Depth of Field and Focus Points - Are they consistent?

[jwbryson1]

I have a question that I pose to anybody with more knowledge than me--probably the majority of you on this forum.

My DSLR has 11 autofocus areas. The question that popped into my heading thinking about this is how do the 11 autofocus points work in unison with my DOF?

For example, let's say I'm shooting an image with a shallow depth of field with the intention of having a blurred out background. When I hold the shutter release 1/2 way to autofocus all 11 of the sensors light up which I understand to mean that I'm in focus "across the board" so to speak.

However, if I'm shooting at a shallow DOF then those background areas should not show up as in focus, should they? Am I thinking about this too much?

 

[Robin Usagani]

If you use all 11 focus point, it will select the ones that lit up CLOSEST to you. Thats why people usually use only one point to really focus on the eyes.

 

[analog.universe]

I once read that autofocus was accurate typically to within 1/3 of the depth of field. So if you have a shallow DOF, and the subject is separated from the background, and you have points lighting up on the subject and the background, then I am as confused as you. My camera seems to estimate pretty well, and only light up the relevant points...

 

[joealcantar]

If the camera is focusing on your primary subject it should be okay, one of those 11 sensors normally highlights to let you know which one was used for focusinng. I don't have the D90 or D40 to go into more detail. Which camera are you having this issue with?
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Shoot well, Joe

 

[Matt24138]

My camera lights up as to where I am pointing if I have a flower close to the lens on the left side it will pick the flower if I am going for something behind the flower moving my aim a little I can get it to pick up the next object. I would check your manual as I believe some have auto focus settings all 11 shouldn't be lighting.

 

[tirediron]

This thread might help a bit.

 

[jwbryson1]

Joe, I am shooting with the D90 primarily today. I keep the D40 as a backup, but I never use it.

Thanks for all the posts. I have shot with single point focus and I like that when shooting eyes.

Happy Weekend!! :lmao:

 

[Big Mike]

The multiple focus points do not have any affect on your DOF.

Your lens can physically only be focused to one distance (or infinity). Your DOF will then be a result of what that distance is, the focal length of the lens and the lens aperture.

When you use the multiple focus points option, the camera will guess what it thinks you want to focus on....and that focus point will light up. If more than one point lights up, it's because those points are over objects that are the same distance away.

So for example, if you are shooting a group of people who are in 3 rows, one behind another. Unless you are really far away, you will never get the points to light up on people in the front row AND people in the back row.
So what you should do, is to specifically focus on someone in the front row or middle row, then ensure that your DOF is deep enough to encompass all the people.

 

[Derrel]

When you use a multi-point focus pattern with a modern, sohpisticated Nikon, and manually select the initial AF point/position, the camera will use RGB color measurements and a reflectivity measurement and the initial distance information to "identify" the subject, and to track it across the frame, no matter where that subject might move to within the frame. If your camera has a Closest Subject Priority setting, the AF system will be instructed to focus on closer items, rather than on background items, which can be pretty valuable in many situations where you KNOW, for sure, that the subject will always be closer than the distant background.

One of the single biggest difficulties with modern AF systems is that there is quite a bit of variation between the systems used in Canon and Pentax and Sony and Nikon. For example, nobody has mentioned Canon A-Dep focus mode, which was a big deal for a while, but has since been left off of some newer models. With your D90, the camera's programming is trying to arrive at "the best focus", based on an analysis of the data gleaned from multiple AF points. It's become pretty complex, the subject of autofocusing, and it takes some reading and study and some actual practice to figure out how the doggone systems work. The camera-model-specific guidebooks from Thom Hogan or those other two companies might be a big help to you in understanding how,exactly, the Nikon D90 works.

 

[Geaux]

I turned off all my AF points when I first got my camera. Single point focus for me.

 

[KmH]

Each of the focus modes, and focus area modes, has a use.

It takes a bit of study to understand which mode to use when.

 

[Derrel]

Each of the focus modes, and focus area modes, has a use.

It takes a bit of study to understand which mode to use when.

Yes, that is true. HOW and WHEN to use the different AF points is a very complex issue with "some" cameras. Using just a single AF point can lead to failure to focus if the subject is low in contrast, especially if the lens has a slow maximum aperture, or the lighting is ALSO flat and low-contrast. Most people who fully explore the multiple AF point systems in Nikons will find that using one or two of the multi-point setups allows them to achieve much faster, better AF, especially with off-center subjects, when conditions are marginal. For tracking moving objects, using one,single AF point means the camera is using data from just one,single point, which throws away all the differential analysis that predictive AF needs to be successful under the most demanding situations. Using what Nikon calls Group Dynamic AF is probably the best way to get a lock-on on a fast-moving subject with the D90, which IMMSMC, was the first amateur Nikon body to get an implementation of 3D Focus tracking from the pro Nikon bodies.

OP: check this page out for a brief primer on the options. Nikon D90 Camera Autofocus - Full Review

And if you really want to know how to use the D90's AF system,and understand the entire camera, buy The Complete Guide To The Nikon D90, here: Complete Guide to the Nikon D90