Nothing spoils a photograph more than a wrongly focused subject. The camera systems are now very advanced when it comes to focusing correctly. The AF sensors are super-sensitive and can work in dim light, low contrast and even through linear polarizers. Understanding how the focus systems work and their limitations can go a long way in getting good images.
Auto Focus (AF)
AF sensors work on various principles. Some send a signal and detect it when it comes back. These are called active systems. In most of the present day DSLRs, the sensors work based on the available information. These are also called passive systems. One of the methods is by using phase detection. The light falls on the AF sensors through two different parts of the lens and passed through a complicated system of mirrors. The camera measures the difference in the phase of these two beams and focuses the lens to reduce the difference. Another method is based on the observation that the focused image has the maximum contrast in comparison to the unfocused images of the same composition.
The AF sensors are located on the lower side of the mirror box in DSLRs and SLRs. The red brackets that one sees through the viewfinder are located on the upper side of the mirror box (above the focusing screen). The camera is made in a such a way, that the AF sensor brackets seen in the viewfinder correspond to the actual AF sensors located in the lower part.
From the initial days of single central AF sensor, the cameras have come a long way. Now they have multiple AF sensors spread across all over the frame. Still the most efficient sensor is the central one, which uses both phase detection and contrast detection. The central AF sensor is also usually a pair of sensors which work in horizontal and vertical orientation. In some cameras, few more sensors apart from the central sensor are also empowered in the same way. Your camera’s user-manual is the best source of information for this.
(A low contrast scene with haze – AF often fails to work in such scenes, especially if the light is low)
The AF system fails commonly when the ambient light is low since the two light beams become very dim. The camera is unable to calculate the phase difference. This is another reason why AF is difficult with slow lenses. In images with low contrast, the contrast detection frequently fails. To overcome this, the camera manufactures have come out with the AF-assist light. All it does is to shine a low powered light on the subject and thereby makes the phase detection beams stronger and contrast clearer. The closer the subject to the camera, the better this light works. It is also quite irritating to people getting photographed. I keep it switched off. A better option is provided by the external flashes of speedlights. They have a red pattern which shines on the subject and helps in AF. The AF illumination light (from the camera or from the flash) is set in such a way that the AF easily works when the central most sensor is used.
Sometimes lenses tend to move differently than what the camera wants them to do. These are AF errors that creep in with time and have to be corrected. This is best done by the lens manufacturers themselves and if you are buying AF lenses, this reason should be sufficient enough to avoid grey market products. The calibration if required. is not something a user should attempt. Some cameras have an option to fine tune the AF within the camera body itself. This works but I recommend avoiding it. Sometimes the same correction will get copied to another lens with perfect AF and thereby cause errors. If the calibration is off, the ideal option is to get it fixed.
What if the subject is moving towards or away from the camera? This is where cameras have made really good progress. Now there are algorithms that work along with AF sensors. These are capable of predicting the movement and projected location as time passes. Spot on AF is now possible by these methods. Going a little further, the same method with continuous feedback is available even in all kind of scenarios where subject is in motion. Nikon calls it AF-C or continuous AF. Canon calls it AI-servo.
How to achieve perfect focus at all times?
- Use autofocus when the light is enough. In dim light, switch to manual focus or use the AF assist lights.
- If the camera is not focusing properly, try tilting the camera body. Sometimes none of the two cross-hair AF sensors can find the subject, tilting the camera helps them ‘see’ the subject properly.
- Use the central sensor whenever possible and recompose after locking the focus (use single point focus). Contrary to what the photographers will have you believe, the change in distance is so minimal that it doesn’t affect the focal plane in all practical solutions. So focus and recompose without any fears.
- If the subject still can not be focused, try focusing on the edge of the subject.
- While using AF-C, try to keep the main subject aligned with whatever AF sensors were being used. Track the subject by using these as guide marks (Further reading – Panning).
- The plane that is perfectly in focus is always one thin plane, but depending on the aperture, the things in front and behind can be in reasonably sharp focus too. So do not use your lens wide open when the focusing is doubtful. Make use of increased depth of field by stopping down the aperture (using higher f-number).
- Focus on eyes if you are clicking portraits or for that matter anything living.
Now the fun part. What about manual focus lenses or when the AF is not working well? Use manual focus!
At the time when film SLRs ruled the world, the focusing screens had various focusing aids built into them. There were split-screens where an image was split into two and by rotating the focusing ring, the split was reduced and then finished. Then there were micro-prisms in which the image was visible all the time but appeared all broken up until focused. Even the focusing screen themselves were made to exaggerate the out of focus effect. Sometimes the focusing screen combined multiple aids. The split-screen was placed in the center, surrounded by a microprism collar and then a matte focusing screen all around. Some cameras also had a digital rangefinder built it that indicated when the subject was in focus.
In the present DSLRs, these focusing aids have sadly disappeared. The only help provided is the electronic range finder that lights up when the subject is in focus. I find it useless. It is easier to observe the overall image in the viewfinder and focus than go by the digital range-finder function. Live-View function is also quite useful in getting perfect manual focus. Zooming in the subject while focusing using Live-View, can help attain that elusive paper-thin but perfect focus with fast teles.
I am old school, so I still prefer the focusing aids provided earlier. I have purchased after-market focusing screens and installed them in almost all my camera bodies. Now I have the comfort of split-screen and microprism collar for achieving precise focus. Installing these focusing screens may void your warranty and can even spoil your camera, so if you want to attempt it, do it on your own risk. Interestingly the Canon and Nikon, two of the market leaders, have the focusing ring orientation opposite to each other. Turning the focusing ring clock-wise on one moves the focus from near to far whereas in the other from far to near. This can be a bit confusing for photographers who are used to one brand of camera and lenses.
So if you are going for replacing the focusing screen, read the instructions on the provider’s website carefully. These are screens which are individually cut to fit the present day DSLRs. Most of them are spares from Nikon and Canon. Consider this before buying –
- Buy a Nikon manufactured screen if you are planning to use Nikon lenses on Nikon body. The focusing will come naturally. The movement of the split image will be in the direction of turning the focusing ring.
- Buy a Canon manufactured focusing screen, if the lenses to be used are Sigma, Tamron etc. on a Nikon body. The movement of split image will be in the opposite direction to the turning of focusing ring with Nikon lenses but with these third party lenses, the focusing will be in sync with the turn of the focusing ring.
- Changing the focusing screen exposes the camera pentaprism, pentamirror assembly to dust. Be ready to ignore a few dust particles on the focusing screen that become visible after this change.
(‘Depth of Field’ scale on a modern prime lens)
Hyperfocal distance focusing-
This is a simple way to ensure that everything within a specified distance range is in focus. Old lenses had lines or marks on their bodies indicating the hyperfocal distance for some apertures. These marks are the ‘Depth of Field’ scale. They gave an approximate idea of the minimum and maximum distance at a given aperture, within which everything will appear to be well focused.
To use this method, first focus on the farthest object that needs to be in focus. Note down its distance using the distance scale. Now focus on the nearest object that needs to be in focus. Again write down this second distance. Now slowly move the focusing ring so that both these values as shown on the distance scale are equidistant from the center. Next, choose an aperture from the Depth of Field scale that covers both the above distances. That is it. Click at this aperture without changing the focus and everything within those first and second distance readings will appear to be in focus.
The depth of field diminishes as the plane of focus is moved closer to the lens. This why even after using a small aperture, the depth of field is so very limited in macro shots. For hyperfocal distance focusing keep this in mind.
This was the method of choice for photographing birds in flight, capturing candid shots and for photojournalism in the days when AF was not there. (Further reading – f/8 and be there)
With the advancements in photo-editors and digital imaging, this is perhaps the most interesting method to hit the scene for getting deep or wide depth of field, especially in macro photography. This is a method of combining multiple pictures of a single subject, with different focus planes, into a single image, so that whole of the subject is sharp.
There are two ways to go about it-
- Use focusing rails and slowly move the camera in relation to the subject. The disadvantage is the change in size and perspective.
- Change the focus manually for different captures, while keeping the camera and the subject stationary. This can also be problematic especially with lenses where the image size changes with focusing.
The multiple images clicked are overlayed over each other using a photo-editing software such as photoshop and combined to create a single photograph with everything in sharp focus. The effort required to create such photos is high and even a slight movement or change in perspective or size can spoil the results. For most purposes, using the Hyperfocal Distance focusing is an easier method than Focus Stacking.
The focus for visible light is different from IR. I’ll talk about IR photography in another article.