In this photography tutorial, I’ll give you a few tricks to find your lens’ sweet spot. First off, what does the “sweet spot” mean? Simply put, the sweet spot is the aperture at which your lens is at its sharpest.
You’ve probably noticed that some of your images weren’t as sharp all the time and you may have shrugged it off as camera shake. Maybe it was, but it could’ve also been softness caused by your aperture setting on your lens. You see, your lens doesn’t have the same sharpness throughout its complete aperture range. So just changing apertures will change you lens’ sharpness. Hence the importance of finding its sweet spot.
Before I get into the details, I would like to mention that, because of the lack of zoom, prime lenses are usually sharper than zoom lenses. Yes, even prime lenses have their sweet spot but the differences are minuscule and can barely be seen by eye (for quality primes). I have 2 prime lenses, the Nikkor 50mm and the Sigma 105mm, and they’re pretty sharp throughout their complete aperture settings. That said, let’s get back to the sweet spot.
There is a general rule that can be applied to most, but not all lenses, when it comes to finding it’s sharpest aperture setting. Generally speaking, your lens’ sweet spot is 2 stops above its widest aperture. I won’t get into the whole f-stop theory here because I’ll bore you to death but it’s important to at least know your full f-stop settings on your lenses. Basically, each f-stop increments reduces the amount of light entering the camera by half which means the circle’s surface area is reduced by half. Here’s a diagram to better explain it.
Ok, now that you understand basic f-stop increments let’s look at an example of the 2 stops theory. If your lens’ widest aperture is f 2.8, your sweet spot should be f 5.6. If your lens’ widest aperture is f 4, it’s sharpest aperture should be…. come on, you know it….. right! f 8. So this is easy, right? Tutorial done, right? You’re now a sweet spot expert, right? Well, not really. All rules are made to be broken. 😉
Once you’re done with the theory, it’s time for the practice. There aren’t any shortcuts for this. If you really want to find each of your lenses’ sweet spots, you’ll have to shoot the same scene at various apertures. I recommend setting up your camera on a tripod in a bright environment and using a remote triger like the MC-30 or the RS-80N3 to prevent camera shake. Put your camera on aperture priority mode and start shooting each frame at 1 stop increments (your camera might be set to 1/2 or 1/3 stop increments). Then open them in Photoshop or another photo software and view the image at 100%. That’s how you’ll find your lens’ sweet spot.
Another option is to look at lens review sites. DPReview has great lens reviews with interactive charts where you can scroll through different apertures and see the various sharpness. It’s so cool! If you know of other great lens review sites that test aperture sharpness, please share them in the comments.
To be completely honest and practical, my first suggestion is for you to buy good lenses right away. This way you won’t have to compromise your aperture for relative sharpness. Remember that you’ll change your camera body but your lenses will stay. I would rather compromise sharpness over aperture any day since aperture is a means to my creativity. Not to mention all the available sharpening softwares out there like Photoshop and Lightroom.