Lens Sweet Spots

What's a sweet spot?

The performance of a lens--including the sharpness--varies depending on the aperture and focal length. Lenses are rarely at their sharpest wide open, and zooms often lose sharpness as they reach the end of their zoom range. The aperture and focal length where a lens works best is called the "sweet spot".

It depends on what you're shooting

At low f-stops (e.g. f/2.8) lenses are usually sharper in the center of the frame and softer in the corners. As the f-stop goes up (e.g. f/8), the corners get sharper and center drops so that the sharpness is more consistent across the frame.

Having the center sharp is important for a headshot style portrait so you get good detail in the person's face, and you probably don't care about the edges of the frame. The "sweet spot" for portraits may be at f/4 to maximize the image quality in the center. However, if you're shooting a landscape then you want the photo to be sharp everywhere. You try to get the best performance in the edges and corners, so the sweet spot for landscapes might be f/8. It's the same lens, but depending on the subject there are two different "sweet spots".

The above chart from Optical Limits is an extreme example. The Canon 11-24mm is very sharp in the center at f/4 but the corners are weak in comparison. As the f-stops climb, the corners get better until at f/8 the corners are at peak sharpness. There is no hard and fast rule, and all lenses are different. There are many examples where peak sharpness in both the center and the corners occurs around the same aperture.

Who needs a sweet spot?

If you look at the graph for the Sony 85mm f/1.8 lens, the sweet spot is at f/2.8. But the scores are so high across the board, it really doesn't matter what aperture you use. On Optical Limit's charts, a value above 3600 is tack sharp even pixel peeping at 100% on a 24 megapixel camera. If you are lucky enough to have a lens like this, you can just ignore the sweet spot and fire away.

Research before you buy

The best time to check a lens' performance is before you buy it. See if the lens performs well in the situations you plan to use it in. I have the Sony Zeiss 35mm f/2.8. It's small and portable and a great lens for street photography. But if you look at the chart, you can see that the corners are good but not amazing. If I'm going out to shoot landscapes, then I'll take the awesome (but also big and heavy) Sigma 35mm Art lens instead.

Keep in mind that standard focal lengths like 50mm or 85mm are going to have the best performance. As you go wider than 35mm, the lens is bending the light at more extreme angles, and the scores are going to drop. The same is true at the long end. As lenses go above 135mm, they get much more expensive and it get's harder to get the same performance.

Avoid weak areas

The best strategy is to turn the problem on it's head. Don't worry about the sweet spot; instead figure out the areas where a lens is weak and avoid using it at those settings. For example, you want to photograph a train chugging up a hill, and you decide the shot works best at 200mm at f/4. You have a 70-200 f/4 lens, but you know it's not great wide open at 200mm. You also have a 100-400mm f/4.5 lens. 200mm is right in the middle of the zoom range and produces sharp images, so that would be a better choice.

Where to Research

There are many review sites, and they all have different ways of testing lenses and showing the results. If you have a lens that you're happy with, you can look at it's chart side by side with a lens you're considering. Here are some favorite sites, but by no means is this an exhaustive list.

• Optical Limits

• DxO Mark

• Imaging Resource

• Lens Tip