Better autofocus with a wide aperture

Monday 17 February 2025, 14:41 by Nando Harmsen | 5,163 views | 8 comments

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

For autofocus to work properly, the system needs to be able to see what it has to focus on. There must not just be light, there must be enough light. A lens with a wide aperture helps when the amount of light is low.

When we talk about a lens with a wide aperture, we often immediately think of a shallow depth of field. And that's right, it gives you the option to use it. But it isn't a requirement.

A minimal depth of field can produce beautiful results, provided you use it well. But you have to like it. This is aperture f/1.8 and a 135mm focal length.
You don't have to use a shallow depth of field, even if you have a lens with a wide maximum aperture. If you never use it anyway, you might wonder whether you need such a lens.

A wide aperture helps you when photographing in dark conditions. A wide aperture lets in more light, allowing you to use faster shutter speeds in low light. There is then less need for a high ISO value.

This was my favourite lens for years. Its light-gathering ability is fantastic, even if you don't use the minimal depth of field available to you.
The resulting shallow depth of field can be a disadvantage, though. Especially when photographing people, such as during a celebration. In that case, a smaller aperture may be a better choice, while increasing the ISO value.

From that point of view too, you might wonder whether a lens with a wide aperture is necessary. The answer is still an emphatic yes. Even if you prefer not to use a wide aperture because you always want a little more depth of field. The reason is autofocus.

A dark cellar in a castle can be a challenge for autofocus. Unless you use a light-sensitive lens. That can make a difference. This was taken with an 85mm lens and f/1.2 as the widest aperture. I used f/5 to prevent the depth of field from becoming too shallow.
An autofocus system needs light, enough light. If there is too little light, autofocus works poorly, or not at all. Look in your camera's manual, and you can find the minimum amount of light required there.

The specifications of the Panasonic Lumix DC-S5 Mark II show that autofocus works from as little as -6EV when the aperture is f/1.4.
For many cameras, the minimum amount of light required is somewhere between 1EV and -4EV. Only more advanced models go below -4EV, which means they can still focus in darker conditions. But note that this value is measured at a particular aperture. Often f/1.2 or f/1.4.

A light-sensitive lens lets in a lot of light, allowing you to keep focusing in dark conditions. Unless it becomes extremely dark, of course.
A camera always measures the exposure, and focuses, at the widest aperture, even if you stop the aperture down. This means that autofocus has to work with the maximum amount of light that can pass through the lens.

Suppose your camera can focus from -2EV of light at f/1.4. If you use a lens with a maximum aperture of f/1.4, autofocus works optimally. If you use a lens with a maximum aperture of f/2.8, the autofocus sensor has no less than two stops less light available.

This Sony FE20-70mm has f/4 as its widest aperture. Although more than sufficient in many cases, autofocus may have more difficulty focusing in dark conditions.
With two stops less light, autofocus doesn't need -2EV of light, but 0EV. With a lens that has a maximum aperture of f/4, that becomes 1EV of light.

This is a nice lens, but not very ideal for autofocus when photographing in dark conditions, especially at a 50mm focal length.
Do you often photograph in dark situations? Then it may be worth investing in a lens with a wide maximum aperture. Even if you never plan to use that shallow depth of field.

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About the author

Nando Harmsen has been photographing for more than 40 years. He specialises in wedding photography and landscape photography. Nando also gives talks, workshops, masterclasses and photo trips, and regularly reviews new and interesting equipment.

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8 comments

W. Debergh wrote on Tuesday 18 February 2025 at 11:19 | reply
The assertion is correct, but with a caveat. With older and lower-quality, fast lenses, sharpness at full aperture is not as good. That sometimes causes problems with focusing correctly.
Translated from Dutch.

Nando wrote on Tuesday 18 February 2025 at 13:18 | reply
You don't have to use the wide aperture. Even if you use f/4 or f/5.6, you benefit from the large maximum aperture. After all, metering and autofocus are always done with the lens wide open
Translated from Dutch.

Hans van Oers wrote on Tuesday 18 February 2025 at 12:24 | reply
Hi Nando, when focusing in low-light conditions or with insufficient contrast, the autofocus fails or keeps searching if the lens isn't fast enough. Try the lens out for this before you go ahead with the purchase. My experience over all these years is that third-party lenses, no matter how good they are on paper, have problems with this. Very good that you make your readers aware of this.
Translated from Dutch.

Nando wrote on Tuesday 18 February 2025 at 13:20 | reply
Third-party lenses can indeed also perform less well. This is because they are reverse-engineered.
Translated from Dutch.

Donald Willemsen wrote on Tuesday 18 February 2025 at 13:07 | reply
Nando, I do agree with you. A larger maximum aperture often also means a larger front lens and not just a larger aperture opening. With a larger front lens, the lack of sharpness in the corners can also be a problem, which you then have to compensate for by stopping down.
Then only the top lenses qualify. With Canon, for example, you are talking about L lenses, and they come with a hefty price tag.
Translated from Dutch.

Nando wrote on Tuesday 18 February 2025 at 13:21 | reply
The quality in the corners is only poorer at the widest aperture, and improves when you stop down. At f/4 you no longer have to worry about it (exceptions aside).
But even if you use f/4, the full aperture is used for metering and autofocus.
Translated from Dutch.

Donald Willemsen wrote on Tuesday 18 February 2025 at 15:27 | reply
Nando, just a quote: 'But even if you use f/4, the full lens aperture is used for metering and autofocus.', end quote.

As far as I know, the light measured is also determined by the aperture that is set. So the light metering does not use the full lens aperture, but the autofocus does.
Translated from Dutch.

Nando wrote on Wednesday 19 February 2025 at 18:27 | reply
Light is always metered with the lens wide open.
The camera converts the amount of light measured at full aperture to your set aperture.
The aperture is only stopped down to the set value when you take the photo. Not before.
Translated from Dutch.

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