Focusing

In order to get the most from your camera it is helpful to understand the basics of how it works.

Here is a drawing of what happens to the light when it enters a typical DSLR.

When light enters the camera it hits the first mirror (magenta lines) and the majority of the light is reflected upwards thru the focusing screen (red lines). However, the first mirror is somewhat translucent and some of the light is allowed to pass thru and reflects off of a smaller secondary focusing mirror onto the autofocus sensor (blue lines). 

It is important to understand that only a smaller portion of the available light is allowed to pass to the autofocus sensor. This is why cameras have a difficult time autofocusing in dimly lit situations. There just isn't enough light getting to the autofocus sensor. In the diagram it shows two separate points hitting the autofocus sensor. By using the two points the camera can determine whether the image is in focus, and which way to move if it is not.

Neither autofocus nor manual focus is actually focusing the image at the image sensor. Manual focus focuses the image at the focusing screen and autofocus focuses the image at the focusing sensor. None of the light hits the image sensor until the two mirrors flip out of the way and the shutter is activated. If the autofocus mirror is out of alignment it will result in front or back focus issues where the camera thinks the image is in focus at the image sensor but it isn't. If the focusing mirror is only slightly out of alignment selecting a single focus point at the top or bottom of the sensor may result in front or back focus issues. If the primary mirror is out of alignment manual focus may result in front or back focus issues.


Diopter Adjustment

Before moving on to the various autofocus modes I should address the focusing screen. You will notice that the image is focused at the focusing screen. This is also where the digital display for the camera meter and other numerical information is projected. If the focusing screen is not in focus thru the eyepiece you will have difficulty in getting accurate manual focus, but it will not affect autofocus. To focus the screen thru the eyepiece you need to adjust the diopter setting.

 Nikon D7000 diopter adjustment

 Nikon D3 diopter adjustment

You turn the diopter adjustment until the focusing screen is in sharp focus. The easiest way to do this is to focus the numerical displays that are projected onto the screen.


The Autofocus Sensor

 

Depending upon your camera, it will have both "cross type" and "linear" sensors. A cross type is like having two linear sensors at the same spot and oriented 90degrees to each other. A cross type sensor has twice the sensitivity and accuracy of a linear sensor. This can be important when manually selecting a focus point; you want to select a cross type if possible. It is also important to know that on most cameras the cross type sensors will be primarily located in the center.


Autofocus Modes

You have two primary modes for autofocus points. Spot focus or multi-point "Matrix" focus.


The Nikon D3 focus mode selector (top down Matrix, Dynamic, Spot)

On my Canon focus mode is selected in the  menus.


In spot focus you select which of the available focus points will be used.
 
The highlighted focus point is the one in use, the rest are being ignored.

 

With multi-point focus you can have all of the points active.

If all of the focus points are active the camera will use additional information from the metering sensor and image database to try to decide where to focus. Because the camera is deciding where to focus for you there is a greater potential for undesired results.

To minimize this you can usually select the number of focus points to be used.
 

 

Single Focus, Continuous Focus, and Focus Tracking

Single focus simply means that the camera locks the focus after it is first achieved. Continuous focus means that if focus is lost the camera will refocus. On some cameras there is a "focus delay" setting which sets how long the camera waits to refocus after initial focus is lost.

Focus tracking is "dynamic focus" and requires continuous focus with single spot sensor set. Set in this manner if the subject moves off of the selected focus point the camera will automatically select a new focus point to use. The camera uses information from the metering sensor to determine where the subject has moved to and which focus point to use. The speed and accuracy of your metering sensor will have a huge impact on how accurate focus tracking is with your camera.

It is also interesting to note that, while the exposure meter is physically separate from the focusing sensor, they are tied together in most other autofocus modes. In full-auto modes, scene modes, and multi-point focusing modes, the camera uses information from the metering sensor to decide "what" the subject is, and "where" the subject is. The camera will also use the information from the metering sensor to compare the scene's color and luminance values against a database of images (30,000 scenes for the Nikon D4) to help it make these decisions. This also provides additional features such as "face recognition".

Camera Choices

Autofocus and focus tracking are two areas where the professional camera bodies shine. Lower end cameras may have very few autofocus points with only the central one being of the more accurate cross type. For example, the Nikon D90 has 11 autofocus points with only the central one being of the cross type. The Canon 1Dx has 61 autofocus points with all of them being of the cross type.

Also a factor here is the metering sensor for focus tracking. Again, more is better. For example, the Canon 60D's metering sensor is divided into only 63 zones. The Nikon D4 functions at the sensors's pixel level of 91,000 zones. The D4 will have much faster, and more accurate, focus tracking.

The question is, do you need autofocus speed and accuracy? If you primarily work with relatively stationary subjects, or if you primarily use manual focus, then the answer is "no."  If you want to take action shots of your kids playing sports, birds in flight, or any other moving subject, then the answer is "yes."




Exposure metering is covered under Exposure- Metering.

Depth of field is covered under Composition.

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