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But care is needed when using the widest aperture settings on these lenses (or any prime lens). The depth of field is narrow and good technique is essential for focusing accurately.
Depth of field calculator
You can work out how much depth of field you have with any given camera, lens and aperture combination by using Depth of Field Calculator on the DOFMaster website.
For example, if you use a full-frame camera with an 85mm lens at f1.8 and focus on something one meter from the camera, then the total depth of field is just 1.35cm.
Now, think about what happens when you use autofocus in one-shot AF mode (I’m assuming that your camera is set to use a single AF point, selected by you, not multi-point AF or eye-detection AF).
• You press the shutter button half-way.
• The camera focuses on the subject, using the active AF (autofocus) point. Focus is locked at this point.
• You press the shutter button down all the way. The camera takes the photo.
If you’re taking a portrait all it takes is for your model (or you) to move slightly between the time the camera locks focus and the instant you take the photo to shift the zone of sharpness away from her eye and onto another part of her face.
This matters, because when it comes to portraits, focus is very important. Most photographers focus on the model’s eyes, or the eye nearest the camera if her face is at an angle (this is just a guideline, and there are exceptions, but it holds true most of the time).
It’s easy to tell if focus isn’t precise, especially at 100% magnification. Take a look at these two examples to see what I mean. The first is an enlargement of the opening photo, showing the model’s eye in focus. The second is a photo taken a few seconds later, showing her eye out of focus (both photos taken using an 85mm lens at f1.8).
Center AF points
There was a time when many cameras had just one AF point, in the center of the camera’s viewfinder. Now, most photographers know that composition is usually better when the subject is placed off-centre. So, to focus with just one AF point, the accepted technique was to cover the subject with the AF point, press the shutter button half-way down to lock focus, then recompose and press the shutter button all the way down to take the photo.
This is fine – until you use wide apertures. A little trigonometry reveals why. Let’s go back to the above example of focusing with my 85mm lens set to f1.8 on something that is one meter from the camera.
Let’s say that you use the centre AF point to focus, then move the camera left to recompose. Doing so increases the distance slightly between your camera and the subject. It also tilts the plane of focus slightly (see diagram below). Add in the fact that it’s difficult to move the camera, even slightly, without moving your body position and it’s now unlikely that your lens is still focused on the model’s eye.
Diagram: The white lines indicate the distance between camera and subject, and the red lines the plane of focus.
Other AF points
Taking all this into account it’s clear that the best way to focus on the model’s eye is to select the closest AF point, depending on the way you have composed the portrait.
But in practice it’s not so simple. For example, when I used my Canon EOS 5D Mark II for portraits I found that the center AF point was much more accurate than the eight outer ones. The reason for this is that the center AF point is a high-precision cross-type AF sensor, and the others aren’t.
Let’s look at this in more detail, because it’s relevant to any digital SLR camera using phase detection autofocus. This is the type of autofocus used by the camera when you take photos while looking through the viewfinder.
Contrast detection autofocus, used by digital SLRs in Live View and by mirrorless cameras, is different, as we shall see further on in the article.
Cross-type AF sensors
The centre AF point on my EOS 5D Mark II is a cross-type AF point. Cross-type AF sensors are sensitive to lines that move both vertically and horizontally across the frame. This diagram shows the camera’s autofocus array.
The other eight are single line sensors (the diagram below shows the difference). They are sensitive to lines that move across the frame in a single line. There are both vertical line sensitive sensors and horizontal line sensitive sensors. It is rare for camera manuals to indicate whether a single line AF sensor is a vertical or horizontal line sensor.
Once I realized that the center AF point on my camera was more accurate than the others I decided to test it properly. I took five portraits at f1.8 with my 85mm lens using each of the camera’s nine AF points. The center AF point focused on five out of five accurately. It rarely missed focus. The others tended to focus just one or two times out of five accurately. This is not good enough. It’s frustrating to capture a great expression only for it to be out of focus.
The end result of this is that on my EOS 5D Mark II I was forced to use the center AF point for portraits. This led to compositions where I ended up placing the model’s eye in the center of the frame.
Here’s an example.
If I wanted to place her eye off-center, like this portrait, I had to take several photos to try and ensure one was sharp.
More cross-type AF sensors
Now, wouldn’t it be helpful if there was more than one cross-type AF sensor on my camera? Yes, it would. Bear in mind the 5D Mark II was released in 2008. In digital terms it’s an old camera. All the latest Canon cameras have multiple cross-type AF points. So do all the latest cameras from every other manufacturer. So it’s not the problem that it once was.
The main point I’d like to make is that it’s important to understand how your camera’s autofocus system works when using prime lenses at wide apertures. Especially for portraits, where focus is critical. Once you understand this, you can adjust your technique accordingly, as I did when composing portraits.
It also helps you understand when you might need to upgrade to a more advanced camera. If you’re constantly bumping up against a limitation of your camera (like I was), then you should consider upgrading. But only once you’ve determined that it’s a genuine limitation of the camera and not your lack of understanding of how it works that’s causing the problem.
Back and front-focusing
I hate to throw another thing at you but it’s also possible that your lens isn’t focusing precisely where your camera thinks it is. This is another limitation of phase detection autofocus systems. I’ll avoid an in-depth explanation of this for the moment, but all you really need to know is that your lens may be focusing slightly in front of or slightly behind of the point where the camera thinks it is focused.
This is not a fault of your lens or your camera, even though it may seem like it at first. It just happens because phase detection autofocus isn’t always as precise as you might need it to be. It’s fast – which is why digital SLRs are still the best tool for photographing fast moving action. But it’s not completely accurate.
Using the example above again, if you focus an 85mm lens set to f1.8 at a point one meter from the camera then you only have 1.35cm of depth of field. If your lens is actually focusing a centimeter behind or in front of that point, you have a problem. It reduces your margin for error dramatically, and you are more likely to have out of focus images through no fault of your own.
Most mid and high end SLR cameras let you tweak the autofocus settings to compensate for this.
This is called Autofocus micro-adjustment by Canon and Sony. Nikon calls it Autofocus fine tune, Pentax uses the term Autofocus adjustment and Olympus Autofocus focus adjust.
Sigma even makes a dedicated USB dock that lets you compensate for focusing errors by updating the lens’s firmware.
Unfortunately most consumer digital SLR cameras don’t have this feature. The only way to adjust the autofocus performance of a prime lens is to take the camera and lens to a technician.
Professional photographers who qualify for Canon Professional Services or Nikon Professional Services can work with those companies’ technicians to fine tune the autofocus systems of their cameras. Both companies send technicians to major sporting events to help out with this.
Contrast detection autofocus
Does this sound like a lot of hassle? It is, and you may be interested to learn there is a better way of doing things. Mirrorless camera owners will know exactly what I’m referring to.
Mirrorless cameras use contrast detection autofocus instead of phase detection autofocus. Actually, more and more cameras use a combination of the two, but contrast detection autofocus is the one we’re interested in from the point of view of focusing accurately at wide apertures.
With contrast detection autofocus the camera reads exposure from the sensor. The advantage of this is accuracy. With my Fujifilm X-T1 I can set my 56mm lens to f1.2 and select any of its 49 autofocus points knowing that it’s going to focus accurately virtually every time.
I tested the X-T1’s autofocus accuracy with the portrait below. I deliberately composed the photo so that there were twigs and leaves between the camera and the model. It still had no trouble focusing accurately on her eye.
The disadvantage is speed. Contrast detection autofocus is slower than phase detection autofocus. The result is that it’s less likely to be accurate when tracking moving subjects. Some mirrorless cameras use a hybrid of both to improve autofocus tracking performance.
Contrast detection autofocus is also used by digital SLRs in Live View (again, some cameras use a mix of contrast detection and phase detection to enable tracking of moving subjects, especially for shooting video footage).
I’ve thrown a lot of information at you in this article, so let’s take a few moments to sum up what you need to know.
To focus accurately on still subjects with prime lenses at wide-apertures you need to use either a digital SLR with plenty of cross-type autofocus points or a mirrorless camera. You may also need to adjust the focusing point of lenses used with digital SLRs.
Mirrorless cameras with contrast detect autofocus don’t have this problem. For this reason they are great to use for portraits. There is no need to worry about which AF points are cross-type points, or whether you need to calibrate your lenses. You can just get on with it, knowing that your camera is going to focus accurately.
If you own a digital SLR it’s a little more complex. Take the time to test your camera and lens combination, much like I did with my EOS 5D Mark II. Learn how to select cross-type AF points, and do some tests to see if your lens is back or front-focusing. Once you have established the facts, you will know which AF points are reliable.
The key is to get to know exactly how your camera and lens combination works so you can trust it when shooting portraits. Or any other type of photo that requires accurate focusing at wide apertures with prime lenses.
If you have any questions about any of this then please let us know in the comments.
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