The bigger the number, the better, right? Wrong! The aperture is a strange thing and one you may find difficult to understand in depth. The first weird thing is that large numbers means a small apertures. It is very counter-intuitive.

In this article, you will learn a couple of quirky details about aperture and why you should avoid shooting in the top range of f/18 to f/40.

The aperture plays a significant role in two different equations. The first one defines the exposure and the other one controls the depth of field.

Changing the aperture will change both the exposure settings as well as the depth of field. In some cases, you can take advantage of that, in particular, if you are a landscape or cityscape photographer.

The advantages of small apertures

Two common goals for a landscape or cityscape photographers are:

1. To get everything within the frame in focus.
2. Get longer exposure times to blur moving objects like water or moving cars.

It happens so, that these two goals go hand in hand with aperture. If you set your camera to a smaller aperture (that is a larger f-number), you will get a greater depth of field. At the same time, you will also get longer exposure times.

The photo below is a photo of a mountain lake in France. It serves as a classic example of what you as a landscape photographer may experience in the field.

You want the foreground to be in focus as well as the mountains in the background. On top of that, you want the water to be smooth. It requires longer exposure times to smooth small ripples on the surface.

To get a longer exposure times, you can attach a Neutral Density Filter on your lens. If your filters are not quite enough, you can also lower the aperture to f/22 or whatever is the smallest your lens can do.

The depth-of-field is maximized at f/22 or smaller if your lens allows it. So this magically goes hand in hand and everything seems great.

However, a couple of things happen, when you stop a lens down all the way to f/22 or even lower.

Problem #1: Small apertures reveal dust on your sensor

The first problem that arises is that the dust spots you have on your sensor becomes painfully visible. Almost any camera, even with a freshly cleaned sensor, will have dust spots.

Dust spots are annoying because you have to clone them out later in the post-processing and if you have many dust spots this is a real pain. For this reason alone, you may want to avoid f/22.

Problem #2: Small apertures lose sharpness

The other problem may a surprise to you. The dust spots are annoying, but not more that. At f/40 you can’t even shoot a sharp photo! But even at f/22, there are problems.

This is a 200% close-up of the unprocessed RAW photo of the French lake above, which was shot at f/22. As you can see the photo is not quite sharp. There is a softness to it and it is not a focus problem, but something entirely different.

This lens, a Nikon 16-35mm f/4, cannot produce anything sharper than this at f/22. You can work on this in the post-processing stage by applying some sharpness, and get something that seems reasonably sharp, but it is not really that good.

Some harsh post-processing has made the image seem sharper. But had the RAW photo been sharper in the first place, this would have been a much better result.

Below are some examples shot using a Sony 24-240mm lens at 240mm on a Sony a7R II body, shot from a sturdy tripod.

This lens is not the sharpest one in town, but for a superzoom, it is one of the best I have seen. At 240mm f/6.3 (wide open – it is no fast lens) through to f/40 (fully stopped down).

Have a look at this series as the aperture lowers. At f/9 the lens is at its sharpest and then sharpness begins to decline. Even at f/13, it is not super sharp, but still fixable. At f/18 the lens begins to lose details and at f/40 you can no longer tell the bricks from each other.

Why they even bother providing f/40 on a lens such as this, is a mystery. So what is going on? This is much worse than a few dust spots and it is NOT fixable.

Diffraction is the problem

What happens is that you run into the laws of physics and there is nothing you can do about it. When you stop down your lens, the hole the light passes through inside the lens becomes smaller and smaller. That’s why it’s called a smaller aperture.

When the hole gets small enough you run into trouble with one of the laws of physics which is called diffraction.

In layman terms, what happens is that the light spreads out a bit when it passes through a small hole. The light intended for one receptor (one pixel) on the sensor spreads a little bit to its neighbors. The result is an unsharp photo.

And the smaller the hole, the bigger the problem, which is exactly what you see at f/40 above. Diffraction begins around f/22, but even as the lens is closing in on f/22 the sharpness is declining.

What is the minimum usable aperture?

So what is the minimum f-stop or aperture you should use? Or phrased not be misunderstood, what is the largest f-number you should use?

All lenses behave differently, but the laws of physics are constant. Some lenses are sharpest at f/5.6 while others may be sharpest at f/9.0, as was the case with the Sony 24-240mm lens. This has to do with the design of the lens.

What is common for most lenses, is that they produce the sharpest photos somewhere in the middle range, from f/7.1 to f/13 (called the sweet spot). What is certain for all lenses is that as the aperture gets smaller (bigger f-number) beyond f/13, the worse the lens performs in terms of sharpness.

Diffraction becomes a problem around f/22 and the lens will become increasingly less sharp. The Sony lens takes diffraction pretty hard while a Nikon 28-300mm I also own is less pronounced.

The title of this article suggests that you should avoid using f/18-f/40. Why do I say f/18?

It is a gradual change, but personally, I have stopped going beyond f/16, simply because I find the photos too soft. You can never make them tack sharp, and you have to process them pretty hard to get something fairly sharp and acceptable.

The best way to find your personal limit on your favorite lens is to put your camera on a tripod and shoot test shots at f/11, f/13, f/16, f/18 and f/22 or even further down if your lens has those apertures.

Look at the photos at 200%. Notice the sharpness difference and decide what your limit should be. Memorize that and just be sure not to go below that aperture.

The compromises

Photography is full of compromises and now you have a couple more you have to make. As I established at the beginning of this article, there are some good reasons why you want to go for small apertures, but they come at a price of lack of sharpness and dust spots.

You may want to reduce the dust spot problem, I know I do. If you stay around f/8 the dust spots will not be very pronounced. However, the shutter speed will be much faster than at f/16 and the depth of field much less as well.

You can affect the shutter speed by attaching a 2-stop Neutral Density filter, which will produce the same shutter speed as f/16 but shooting at f/8.

Another solution

You can solve the problem of getting everything in focus by shooting more than one photo. One having the foreground in focus and one having the background in focus and then blending these two photos.

This technique is called focus stacking. Whether that is easier than fixing dust spots is something you will have to decide for yourself.

In photography, there are always compromises you have to make. How will you overcome the urge to shoot at f/22 and beyond?

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