I love my Canon Rebel T6 and take portrait pictures in Av mode with a 50mm in 1.8 Av. I'm using auto focus in the 'focus on center point' mode. I make sure that the center point focuses on the face so the face should be the sharpest part of the picture. However, it isn't. Usually the auto focus misses by a couple of inches (focus and sharpness is best about 2 inches closer to the lens) when it comes to the sharpest part of the picture. So the face is never 100% sharp. Any thoughts on what I'm doing wrong?
You're not doing anything wrong. You're just finding the limits of the camera/lens combination you are using.
The EF 50mm f/1.8 (in various versions) has been known as the "plastic fantastic" for a long time. For what it can do at what it costs, it is a fantastic value. But it isn't really a fantastic 50mm prime lens when compared to many others that, admittedly, cost a little to a lot more.
One of the weaknesses of the EF 50mm f/1.8 II is that it has fairly large steps between each focus motor position when compared to other Canon lenses. In fact, it has the largest steps of any Canon EF lens on the market. This means if you are trying to focus on something that is in between one step and the next, the camera has to decide whether to go slightly past your subject or slightly in front of your subject.
- Another weakness of the EF 50mm f/1.8 family is that they need to be stopped down a bit to reach their maximum center sharpness. Wide open at f/1.8 even the center of the image is a little soft compared to when the lens is stopped down a few clicks.
- Your Canon EOS Rebel T6/1300D is another piece of Canon gear that is a pretty good value but the low price comes with the caveat of giving up some of the things higher priced cameras offer. One of those features is AFMA (Autofocus Micro-Adjustment) that allows the end user to calibrate their cameras AF system to a particular lens. If a lens consistently misses in front of the desired subject, AFMA can be used to tell the camera to focus a little further than what it otherwise would.
If you are shooting from a tripod¹ there is a way to increase the accuracy of your camera's AF, but it comes at the cost of slower AF and using the rear LCD in Live view to compose your shot instead of being able to use the optical viewfinder. When you are shooting in Live View the camera uses contrast based autofocus from the signal it is receiving from the main imaging sensor. When you shoot through the viewfinder the camera uses a dedicated phase detection AF sensor because the mirror that reflects the scene into the viewfinder blocks light from reaching the main imaging sensor.
Another thing you can try to do is stop down to f/2.2 or f/2.8 where center sharpness increases significantly from f/1.8. At f/2 the lens is a little better than wide open, but it's still noticeably softer than even narrower apertures. You're still going to have fairly shallow depth of field when shooting at "head shot" or "half body" portrait distances with a 50mm lens at f/2.2 or f/2.8.
For a more general survey of the various things that cause blurry images, including focus issues as well as other things, be sure to check out: How do I diagnose the source of focus problem in a camera?
¹ So that you don't risk camera movement that changes the camera-subject distance between focusing and exposure, which is all too easy to do when holding a camera away from one's body to use Live View handheld.
It might be best if you post an example image, but in any case, there are a few things to bear in mind.
First, shooting at such wide aperture provides very little leeway between the near and far limits of depth of field. Using a depth of field calculator, you will find that at 6 feet from the subject, only 4 inches will be in focus. Should your lens be off, or you focus on the nose, you risk having other parts of the face potentially out of focus. Stand a little further back and you can gain an inch or two. In any case, f/1.8 is likely much too wide open for best results.
Second, your shutter speed may be impacting your image quality as well. Be sure to have a fast enough shutter speed to correct for hand shake, generally 1/60- 1/80 is a lower limit without any image stabilization.
Finally, I find that the best images of a face start with the eyes in focus. This is where we naturally look, and having the eyes out of focus will impact the image quality tremendously. In this case, you should use a focus point on the eye, and open the aperture enough to provide some reasonable depth of field, allowing the nose and ears at least to also be in focus. Don't focus on the nose, focus on the eyes. It might help to choose a single focus point, rather than a group, which might cause the camera to choose a focus you don't actually want.
Typical AF problems (classic, not live view AF):
- body and lens, while both within specification, are badly matched, exacerbating each other's problems.
- => use a body with microfocus-adjustments.
My 70-200 f/2.8 was always soft at f/2.8 (and I often have a bare minimum of light), but a new body with micro-focusadjustments made it a whole new lens with a really enjoyable f/2.8.
- => send the combination in for tuning (as a combination)
- => use a body with microfocus-adjustments.
- Focus and then rotate the camera to frame the photograph causes the plane of focus to move away.
- => use a different AF point with less rotation needed.
- => restrict the technique to long lenses, where a 1°-5° rotation already moves everything out of frame
- Assuming the AF active area is just as large as the marker in the viewfinder.
- => Understand that the AF area is likely 3 times as wide and as high.
- => Understand that the AF marker in the ground glass is an approximation, especially in more affordable cameras.
- Focussing on the wrong spot in the face, often in combination with the large AF area.
- => Try focussing on the leading eye, as that is the most important part of the image to be in focus.
- => Make sure you do not accidentally get the nose: the AF is likely to choose the closest object it can see. With f/1.8 the nose can really move your focus far away.
- Too long of an exposure time causes object movement and/or camera shake, making the photo look soft/out of focus.
- => Try 1/200s and 1/500s at 50mm f/1.8. If that improves things, you have found one (partial) solution. You probably will prefer sharp but a bit noisy to little noise--all blur.
- => Flash (without the HS/Highspeed mode) can also help freeze movement. The less the flash power asked for, the shorter the flash is on. However, flash photos that look natural is a completely new skillset.
The glass of the lenses of your 50mm itself is clean, I assume?
The 'ultimate' test: tripod, static target, lots of light, moving the focus on the lens little by little by little, taking photos. Find the sharpest one (JPEG image size can be a rough estimate, blurry photos have less info and therefore need less space.) That would be about the sharpest the lens can provide in these circumstances. It may just not be "good enough" anymore for what you want it to do.
And you could try focus stacking, but that only works properly with stuff that does not move. Photographers using macro or loupe lenses tend to use that, as your field of depth is incredible to theirs --- a human hair width can be all the "in focus" area they can get from a single exposure ...
Personally, I like tight portraits ("just the face" kind) but from a longer distance away, using a tele lens. That is not everyone's cup of tea, obviously.
There are a lot of good detailed answers here, and they are correct, but I'd like to key in on a simpler truth. At f/1.8, there isn't enough depth of focus to get your subjects whole face in focus. If you focus perfectly on their eyes, the image will look good, but if you are focused a little in front or a little behind, it will be obvious that you've missed focus.
It is simply easier to get your subject properly in focus if you stop down to f/2.4 or higher. There are plenty of times when shooting at the widest aperture possible is good, but more often than not stopping down a little will give you a more consistent result.
Try to get further from the subject. It increases focus area, in comparison to shorter distances.
Usually the auto focus misses by a couple of inches (focus and sharpness is best about 2 inches closer to the lens) when it comes to the sharpest part of the picture.
Based on your description, the lens is "front focusing". You can determine by how much) see below) and MAY be able to alter it.
To determine if a lens is front or back focusing, and by how much:
Find a scene / object / surface with a regular pattern that extends away from you over a 'suitable' distance. This may be a wire mesh or a chain made of links or a cobble surface or a woven material or ...
Note a focus point about half way along the surface (or at a distance that suits once the concept becomes clear).
Aim at the surface at an angle of around 20 to 30 degrees (or whatever suits once the concept becomes clear) so that parts of the surface cover a range ahead of and behind the focus point.
Focus on the selected point and take a photo or few.
Examine photos and determine where the focus point is compared to where you expected.
Calibrate camera if you can. Allow for it if you can't.
Note that lens demounting-remounting will affect the result - hopefully but not necessarily only slightly. Seating the lens home vigorously as you mount it can help achieve consistency. The REALLY enthused MAY consider a suitably thin shim on the mount or other invasive solutions, but this would usually not be considered.
Adding a thin paper shim (rice paper or similar as a test) with suitable holes in it will probably make the front focus extent worse, but give you some idea of what sort of changes are required. The camera may have an in-body adjustment to allow the focus sensors to be moved. Or the lens MAY be able to be persuaded to very slightly reduce the seating palne to lens distance. Not something most are likely to wish to 'play' with.
Focus demonstration of a food grater using the method described above.
The focus point is very clearly seen, and can be checked against what was intended.
Full frame. f/2.8, 70mm.