What is a lens’s “sweet spot?”
Everybody wants sharp and focused photos. You can get good sharpness near to far in a photo by hyperfocal focusing, but what are the limits of sharpness of your lens? At what aperture (f-stop value) will you get the sharpest photos? This is also known as the “sweet spot” of the lens. Here is a simple test you can do to find out where your lens is sharpest. But first, some basic rules…
General Rules for Lenses
The rules are about the same for every lens. Here are the general ones:
- Sharpness is best in the center of the lens (center of your photo). At the edges, there is distortion because of the curvature in the lens. It is unavoidable.
- At high f-values (stopped down, small aperture opening) there is not much light entering the lens. So, the edges are blurry.
- At low f-values (stopped up, large aperture opening) there’s a lot of light entering through the lens. The image is then brightest and sharpest at the center and less so at the edges.
What is my lens’s sweet spot? How to find it:
You should know how sharp your lenses are and it’s easy to test them to see for yourself.
What you will do is shoot several photos of the exact same thing, focused at the exact same point. You’ll use different f-values of your aperture and then look at each photo to see how sharp they are. Simple, right? Right.
- In manual (M) mode, set a low ISO (100) and a fast shutter speed (1/500 s).
- Pick a detailed subject to shoot that’s about 50 to 150 feet away. Set up your camera on a tripod.
- Set your aperture f-stop to the lowest and take a photo. Increment the f-stop by one stop and repeat. Continue until you get to the highest f-stop.
- Review each photo and look for sharpness at the center and the edges.
Here’s what you are going to find:
- Sharpness will increase at about double your lowest f-stop. Here’s an example. For a prime lens with largest aperture of f/2.8 (lowest f-stop you can go), the best sharpness will be at about f/5.6 (f/2.8 x 2 = f/5.6). This is the “sweet spot.”
- At this sweet spot and in a range to double the sweet spot, according to the f-stop value, everything will be tack-sharp. Using the previous example, with a sweet spot of f/5.6, things will be tack-sharp up to about f/11 (f/5.6 x 2 = approx. f/11).
- Above this range (f/11 in the previous example), the sharpness will decrease. It continues to decrease until much less at higher f-stops (like f/22 for example), and the tack-sharp image is gone. This is sometimes referred to as “lens softness.”
The lesson here? If you want the best sharpness, shoot at apertures that are at the sweet spot. You can go from the sweet spot to double the f-stop and still have sharp images. This will vary somewhat for your particular lens, but if you do the calculation, you will know what your lens’s range is. To find out more about any particular lens go to the DxO Mark website and see if your lens is there. There you will find lots of lens information including where the sweet spot is. Or, you can find it yourself by my method above.
Also, if you want to maximize what kind of sharpness and focus you can get out of your lens, then use DxO Optics Pro. This program has a lens softness correction feature that is particularly good on RAW files. This feature is far beyond other programs abilities and will make your images tack sharp and crisp. I use DxO as a RAW file processor before and in tandem with Lightroom.
If you notice blur or focus problems in a photo, use Focus Magic software. It applies a correction that is not found in other software. It will repair photos that you thought were blurry and beyond repair.
Thank you for reading what I wrote — I hope you enjoyed it.