Thursday, October 31, 2013

Understanding and Achieving Image Focus and Sharpness: Part I

Nikon 51 point Autofocus System layout in the Nikon D4 DSLR, photo courtesy of Nikon CorporationImage sharpness depends on multiple factors including focus.

While it's true an image can't be sharp unless the subject is in focus, the converse is not necessarily true, that an image which is in focus, will necessarily be sharp. Image sharpness is fundamentally controlled by focus, but there are other important factors which enable in-focus images to be sharp.

In this multi-part series, we'll discuss what it takes to make a photograph look sharp. We'll look at how to successfully focus your camera on your chosen subject, and how to make your well focused image actually look sharp.

In Part I we'll look at how modern digital cameras autofocus (AF), and what factors influence AF systems' ability to focus quickly and accurately.

Autofocus cameras either use “active AF,” emitting a signal to estimate the distance to the subject, or “passive AF” using either contrast or phase detection, which both rely on reading the contrast at your focus point(s) for achieving accurate focus.

Active AF systems measure the distance to the subject independently from the camera's optical system, and use the distance information to adjust the camera's optical system to the correct focus. Active AF systems typically measure distance, via emitted ultrasonic sound waves or infrared light. Many compact digital cameras use Active AF.

Passive AF, contrast detection uses the camera's main sensor to autofocus which is a disadvantage, as this method can't determine if the camera lens is front or back focused, only that it is out of focus. As a result this method is iterative, and therefore slower than phase detection AF. Contrast detection is essentially fail proof, however, as it doesn't rely on special sensors which must be pre-calibrated.
Some compact digital cameras use contrast detection. DSLR cameras which have “live view,” often use it for that feature.

The preferred autofocus method for DSLR cameras is passive phase detection, because it's faster than other methods, and highly accurate.

Phase detection requires autofocus sensors, which are tiny rangefinders laid out in arrays across the camera's field of view. Each sensor measures the amount of front or back focus by assessing contrast at their respective point (focus point) in the scene. (That's why phase detection autofocus is so much faster than contrast detection.)

Typically, phase detection AF is achieved in less than a second, but sometimes, autofocus lock is hard to achieve, and the lens will noticeably focus back and forth (hunting) before finally achieving focus lock, or possibly fail. This is also true for contrast detection AF.

Generally, phase detection AF is more powerful and better able to achieve quick, accurate autofocus in the variety of conditions encountered by photographers. It's ability is in large part dependent on the number of focus sensors the system contains, and the number of cross type, versus vertical or horizontal sensors in the system.

AF sensors are actually all linear. Cross type sensors are paired linear sensors, one vertical and one horizontal. Cross type sensors are more powerful because they can detect contrast patterns in multiple directions. The center sensor, in all DSLRs using phase detection, is a cross type sensor. Having multiple sensors, especially if cross type, are especially helpful when focusing on moving subjects.

Now that you know how autofocus works, it's time to discuss what factors influence autofocus, performance. You can use an understanding of these factors to your advantage in achieving satisfactory focus.

There are three main characteristics which control the passive autofocus system's ability to achieve focus:
  • light level,
  • subject contrast,
  • camera and/or subject motion.
While a well lighted subject is easily “seen” by the AF sensor(s), a dimly lighted subject will be difficult for the AF sensor(s) to read.

While much of the time, you can't control the amount of light falling on the subject of your image, you can utilize a “fast lens,” one which has a very wide maximum aperture.

The amount of light illuminating the passive autofocus system can be directly affected by the maximum aperture of the lens used. Most lenses today have an aperture which only closes to its chosen setting at the time the photograph is made, when the shutter release activates it, or when it's activated to preview the image. During autofocus, the aperture of most lenses remains set to its maximum opening. The larger the maximum aperture a lens has, the more light that can get through it for the camera's AF sensors to use.

While you can rarely control the contrast of the subject on which you wish to focus, you can choose the highest contrast areas of the subject on which to focus. Choose areas having a sharp edge, a pronounced texture, or large tonal difference, in that order, to maximize your autofocus system effectiveness.

Motion of the camera or the subject will make focusing more difficult. Today's modern digital cameras/lenses have methods to help counteract camera/lens motion when you're making the image, such as vibration reduction/optical stabilization, but these don't help autofocus. The cameras also have continuous focus settings to account for movement of the subject. (More on continuous focus in another part of this series.)

If you're having focus problems due to camera/lens movement, then you need a monopod or tripod, in my opinion.

In Part II, we'll discuss choosing the best focus mode for a particular subject, manual or autofocus, or using them together, as well as single versus continuous focus, all to achieve a solid focus for a variety of situations common to travel photographers.

5 comments:

Phillipa-London, UK said...

I had no idea the speed of the lens could affect my camera's ability to autofocus. I guess I should have known.

Peter - Pittsburgh said...

So, it sounds like you recommend cameras that use phase detection vs. contrast detection for autofocus?

Ned S. Levi said...

The focus method a camera uses is only one characteristic of it one should note when making a camera purchasing decision.

While it's true, that generally phase detection is the faster autofocus method, between the two, and generally more flexible, there are times each are good choices, and in fact, there are many high end DSLR's these days that use both methods; phase detection for most still images, and contrast detection for still images during live view, and videos.

Rather than advocating purchasing cameras with particular autofocus method, I'm advocating learning what method your camera has, and learning how best to use that method. With that understanding you will be better prepared to achieve good focus with your camera, even when conditions for using autofocus are less than ideal. Additionally, that knowledge will help you understand when switching to manual focus is in order.

Peter - Pittsburgh said...

Thanks for the detailed explanation Ned.

Zack - Des Moines said...

Great article. Now I understand why that center focus point works so much better than some of the other focus points in my D5000.

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