What's the need for so many focal points? Isn't just one enough?

[raaskohx10]

I don't know whether or not this question has been asked before. Just wondering why do modern DSLRs have so many focal points (my 550D has 9). I use the one at the centre, aim at my subject, lock the focus, recompose (if needed), and shoot. Simple. Any idea why camera manufactureres give so many focal points? Just to save our time from re-composing after locking focus? This question has come to my mind after reading complaints from 5d mark III users who say their pictures are not coming sharp (and they later find out that it was due to automatic selection of focal points by the camera). How do you guys shoot? Select the middle focal point (as it focusses both vertically and horizontally) or let the camera decide which one to choose?

 

[Benco]

One reason is so that the camera can do focus tracking of moving objects.

 

[SCraig]

I never, ever allow my camera to decide which focus point to use. The first thing I do when I get a new body is figure out how to disable any kind of bounce-around 3D let-the-camera-pick-the-focus-point options it may have. I do use different focus points, such as when I want the focus to be in a different area of the frame, but I move the focus point myself. I do that as opposed to recomposing because I shoot birds and wildlife a lot and they tend to move a lot. Moving the focus point allows me to use continuous focusing as opposed to locking the focus and recomposing.

 

[OLaA]

It's all a matter of preference. All modern cameras have the ability to select a single focus point as well. The reason some would want the option of many with a bunch of cross types is simply because they're more than likely shooting live action. Capturing a moving subject doesn't always give the benefit of time to focus before recomposing the shot.

Wild life and sports would be prime examples. But even in a studio setting when you have a model rapidly running through poses and expressions single point focus doesn't yield the best results

 

[Rick58]

Beats me. There is only one focus plane. Everything else falls under a degree of DOF, so I really have no idea...says the old fart.

 

[Overread]

Recomposing can fail - in fact many times it will fail.


Imagine the plane of focus as a sheet of paper parallel to the front of the lens. Now you can make that paper thicker or thinner by changing the aperture and you can change where the paper is placed (ie the distance from the camera) by changing the focus. Now if you focus the camera on a specific point and then change the composition whilst keeping the paper still (ie whilst keeping the focus untouched) what can happen is that the plane of focus will move and the plane will shift partly or fully off the original point of the subject (depending on the angles involved).

This means that what was a sharp focused photo is now a soft photo.

To correct you can manually adjust the focus, however manual focusing on a DSLR is harder than in the film days - there are no built in manual focusing aids and the viewfinder is darker and often smaller making it harder to see. This problem is especially evident when shooting with wider apertures.


Thus having more focusing points allows you to be much more free with your composition options without having to always be re-focusing the whole time. In addition consider fields of interest where you can't focus and recompose - such as sports or wildlife. Centre point works well, but if you want to focus off-centre on a moving target you'll love those off centre AF points!

Also don't forget that AF is quite advanced, many of those extra AF sensors will be used to help the AF sensor you've selected (unless you specify full single spot only) - this help can increase accuracy and speed of getting the AF lock.

 

[Rick58]

I never, ever allow my camera to decide which focus point to use. The first thing I do when I get a new body is figure out how to disable any kind of bounce-around 3D let-the-camera-pick-the-focus-point options it may have. I do use different focus points, such as when I want the focus to be in a different area of the frame, but I move the focus point myself. I do that as opposed to recomposing because I shoot birds and wildlife a lot and they tend to move a lot. Moving the focus point allows me to use continuous focusing as opposed to locking the focus and recomposing.

THIS makes sense to me.

 

[480sparky]

.............there are no built in manual focusing aids............

Both my D7000 and D600 have a rangefinder focus aid built into the VF. It will indicate which way to rotate the focus ring of the lens and then indicate when focus is achieved.

 

[SCraig]

I have tried to use that 3D smart deductive "Smart" focusing on numerous occasions and have NEVER gotten it to do what I think it should have done. For example, I was shooting some gulls in flight right after Christmas and decided to try it again. There were several of them wheeling around so I picked a gull, focused on it, tracked with it, and the focus point was all over the place. It even switched to a different bird from the one I had picked when the one it picked flew in front of the one I was tracking. Every shot I took while trying to use it was garbage.

Some people may like it and have good results from it. If so, more power to them. I personally have no use for it and will not use it.

 

[raaskohx10]

Recomposing can fail - in fact many times it will fail.


Imagine the plane of focus as a sheet of paper parallel to the front of the lens. Now you can make that paper thicker or thinner by changing the aperture and you can change where the paper is placed (ie the distance from the camera) by changing the focus. Now if you focus the camera on a specific point and then change the composition whilst keeping the paper still (ie whilst keeping the focus untouched) what can happen is that the plane of focus will move and the plane will shift partly or fully off the original point of the subject (depending on the angles involved).

This means that what was a sharp focused photo is now a soft photo.

To correct you can manually adjust the focus, however manual focusing on a DSLR is harder than in the film days - there are no built in manual focusing aids and the viewfinder is darker and often smaller making it harder to see. This problem is especially evident when shooting with wider apertures.


Thus having more focusing points allows you to be much more free with your composition options without having to always be re-focusing the whole time. In addition consider fields of interest where you can't focus and recompose - such as sports or wildlife. Centre point works well, but if you want to focus off-centre on a moving target you'll love those off centre AF points!

Also don't forget that AF is quite advanced, many of those extra AF sensors will be used to help the AF sensor you've selected (unless you specify full single spot only) - this help can increase accuracy and speed of getting the AF lock.

This is exactly what I was thinking about getting soft imges (which I belive can be overcome by selecting a tighter aperture like f/11 or f/16). Now another problem comes up and that is which focal point to select as they focus differently (some focus horizontally and some vertically and the central one focusses in both directions). This chart adds to the confusion.

 

[curtyoungblood]

In my experience, with a 5d II, which has the same (at least a similar) 9 point system, the cross-type center point is the only one that works really well. The other points are iffy, at best. I still get better results recomposing than using the other points, unless there is particularly good light and I am paying extra close attention.

 

[cgipson1]

This is a worthwhile read: Understanding Camera Autofocus

I use single point AF (Spot Focus) exclusively for most of what I shoot, and I do move the focal point around to get the framing I want. I have found recomposing to be less than acceptable most of the time. I find even the Vertical Sensors are typically more accurate than recomposing...

 

[Overread]

The horizontal/vertical and cross type are different kinds of sensor - they will all focus on the same things, however the cross type are the most accurate of the three types and the most reliable.

In cameras with weaker AF systems, such as the rebel series cameras and the 5D and 5DMII the cross type middle sensor is the most powerful and often the only cross type in the setup. As such many people will favour it and then either compose with cropping in mind or will recompose - adjust the focus manually and then shoot.

Higher end AF systems such as the one in the 7D or 5DMIII have many more cross type AF points on offer and thus the viability of those external points is increased.

 

[raaskohx10]

The horizontal/vertical and cross type are different kinds of sensor - they will all focus on the same things, however the cross type are the most accurate of the three types and the most reliable.

In cameras with weaker AF systems, such as the rebel series cameras and the 5D and 5DMII the cross type middle sensor is the most powerful and often the only cross type in the setup. As such many people will favour it and then either compose with cropping in mind or will recompose - adjust the focus manually and then shoot.

Higher end AF systems such as the one in the 7D or 5DMIII have many more cross type AF points on offer and thus the viability of those external points is increased.

So, does it mean I'm doing the right thing by slecting the central AF point for locking focus and then re-composing for taking the shot (as I have a 550D)?

 

[Overread]

The horizontal/vertical and cross type are different kinds of sensor - they will all focus on the same things, however the cross type are the most accurate of the three types and the most reliable.

In cameras with weaker AF systems, such as the rebel series cameras and the 5D and 5DMII the cross type middle sensor is the most powerful and often the only cross type in the setup. As such many people will favour it and then either compose with cropping in mind or will recompose - adjust the focus manually and then shoot.

Higher end AF systems such as the one in the 7D or 5DMIII have many more cross type AF points on offer and thus the viability of those external points is increased.

So, does it mean I'm doing the right thing by slecting the central AF point for locking focus and then re-composing for taking the shot (as I have a 550D)?

Depends.

In good light and with a subject that has clear contrast the external points shouldn't have much trouble getting a lock on. They are weaker but certainly not hopeless. Recomposing also requires the adjustment to the focus manually unless you stick to smaller apertures (which limits you creatively speaking).

All the time manual focusing lenses are fantastic for this since you can use the AF to get a lock, then recompose and adjust focus without having to hunt for the AF on/off switch on the lens.