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 subject distance.
The resolved detail is distinct from the resolution. An out of focus image shot with a modern DSLR will have a very large resolution but no resolved details. If you need advice on your wedding photography, check out our photography packages and services at Wild Romantic Photography.
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!
How to Choose the Sharpest Aperture
Photographers have a dilemma. If you want your photographs to have the largest possible depth of field – from the foreground to infinity – a small aperture is absolutely necessary. At the same time, though, a small aperture causes your photograph to lose sharpness from diffraction. So, where’s the sweet spot? In this article, we will cover how to choose the sharpest possible aperture for such a photograph that is easy to use in the field.
Before that, though, please note that this article only applies if you want everything from the foreground to the horizon (infinity) to be sharp in your photographs. If you are simply interested in the sharpest aperture on your particular lens, this is the wrong article; check out our lens reviews instead, each of which includes sharpness tests. In an ideal world, you would always be able to use the sharpest aperture on your lens. In practice, though, you will find yourself stopping down to smaller, diffraction-prone aperture values if you need more depth of field.
There’s an old photographer’s rule of thumb that states the sharpest aperture on a given lens can be found about three stops from wide open. That means on a lens with a maximum aperture of ƒ/2.8, the sharpest aperture is likely to be around ƒ/8. In practice, it’s different with every lens, but it’s always somewhere near the middle. All things being equal, those apertures in the area of ƒ/8 or ƒ/11 are going to be sharper than the apertures at the ends of the dial—both wide open (whether that’s ƒ/2 or ƒ/4) and stopped down (like ƒ/22 or ƒ/32). So, if you don’t have any specific depth of field needs, you’d be well served to simply set your lens to ƒ/8 and go from there, knowing you’re going to produce an image where the sharpest areas are really sharp.
But what if you want to know the exact sharpest aperture on each of your lenses? For that, you can do a simple test to determine for yourself, for instance, that your wide-angle is sharpest at ƒ/5.6 and your normal prime and telephoto zooms may be sharpest at ƒ/11. All you need for the test is a tripod, any light and a newspaper.
First, set your camera on a tripod, and get it level and centred, pointing at the wall from about five feet away. Get closer for a wide-angle lens, farther as needed for a telephoto, then tack or tape a page from the newspaper to the wall. Choose the financial report or something with a sea of tiny type. This will be the fine detail you will compare from shot to shot at various apertures in order to determine which one is sharpest. (You could purchase a resolution chart for the test, but newsprint will accomplish much the same thing at a fraction of the cost.)
With the newspaper on the wall and the camera focused on its type, employ mirror lockup, and/or a cable release to ensure you eliminate any camera shake that could cloud the test with blur. Set your lens’ aperture to wide open.
Position your light source so that it doesn’t cause lens flare or glare on the shooting area. Placing the light beyond 45 degrees to the side and a good distance from the newspaper is a good way to minimise the chances of glare, just like in traditional copy work. You can use an aperture or a continuous light source; the only difference will be how you adjust your exposures.
With a continuous light source, take a meter reading wide open and set the exposure appropriately. Let’s say at ISO 100, the correct shutter speed at ƒ/2.8 is 1/250th of a second. Take a picture at this camera setting, and then adjust the aperture to the next stop—ƒ/4. Adjust the shutter speed to maintain the correct exposure (in this example, to 1/125th) and take another picture. Move to ƒ/5.6 and 1/60th and shoot again, then continue all the way to the minimum aperture and slower shutter speed. Here it’s especially important there’s no camera shake, so do use whatever technical means to ensure you eliminate all traces of shake. Looking for wedding photography Melbourne? Look no further! Wild Romantic Photography has you covered.
If you’re using an aperture, set it to manual power and its lowest output. Position it (and/or modify it with a scrim) as needed to get the correct exposure down to ƒ/2.8. Because the shutter speed won’t alter the aperture exposure, moving across the dial to ƒ/4, ƒ/5.6 and onward to ƒ/32 will require adjusting the aperture’s output. Since you started wide open and at the lowest flash output, you’ll likely need to up the output in full-stop increments with every adjustment to aperture. For instance, from ƒ/2.8 and 1/128th power, you’d go to ƒ/4 and 1/64th, then ƒ/5.6 and 1/32nd, and so on. If you end up at full power but not the minimum aperture, you’ll have to start increasing the ISO to maintain the correct exposure.
In each case, you’ll end up with a series of pictures of newsprint that all appear to have a consistent exposure. But, once you open these images on the computer and see them all side by side at 100%, you’ll start to notice not-so-subtle differences in the sharpness of the newsprint. It’s likely that at ƒ/2.8 and ƒ/32 the text could be downright blurry, whereas in the middle of the range, around ƒ/8 and ƒ/11, the print may be so sharp that you can see the tiny inconsistencies in the way the ink was laid down on the paper. (You also might notice issues like edge distortion and vignette, particularly at the wide-open aperture—seen in the examples here as what appears to be a slightly darker exposure wide open.) The difference between the sharpest and the blurriest apertures should be easy to pinpoint; in some cases, it will be subtle, but they’re there when you look close.
For many, an approximation of the sharpest aperture is all the information they need. But if you want to be absolutely sure, you’ll want to toggle back and forth between the exposures that appear sharpest. If your file browser doesn’t make this easy, create a two-layer image in Photoshop and drop one file onto one layer and the other on another. (Consider naming the layers with their appropriate apertures, as it can quickly become difficult to remember which one was which.)
If you want to be very specific, you can always carry out the test a second time. After determining, let’s say, that ƒ/8 is the sharpest of the apertures you tested, you could repeat the process and adjust the aperture in third-stop increments from ƒ/5.6-and-a-third to ƒ/8-and-two-thirds (that’s ƒ/6.3 and ƒ/10, respectively). Maybe you’ll determine that for a given lens the sharpest aperture is in fact ƒ/7.1. You can then use this as your default aperture for every shot. Unless you have a specific need for more or less depth of field, why not work with your lenses’ sharpest apertures so your camera produces the sharpest pictures possible?
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The method above is far from the only way to choose the sharpest aperture for a scene. Although it is mathematically accurate, it takes some time for every photo; there are simpler techniques as well.
For one example, you could focus stack a difficult landscape. This isn’t particularly quick, but it results in photos that are incredibly sharp. Simply set your lens to its sharpest aperture, then take a series of photos focused at different distances. (Again, I recommend the FocusStacker app.) You can combine the resulting images in a program like Photoshop or Zerene Stacker, which outputs a single photograph with the sharpest portion of every individual shot. This method gives you more detail than any single photograph, no matter which technique you use. Unfortunately, it tends to work well only if your subject is completely still.
If you can’t focus stack, another method is to focus at the hyperfocal distance and estimate the best aperture based upon your prior experience. For example, if you are photographing a scene with tremendous depth, you may choose to shoot at f/16 simply from experience. This technique is incredibly quick, but – even for expert photographers – it won’t always be the sharpest aperture possible.
Reviewing the LCD:
If a photo’s sharpness is particularly important, you may simply decide to review the image on your camera’s LCD screen. You can make changes from there, setting the widest aperture that still gives the depth of field that you need. However, as useful as this method can be, a three-inch LCD is far from the best way to check the critical sharpness of a large photograph.
Each of these methods can be valuable, and a “mathematically perfect” aperture is overkill for many photographers. This is especially true if you create smaller prints, or if you primarily display your images online. It all depends upon your specific situation.
You now have the most powerful tool there is for technical image sharpness. What are you going to do with it as an artist?
This technique optimises sharpness at each end of your range of interest. It just so happens that the optical sharpness in the middle of your range is exactly twice what it is at the ends of the range, but with the finite resolution of the film and the printing process, this will be invisible, especially if the scale tells you that apertures like f/11 and f/16 are needed at which point the definition throughout the image is very high.
If you have a very deep subject and your scales tell you to use teeny apertures like f/64, then the optical image is not too sharp anywhere but is still the best you can get. Remember that at smaller apertures diffraction limits the image sharpness, even if it is in perfect focus. At small apertures like f/64, you may start to note that the image at the centre of the range is twice as sharp as the image at the endpoints of the range. This is because at f/64 the best sharpness is low enough potentially to be visibly degraded and allow you to see the differences in sharpness throughout the image. If you are photographing a flat enough image to allow f/stops in the f/16 range then everything is so sharp that you won’t notice this delineation.
If you would prefer to have the centre of the distances in your photo rendered a bit more sharply than the far and near points, then you may want to open a stop. If you do you’ll lose some sharpness at the far and near points and gain a bit in the middle. This is probably a hard concept to grasp and to see, however for the technically fluent photographer this is a tool to fine-tune the distribution of critical sharpness.
If one’s setup dictates a small aperture one may want to forgo optimum sharpness over the entire scene and use a wider aperture to allow a main part of the subject to be rendered more sharply if there is the main subject. Here’s where you are back to artistic experimentation to get the look you want.
Remember that these effects will only be obvious once you have seen just how sharp the sharp parts of your images are at apertures like f/16 as opposed to the diffraction-limited ones made at f/64. We have the best wedding photographer in Yarra Valley to capture your beautiful moments on your wedding day.
Why You Shouldn’t Worry About Sharpness
Before diving in, we still 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.
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. At Wild Romantic, we have the best wedding photographer in Mornington Peninsula to capture every single moment on your wedding day.
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.
This entire article boils down to a single question: When you are taking pictures with a large depth of field, how do you choose your aperture setting? If you prefer to leave the camera at, say, f/11 – no matter what the subject is – there is nothing wrong with that at all. Your resulting photos may not be as sharp as possible, but they typically will be good enough. However, if you want to spend time making sure that your aperture has the best possible ratio of the depth of field to diffraction, the tips covered in this article are the most accurate methods available.
Photographers like to chase technical perfection. It is wonderful to drive ourselves to create the best possible photographs; if nothing else, it shows that we care about our work. However, mathematical perfection is not always necessary, and sometimes it is not even desired. If we lose sight of a photograph’s innate, aesthetic qualities, none of these technical settings will make it a success.
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So, if you choose to use the aperture charts or formulas, do so for a reason. “Perfect” sharpness is not itself an end; it is a tool to help convey your photographic message. If you are making large prints of a dramatic landscape, these techniques absolutely may be worthwhile. However, if you are simply taking pictures for fun, there is no reason to abandon the methods that you already enjoy.