A large aperture is not just for a shallow depth of field

Wednesday 8 June 2022, 15:30 by Nando Harmsen | 5,041 views | 1 comment

This article was translated from Dutch with the help of AI. Read the original.

You know those beautiful and often very expensive lenses with huge apertures. If you already consider f/2.8 large, it is nothing compared with f/1.4 or even f/1.2. They give you an extremely shallow depth of field.

But what use is such a large aperture if you always shoot at f/8? I explain what else a large aperture is useful for.

Not everyone wants to use an extremely shallow depth of field. That is understandable, because it is not suitable for every type of photography.

Photographers who almost always use an aperture between f/5.6 and f/11 in particular will not quickly see the point of f/1.4.

A studio portrait is often photographed with a larger aperture. In these circumstances, you do not need a large aperture.
Yet a very large aperture can be very useful. Not to achieve a shallow depth of field. Nor to let enough light through the lens in dark conditions so that you can keep shooting. It has everything to do with focusing.

We do not often stop to think about it, but autofocus has its limitations. Contrast is needed to focus.

If there is no contrast, focusing will fail. The autofocus will hunt. If there is contrast, there still needs to be enough light to see that contrast.

A lens like this allows you to get the maximum performance from autofocus. It is only useful if you also shoot in those conditions.
Every camera needs a minimum amount of light to focus. This is shown as an EV value.

For example, a camera's autofocus may work down to -2 EV. If it is darker than -2 EV, the AF is no longer usable.

A large maximum aperture lets an enormous amount of light through. Autofocus can benefit greatly from this.
This figure of -2 EV is completely arbitrary in this case, just an example. Each camera has its own minimum light level listed in the specifications. For some it is -1 EV, while others go down to -3 EV. Some even go down to -7 EV. In this case, I will stick with -2 EV as an example.

The amount of light corresponding to -2 EV is the amount of light from a full moon in a snowy landscape. There is one catch, though. The camera can only focus in these conditions with an f/1.2 lens. That is a requirement because this lens can let through enough light.

If you had an f/4 lens, focusing under a full moon in a snowy landscape would no longer work. After all, an f/4 lens lets through no less than 3.3 stops less light.

This means that with a lens with a maximum aperture of f/4, you can only focus at around 1 EV instead of -2 EV.

Autofocus can struggle in very dark conditions. If not enough light comes through the lens, autofocus cannot see what it needs to focus on.
To give you an idea, 1 EV of light roughly corresponds to the night-time skyline of a city in the distance. If you have a lens with a maximum aperture of f/5.6, you will need at least +2 EV of light. That corresponds to shooting in a city with street lighting.

If you shoot a lot in dark conditions, you can certainly benefit from a lens with a large aperture. Focusing can be much better, faster and smoother in those situations. The risk of the focus hunting is much smaller.

A wedding photo with an off-camera flash in the evening. You do not need a large aperture for a good exposure, but it can help the camera focus properly and quickly.
Does this mean you always need a lens with a large aperture? Of course not. It is only useful if you shoot in conditions where the light is anything but optimal. Or if you like an extremely shallow depth of field, because that is an option too.

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