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Photography

How to Calculate the Sharpest Aperture for Any Lens

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This post is part of a series called Lenses.
Mastering Two Alternate Focus Techniques
Everything You Need to Know About Lens Quality

When we talk about the sharpness of a lens what we are actually considering is its ability to resolve detail. When an in-focus lens is capable of reproducing more distinguishable details in an image we call that lens sharp. The sharpness of a lens isn’t constant; it changes with aperture, focal length (if it’s a zoom lens) and the subject distance.

Resolved detail is distinct from resolution. An out of focus image shot with a modern DSLR will have a very large resolution but no resolved details.

Sharpness is typically measured in line pairs per millimetre (lppmm) or by the size of the blur on the sensor in microns. The exact specifics of these measurements are unimportant for this tutorial, just know that there is a scientific and well understood means for determining sharpness that involves a little more than just eyeballing it!

In this tutorial we'll look at how to calculate the optimal aperture for any lens to maximise the resolved details in an image.

Why You Shouldn’t Worry About Sharpness

Before diving in, I want to talk about why seeking maximum sharpness rarely matters all that much.

In any situation there will be a maximally sharp combination of aperture and focus point, however there will be countless other combinations that will return acceptable results. Only by zooming in extremely close and “pixel peeping” will it be possible to determine the difference between the sharpest possible image and one that is sharp enough.

There is so much more that goes into making a great image than just ensuring it’s sharp. An image can be technically perfect and have absolutely no artistic merit—all those extremely sharp, extremely boring pictures of test charts—while a stunningly creative image might barely be sharp at all. The subject, the tones and all the things that make photography so wonderfully subjective play a much more important role.

out of focus wheat field
Even the in-focus areas of this dreamy image of a wheat field aren't particularly sharp. Wheat Field. DanielNanescu/PhotoDune.

There are certain situations where focus matters, which is in most ways a proxy for sharpness. In portraits for example, the majority of the time you want the subject’s eyes to be the point of focus and thus the sharpest point in the image, but, by and large, as long as the image is sharp-enough other things become a lot more important.

So bear all this in mind as you read through this tutorial. Knowing how to calculate the optimal aperture and focus for a maximally resolved image is useful for photographers—especially landscape and architecture photographers—but it is only one of many parts that go into making an image. For most situations, the rules of thumb I’ll cover here will be all that’s needed to get an acceptably, even extremely, sharp image, even if it’s not the sharpest possible one.

The Sharpness Balancing Act

There are two main things that affect how a lens resolves details, defocus blur and diffraction blur. Defocus blur occurs most at wide apertures while diffraction blur occurs most at narrow apertures. It is balancing these two competing factors, as well as the apparent depth of field required, that leads to the optimal aperture for a given scene.

Astigmatism

Modern lenses are amazing, but, because of the way optics work, most lenses still have at least a little bit of astigmatism. This means the focal plane is not the perfectly uniform two-dimensional invisible wall we like to imagine it to be. You can see this with large apertures especially. It's easy to test, too! Set up your camera a meter or two from a brick or stucco wall and make a series of properly exposed images while varying the aperture. With most lenses you'll be able to see that the center of the image is noticeably sharper than the edges at wide apertures.

In any case, blur is in many forms of photography actually a rather pleasing effect. A blurry, bokeh filled background is often a desirable part of a portrait. In other cases, such as shooting at night, blur is the cost part of using a wide aperture to get a decent exposure, but it also gives the viewer a useful visual clue to the type of lighting situation.

There are situations where you need a very sharp image though, corner to corner, astigmatism be damned. That's what this tutorial is about, so let's continue.

Depth of Field: Circles of Confusion

When you take a picture, light passes through your lens and onto the the image plane, be it film or sensor. The lens focuses the light so that objects at the plane of focus are reproduced in sharp focus on the image plane. Points not on the plane of focus, either closer or further away, are reproduced as slightly larger dots, or "circles of confusion."

Reducing the size of the aperture reduces the size of the out-of-focus dots. If you reduce the size of the aperture enough, the dots can get small enough that the technically defocused circles of confusion become so small that they are indistinguishable from truly in-focus points. The effect is an increased apparent focus area at smaller apertures: depth of field.

Sharpness and Depth of Field

The wider the aperture is open, the less of the scene that can fall in the depth of field. With a 50mm lens open to f/1.4, you can focus to infinity but anything nearer than just under 45 metres will be out of focus; the mountains in the distance will be sharp, but you won’t be able to see anything in the foreground. It doesn’t matter how sharp the in-focus areas of the image are, anything that falls outside the plane of focus will be blurry.

out of focus portrait
While my eyes are tack sharp in this image, the focus falls away in a pleasing manner. More depth of field isn't desirable in this situation.

If you want to get everything from just a few centimetres in front of the camera to the mountains in the distance in focus, you need to stop down your aperture. The further you stop it down, the more apparent depth of field you’ll have. The narrowest aperture on a given lens—often f/22 on modern lenses—will give the greatest depth of field. The problem is that at narrow apertures, diffraction begins to have a detrimental effect on the maximum achievable image resolution.

Balancing these two factors is the key to maximising the sharpness of your images.

Finding the Sweet Spot

For every lens there is one aperture that gives the maximum resolution of the in-focus areas. If you’re shooting a two-dimensional test chart, it’s the one to use every time.

The simplest way to find this “sweet spot” is to Google it. Every popular lens is reviewed and its performance measured on countless websites.

If you want to calculate it yourself, however:

  • Put your camera on a tripod and place a sheet of newspaper in front of it at a distance that allows you to see the whole paper in the frame.
  • Focus on the newspaper and then take a photo at every aperture the lens allows.
  • Import the images to your computer, zoom to 100% on each one and then compare them.
  • The image with the most legible text will be the one shot with the lens’s sweet spot.

In general, this sweet spot is between two or three stops down from the maximum aperture. For example, for a lens that has a maximum aperture of f/1.4, the sweet spot normally falls between f/2.8 and f/4. For a lens that has a maximum aperture of f/4, it’s between f/8 and f/11.

Although there is only one sweet spot for each lens, in practice, the difference in resolution between f/8 and f/11 on a lens that opens to f/4 is minimal. If sharpness is all you care about, just stop your lens down by two or three and shoot away.

Sharpness in the Real World

In the real, world sharpness isn’t your only concern. If you’re shooting a landscape rather than a test chart, depth of field is critical. Stopping the 50 mm lens down from f/1.8 to f/5.6, which is roughly its sweet spot, moves the nearest point of focus to 15 metres—an improvement from 45 metres but it still doesn’t give much of a foreground. Stopping the lens down further would bring everything into your depth of field, but if you stop it down too far diffraction will reduce sharpness.

aerial photo
This photo is sharp throughout. I used a Canon 17-40mm f/4 with an aperture of f/8 to get everything from 10 metres to infinity sharp. As I was shooting from a helicopter, nothing was ever closer.

What we want to do is find the aperture that gives us the maximally sharp image that has the depth of field we want. Published depth of field tables go part of the way towards this. They tell what the widest aperture that will provide the depth of field we want is, however that is seldom the aperture that will provide the sharpest detail throughout the in-focus range.

The maths to make the calculations gets quite complex. Ken Rockwell has written at length about how to do them but there is no real need to do so yourself. George Douvos—who’s also written about how to calculate the sharpest possible aperture—has developed an iPhone app called OptimumCS-Pro which makes all the calculations for you. If you want to find the optimal aperture for maximising sharpness, it’s by far the best way to do it.

The other alternative are to work through Rockwell’s formulae or use the rules of thumb in this tutorial.

The Rules of Thumb for 35mm Cameras

The following are some rules of thumb for using, if not the optimal aperture, one very close to it. A half-stop or so either side won’t have a very noticeable effect on an the degree of sharpness in an image. They only apply to 35mm cameras or smaller. Medium and large format cameras often use relatively far narrower apertures which further complicates the calculations.

  • If you want maximum sharpness with no concern for depth of field, use your lens’s sweet spot. If in doubt, assume it’s f/8.
  • If you are shooting with a wide angle lens (24mm focal length or less) and want everything from one metre to infinity to be sharp, focus at a point two metres away and use an aperture of between f/11 and f/16. The wider the lens, the wider the aperture you can get away with.
  • If you are shooting with a 35mm lens and want everything from two metres to infinity to be sharp, use an aperture of f/16 and focus at a point four metres away. These are great settings for street photography.
  • If you are shooting with a normal lens (50mm) and want everything from two metres to infinity to be sharp, use an aperture of f/22 or higher.
  • Lenses with focal lengths longer than 50mm cannot have both a sharp foreground and background. With an 85mm lens, an aperture of f/5.6 will, at best, have everything from one hundred metres to infinity in focus.

With these rules of thumb, you should be able to get extremely sharp images every time. They mightn’t be the sharpest possible image, but they will be very close. To find out the perfect aperture every time, use OptimumCS-Pro.

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