Understanding EOS Autofocus: Focusing at Wide Apertures

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Portrait at f1.8

This is the first in a series of articles about the Canon EOS autofocus system.

A few weeks ago I wrote about my two favourite portrait lenses, the EF 50mm f1.4 and EF 85mm f1.8. One of the reasons I like these lenses is because of their wide maximum apertures. These let me take photos with shallow depth-of-field and beautiful bokeh (the opening photo was taken with my 85mm lens at f1.8). However, care is needed when using the widest aperture settings on these lenses (or any prime lens) as the depth-of-field is so narrow that accurate focus is essential.

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 Canon’s depth-of-field calculator.

For example, if I use a full-frame camera, set my 85mm lens to f1.8 and focus on something one metre from the camera, then the total depth-of-field is just 1.37cm. That’s not much at all.

Now, think about what happens when you use autofocus in one-shot AF mode:

  • 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. You should 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 is usually 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):

In focus eye at f1.8

Out of focus eye

Now, I’ve been taking a lot of portraits recently with my 85mm lens set to f1.8. A surprising number of them have been out of focus, so I decided that I should get to know my camera’s autofocus system better. I ran some tests and I realised that my portraits are more likely to be out of focus if:

  • Light levels are low.
  • I use any autofocus point other than the central one on my camera.

The first point makes sense. The camera’s autofocus system needs light to work, and may struggle in low light. It may also struggle if the contrast of the subject is low, which it often is in low light.

The second point also makes sense when you look at the make-up of the autofocus points on the EOS 5D Mark II. Not all the AF points are equal – the centre AF point is more accurate than the others. I found that it focuses accurately nearly all the time, even in low light, when I use my 85mm lens at f1.8. There are good reasons for this, and I will look at them in detail in a subsequent article.

Also, autofocus point distribution varies between EOS cameras, so don’t assume that yours works the same way as my 5D Mark II. I will explore this in more detail in subsequent articles too.

Centre AF points

There was a time when many cameras had just one AF point, in the centre 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 metre from the camera. Let’s say that I use the centre AF point to focus, then move the camera left to recompose. Doing so increases the distance slightly between my 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 my body position and it’s now unlikely that I am still focused on the model’s eye.

Focusing distance diagram

Diagram: The white lines indicate the distance between camera and subject, and the red lines the plane of focus.

One solution, for the time being, is to use the central AF point to focus whenever I use a wide aperture and the light levels are low. Here’s an example of how this affects composition, given that I can’t move the camera after I have achieved focus. You can see the effect on the image (I could always crop the second image if I wasn’t happy with it):

Off centre AF point

Off centre AF point used (above)

Centre AF point used

Centre AF point used (above)

 

But what if I really need to use an off-centre AF point because of the composition? In that situation I know that I need to take more photos, to help ensure that at least one of them is sharply focused.

I’ve also noticed that my camera’s autofocus is more accurate the closer I am to the model. I think that’s because when I am close, the active AF point covers her eye, and knows where to focus. When I am further away the active AF point covers her eye, eyelashes and part of her face. There’s no guarantee it will focus precisely on her eye.

Another solution is to stop down to something like f2.8 or f4. The depth-of-field is wider at these apertures, giving me more margin of error when it comes to focusing, but still small enough to defocus the background.

Conclusion

The aim of this article is to point out some of the practicalities that you will encounter if you take portraits (or other photos) with a prime lens set to its widest aperture.

We tend to take autofocus for granted (after all, it works well most of the time) but when it doesn’t work we tend to blame the camera’s autofocus system rather than look at our technique. If you take the time to understand your camera’s autofocus system, you will be able to get the best out of it you can. If you upgrade, it should be because you genuinely need a camera with better autofocus, not because you don’t understand how to use the autofocus on the camera you already own. This series of articles about the Canon EOS autofocus system will help you do that.

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2 Responses to “Understanding EOS Autofocus: Focusing at Wide Apertures”

  1. Anna says:

    Andrew, thanks so much for this information! It’s very helpful. I am relatively new to photography and mainly shoot macros using manual focus and Live View, tripod mounted. I would like to do more portraiture, thus your articles are of great interest to me. Looking forward to the rest in this series!

  2. […] Understanding EOS Autofocus: Focusing at Wide Apertures –> read it here […]

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