Why You Should Avoid Shooting at Small Apertures Like F18 to F40

Why You Should Avoid Shooting at Small Apertures Like F18 to F40


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.

ocean view and cliff - Why You Should Avoid Shooting at Small Apertures Like F18 to F40

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.

mountain scene France - Why You Should Avoid Shooting at Small Apertures Like F18 to F40

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.

Why You Should Avoid Shooting at Small Apertures Like F18 to F40

Dust spots clearly visible due to a small aperture.

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.

Why You Should Avoid Shooting at Small Apertures Like F18 to F40

200% crop of a blurry image at f/22.

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.

Why You Should Avoid Shooting at Small Apertures Like F18 to F40

200% crop of the final processed image.

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).

Why You Should Avoid Shooting at Small Apertures Like F18 to F40

1/320th at f/6.3

Why You Should Avoid Shooting at Small Apertures Like F18 to F40

1/160th at f/9.0

Why You Should Avoid Shooting at Small Apertures Like F18 to F40

1/80th at f/13

Why You Should Avoid Shooting at Small Apertures Like F18 to F40

1/40th at f/18

Why You Should Avoid Shooting at Small Apertures Like F18 to F40

1/15th at f/29

Why You Should Avoid Shooting at Small Apertures Like F18 to F40

1/8th at f/40

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.

Why You Should Avoid Shooting at Small Apertures Like F18 to F40

Shot at f/22 on a full frame camera. Sharpness is not optimal.

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.

Why You Should Avoid Shooting at Small Apertures Like F18 to F40

15mm at f/8 on a full frame camera.

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.

Why You Should Avoid Shooting at Small Apertures Like F18 to F40 - park bench and scenic view

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.

Why You Should Avoid Shooting at Small Apertures Like F18 to F40 - large chain links

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.

Why You Should Avoid Shooting at Small Apertures Like F18 to F40 - forest and pathway

This is a focus stacked photo shot at f/11 and 134mm on a cropped sensor.

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

Read more from our Tips & Tutorials category

Jacob Surland is a fine art photographer based in Roskilde, Denmark, who travels around the world. Jacob specializes in dramatic cityscape and landscape photos and he has a strong focus on image editing. He is a bit unusual as a photographer because it is the post-processing part that is his true passion. Recently he won a prize at the Florence Biennale 2017 based on two of his photos. See more of his work and follow him on his blog.

  • CEpheide

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

    You can have a hole of 8 meters of diameter and have diffraction. But you start to see the diffraction pattern only when it becomes larger than a pixel. The size of the pattern being inverse proportional to the aperture size it becomes only visible (larger than one pixel) at smal aperture.

  • Jacob Surland

    Hi CEpheide,
    Thanks for adding details 🙂
    Kind regards

  • caeklund

    Does the effect change as aperture size gets smaller and smaller. Ins’t a pinhole a perfect lens. I should know this (PhD in physics)–does a pinhole suffer from diffraction?

  • Al Hughes

    The technical aspect is a bit heady but the information is quite clear and helpful. I hadn’t considered my lenses having a sweet spot for sharpness but it makes sense. I’ll have to perform some tests on my lenses as instructed and find those sweet spots. Thank you for the article.

  • You may benefit from taking a look at PhotoPills’ diffraction calculator: https://www.photopills.com/calculators/diffraction

    Also, DxO Mark is a great resource for helping photographers see, visually, the strengths and weaknesses of the equipment they use or are considering acquiring. https://www.dxomark.com/

  • Eric

    Hello, I totally agree with this very interesting article. I knew about the reason why not shooting lower than f 1/16 (my personal limit) but I reckon the explanation is very clear and understandable (text + photo). Well done and thank you !!

  • David H

    I wondered about the utility of reducing ISO to aid widening the aperture towards the sweet-spot?

  • David H

    PS Very informative article… thank you!

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  • Sam Wood

    Great article which explains why I’m not able to achieve the sharpness I’ve been after. I was not aware of the loss of sharpness due to diffraction. I am well aware of the ‘sweet spot’ but I did not realize the loss of sharpness at the smaller apertures.

  • Matt

    Very helpful article. With my Canon 5DM2 and M3, I usually cap off at f/11.

  • Jerry Granaman

    I stopped shooting above f8 along time ago for these very reasons, I do a lot of long exposure photography which has issues of it’s own such as noise, I use -6 to -16 stop ND filters which covers most light problems.

  • joelluth

    If you’re trying to maximize depth of field without using too small of an aperture, it’s also helpful to use the hyperfocal distance.

  • Jack Doy

    Nice one.

  • Jacob Surland

    Absolutely true Joe. Hyperfocal is the last step before focus stacking. I use the hyperfocal distance a lot in my photography and there are several apps to calculate it.
    Thanks for the input!

  • Jacob Surland

    Hi Jerry,
    I haven’t stopped shooting beyond f/8. But I stick around f/8 for as long as I can. And I never go beyond f/16.
    I too have started to use the ND filters to extend the exposure time. My favorite combo is ND6 + ND3, because I have the flexibility to remove the ND3 when the light slowly disappears.

  • Jacob Surland

    Thanks Matt – sounds like a good choice.

  • Jacob Surland

    Hi Sam,
    There are so many ins and outs in photography – great that you learned something new 🙂

  • Jacob Surland

    You’re welcome.

  • Jacob Surland

    Hi David,
    Depending on the conditions you are photographing in, you may not be able to reduce ISO enough. Many cameras does not go below ISO 100 and if you are already at ISO 100 and want longer exposure times, you will have to go for either either smaller apertures or for the better: attach an ND filter.

  • Jacob Surland

    Hi Eric,
    You are welcome 🙂

  • Jacob Surland

    Good input – thanks.

  • Jerry Granaman

    Thank you for the reply I enjoyed the article, I think it touches on several points a lot of people are unaware of.. I have an ND 3 and 10 as well as a couple of grads, ( too easy to wrap a lot of cash into good ND filters) I use from time to time as well and I try to keep my ISO near 100, I use prime and fixed lenses when possible to avoid dragging more dust into my sensor. I try and shoot early am and late afternoon to evening. This system seems to work well with my Nikon for long exposure times. If doing infra red then of course when lots of light is available but that is another story.

  • Jacob Surland

    That is one the fascinating things about photography. You can keep optimize they way you set up, shoot etc. A zoom is like a pump in regards to dust. You avoid that pump by using primes. There are so many possible priorities and compromises to make, which I find very fascinating. Personally, I often prefer the flexibility of a zoom, over dust pump issues, though I do use primes. And that is a compromise. Have a nice day 🙂

  • CEpheide

    Hi nice question. Yes a pinhole is suffering of diffraction as well. Your camera is a machine that transform angles to distances. Two point separate by an angle of 1 degree will be at a given distance on your sensor. This distance depends obviously on the focal length.
    Their is a natural lower limit of angle for which you can resolve things. This is call the diffraction limit and this angle is 1.22 l/d (l being light wavelength and d the diameter of the pupill).
    So the higher the pupill size the lower this angle will be. (This is part of why we are building big telescope). As I say this angle will be transformed to a distance on your sensor depending on the focal length. But given the definition of the f-number (focal divided by pupill size) it appear that the size of the diffraction pattern on the detector is dependent to the f-number only. The larger the f-number the larger the pattern.

    For the pinhole this is the same thing. But what is hidden in the weeki pedia sentence is that when you get the pinhole smaller you also need to change the distance pinhole to detector (the focal length) to make it sharp and you change your field of view or your angle to distance conversion factor. You keep the diffraction pattern small enough. But you increase the sharpness because the circle of confusion (different than diffraction pattern) get also small.
    So for a sharp pinhole photography you will have a smaller field of view until, as your article says, that only oblique light is getting in -> infinite small field of view.

    I hope I didn’t say stupid stuff. Sorry that an iPhone typing reply.

  • CEpheide

    Ho and as you have a PhD in physics you will understand this (sorry for the other):

    When you solve the light propagation equation you realize that the light propagated from one obèrent is the Fourier transform of the object.

    A camera, eyes, or a telescope is an inverse Fourier transform (that reconstruc your image). But before doing a inverse Fourier transform it dos filter the high spatial frequencies, this filter depend on the pupill size. The smaller the pupill the more agressive the filter will be. So you loose high spatial frequencies and when your device does the inverse Fourier transform this lead to a lost in resolution.

  • caeklund

    Interesting. And make sense. Thanks.

  • caeklund

    No stupid stuff. Thanks. What surprised me about the original article was the magnitude of the effect as you change the lens opening. An eye opener, so to speak…

  • My pleasure.

  • Akay

    Will I be correct to add that to use “principle of hyper focal length ” one HAS to resort to manual focus mode …..

  • Akay

    “when light slowly disappears” – can variable ND filters be used in such conditions ?

  • Akay

    Diffraction , aka Optical turbulence , comparable to turbulence in flow of liquids – U suddenly drove me down the memory lane , may be 55 years, Physics/Optics lectures followed by laboratory experiments, Newton rings using ‘ bi-prism ‘ , and all that …. thanX

  • Jacob Surland

    Hi Akay,
    Yes you have to focus manual. Some prime lenses have a mark to indicate the hyper focal distance.

  • Jacob Surland

    Hi Akay,
    Yes they can. The variable filters I have tried, have not been ‘even’ at wide angle (less than 24mm). I shoot a lot around 16mm and an variable ND filter, however practical it may be, has not been an option for me.

  • Jacob Surland

    You are welcome 🙂

  • Akay

    Wide angle concept raises another query .. I have 52 mm n 77 mm variable ND filters plus two full sets of step up-step down rings to suit 26 mm to 95 mm range. Of course I have some fixed intensity ND filters also , from ND-2 to ND-too much (20 stops of 95 mm)
    My arsenal covers from 37 mm (camcorder) to 52 mm, 58 mm , 67 mm, 77 mm n 95 mm filter dia , my lens range covers 10-18, 18-55, 18-135, 24-105, 70-200 and 150-600 plus some primes like 35, 50, 600
    NOW pl advise if use of these variable filters with step up – down rings be advisable ? Excluding 600 mm lens of course
    Perhaps I am one of the worst case of theoreticians !
    It may surprise U that inspite of so much equipment I have never used any of my ND filters ..

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