How to Find Your Lens's Sharpest Aperture or Sweet Spot

Knowing your lens’s sharpest aperture—its “sweet spot”—is one surefire way to maximize image sharpness. Image sharpness is the goal of many photographers—regardless of the subject matter. Sharpness is also the catalyst for the dozens of lens-testing and review websites that populate the Internet, as well as the impetus for endless snobbery and debates over which lenses reign supreme in the quest for ultimate clarity. In my article, 21 Tips for Getting Sharper Photos, I talk about many potential ingredients in the search for sharp images, one of those being the sweet spot. Let’s take a closer look at how you can determine your lens’s sharpest aperture or its sweet spot in this article.

Images © Todd Vorenkamp 

The Sweet Spot

If you have taken enough photographs, you may have noticed a difference in sharpness while your aperture changes. If you have never noticed this, welcome to a period of important image quality enlightenment! 

The optical fact is that a variable-aperture lens will produce images of varying sharpness as the aperture diaphragm closes from wide open to its smallest opening. Generally speaking, for almost every lens, you’ll get a sharper image―with all other factors being equal―at the middle apertures and not at its widest or smallest aperture.

Let’s dive deeper into why that is, and how you can figure out your lens’s sweet spot.

Aberrations

It is a mathematical fact that increasing your lens aperture to or toward its maximum opening increases optical aberrations. You can take a deeper dive in this article on lens aberrations, but know that, as you stop down your aperture (make the opening smaller), you will start to reduce different types of lens anomalies.

Diffraction

OK, so if we make the aperture smaller, we reduce aberrations and get a sharper image. So, why don’t we just crank our aperture diaphragms closed when we need ultimate sharpness? 

Because, as our apertures get smaller, the lens experiences the phenomenon of diffraction. When light passes an object, it bends. And, when light is forced to pass through a tiny hole in your aperture diaphragm, that unwanted bending becomes more prevalent and results in a less-than-sharp image.

We take a closer look in this Explora article on diffraction.

The Middle Ground

Based on the above information, you can see that having your lens aperture wide open increases the presence of aberrations that can cause softness in the image. Contrarily, stopping the aperture diaphragm down increases diffraction, which also causes softness.

Aperture too wide? Aberrations. Less sharpness. Aperture too small? Diffraction. Less sharpness.

Where does that leave us? The middle apertures. That is where you find your lens’s sweet spot. So, how do we determine which, if any, middle aperture is the absolute sharpest for your lens? 

Lens Testing

I find that the best way to determine your lens’s sharpest aperture is to do a simple, controlled, bench test of your lens. How do we do that? Check out this article for an in-depth look at a simple, but effective at-home lens test.

In years of doing these non-scientific at-home tests on dozens of lenses, I find the following general results: 

Maximum Lens Aperture	Sharpest Lens Aperture
f/1.2 or f/1.4	f/4 or f/5.6
f/1.8	f/5.6
f/2	f/8
f/4	f/8
f/5.6	f/11

 

Rule of Thumb

You might notice that those test results show that the lens’s sharpest aperture is 2-3 stops closed from the maximum aperture of the lens. This creates an easy rule-of-thumb that you can follow with any lens with which you are out in the field—regardless of your experience with a particular lens.

Center vs. Corners

When you test a lens, you need to pay attention to the difference in sharpness at the center of the image at different apertures, as well as the corners of the image.

You will find that center sharpness arrives much faster than corner sharpness—sometimes the center can be almost as sharp at wide-open apertures as mid-range apertures, but the corners lag behind the center in their journey to sharpness.

When I determine the sweet-spot aperture, it is almost exclusively based on corner performance.

Lens Test Diversions

A few things to note about lens tests—at home and online: no two lenses are made equally. With computer-controlled manufacturing, the differences between two identical lenses from the same production line are much less than the days of handmade glass, of course, but there are still variations. You might buy a brand-new lens and refer to an online lens review of that optic that shows an MTF chart illustrating a certain aperture is sharpest, but your example of that lens might perform differently.

So, the above numbers, based on my experiences and tests, can be a fairly accurate guide, but you must test your own optics to get the best solution.

PS: I only test lenses using full aperture stops. I generally only take photos at full aperture stops, as well. (I guess I should discuss that with my therapist.)

PPS: Often the difference in sharpness between midrange apertures is so imperceptible that it really makes no difference. If the difference in sharpness between f/5.6 and f/8 requires 300% zoom pixel peeping, I can’t imagine you’d see a visual difference, at any magnification between, say, f/7.1 and f/8.

Zoom Lenses

In the same way that a lens’s sharpest aperture is in the middle range, a zoom lens is often sharpest in the middle of its zoom range—not out at the focal length extremes. You can also bench-test this by doing multiple tests at different focal lengths (I recommend testing the extremes plus “traditional” prime focal lengths for your test… test a 24-70mm zoom at 24mm, 35mm, 50mm, and 70mm, etc.) to keep things simple.

You may find that on, for example, your 24-70mm f/2.8 zoom lens, that your sharpest aperture at 24mm is f/5.6 but your sharpest aperture at 50mm might be f/8.

Hyperfocal Distance

One tool in the quest for scene sharpness, especially when photographing landscapes with near-foreground objects and distant backgrounds is to calculate the lens’s hyperfocal distance. Setting both the aperture and the focus ring at a certain point (a particular distance) ensures that the scene will have a certain level of sharpness.

As a discussion of hyperfocal distance is a bit of a detour from our lens’s sweet spot (not to mention that it involves math!), I have taken a closer look at this topic in this companion article.

Practical “Tests”

You are now standing at Tunnel View, in Yosemite Valley. The light is perfect and the scene before you is breathtaking. You grab a prime or zoom lens and frame the best composition. Your goal is ultimate sharpness.

What aperture do you use? Well, if you tested your lens and remember the data (or saved the info on your smartphone), you can dial up that magical aperture and start shooting.

But, what if you don’t remember the sharpest aperture? Or, what if you never tested this particular lens? Don’t panic. You can use the above numbers or 2-3 stops down from your maximum aperture rule-of-thumb I mentioned earlier. Or, guess what? Digital images are basically free, so shoot the scene at f/4, f/5.6, f/8, and f/11 and then compare the results of this in-the-field-bench-test later! 

When Not to Use Your Sharpest Aperture

I will admit that, when I started doing these lens tests years ago, I launched into the world shooting at the sharpest possible aperture almost all the time and not giving much thought to depth of field, or the scene I was trying to capture.

You already know from your images that the center of your lens is almost always sharper than the corners, and your testing revealed that the center sharpness arrived several aperture stops before the corners become sharp. With this knowledge, you can safely shoot portraits, or other kinds of photographs where the viewer’s attention is toward the center of the frame or you prefer shallow depth of field and blurry background, at wider apertures where you don’t need maximum corner-to-corner sharpness.

For example, when photographing my son, I know that my 35mm f/1.4 lens performs pretty well in the center at f/1.4, so I can shoot there all day long and maximize my light gathering to maximize my shutter speed to try to freeze his perpetual motion. If I have extra light, I stop down to f/2 for a bit more center sharpness, but the shallow depth of field still means that corner sharpness isn’t a needed consideration. For family shots where people are at different distances, I will shift to f/5.6 or f/8 to get more depth of field and a wide area of sharpness. For static landscape shots with that same lens, I dial up f/5.6 or f/8, as well.

Sweet Spot Sharpness

In summary, you can bench-test your lenses to find their sharpest aperture, use the rule of thumb, or simply shoot at different apertures to see what shot works best in the field. If you want to ensure that foreground and background objects are in focus, you can crunch the hyperfocal focusing and use math to your advantage. Don’t get stuck on shooting all day long at that sweet spot aperture if your scene dictates a different look and feel! 

Do you have questions about your lens’s sharpest aperture? Let us know in the Comments section, below!