Auto-Focus Points - how do I use them?

[JHF Photography]

I shoot with a D50, it only has five focus points... but I select the point closest to what I want to focus on with my selector first, then if it's not quite where i need it, I focus and recompose.

I use the centre point and focus & recompose. On my camera, only the centre point is a cross type sensor, so I trust that one to work better than the others.

Now that's where I'm fuzzy on understanding auto focus, what exactly does it mean that it's a cross type sensor?

 

[Garbz]

When doing macro I use specific focus points. Other than that I always shoot and recompose. The reasons are, the centre focus point is the best electrically, the centre of the lens is the best optically (bearing in mind that all lenses are wide open it means that around half the time using the Nikkor 50mm f/1.8 the edge points fail to focus completely), and the AF assist light only works when the centre point is selected.

Technically you could say I do use the other points too when I shoot sports since the predicative focus uses information from all points to follow options.

On my Elan 7 it has a laser that reads my eye to see what I am looking at and focuses on that. Its like a 2 for 1 deal it focuses and gives free Lasik:thumbup:

Does it focus all the way back to the exposure display in the bottom of the viewfinder?

 

[Big Mike]

Now that's where I'm fuzzy on understanding auto focus, what exactly does it mean that it's a cross type sensor?

Autofocus works by comparing contrast in a small area. A regular linear type AF sensor will only detect contrast in one direction...for example, vertical but not horizontal. So this AF point would have trouble focusing on something that has no vertical contrast...vertical stripes for example. The opposite for a horizontal type sensor.

A cross type sensor will detect both vertical and horizontal contrast, which means it will have an easier time detecting contrast and attaining focus.

Some cameras, like mine, have only one cross type sensor...and actually, it only works as a try cross type when used with a lens with a max aperture of F2.8 or larger.

 

[JHF Photography]

Thanks!

 

[Garbz]

Autofocus works by comparing contrast in a small area.

No that's the way point and shoot's do it and SLRs with Liveview do it when liveview is on. It's akin to getting a histogram of the area and moving the lens till the histogram is at the widest point.

SLRs use a phase detection system where prisms split the light from opposite points of the AF sensor onto a light sensor where the the light is analysed to find out which direction the lens needs to focus to make both points identical in brightness and phase.
Cross type AF points use 4 samples (2 horizontal and 2 vertical)

 

[Neuner]

I'm really lost with this feature and my D80 as well. I get aggravated with the 3D Matrix setting and what it decides to focus on so I mostly use the center point setting. The problem is that even with the center point it still goes beyond the center and focuses on something around it. I may have to trigger the camera to focus several times while slightly moving it around the subject before I get it to focus on the area I want. Very frustrating when you have minimal time for a shot. I've never gotten my joystick selection to work.

 

[usayit]

I leave all of them on 1dMII for most situations. The viewfinder lights up all the focus points that are considered in focus and I quickly make an assessment. If not happy, I'll switch to a single center point with a simple push of a button (user defined).... and do the focus and recompose.

During the occasional head portrait, I'll switch to less focus points a select a particular point where I want the eyes to be framed. Makes for a more fluid workflow rather than continuously recomposing from the center point. Switching to less focus point point will make selecting the "nearest" point easier as there are less to scroll through.

 

[Bifurcator]

No that's the way point and shoot's do it and SLRs with Liveview do it when liveview is on. It's akin to getting a histogram of the area and moving the lens till the histogram is at the widest point.

SLRs use a phase detection system where prisms split the light from opposite points of the AF sensor onto a light sensor where the the light is analysed to find out which direction the lens needs to focus to make both points identical in brightness and phase.
Cross type AF points use 4 samples (2 horizontal and 2 vertical)

Yep! I was just going to say the same thing till I read yours. :thumbup:


@Iron Flatline
I do both. I use the center when in a hurry and I select the nearest when I have time as it's not going to suffer from lens curvature. Recomposing after a center AF will almost always be wrong to some degree at least.

I like the single positionable AF point found in bridge cameras best of all though. This is also available in some modes of some dSLRs too but it just rocks! Next best is the tracking servo AF point. Place the tracking point on the subject and recompose at will - all the while the AF sensor maintains correct focus on the subject. Sweet! Positionable AF points and Trackig! :hug::

 

[Garbz]

I'm really lost with this feature and my D80 as well. I get aggravated with the 3D Matrix setting and what it decides to focus on so I mostly use the center point setting. The problem is that even with the center point it still goes beyond the center and focuses on something around it. I may have to trigger the camera to focus several times while slightly moving it around the subject before I get it to focus on the area I want. Very frustrating when you have minimal time for a shot. I've never gotten my joystick selection to work.

3D Matrix has nothing to do with exposure. This feature simply uses the range information from the lens (after the exposure) and the phase information from the other exposure points to determine the focus.

With that corrected it does sound like your D80 is playing up a bit. It isn't too uncommon for SLRs to have focusing problems, mostly they are because of user error, but frequently too the body is to blame, and sometimes even the lens. Does it do it for all lenses? It may be worth contacting Nikon about it.

 

[Hawaii Five-O]

Does it focus all the way back to the exposure display in the bottom of the viewfinder?

It does!

 

[Joves]

I use all of my points and shift it depending on what I want the exposure to look like. I also have mine set to grid so I stay all nice and squared up. I dont like to compose a shot using the lock function then recomposing. To me it is like doing things twice.

 

[TamiyaGuy]

I must admit, I don't really see much use for them (although I only have 3 on my D40). Almost all the time, I use the center focus point, then re-compose after focusing. But it is useful for fleeting shots where you want to use the "rule of thirds", and for action photos where you wish to leave "looking space".

 

[tirediron]

Doesn't the D300 only have so many actual AF points and the rest are helpers? I didn't think you could pick through all 51.

Kundalini answered the question, the '51' was meant tongue in cheek - it sounds way cooler than 15. (But that is how I focus).

 

[Antithesis]

I usually decide on a composition really quick, hit the af point select button and use the little joystick on my 5D to select the one in the area I want to be in focus. One of the big complaints about the 5D is that it only has 9 points, and they are located in pretty close to the center of the finder, limiting composition. I personally hate using the lock button, because moving subjects tend to leave your focal plane, and I keep my camera on continuous most of the time anyways, so it's useless.

I am really looking forward to another camera with more points because my portrait comp's always look all weird and off kilter because of the location of the points. I'll probably always keep this one for certain things, but the points are definitely one of my peaves.

 

[Rachelsne]

I use my focus points all the time,