Bill Pierce
Well-known
With mirrorless cameras now competing more and more with DSLRs in the hands of serious photographers, I was recently asked about the difference between the contrast detection auto focus found on mirrorless and the phase detection on DSLRs. (Actually, I was asked, “Which is better?”)
Phase detection is pretty complicated, but, to oversimplify it, light rays from opposite edges of the lens are evaluated by a sensor. If the lens is in focus, the rays will be at a set distance from each other. If the focus is off, the light rays will be closer or farther apart. Thus the system not only knows if the image is out of focus, it knows whether to move/focus the lens forward or backward. Phase detection is fast.
On the other hand, contrast detection just moves the lens back and forth sensing when it reaches its highest contrast. The contrast is getting higher, let’s keep moving in that direction. UhOh! It’s starting to get less contrasty. Let’s go in the other direction. It’s sort of a hunt and peck system, but it gets the job done.
It sounds like phase detection, which is the system we see on most DSLRs, is the clear winner! Not always… In DSLRs, the autofocus sensors can’t be on the sensor; it doesn’t see any image when the mirror is in place. If that sensor is to mimic the plane of focus, it and the mechanisms that direct light to it have to be placed very accurately, especially when some of the new high megapixel sensors can show the loss of quality that come from a slight misfocus. Add to that the fact that you have just read the most oversimplified description of phase detection ever. Those little camera computers are reading a lot of info from the lens and focusing motors.
Contrast detection is comparatively simple. It doesn’t require additional sensors or as elaborate computer processing of the information. That means smaller and cheaper - ideal for those mirrorless cameras that when they were first introduced were thought of as “amateur.” ( And if the back and forth focusing action looking for the contrasty or sharp image seems somewhat less than cool and hip, remember it’s exactly what a human being does when they’re focusing.)
Please note that it is also difficult for contrast detection and humans to focus on moving subjects. Phase detect does that better. Nor can big telephotos easily make the many little focusing movements often required by contrast detection. So, even though the popularity of mirrorless is resulting in real improvements in contrast detection focusing, you’e still likely to see those big DSLRs on the sidelines of football games for awhile.
However, most folks are not going to have to choose the camera they use because of its focusing system. Both systems are fine for most folks. Big cameras with big lenses will probably use phase and little cameras with little lenses will probably use contrast. But photographers are going to pick the camera they like for a complex barrage of reasons. And for most folks, it’s not going to be too important whether their camera uses phase detection, contrast detection or the newest mirrorless rage - phase and contrast detection.
Your thoughts?
Phase detection is pretty complicated, but, to oversimplify it, light rays from opposite edges of the lens are evaluated by a sensor. If the lens is in focus, the rays will be at a set distance from each other. If the focus is off, the light rays will be closer or farther apart. Thus the system not only knows if the image is out of focus, it knows whether to move/focus the lens forward or backward. Phase detection is fast.
On the other hand, contrast detection just moves the lens back and forth sensing when it reaches its highest contrast. The contrast is getting higher, let’s keep moving in that direction. UhOh! It’s starting to get less contrasty. Let’s go in the other direction. It’s sort of a hunt and peck system, but it gets the job done.
It sounds like phase detection, which is the system we see on most DSLRs, is the clear winner! Not always… In DSLRs, the autofocus sensors can’t be on the sensor; it doesn’t see any image when the mirror is in place. If that sensor is to mimic the plane of focus, it and the mechanisms that direct light to it have to be placed very accurately, especially when some of the new high megapixel sensors can show the loss of quality that come from a slight misfocus. Add to that the fact that you have just read the most oversimplified description of phase detection ever. Those little camera computers are reading a lot of info from the lens and focusing motors.
Contrast detection is comparatively simple. It doesn’t require additional sensors or as elaborate computer processing of the information. That means smaller and cheaper - ideal for those mirrorless cameras that when they were first introduced were thought of as “amateur.” ( And if the back and forth focusing action looking for the contrasty or sharp image seems somewhat less than cool and hip, remember it’s exactly what a human being does when they’re focusing.)
Please note that it is also difficult for contrast detection and humans to focus on moving subjects. Phase detect does that better. Nor can big telephotos easily make the many little focusing movements often required by contrast detection. So, even though the popularity of mirrorless is resulting in real improvements in contrast detection focusing, you’e still likely to see those big DSLRs on the sidelines of football games for awhile.
However, most folks are not going to have to choose the camera they use because of its focusing system. Both systems are fine for most folks. Big cameras with big lenses will probably use phase and little cameras with little lenses will probably use contrast. But photographers are going to pick the camera they like for a complex barrage of reasons. And for most folks, it’s not going to be too important whether their camera uses phase detection, contrast detection or the newest mirrorless rage - phase and contrast detection.
Your thoughts?
Pioneer
Veteran
If you have actually used a good phase detect system to photograph moving subjects then it can be very frustrating to put up with a mirrorless camera.
xenohip
Established
Very slight focusing errors at large apertures can be unduly distracting, so in some cases contrast detect is more desirable.
Suppose for example a low light portrait, model's shoulder in toward the camera, face turned to the camera, wearing a sweater with a delicate ribbing, focus on the near eye. Now if the focus is slightly off, the near eye will be slightly soft. This might be fine... But by contrast (ha ha) the sweater presents a continuous range of focal distances from the image plane. At some place, the ribbing on the sweater will be _exactly_ in focus. The viewer's eye is naturally drawn to the exact focus on some random part of the sweater, and away from the face.
Not an academic example, by the way. Happened not just rarely, and part of the reason I sold an earlier dslr.
Suppose for example a low light portrait, model's shoulder in toward the camera, face turned to the camera, wearing a sweater with a delicate ribbing, focus on the near eye. Now if the focus is slightly off, the near eye will be slightly soft. This might be fine... But by contrast (ha ha) the sweater presents a continuous range of focal distances from the image plane. At some place, the ribbing on the sweater will be _exactly_ in focus. The viewer's eye is naturally drawn to the exact focus on some random part of the sweater, and away from the face.
Not an academic example, by the way. Happened not just rarely, and part of the reason I sold an earlier dslr.