How do you find out the "sweet spot" of a lens?

Question

I've tried googling this, but have never found a satisfactory answer.

I've heard the term "sweet spot" thrown around by some photographers to mean the f-stop of a lens which results in the highest sharpness that lens can achieve.

A few questions on this:

1. General photographic knowledge states that the higher the f-stop (the smaller the aperture), the largest depth of field you will achieve. This seems to "suggest" the higher the f-stop, the sharper your image will be (all other factor being equal of course). Does the idea of a "sweet spot" trump this rule? (so, theoretically a f11 could be sharper than f22)

2. Is the "sweet spot" an optical algorithm that can be applied to any lens, or is it to do with the particularities of the manufacturing of some lens?

3. Finally, how can I determine the "sweet spot" of my prime lenses?

Note: I know other things come into consideration in sharpness, like ISO, light, glass (lens) etc etc, but please ignore these and assume these are equal for every different lens.

For context, I'm mainly trying to achieve ultimate sharpness in architectural (indoor and outdoor) photography, and urbanscape, where usually I tend to shy away from small depth of field.

Answer 1

The sweet spot of a lens is probably just as dependent upon the type of image capturing surface used as the lens itself. Both film and digital sensors have a limit of detail they can resolve (although large-format film has the tendency to capture FAR more detail than 35mm or digital sensors at much tighter apertures, around f/22.) Assuming you have a lens with the best resolution imaginable...it will ultimately be limited by the imaging material. This is due to the "diffraction limit" of the film or sensor.

The mechanics behind finding the "sweet spot" of a lens can be fairly complex, as it is very mathematical. To simplify this for consumers, the MTF (modulation transfer function) chart was born as a way to provide clear, mathematically derived information about the sharpness, or resolution, of a lens, film, or sensor. If you are interested in the underlying theory, this article is a good read: Understanding image sharpness.

In simpler terms, assuming you want the maximum clarity for the sensor size and density you are using, for most DSLR image sensors the "sweet spot" of most lenses of decent to high quality is between f/8 and f/11. Entry-level DSLR's, which tend to have smaller sensors with smaller photosites of greater density, are diffraction limited at around f/8 or f/9. Higher-end DSLR's, which tend to have larger sensors with larger photosites and lower density, are diffraction limited around f/11.

Outside of having a really crappy lens that does not have the greatest intrinsic resolution, most lenses can resolve a high degree of fine detail. Most lenses on the market these days have their own MTF chart that can be helpful in knowing the lenses "sweet spot" in and of itself. Most digital cameras have information about when the sensor becomes diffraction limited. Review sites such as DPReview.com, the-digital-picture.com, etc. will also state the apertures at which the sensor becomes diffraction limited for most cameras. I don't do much film myself, so I can't offer you much regarding when various types of film may become diffraction limited.

It should be noted that the diffraction limiting aperture (DLA) is only when diffraction starts affecting quality, but not when it has reached its maximum effect (which is usually several stops beyond the DLA.) Visible image softening from diffraction will usually not be apparent until a couple stops beyond the initial DLA. For sensors of a given size (i.e. APS-C), a higher-density sensor will start revealing diffraction earlier, however the lower-density sensor will be incapable of resolving detail as high as the sensor of greater density. For any given megapixel size (i.e. 18mp), a sensor with larger physical size will usually provide better results. Diffraction affects image quality due to light dispersing beyond a single photosite and affecting others. As larger sensors (i.e. Full-Frame vs. APS-C) have larger photosites, they become diffraction limited at tighter apertures than smaller sensors.

The real trick is finding the overlap between the point of peak sharpness for the lens, and the point at which an image sensor is able to resolve clear detail without visibly softening it due to diffraction. An aperture setting in the overlap area will be the true "sweet spot" of the camera and lens you are using. On the flip side, if depth of field is more important than ultimate sharpness, then a higher aperture may provide a sweet spot more appropriate to your work.

Answer 2

With primes I always put a page of text up on the wall, put my camera on a tripod with a remote trigger (the self timer works too) and take a couple of photos at each major f-stop: 2.8, 4, 5.6, 8, 11, 16, 20 and then I compare them for sharpness at the center, edges, and corners. You'll see that there's a range that's sharpest and I use a label maker and print "8-11" to put on the lens itself so I know for each lens.

With zooms it's harder because the sweet spot will change with the focal length, so for a 70-200mm lens you'd want to do it incrementally like 75mm, 100, 125, 150, 200 maybe.

Just keep in mind that even if the text isn't perfectly sharp at any focal length/aperture, we don't generally photograph text and differences in sharpness can be seen with text that you'd never see in a landscape, for instance.

Answer 3

I think "sweet spot" is a rather poorly-defined term in general use, and in fact you'll see some people talk about the sweet-spot of a lens with regard to the sharpest aperture setting, and others talking about the sweet spot of a lens' image circle (e.g. using a full-frame 35mm lens on a cropped-sensor DSLR).

You cannot generalize and say "50mm primes have a sweet spot at f/8". Different lens designs perform differently and will make different trade-offs. So not all lenses of a given type will have the same sweet spot.

With regards to sharpness and aperture, modulation transfer function (MTF) charts will give you a good picture (albeit theoretical), if they're published for the aperture settings you're interested in. But MTF charts can be difficult to find for some lenses, and usually will show just one or two aperture settings.

The empirical way of determining the sweet spot for a lens that you personally own is to take test shots at different apertures, preferably of a flat-field scene with both fine details and high-contrast edges. Then compare the images and draw your conclusions. It may not be clear-cut, depending on what your criteria are. For instance the aperture where the corners sharpen up may be different than the aperture where the center of the images is sharpest. Obviously shooting technique is important for this, so using a tripod with mirror-lockup and a cable release is ideal to remove camera shake as a factor.

While the f/8-f/11 range is generally considered a safe choice, I wouldn't say it's universally true. Higher-quality lenses will already be starting to see the effects of diffraction by f/8, especially on high-resolution camera sensors. For instance, many late-model pro-level lenses will hit their sharpness sweet spot around f/4-f/5.

Answer 4

Regading photography, there are two points that limit the resolution of an image: one is the Depth of field (look at Wikipedia yourself, I'm not allowed to post two links), the other the physical resolution of the lens (Rayleigh criterion of maximum resolution).

A large depth of field is generally obtained with a small aperture (f/11 has a smaller depth of field than f/22), while a large aperture leads to a smaller diffraction limited spot size for these areas of the image that are in focus.

For an ideal picture there are two contradicting goals: large aperture (small f-number) for the points in focus, small aperture (large f-number) for for a large depth of field. Depending on the lens, the film/ccd detector used and what you intend to picture, different settings are optimal.