How do you find the sweet spot and sharpest aperture of any given lens you’ve got? Well, the cynic in me would say, “Google it.” And there’s some sense there because for just about every lens that exists there also exists someone who’s done the work for you; someone who has gone through the trouble of figuring out sharpness on a micron level.
That said, however, there’s intrinsic value to be derived from going through and understanding the process required to find the ‘real world’ information (vs something like lppmm); doing rather than reading.
And beyond that too we must also address the matter of performance deviation from unit to unit – ‘statistical variance,’ if you will. Your Nikon 85mm 1.8 may perform slightly differently than mine, and if you’re the kind of person to whom the optimal sharpness of a lens matters, then this too matters to you.
Distinguishing Sharpness From Resolution
So to address this properly let’s touch on a few minor points like what sharpness actually is and what else to consider when determining your lens’ ‘sweet spot’.
Above all else, when we refer to lens sharpness what we’re referring to, is, a lens’ ability to resolve detail at a given aperture. A lens that can resolve high detail at a particular aperture is considered ‘sharper’. that is not, however, to be confused with being ‘in focus’, which is something different altogether, even though in more casual settings and in basic photographer diction the two have become proxy for each other.
*Speaking of confusion, ‘resolved detail’ is also not to be confused with sensor resolution. Your sensor will deliver its resolution regardless of how sharp the image is that is rendered onto it via the lens.
So with that in mind, from a practical standpoint what you should understand about the sweet spot of a lens is that what is, is the aperture of the lens that gives the highest amount of resolved detail of the area of the image that is in focus. That’s it, but this also has other consequences too, because that particular spot of the lens tends to also have the least amount of chromatic aberration and diffraction issues, the controlling of which is one of the only ways we can manipulate a lens to affect the color of an image.
The Sweet Spot
There is some generally accepted rule of thumb that to find the sweet spot of a lens you must stop down about 2 stops from the widest aperture, perhaps even three. So if you’re lens wide open is at f/4, the theory states your sweet spot is about f/5.6 or f/8, and then up to f/11 you should be fine with diminishing returns after that due to diffraction.
This is partially this why some professionals really do push for the wider aperture lenses like 1.2 or 1.4; not to necessarily shoot at 1.4, but because it gives them a sweet spot that’s still a wider aperture than say a less expensive 1.8. The 1.4’s sweet spot (using this conventional wisdom) would be f/2.8 or f/4, and the 1.8’s sweet spot would be f/3.5 or f/5 or thereabouts.
In practical use this is probably not a bad thing to go by, especially if you’re not using tip-of-the-spear lenses. With higher-end lenses, however, there can be less a need to stop down, which is partially what you’re paying for. Using some high quality medium format glass, for example, your sweet spot could be around f/6.3 for a max 2.8 lens versus f/8 for more standard 2.8 DSLR lenses.
All that said and done, doing a basic controlled test, as alluded to above, can be an eye opener, and it’s simple to do. In a controlled lighting environment, shoot the same scene all the way through your various apertures, then upload to a computer at 100% crop and have a look. that’s all there is to it and you’ll probably be surprised at what a difference a third of a stop can make to your image. Inversely, if you’re not of a pixel-peeping persuasion and shoot wide open for artistic purposes then you may not see a big enough difference to really alter how you shoot. Either way though, it’s got inherent value.
Check out the video below from Pixel Village for a quick look at how to do one of these quick tests, and Radha has provided the files from his test for you to analyze yourself. And if you’re interested in getting up to speed, and fast, our foundational courses in SLR Lounge Premium is the perfect way to to do it, and get to where you want to be in the shortest time.