What are the benefits of more than one focal point?

[wulf]

I love manual lenses with manual focusing and think that a split prism focusing screen would be ideal for me. Therefore, I'm not too bothered about the fact my D40 only has three focusing points (although I do occasionally swap between them to make things easier when using the "green blob" focus indicator to help out my eyes).

How do things work on cameras with loads of focus points - eg. 50+? The lens can only focus at one distance so how do you benefit from all those choices and, when in autofocus mode, avoid the problem of accidentally focusing on the wrong subject?

Wulf

[OsmosisStudios]

A d300 has 51 focus points. The centre one (which is most often used) and 50 supporting points.

These supporting points (or cross-points) help in getting an accurate autofocus (the system uses all 51) and helps in getting a faster autofocus too.

Its not that theyre necessary, its that they help the main point.

[wulf]

Are there primary and subsidiary points? Is it in effect 1 main focus point and 50 which are half engineering and half marketing?

Wulf

[jprime84]

My XSi has 9 points, and there is a mode called Auto DOF where the camera automatically adjusts settings such that as many of those points are in focus as possible.

I have never used it, but I guess the idea is you are taking a picture of a group of people and they all need to be in focus even though they are different distances away?

Also, when I am manually focusing I get a beep and the corresponding focus points will light up when I get something in focus manually. I guess having more of these points monitors more of the image for when it is in focus.

[wulf]

I wondered if some kind of Auto DoF / aperture adjustment might be part of the deal although, as a manual fan, I'm not sure I would get much use out of it.

Having focus points light up as they detect focus would be a neat feature; I could see myself using that.

Wulf

[oldwolf]

You can usually choose which focus points you want to use. I normally have mine set to the center focus point, press the shutter release halfway and see the little red light blink real quick, reframe and then press the shutter release the rest of the way to get the shot.

I think the multiple focus points helps if you don't want to use just one and have the camera guess at what you want in focus. Not great if you don't know what you want in focus but very helpful if you're shooting from the hip or putting the camera on the ground and aiming up to get a picture. I've done that a few times and have thanked my camera for it hehe.

[LoveDSLR]

Let's concentrate on the latest Nikon models right now.

D40/D40x/D60: 3 focusing points with one cross-type sensor (center)
D90/D5000: 11 focusing points with one cross-type sensor (center)
D300: 51 focusing points DX mode with 15 cross-type sensors
D700/D3/D3x: 51 focusing points with 15 cross-type sensors (FX and DX modes)

One advantage of having more focusing points and cross-type sensors is to track moving objects while in servo AF tracking mode. A plus for sports or grab and point situations.

51 focusing points increase the possibility of a point covering the intended focus area (e.g., an eye). You can manually select that point if auto AF does not. It can be an improvement over focus and recompose technique for some people.

Focus and recompose with center focusing point (same technique used in the days of SLR) introduce a margin of errors when you recompose (arc movement) especially using fast lenses with shallow DOF. There are people critical of this fact in the DSLR world and I'm not one of them (unless I shoot at f/1.2).

If more than one point confirming focus then I believe it picks the closest one. Depending on the DOF, you can either use the confirmed points or manually select the right one.

Furthermore, 15 cross-types improve low light focusing too.

Note that there is a difference between FX and DX. In DX mode, the 51 focusing points cover a wider area than FX. A welcome feature in portrait photography.

P.S. With the D40, I only use the center point too.

[sybren]

My 350D has 7 AF-points. Like you I use the centre one mainly. However, when shooting portraits I sometimes pick a point that's closer to the eyes of the model. It makes the focus-compose-shoot cycle faster. Another advantage is that it's less likely that I accidentally move forward or backward during recomposing, throwing the frame off focus.

AFAIK the huge number of points (>50) are mostly used for AF tracking, where the subject moves through the frame and the camera automatically moves from one AF point to the next.

[LoveDSLR]

Originally Posted by sybren

My 350D has 7 AF-points. Like you I use the centre one mainly. However, when shooting portraits I sometimes pick a point that's closer to the eyes of the model. It makes the focus-compose-shoot cycle faster. Another advantage is that it's less likely that I accidentally move forward or backward during recomposing, throwing the frame off focus.

AFAIK the huge number of points (>50) are mostly used for AF tracking, where the subject moves through the frame and the camera automatically moves from one AF point to the next.

You are correct in your technique with the 350D. Often it is not the number of focusing points but the number of cross-type sensors.

Nevertheless, Nikon 51 and Canon 45 focusing points is a time saving feature for sports and shooting gatherings when you don't have time to manually select which focusing point (except focus and recompose). That's why Nikon 51 focusing points are reserved for semi-pro and pro models only (other than cost).

[Sixpak]

Thanks again for pointing me here LoveDSLR...

Anyway... I understand having all those focus points as being pretty handy when shooting fast action (some reason toddlers come to mind or sports. Especially with having continuous focus on and tracking.

Now when I shoot group shots, I usually manage to pick out a few heads in the various points. Camera actually seems to recognize a head? I found it as one of those, "oh hey, neat!" kind of things, but didn't give it much thought.

When I shoot a single person (portrait) I notice the focus points always seem to go after cheekbones, chin, nose, an ear, anything but the eyes... This is pretty much the reason why I've just gone to focus center where I want (the eyes normally), then recompose.

I can't even imagine trying to move my selector over 50 different focus points. It just doesn't seem to be a very easy or fast way to get the shot?

What about candids? It's not as fast as sports, but requires you to be fairly quick.

I'm just wondering if there's something here I've been missing and I need to dig a little deeper or perhaps I simply have misunderstood some capability.