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I've bought a Canon EF 50mm f/1.8 to use with my EOS 200D and while I love the wide aperture, it appears to be quite difficult to achieve sharp focus on people's faces when shooting at f/1.8. I've tried both using Dual Pixel focus in Live View and using the center focusing point in the viewfinder, but it doesn't get me perfect results.

Is this simply a characteristic of cheap wide-aperture lenses? Or is it impossible to get sharp focus with a fully open aperture?

When I'm at at home using a tripod and focusing on a still subject I do get good focus (so it's not a camera or lens issue). But it's hard to do with people who are slightly moving between shots.

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You've indicated in a couple of comments that you find the focus/sharpness acceptable when shooting a static object from a tripod and manually focusing. That eliminates the softer image you get from this lens when used wide open as the issue you are trying to identify.

You've also indicated in another comment that you're using it with an EOS 200D (a/k/a Rebel SL2). This entry level camera has a fairly basic AF system. Only the center AF point is a cross type and more sensitive with lenses wider than f/2.8. The 200D does not have user accessible AF Micro Adjustment capability.

Whether due to hardware issues or technique, it seems you're missing focus in front of or behind your intended target. With a 50mm f/1.8 lens wide open at distances close enough to fill the frame with a face, your depth of field will be less than one inch (or 2 centimeters). That's very little to no room for error. Here are the issues you must weigh when deciding what AF points and compositional techniques to use:

  • The center AF point on your 200D should be the most accurate one as it is the only cross-type AF point and is the only one more sensitive when using lenses with a maximum aperture wider than f/2.8.
  • Using the center AF point to focus and then recomposing to place the point you wish to be in sharpest focus elsewhere in the frame is known as 'focus and recompose.' This can introduce errors as the distance from the lens' optical center to the focus target can change as the camera is rotated away from the focus target. Most of the error is usually because the photographer rotates the camera around an axis in the center of the photographer's body, rather than rotating the camera around the optical center of the lens.
  • Lenses with a very flat field of focus demonstrate the most error from 'focus and recompose.' The EF 50mm f/1.8 STM has the same optical formula as the EF 50mm f/1.8 and EF 50mmf/1.8 II. They all demonstrate a slightly curved field of focus. While this makes the corners softer when shooting flat test charts with the lens focused at the center, it actually works to your advantage when practicing correct 'focus and recompose' techniques.

In the end you need to weigh the possibility that the center AF point may be more accurate than a peripheral AF point against the advantage of not having to move the camera when using a peripheral AF point.

Some things you can try to help improve the chances of getting the shot 'in focus.'

  • Focus on the nearest eye. Our brains are wired to view the eyes as the most important spots on the face. If the near eye is in focus the face will usually look focused, even if some parts nearer and further than the eye are blurrier. If the nearest eye is not in focus the entire face will not look right, even if other parts of it are in focus.
  • Stop down a little to increase the depth of field. As already mentioned, at three feet subject distance a 50mm lens at f/1.8 has less than an inch of DoF. And that's assuming the image will be viewed no larger than 8x10 inches. If we change the display size and/or viewing distance of an image, we also change the depth of field of that image. Keep in mind that pixel-peeping a 24MP image file on a 22" HD (1920x1080) monitor at 100% is the equivalent of looking at a part of a 60x40 inch print!
  • Use a tripod to stabilize the camera. Often what we think is missed focus can actually be blur caused by camera movement.
  • Use a high enough shutter speed that subject movement isn't an issue. If you are indoors this usually means adding flash or more continuous lighting to the existing ambient lighting in order to have a bright enough scene. Bumping up the ISO doesn't add any light, it just amplifies what is there. But it also amplifies the noise in the image and using noise reduction also reduces image detail. Even with static subjects, a well lit scene will look 'sharper' that a poorly lit one. The extra light allows more contrast between the details in the scene.
  • Don't focus so long before you take the shot that your subject has too much opportunity to move closer or further away from the camera. If you use the 'focus and recompose' technique, be sure your subject understands the need to stay perfectly still during the interval between focusing and exposing the shot.

From a comment by the OP:

Canon 200D, one shot AF with central focus point in viewfinder mode and face tracking mode in live view mode. I do get slightly better results in live view thanks to dual pixel technology but not 100% sharp.

If you're using Live View it is not an AF calibration issue between the lens and the camera. You're either dealing with camera movement, subject movement, or the limits of the lens' incremental AF steps. Movement by the camera or subject between when camera focuses and when you take the picture will result in a change in distance not reflected by a change in focus. Movement during the time the shutter is open (camera, subject, or both) will result in motion blur.

The STM lenses have discrete steps that they move, even when manually focused. There is no way to focus the lens between what we might call "position 5078" and "position 5079". The few non-USM lenses (pre-STM) in Canon's lineup are similar with AF (as of a couple of years ago the EF 50mm f/1.8 II had the widest steps between AF positions of any current Canon lens), but can manually be focused via mechanical linkage to any of an infinite number of positions (if you can precisely move that crazy focus ring on the front of the old 50mm f/1.8 II without getting your fingers in the shot). The STM lenses are 'focus-by-wire' and even when you move the focusing ring the stepper motor is what moves the lens elements. (There are also a few USM lenses that are 'focus-by-wire' with discrete minimum steps of movement even when focusing manually).

9

Is this simply a characteristic of cheap wide-aperture lenses?

It is a characteristic of this particular lens, and in general for most fast lenses, that they tend to exhibit more softness when wide open than when stopped down. The EF 50mm f/1.8 STM has its "sweet spot" in the f/4-5.6 range.

That doesn't mean you shouldn't ever shoot the lens wide open, but you do need to understand you're making compromises on sharpness, vignetting, and chromatic aberration when you do. And you probably shouldn't shoot with it wide open by default. Especially as the wider the lens is open, the thinner your depth of field becomes, and the harder it will be to accurately autofocus the lens.

See also: Why are my photos not crisp?

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You are right, it is quite difficult. There is no mode or technique that will give you perfect results, there are only modes and techniques that will improve your results.

By very definition a wide aperture will give you a shallow depth of field, and a shallow depth of field makes it even more apparent when the desired subject is not precisely within the narrow depth.

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it appears to be quite difficult to achieve sharp focus on people's faces when shooting at f/1.8

If you're trying to fill the frame with someone's face, you're probably quite close to your subject. Shooting wide open when the subject is close gives you very small depth of field. You can use a [depth of field calculator] to figure out exactly how small; for example, a 50mm lens at f/1.8 with a subject distance of 3 feet gives a depth of field of 0.07 feet, or 0.84 inches. If you're focusing and then reframing, you could easily be moving the camera enough to put the parts of the image you care about out of focus. Or, you might be getting the tip of the nose in focus, but still have the eyes and mouth out of focus.

Using a smaller aperture (larger f-number) and increasing the distance to the subject will both help. As inkista already pointed out, reducing the aperture will also improve sharpness.

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What point of the person's face do you focus on?

I have the same lens, on a 7Dmk1 (mentioned this to tell you that I am using it on an APS-C sized sensor, and not FF), and I focus on one eye, then re-compose to the required frame and click. I have not had a dreadfully soft pic yet. Though I mostly use this lens with a tripod, handheld pics are also great. Yes I have clicked wide open, and then stopped down. It does better when stopped down. I recommend stopping down the aperture for portraits, and using wide open for bokeh.

You might need to experiment with stopping down the aperture till you get reasonably good AF hits and sharp final image.

As a side note, since you haven't mentioned which body you have it on, have you tried any micro-focus adjustments? Maybe with this lens or others?

  • Its a Canon 200D so a crop as well. I usually focus on the face in general and then reframe. I will try focusing on the eye from now on. – JonathanReez Supports Monica Oct 11 '17 at 12:27
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When shooting a portrait I tend to keep the focus on the target's eyes, mainly because I feel eyes are the most story-telling part of a person's face. However, I don't usually use the widest aperture for couple reason (I use either the Sigma ART 35mm or 50mm f/1.4).

  1. When using large aperture, other body parts of your target can be out of focus. That may be something you want to achieve, but in order to isolate your main target from the surroundings, I would prefer majority of the target stays in focus.
  2. With the slightest movement, your focus can go off and only realize the target's ear or nose is in focus in post production.
  3. You need to start learning utilize all the elements instead of only adjusting aperture to achieve bokeh.

When shooting portrait, I will try to adjust my setting to allow shutter speed at around 125 to 500, aperture around 2, and keep my ISO lower if possible. Of course the settings need to be adjust depends on the environment, but that is the base I will work from.

But to answer your question, I would suggest to forget about composition, move away from the target, try to use the better focus point your camera has to offer, utilize that large CMOS to crop in post production. You may lose some depth but greater chance of keeping your target in focus.

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