Working With AF Modes – The 7 Essential Things That You Need To Know

What is Auto-focusing?

Auto-focusing is the process of locking focus or creating a sharp image of a subject on to the imaging sensor. This is achieved via an array of sensors and focusing motors located inside the camera and the lens working in tandem. The main architect of ensuring that a camera is auto-focusing without issues are the auto-focusing sensors. These sensors are located at the base of a DSLR camera. In modern mirrorless cameras, the main sensor performs the dual role of auto-focusing as well as recording of images.

Types of Auto-focusing

There are mainly two types of auto-focusing technologies in vogue. One is Active auto-focusing and the other is the Passive auto-focusing.

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Passive Auto-focusing

Passive auto-focusing is the most widely used auto-focusing mechanism on DSLRs as well as other types of cameras. Passive auto-focusing uses two different technologies for calculating the distance between sensor and subject and then accurately locking focus. These are –

Phase Detection Auto-focusing

Phase detection auto-focusing works by employing a secondary phase-detection mirror behind the main reflex mirror. This mirror splits the light beam into two distinct beams. The two light beams create two pictures which are then analyzed to check for distance and other parameters. A motor then drives the focusing elements on the lens and attempts to bring the two images together. That in the process creates a sharp image on to the main sensor.

There are many different types of phase-detection auto-focusing mechanisms available in the market. But the two main types are – one where an array of sensors is located at the bottom of the main mirror and the second where the actual pixels on the main sensor doubles up as phase detection sensors.

Contrast Detection Auto-focusing

The second type is Contrast Detection Auto Focusing. Contrast detection auto-focusing is a slightly different technology. It still works on the fundamentals of auto-focusing, but here instead of splitting the light beam into two and then analyzing those two images, the sensor inside the camera analyses contrast. It does so by pushing the focusing mechanism back and forth. Ever so slightly. Then analyzing the improvement or detriment achieved in contrast. You would find your camera ‘hunting’ for focus as the point of sharpest focus is achieved and then the image getting blurred to back again achieving sharpest focus. The fundamental thinking behind this technique is that focus is sharpest where contrast is the highest.

Auto-focus Modes

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There are three Auto-focus modes that are readily available on most cameras. These are Single-shot Auto-focus, Continuous Auto-focus, and Hybrid Auto-focus.  Single-shot Auto-focus denotes that the camera locks focus just once. This is useful for portraits, landscapes, architecture and those sorts of subjects which are not moving about. Continuous Auto-focusing is used for subjects that are moving about. These would be kids playing in the park, wild animals, birds, fast action, etc. The final mode is the Hybrid mode. In this mode, the camera starts off in single-shot auto-focus mode. Now, if the subject moves the camera automatically switches to Continuous auto-focusing. The advantage of this mode is that the camera automatically makes the decision based on the data already inside it. This eliminates any human errors like not switching to the right mode when it is necessitated.

Active Auto-focusing

Examples of Active auto-focusing are very few as they employ actively measuring the distance between the sensor and the subject being photographed. They are very difficult to be implemented in a cost-effective manner in modern digital cameras. But some cameras do have them such as the ones which have an active IR light emitting from the camera to determine the distance between the subject and the sensor.

Manual Focusing Override

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Manual focusing override is an important part of the auto-focusing mechanism. It allows the photographer to manually adjust the focus by turning the manual focus ring even though auto-focus is engaged. You will find some manufacturers labeling this technology as FTM or full-time manual focusing override. Of course, this depends from manufacturer to manufacturer. 

Focusing Delimiter

The Focus Delimiter button is very helpful in tele-lenses. Tele-lenses because of the nature of these lenses have a long focusing range. A subject at infinity such as a wild elephant standing at 100 feet would always have to be focused at infinity. A standard telephoto lens like would come with three focusing ranges. One that would allow the lens to look for focus across the focusing range, and two others that would split the focusing range into two. if you know at what distance your subject would be it is better to choose a limited focusing range because it helps in speeding the whole process of focusing.

Focus Lock

The focus lock button is denoted by the alphabets AF-L. Or something in that line depending on the manufacture. The Focus Lock button has one main purpose. That is to lock focus after you have manually or by half-pressing, the shutter release achieved focus where you want to. This is useful for product photography as well as for macro photography and for landscape. There is usually a lever at the back of the camera with the letters AF-L or equivalent that allows you to do that. An alternate option would be to switch to manual focusing after you have achieved focus.

Cheap lens series: The 35mm prime

What is a 35mm prime lens?

In the world of DSLR lenses the 35mm prime lens is a widely revered one. It is a standard fixed focal length lens with sharp image quality and practical everyday use. Mind you this is not a zoom lens. So, the only way you could zoom is if you move your feet. Too tight a frame? Step back a few paces. Too much negative space around the subject? Step forward for a tighter composition. As you can imagine this is not the type of lens that would make zoomers happy. Prime lenses are designed for optical superiority and they are widely considered as a hard working photographer’s lens.

Prime lenses vs. zoom lenses

You may have heard protographers going ga-ga over the quality of prime lenses. This debate of prime vs. zoom will always find arguments on either side. We are not going to delve too much into this. One thing I would, however, mention is primes are primed (pun intended) for optical superiority. Zoom lenses have too much on their plate and that sometimes tend to weigh down on their performance.

Advantages of the 35m prime

The 35mm prime is considered as a standard prime. The 50mm and the 40mm prime lenses are also considered as standard primes. Standard being close to the focal length of the human eye. While that is just a statement and warrant a much deeper discussion (beyond the purview of this article), consider just this – a standard prime gives us the same angle of view which is roughly the equivalent of the human eye.

35mm prime lenses come optimized for both full-frame 35mm as well as crop sensor powered cameras. Canon currently, however, don’t sell a lens specifically designed for its smaller crop sensor cameras. Nikkor on the other hand does. Having said that EF-S mount cameras can use lenses designed for the EF mount. Thus Canon’s 35mm lenses designed for full-frame cameras work on its smaller sensor cameras.

A 35mm lens when mounted on APS-C sensor based digital cameras gives a slightly smaller angle of view. This happens because of the crop factor. For those who are not aware of this, very quickly, the crop factor denotes what the effective focal length will be when a lens designed for a bigger camera is used on a smaller camera (given the lens mount is compatible). This happens because the sensor behind the lens is only going to use a small portion of the image coming through the lens.

On a Nikon DX format camera the crop factor is 1.5. Which means a 35mm prime becomes a 52mm prime. On a Canon the crop factor is 1.6. It results in the angle of view becoming the same as a 56mm lens mounted on a 35mm camera. Focal lengths (and effective focal lengths) are always expressed in 35mm format terms.

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Ergonomics, weight and build quality

35mm lenses are no-fuss optical tools. They are, as mentioned before, designed for optical superiority and that gives them a real edge compared to zoom lenses. Even the cheapest prime lens will give a well-made zoom lens a run for its money.

35mm primes are lighter compared to zooms that cover the same focal length. This is because zoom lenses are composed of a number of lens elements which enable it to alter the focal length. It also has focusing elements and in some cases image stabilization elements too. On top of it modern auto-focusing lenses have AF motors built into them. All that adds to the bulk. Comparatively, older lenses such as the legendary Nikkon 35mm f/1.4 manual focusing lens has no image stabilization, no auto-focusing motor and no zoom lens elements. It is lighter but is designed with a single purpose – optical superiority. You won’t get anything as sharp and with such beautiful color rendition, even though the lens is more than three decades old!

Maximum aperture

Another benefit of the 35mm prime and for that matter all primes is that they are blessed with fast apertures. Faster apertures does two things. A - They gather more light, which, especially in low light situations, can help you to capture better images sans noise. The second benefit is you can really close down the depth of field to a narrow slit of focus obliterating everything else in the frame. Such shallow depths of field has its own uses in photography. It is capable of producing beautiful bokeh; something that is imperative for isolating a subject from its background.