distance and when you recompose that distance has now changed, it could be slight, it could be great, but it has changed.

see here:

//digital-photography-school.com/the-problem-with-the-focus-recompose-method

you are much better composing your shot first and then selecting an AF point over the are you want your focus to be. this is why it it is very easy to select AF points on the fly with the joy stick on the back.

Message edited by author 2012-05-08 07:20:40.
05/08/2012 09:38:00 AM · #7
Great, some good feedback here. So i understnd for fast moving subjects multiple focus points come in handy. Not sure why using center point an focus/recompose would be an issue as that's what i do all the time and get perfct shots at wide open with fast primes.
05/08/2012 09:46:50 AM · #8
Originally posted by MEJazz:

Not sure why using center point an focus/recompose would be an issue as that's what i do all the time and get perfct shots at wide open with fast primes.


It's what i do a lot as well but it is definitely an issue. If you centre focus on something at a wide aperture of f1.8 or f1.4 say, and then recompose so that it is then in the corner of the composition then unless it is still in the very narrow field of focus then it is going to be out. Even if by just shifting your camera a tiny amout. Stands to reason really.
05/08/2012 09:47:54 AM · #9
Ditto what Wendy said. The D7000 even has "predictive focusing" which works out which way the found focus point is moving in the frame, then works out where it should focus when the shutter opens. It's amazing for flying bird shots.
AF seems ok when shooting people at smaller apertures, but not the right approach for shooting "wide open" with fast glass. Focusing a 1.4 in low light is a learned art.
05/08/2012 09:50:53 AM · #10
Originally posted by MEJazz:

Great, some good feedback here. So i understnd for fast moving subjects multiple focus points come in handy. Not sure why using center point an focus/recompose would be an issue as that's what i do all the time and get perfct shots at wide open with fast primes.


well if it works for you than great. i use it most of the time as well, but if you start missing focus, especially at close distances, you'll know why.

it has to do with geometry and triangles:



Message edited by author 2012-05-08 10:06:06.
05/08/2012 11:27:59 AM · #11
Just a few words about focus/recompose error. Although the geometry-related error can be very significant, it is often not the most important error. See the graph below:



Notice that for angles of shift of about 5 degrees or less, the focus error is less than 7.5mm. For narrow DoF work this can still be a factor, but more important is our tendency to rock forward and backward. During the time delay between the initial focus acquisition and pressing the shutter button, any rocking motion will impart focus error. The rocking motion can easily exceed a couple centimeters, making the geometric error a less important factor. If you brace yourself against a solid object, negating any rocking motion, now the geometric issues come to the forefront.
For reference, 5 degrees is a shift of about 1/3 of distance from the center to the edge of the frame at 85mm.
06/09/2012 03:00:25 PM · #12
There are two separate questions on the use of multiple focus points. I would like to hear answers for both:
1. When do you need your camera to have multiple focus points. One answer: to allow you to focus on off-center objects without having to reframe. Are there others?
2. When do you need to use multiple focus points concurrently? Many cameras allow you to select group of focus points, or all focus points. When do you use this mode? Can it cause the camera to chose the wrong points to focus on? How likely is that? One answer spoke about the focus on moving objects being transferred across focus points in continuous focus, but there seemed to be disagreement whether this happens.
06/09/2012 10:13:12 PM · #13
If the camera is in a-dep mode( or similar) possibly tv. If you have multiple af points set, it will help the camera in choosing an aperture to get them all in focus.
06/09/2012 10:39:09 PM · #14
Re the two diagrams; Is there a difference in the "recompose error" when using a "flat field" lens like a Micro Nikkor 55 or 105 and the way focus would work with a "curved field" lens like the 50 f1.4 Nikkor. It seems to me that chart 1 would be true with a "flat field" lens, and not so much with a "curved field" one.

I use the old manual 50 1.4 in low light to get candid and informal existing light portraits, and have found that it's helpful to pick anything shiny, like a necklace for instance, to gauge where the focus point is, then if there is enough time and light, tweak the focus on the desired eye.
The focusing system on the D7000 is impressive for birds and other active shooting situations. The "predictive focusing" feature is one that actually works pretty well for birds in flight.
06/07/2018 05:21:22 PM · #15
Try Multifocus tool from Altami Studio. It's free and has a simple interface.
06/08/2018 03:53:27 AM · #16
Thanks very much
Pages:  
Current Server Time: 03/28/2024 08:20:38 AM

Please log in or register to post to the forums.


Home - Challenges - Community - League - Photos - Cameras - Lenses - Learn - Prints! - Help - Terms of Use - Privacy - Top ^
DPChallenge, and website content and design, Copyright © 2001-2024 Challenging Technologies, LLC.
All digital photo copyrights belong to the photographers and may not be used without permission.
Current Server Time: 03/28/2024 08:20:38 AM EDT.