A Viewfinder Darkly

Photography tips and tutorials

How to Use DSLR Autofocus

Autofocus in DSLR cameras is a valuable tool, but photographers need to choose the right AF mode and configuration to ensure the area of interest is in focus.

The sophisticated optics and large digital sensors in DSLR cameras offer greater potential for sharp images  than phone  cameras, requiring accurate autofocus (AF)  systems. This is balanced by the need when photographing  moving subjects for fast AF.

Speed boats

A power boat race start provides fast moving subjects for the AF system to track

Focus Modes

Auto AF mode 

This is a combination of the two main two AF modes. The camera starts off in Normal mode and if the subject moves fast enough it switches automatically to the Continuous focus mode.

Normal AF mode

This is the general purpose AF mode  covering most situations where the subject  is stationary or slow moving.

While taking photos using the optical viewfinder DSLRs use a phase detect AF method. This is significantly faster than the contrast detect method used in live view mode and phone cameras.  The fast AF means the delay between pressing the shutter button and taking the photo is almost imperceptible.

When the shutter-release button is pressed halfway, the AF system is activated and chooses from a number of focus points, focuses the lens, and then locks the focus until the shutter button is released. Keeping the shutter button halfway depressed locks the focus and allows photographers to recompose the shot.

In normal AF mode, the camera will not  shoot until focus lock is achieved. This is to help make sure every photo is focused.

AF switch

AF mode is selected using a switch or a menu item

Where the subject is not obvious, or the photographer wants a specific part of the image in sharpest focus, the photographer has the option of selecting which focus point to use, rather than using the camera’s choice. AF systems have a number of sensors because the main point of interest is not in the same position in every scene.

Continuous Focus

This is the most complex and varied implementation of Autofocus, with a large difference in capabilities between cameras and manufacturers.

In high-speed action shots, even the almost imperceptible DSLR shutter lag in normal focusing often results in subjects that are out of focus, or missing the key moment.  The autofocus system starts to work when the shutter-release button is pressed halfway down, and while the button is kept half pressed continues adjusting the focus as the subject moves.

Advanced DSLRs often have a separate AF button conveniently placed on the rear so photographers operate it with their thumb. This button activates the autofocus system independently of the shutter button. This is much easier than trying to keep the shutter release button half pressed while tracking a moving subject.

Photographers have the option in Continuous AF mode to allow the shutter to fire even if the image is not in perfect focus.

If the subject briefly moves away from the chosen focus point the camera will use information from surrounding points to try and keep the subject in focus.

af button

The AF button on the back of a DSLR with integral sensor point selector

Photojournalist cameras such as the new Canon EOS-1D X Mark II  have a range of settings  in their Continuous AF modes to  suit the type of motion expected in the scene. For example, there is one labelled for photographing tennis players that continues to track subjects while ignoring possible obstacles that may briefly come into view. Another choice is for figure skaters, or other subjects,  where they change direction suddenly.

The main driver for the continual improvement in AF tracking ability is the development of faster and more powerful microprocessor chips for digital cameras. These enable the camera to process the increased data from more AF sensor points while keeping the lens focused on a fast moving subject.

Manual focusing

In this mode, the autofocus system acts as an electronic rangefinder. The focus confirmation indicator in the viewfinder still operates when the lens has been manually focused on a point.

Despite all the computing power and multiple focus point in some difficult situations  where AF systems  do not work, although these are getting fewer.

Choosing a focus Point 

When the shutter release button is half depressed the position of the active AF sensor, or sensors, is indicated in the viewfinder.

The AF system’s first choice for the focus point is in the centre of the view. This is not always the same position as the main  item of interest.  This difference can be an issue depending on the depth of field.  This means the areas just in front and behind the point of focus appear reasonably sharp, the further you move away from the focus point the apparent sharpness decreases.

There is more than one  way around these situations. One is the option of selecting another AF sensor as the main focus point.

landscape focusing

For landscape photography getting the whole scene in focus is important

Another is to move the camera so its choice of focus point is aimed at the main item of interest and keep the shutter release button half depressed. This locks the autofocus at that distance, then recompose the  photograph and fully depress  the shutter release button to take the photo.

Read the manual

When you buy a new DSLR you really do need to read the AF section of the manual.

Manufacturers keep updating the AF system of every new model DSLR. Couple this  with the wide variations in AF capabilities between DSLR models means you need to understand the fine detail to get the best out of the AF system. Because of these variations, this article is only an overview of general terms. The aim is to help you understand the basic concepts of AF systems to help you interpret the camera’s user manual.

Don’t just leave it on automatic AF and hope, take control of your camera.


Pentax K-1 a  full-frame DSLR at last

The new to Pentax full frame sensor

The Pentax K-1 is DSLR with a digital sensor the same size as the old 35mm film. The new sensor is probably the same Sony  unit that is used in  Nikon’s D800 series cameras.

Pentax has been promising their  full-frame DSLR  for several years, but the details from Pentax suggest the  wait might have been worthwhile.  Part of the delay is probably due  to Nikon getting preferential treatment from the sensor manufacturers, Sony. This is a familiar scenario with Pentax playing second fiddle to Nikon, who buy many more sensors from Sony  than Pentax.

Even though they may share hardware, each camera company have their own processing methods in their cameras.  Pentax has a history of  achieving very good skin tones, dynamic range, and noise performance with their systems.   It could be that Pentax used this  extra time  to develop their processing algorithms. In the K-1 this is the PRIME IV imaging engine, these interpret  the electrical signals from the digital sensor to produce the image file. (more…)


Canon announces the EOS-1D X Mark II professional digital camera

Canon EOS1DX MarkII body

Canon’s improved MkII version of their EOS-1D X Digital SLR camera will soon be available.

Canon’s move  follows Nikon’s recent announcement  of the imminent release of their new D5 , a direct competitor for Canon’s top camera. Because photojournalists are the main users of these high-end digital cameras the specifications reflects their needs, rather than ultimate image quality.

Neither of the new cameras are available yet, but it seems both companies  wanted to avoid comparisons between the other’s new model, and their older version.  So it is useful to analyse the Canon EOS-1D X Mark II promised specifications with the  Nikon D5 in mind.

Both new cameras have new 20 Megapixel full 35mm frame digital sensors. The  Canon has a standard ISO speed range of 100 – 51,200 with extended  ISO speeds  up to 409,600 available. This falls short of the Nikon D5’s standard ISO range of 100 to 102,400 with an extended range up to ISO 3,280,000. While the extreme ISO ranges may sound impressive it is really a matter of digital noise performance in the normal ISO ranges that are important to most photographers. The DxOMark sensor testing will give a good indication of the relative noise performance over their ISO ranges. (more…)


Nikon’s new flagship DSLR the  D5 

D5 front

Nikon recently announced the upcoming release of their new flagship DSLR camera,  the D5, that includes a Nikon developed 20.8-megapixel CMOS sensor,

Nikon design their top of the range DSLR’s as photojournalist tools. Nikon’s aim is to produce a camera that will capture a moment whatever the light or subject speed.

“The D5 doesn’t simply get the shot that others might miss– it helps get the shot that others just simply cannot,” said Masahiro Horie, Director of Marketing and Planning, Nikon Inc.

Making the most of those fleeting opportunities is more important to  photojournalists than ultimate image quality. The Nikon D5 compromises on image quality to achieve remarkable low light and fast shooting speeds. Not the best image quality, merely excellent. (more…)


Digital Photography a Golden Age

Compact digital point and shoot or a phone camera is easy to carry

Digital  photography provides exceptional tools for taking and sharing photographs,  making this a golden age for photographers.

The advent of digital cameras means  just about anyone can take a reasonably exposed and focused photograph and produce quality prints, even on typical home office equipment. This has disrupted the business model of many professional photographers, making it difficult for them to stand out from the madding crowd. They have to be exceptional, not just competent. (more…)