3 Reasons Why Mirrorless Cameras are Better than Digital SLRs for Focusing
A lot has been written about the drawbacks of autofocus performance from mirrorless cameras. Most of this focuses on the tracking of moving subjects – an area where the phase detection autofocus found in digital SLRs is still superior (although the gap is closing).
But when it comes to focusing on still subjects, the mirrorless camera is a better tool. Surprised? If you’ve never used a mirrorless camera, you may be. Let’s take a look at the reasons why.
1. Phase detection versus contrast detect autofocus
Mirrorless cameras have a different autofocus system than digital SLRs.
In a digital SLR most of the light coming through the lens is reflected up by the mirror, into the pentaprism and through the viewfinder. A small part is deflected downwards to a dedicated autofocus sensor. It uses a system called phase detection autofocus to calculate the camera to subject distance, and tell the lens where to focus.
The red lines in this diagram show the path that light takes through an SLR camera with the mirror in the down position. Most of the light is reflected into the pentaprism and the viewfinder. Part of it is reflected downwards towards the autofocus sensor.
The advantage of phase detection autofocus is that it’s fast (generally speaking – but it also depends on which camera you have) and very good at tracking moving subjects. It’s the best system anyone has managed to come up with for an SLR camera.
However, phase detection autofocus has a significant weakness – lack of accuracy.
There are two main reasons for this. The first is that most digital SLRs have a combination of cross-type and single line autofocus points. Cross-type autofocus points are the most accurate, and should always be used when focus is critical (for example, when using a prime lens at its widest aperture), otherwise the camera may not focus where it is supposed to. Your camera’s manual will tell you which of its AF points are cross-type.
Whenever you use a non cross-type autofocus point, you cannot rely on the camera to focus accurately. This is fine when using small apertures, which give you plenty of margin for error, but not when focus and accuracy is critical.
The second reason is to do with camera and lens calibration. Even when you use a cross-type autofocus point your camera may not focus exactly where it is supposed to. For accurate focus, every part of your camera setup – from the autofocus sensor, to lens and autofocus motors that tell the lens where to focus – must be working in perfect harmony. It only takes a small degree of misalignment to throw the accuracy of the system out.
Most of the time you won’t notice, because there is sufficient depth-of-field to make the focusing inaccuracies irrelevant. But if you use a wide aperture, especially with a telephoto lens, then depth-of-field is measured in millimetres, and accurate focus is essential.
For example, if you are taking a portrait then it is conventional to focus on the model’s eyes. If you miss focus, and her eyes are soft, then people will notice and the portrait will lose its impact.
I made this portrait with an EOS 5D Mark II and 85mm lens set to f1.8. With this camera it is necessary to measure and calibrate the autofocus system to ensure accurate focus at wide apertures.
Most mid-range and high-end digital SLRs have a feature that allows you to measure and compensate for inaccurate focusing. Manufacturers have different names for this – Canon and Sony use the term Autofocus micro-adjustment, Nikon calls it Autofocus fine tune, Pentax uses the term Autofocus adjustment and Olympus Autofocus focus adjust. It’s bit of a long winded process – you have to test your lenses by focusing on a ruler, or a purpose made scale, to see if the focus is accurate, and make adjustments if it isn’t.
You can also get your camera and lenses calibrated at a service centre. This is the only way to calibrate an SLR camera that doesn’t have the above feature built-in.
That was bit of a long explanation, but crucial if you are to understand why phase detection autofocus is not as accurate as it should be.
How are mirrorless cameras different?
So, how do mirrorless cameras differ? As they don’t have a mirror, there is no way of deflecting light to a dedicated autofocus sensor. The solution is to take a reading from the sensor. The camera looks at the point on the sensor which is meant to be in focus, and adjusts the lens until maximum contrast is achieved. This is called contrast detect autofocus.
This system is slower, because the camera has to move the lens first one way, then the other, to find the sharpest point. But, it is much more accurate (for still subjects).
With a mirrorless camera autofocus micro-adjustment is redundant. You don’t need it, and you will never have to measure or calibrate the camera’s autofocus system. It also doesn’t matter which autofocus point you use, as they all work equally well. That is why, for still subjects, autofocus in mirrorless cameras is superior to that of digital SLRs.
I made this portrait with a 56mm lens at f/1.2 with my Fujifilm X-T1 mirrorless camera. With this camera it is easy to focus on the model’s eye. There is no need to calibrate the camera’s autofocus system.
2. Manual focusing
Mirrorless cameras are also a better tool for utilizing manual focus lenses.
Modern digital SLRs are not designed to be helpful with manual focus lenses. The split prism focusing screens of the past are gone, and assistance is limited to a light that comes in the viewfinder when the subject underneath the selected AF point comes into focus.
Mirrorless cameras are different. They have a tool called focus peaking, which is specifically designed to help you manually focus a lens. The camera highlights the parts of the scene that are in focus, so that you can see which areas are sharp. You can also magnify the image at the touch of the button, making it even easier to see whether the subject is sharply focused.
This feature works best when using lenses at wide apertures. Both tools take advantage of the camera’s electronic viewfinder, a feature that most digital SLRs don’t have.
This mock up shows how focus peaking works. I made the portrait with a Helios 58mm manual focus lens at its widest aperture setting of f/2. The red lines indicate how focus peaking shows you what is in focus.
3. Hyperfocal distance
Fujifilm cameras have another tool that will be of interest to landscape photographers as it helps you instantly find the hyperfocal distance without referring to tables or smartphone apps.
The viewfinder has a depth-of-field scale that shows you the point you are focused on and the area in focus on either side, according to the selected aperture. If you move the focusing ring until the depth-of-field scale touches the infinity mark at one end, you have found the hyperfocal distance point. It’s quick and easy.
To be honest, I don’t know if this feature is available in any brand of mirrorless camera other than Fujifilm. I’d be grateful if Sony/Olympus/Panasonic, etc., owners would let us know.
This diagram shows how the depth-of-field scale works. The bar shows the point the lens is focused on (white), and how much of the scene is in focus (blue). The lens is focused on the hyperfocal point in this made-up example.
Since buying my first Fujifilm camera a little over 18 months ago, I’ve been pleasantly surprised by what a great tool mirrorless cameras are for photography. They are much better than my old digital SLR for focusing on still subjects, or for using manual focus lenses.
The difference is so great that I predict that one day most photographers will use mirrorless cameras, and digital SLRs will be a niche item built for photographing sports and wildlife.
But what do you think? Please share your thoughts, or ask any questions about focusing, in the comments below.
If you want to know more about lenses and autofocus check out my ebook Mastering Lenses: A Photographer’s Guide to Creating Beautiful Photos With Any Lens.
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