Benefits of large number of autofocus points?

[davidedric]

I must be missing a trick, but what are the benefits of large numbers of autofocus points?

My G80 has forty nine - but I never use them all. Isn't that inviting the camera to decide where to focus?

Generally, I just use a single, fairly small (the size is adjustable) point in the centre of the screen. I usually focus and recompose, though it's easy enough to move the focus point around the screen.

I also have a 3x3 custom setting in the centre of the screen for moving wildlife.

What am I missing?

Dave

[tclune]

Two things come immediately to mind. First, if you are shooting BIF and the like, you want a number of points available to track the focal point through motion. And second, even if you use only one focal point (the usual situation for me), the large number of possible locations for that focal point make it easy to get the focus where you want it without focus-and-recompose, which can be inaccurate when focus is critical. I suppose, if you want the camera to choose a focal point for you, it would still be advantageous for there to be as many possibilities as possible -- although I personally have never turned that decision over to the camera. FWIW

[Shadowman]

Autofocus or just focus points in general? You could do some serious focus stacking with 51 or more focus points.

[davidedric]

Autofocus, John

Autofocus or just focus points in general? You could do some serious focus stacking with 51 or more focus points.

[xpatUSA]

I must be missing a trick, but what are the benefits of large numbers of autofocus points?

My G80 has forty nine - but I never use them all. Isn't that inviting the camera to decide where to focus?

I looked in the handbook for one of my cameras which has a whopping 5 AF points (4 more than the previous model!). I had to look because I rarely use AF and even then only the center point. Sure enough, there is indeed an "Automatic AF point selection" mode.

How on earth the camera can decide which of those five points is appropriate for a scene is totally beyond my ken!

So, I'll stick with manual focus since my subjects rarely move at all . .

<>

What am I missing?

Dave

From my POV, Dave, you ain't missing much . . .

[george013]

Auto focus is a technique. A focus point is a physical place corresponding with a place on the sensor.
I think the benefit of more focus points is with dynamic focusing, moving objects as said before. The smaller the distance between the focus points, the more accurate. I never used it for my shots so I can't speak from experience.

George

[xpatUSA]

I looked in the handbook for one of my cameras which has a whopping 5 AF points (4 more than the previous model!).

I just now discovered that only the center point is of the "cross" type (shock, horror), so my decision to only use that one if needed was quite intuitive . .

[IzzieK]

Dave...it is used mostly on action shots e.g. sport and BIF or running animals and kids. I am putting away some money for a D850 now...while I master (?) my D810.

[NorthernFocus]

I think the biggest advantage is marketing/bragging rights for the manufacturer.

[george013]

...
Generally, I just use a single, fairly small (the size is adjustable) point in the centre of the screen. I usually focus and recompose, though it's easy enough to move the focus point around the screen.
...
Dave

I missed this one.

Recomposing might bring your subject out of focus.

George

[Stagecoach]

I missed this one.

Recomposing might bring your subject out of focus.

George

The options you have so that this does not happen are AF/L (AF lock) or to use BBF (Back button focus) in AF-C (continuous) mode and remove your finger before re-composing. If these options are available on your camera.

[Stagecoach]

I must be missing a trick, but what are the benefits of large numbers of autofocus points?

What am I missing?

Dave

Dave, I suppose if you have the camera on a tripod and your framing is tight and you don't want to move it you have a better chance of finding a focus point that aligns over an object you want to AF on.

Personally I never use the 51 I have and always use it on 9 which means its faster to move around the full area if needed.

[george013]

The options you have so that this does not happen are AF/L (AF lock) or to use BBF (Back button focus) in AF-C (continuous) mode and remove your finger before re-composing. If these options are available on your camera.

No. The reason I say so is something else.
I just made a drawing to explain what happens. If you use the center focus point, the black drawing, and than recompose, the recomposing causes that the focus plane will be in front of the subject. If you can see this in the image depends on the focal length or the AOV and the f-number and the angle of recomposing. I do recompose myself sometime, keeping the release bottom half pressed, but I'm always aware of it.



George

[William W]

I must be missing a trick, but what are the benefits of large numbers of autofocus points?

Speaking in regard to Canon EOS System – I assume the same applies to Nikon et al.

A greater number of Auto Focus Points increases the keeper rate (apropos nailed Auto-focus) when the subject moves in the FRAME at a relative fast speed.

Hence, most EOS 1 Series (and now quite a few more) feature a 45-point AF system – and these cameras are used by many Photographers who like capturing (fast) sports and (fast) wildlife action.

WW

[Stagecoach]

No. The reason I say so is something else.
I just made a drawing to explain what happens. If you use the center focus point, the black drawing, and than recompose, the recomposing causes that the focus plane will be in front of the subject. If you can see this in the image depends on the focal length or the AOV and the f-number and the angle of recomposing. I do recompose myself sometime, keeping the release bottom half pressed, but I'm always aware of it.



George

George,

It is obvious that when we focus and recompose the camera to subject distance will change but I doubt in the vast majority of situations this is going to make any appreciable difference.

In real world terms, if you shoot a portrait with a 75mm lens at f/2.8 with the camera focused on the eye of your model at the centre of the frame at 3 metre subject distance and then re-compose so that your models eye is to the left or right on the 1/4 line, the subject distance will change by around 19mm. The DoF at those conditions is 270mm (approx half front,half behind).

[davidedric]

Yes, I understand that, but I'm mostly using it with wildlife and a long telephoto, typically to avoid the subject being dead centre of the frame. I haven't done the maths, but I'm pretty sure the effect is very small.

Dave

No. The reason I say so is something else.
I just made a drawing to explain what happens. If you use the center focus point, the black drawing, and than recompose, the recomposing causes that the focus plane will be in front of the subject. If you can see this in the image depends on the focal length or the AOV and the f-number and the angle of recomposing. I do recompose myself sometime, keeping the release bottom half pressed, but I'm always aware of it.



George

[William W]

Regarding the comments and the discussion about "Focus and Recompose".

I agree with Tom (post #2) "the large number of possible locations for that focal point make it easy to get the focus where you want it without focus-and-recompose, which can be inaccurate when focus is critical."

However, in practical terms, focus (DoF) is not very often that critical - consider that IF one chooses to Recompose by TILT or TWIST, then, most of the times when the focus would be "critical" is when the Shot is Framed very Tight and the Lens's aperture is very large. For example a Very Tight Head Shot at F/1.4 - BUT (this bit is important) - if it is a Very Tight Head Shot and (for example) we focus on the eye the amount of TILT or TWIST to recompose would be a minimal movement and the Plane of Sharp Focus is (usually) insignificant. This point is made by Grahame in Post #15.

Which brings me to George's diagram (post #13) describing exactly that technique of Recompose by TILT or TWIST.

The salient point is, that technique of TILT or TWIST is only one of two techniques for Recompose which are available to use: the second is to Recompose by moving the camera PARALLEL to the Plane of Sharp Focus – and this Recompose technique is preferable (and quite easy to master) when the framing is very tight, for example the Tight Head Shot using a Lens with a Large aperture.

***



Typically I use Centre Point AF (BBF); Lock Focus; then Recompose.

For those interested in the technical: the above was made with an EOS 5D and an EF 85/1.8, at F/1.8. I chose this as an example because it was similar to the example that Grahame gave, but at F/1.8 and not F/2.8. It is a full frame crop. The SD was about 6ft (let's say 2000mm); at 2000mm SD and F/1.8 the DoF is about 45mm (CoC = 0.025mm). Moving the camera parallel to the Plane of Sharp Focus is really not a difficult technique to master - and as can be seen by the sample image one doesn't have to move all that much - the other point to consider is, even if I did TWIST the camera to Recompose - I think that it wouldn't have mattered, anyway, it would only have been a very small twist: but I am too tired to do the geometry to prove that thought correct.

WW

[James G]

Keeping it simple, I use spot and a small group in centre of the frame. I do not shoot sports action but do photograph aircraft, wildlife and insects (macro).

I have experimented with using the all/many of the autofocus points, but found them distracting and largely of little actual advantage (to me!).

When I'm capturing landscape, or engaged in astrophotography/ moon shots, I focus manually.

[george013]

George,

It is obvious that when we focus and recompose the camera to subject distance will change but I doubt in the vast majority of situations this is going to make any appreciable difference.

In real world terms, if you shoot a portrait with a 75mm lens at f/2.8 with the camera focused on the eye of your model at the centre of the frame at 3 metre subject distance and then re-compose so that your models eye is to the left or right on the 1/4 line, the subject distance will change by around 19mm. The DoF at those conditions is 270mm (approx half front,half behind).

A zero to much: 27mm. Before 13mm and behind 14mm. For a FF. For a DX it's 18mm, before 9mm and behind 9mm. I didn't check the 19mm but if this is true, you're out of focus.

Don't misunderstand me, I wrote "Recomposing might bring your subject out of focus."

Simple said recomposing can be done with max half of the horizontal AOV. A 70mm lens has an AOV of 29 degrees, maximal recomposing will be 14.5 degrees. A 400mm lens has an AOV of 5 degrees, max. recomposing will be 2.5 degrees. But when shooting wide angle 14mm 102 degrees you can go up for 51 degrees. Think of a group of trees in a lane and you want the first tree in focus.

George

[DanK]

For a couple of years, I have been using a Canon 5DIII, which has 61 AF points, of which 41 (I think) are cross-type.

I don't do much action photography, so I can't say much about that, but when I do that kind of photography, I typically use either 5 or 9 AF points--one primary one, and either 4 or 8 around it to catch focus if the object moves outside the area covered by the primary one.

For me, the main advantage has been the ability to chose the AF point or region. The 5D is highly customizable, and I have AF selection on the joy stick, so I can quickly move it while working. For certain purposes, it is handy to have the AF point not in the center. For example, for candids of people, I put the AF point toward the top, where the face normally would go. The 5DIII remembers the location separately for portrait and landscape modes, which is handy. I also have one of the custom settings set for indoor candids--the aperture and shutter speed I normally use with bounced flash, and both AF points at the top of the range. I find it saves me time and avoids the recomposition errors that George pointed out.

For that use, the number is perhaps less important than the spread--the portion of the field that the AF points cover.