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Look through the viewfinder of any DSLR camera and you will see several dots, or squares, that represent individual points at which the camera is capable of focusing. The purpose of these focusing points may seem fairly obvious, but not all of them are created equal. When you press the shutter button (or back button) halfway, some of these points will light up, indicating that everything at that specific spot is crystal clear and your photo will be nice and sharp.
However, the speed at which your camera can focus on one of the points, as well as how accurate the focus will be, depends greatly on whether the individual focusing point is a single or cross-type. Understanding the differences in how they operate can help you decide which ones to use to take better photos.
Most DSLR cameras use what’s called a phase-detection focusing system – whereas most mirrorless cameras, point-and-shoots, and mobile phones use a separate system called contrast-detect. In a DSLR, most of the light coming through the lens is reflected upwards by the mirror, to the optical viewfinder, which lets you see precisely what the camera lens sees.
However, a tiny bit of light is also sent downward to a series of sensors that are capable of figuring out whether the image is in focus. The science behind this involves splitting the incoming light, and comparing two beams, to essentially see if they match up. If not, an electronic signal is sent to the focusing motor, to adjust the lens until the image is in focus. All this happens in a fraction of a second, but these fractions matter in photography, and can often be the difference between a tack-sharp image and a blurry shot.
The problem with traditional phase-detecting systems is they get a bit stumped if there are a lot of vertical lines in the spot where they are trying to focus. To see how this works for yourself, print a sheet of paper on your computer, with nothing but vertical lines. Tape it to a wall, and try to focus on it with your camera. If you are using one of the focusing points on the outside edge of your camera’s viewfinder, your lens will likely spend a few seconds hunting for focus but will probably never find it. However if you turn the paper sideways and try again your camera will likely get things focused fairly easily. This is because when light is sent to the phase-detection sensors in your camera, the sensors don’t have enough information to determine focus, if all it sees is vertical lines.
While most of the time when you are out taking pictures, you are probably not shooting images of vertically-lined paper, this example does illustrate how your camera’s autofocus can get slowed down, and become unreliable under certain conditions. Ironically, in this test, your camera will find focus much better if you use the live view function. That employs a contrast-detection focusing method which is also used in most mirrorless cameras, and while it is a bit slower, can have some advantages over traditional phase-detect systems.
To address this issue, most camera manufacturers have implemented cross-type focusing sensors that work fine when focusing on images with horizontal and vertical patterns. On high-end models (like the Nikon D5 or Canon 5D Mark III) there are several clusters of cross-type focusing sensors, but lower-end models (like the Nikon D3200 and Canon Rebel T3i) usually have just one, right in the center. This means that the center autofocus point will likely be significantly faster, and more reliable, than the points on the edge. You can see the results yourself by repeating the test from earlier with the center focus point, instead of one on the perimeter of your viewfinder.
The real-world implications of this are quite significant, and may very well change how you approach your photography. Many people use an automatic setting that allows their camera to look at all the available focus points, and determine which one should be used to set the focus. But, if you know that the the cross-type points will give you consistently better results, you might try using them more often.
This is especially useful with sports and fast action, but other types of photography situations can benefit from utilizing cross-type points also. Portrait, family, and wedding photographers often utilize the focus-and-recompose method to nail focus with a cross-type sensor, then shift their camera’s field of view to get precisely the composition they want. If you shoot landscapes you might not need speedy autofocus, but using your camera’s cross-type sensors may help your focus be more accurate.
Of course all this doesn’t mean that the normal focusing sensors on your camera are worthless, just that knowing which ones are cross-type can often give you an advantage you might not have otherwise had.
One other point worth noting is that mirrorless cameras use phase-detection focusing more than they used to, and some are implementing cross-type sensors too. Just because this technology started with DSLRs does not mean it will be forever limited to these types of cameras, and as manufacturers continue to innovate we will likely see more, and better, focusing options in the years ahead.
To help you figure out how many cross-type focusing points your camera has, you can do a little searching online, or look at the list below. I have compiled some data based on the more popular camera models for you:
Have you ever tried using cross-type focusing points? What have your results been like, and what other tips and tricks do you have for other dPS readers? Leave your thoughts in the comments section below.
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