Contrast Detect AF (CDAF):
Contrast Detect AF relies on assessment of the image contrast at the sensor and then uses a series of iterations of lens element movements so that it can reassess the level of contrast and determine the point of maximum contrast.
The capability of this mechanism to achieve fast AF is limited by the weight of the lens elements involved in focus, the AF motors, the data communication band width between the camera and lens, the AF computer algorithms, subject contrast and movement.
At present technology, even with Panasonic adding 2 extra lens coupling pins for the Micro Four Thirds standard and developing smaller lenses optimised for CDAF, while the AF is very fast for slow moving or stationary subjects, even in low light, even with this optimisation, it has great trouble locking onto faster moving subjects, and thus has limited applicability to sports photography.
When using non-CDAF optimised lenses, AF can be VERY slow, if it works at all (generally will not work on Panasonic GH-1, G1, GF-1 models but will work slowly on later Panasonic models or on Olympus models).
CDAF does have a number of significant advantages over phase contrast AF such as:
- allows almost any area of the image to be the AF point instead of specified AF sensor sites
- works even at small apertures as long as there is enough light coming in
- allows face recognition AF
- allows AF tracking of slow moving subjects of a specified appearance just by selecting a subject to lock onto
- eradicates the perennial problem of AF calibration errors
Phase contrast autofocus:
All current dSLRs use phase contrast AF sensors as the primary mechanism.
The AF sensors are generally located under the SLR mirror and some light passes through the mirror then through light splitting prisms to reach the bank of AF sensors.
The light is split so that each sensor detects only light coming from one side of the lens.
Basic AF sensors detect the lateral displacement of a vertical line in the image when the image is out of focus, with the line being displaced to opposite sides of centre depending on which side of the lens the light is coming from.
The camera computer can then use this information of lateral displacement to accurately determine the correct position of the lens focus element required in order to achieve focus.
This makes AF very fast and even predictive continuous AF is fast and can be quite accurate.
Unfortunately, if there is no vertical line, such as sensor will not function, and thus most newer and more expensive dSLRs use AF sensor with horizontal and vertical capabilities (“cross hair” sensors) which increase the chance that it will be able to use part of the subject to AF upon.
As the distance from the lens to the sensor is potentially different to the distance from the lens to the imaging sensor, there is a potential that different lenses, different temperatures, etc can result in minute changes to these distances and thus the potential for consistent back-focusing or front-focusing to occur which requires AF calibration to correct. Fortunately this is now possible by the end user with newer camera models – previously you would need to send the camera and lens to the manufacturer for calibration.
Another problem is that due to cost issues, generally only the centre AF sensor is made sensitive enough to allow lens apertures up to f/8 while other AF sensors may only work at wider apertures.
This means that if you try to use a 2x teleconverter with an f/5.6 lens, phase contrast will NOT work.
Hybrid AF systems:
Most dSLRs made in the last few years have “Live Preview” mode in which the mirror is temporarily raised and light hits the sensor directly, thus bypassing the phase contrast AF sensors, and thus CDAF must be used. Unfortunately, these systems are not generally optimised for CDAF and thus AF in live preview is quite slow.
A handful of cameras allow phase contrast AF during Live Preview whilst having the mirror in the normal down position by having a separate image sensor in the pentamirror compartment which sends the video feed to the LCD screen. This was first seen in the Olympus E330, and has since been taken up by Sony in some of its models.
Mirror-less camera systems:
Fuji has just announced a new “hybrid AF” technology which essentially converts strips of image sensor photosites into phase contrast AF sensors.
This is an exciting development which may allow cameras like the Micro Four Thirds to have fast and accurate action AF without having to worry about mirrors, and then perhaps we will see the development of very fast burst rates of much more than 10fps as there is no physical limitations of moving the mirror up and down.
Olympus is taking their time in developing their new Four Thirds dSLR and this is rumoured to be quite different to previous models and likely to allow both fast CDAF and phase contrast AF – and perhaps optimised for either Micro Four Thirds or Four Thirds lenses – perhaps we will see something in the next few months.
Creating AF lenses from legacy manual focus lenses:
As mentioned in a previous post here, Olympus appears to be working on a new Olympus OM adapter for Micro Four Thirds which not only includes a 0.5x wide converter to give the natural field of view of these lenses but which would also allow fast CDAF.
Also, as mentioned in this post, Panasonic appear to be working on an adapter which includes a pellicle mirror and phase contrast AF sensors which would allow phase contrast AF with Four Thirds lenses and perhaps other legacy lenses when mounted on Micro Four Thirds.
Now these would be a very exciting developments indeed!.
Whatever happens, the improving sensor technology along with improving AF technology will only make the Micro Four Thirds cameras even more compelling as THE camera to take with you every where.