040 - How to Find your Lens’ Sweet Spot
Today, we are talking about something that some photographers - regardless of level - don’t really seem to know. And that’s that lenses are not uniformly sharp at every aperture. And in the case of zoom lenses, not equally sharp across the zoom range. Some lenses just perform better at certain apertures and at specific points in their zoom range. To find where a lens performs at its sharpest, you need to find its sweet spot.
The sweet spot on a lens is where the lens will be at its sharpest. Meaning it will offer the most detail for what is in focus. So, essentially, the in-focus part of your image will be at its sharpest when you find the sweet spot on your lens. Now, don’t confuse sharpest with depth of field. They don’t mean the same thing. Depth of field means what appears to be in focus in the image. A large depth of field will mean more of the image will appear to be in focus. Whereas a lens’ sweet spot may not offer the greatest depth of field, it provides the sharpest image for what is in focus in your photo.
We’ve talked about greater depths of field using your aperture and some of the problems that can occur before. Check out our episodes on the Exposure Triangle (episode 12) and Landscape Photography (episode 22) to refresh your memory on this subject.
Today, let’s focus on getting the absolute sharpest image that we can from a lens. Now, of course, some lenses are inherently sharper than others. It is the way they are built. Typically, high-end lenses will be sharper than lower quality ones (it’s one of those things you pay for), but not always. There are some superb budget lenses out there that are beautifully sharp. So, try this out no matter what lens you are using, and you may be pleasantly surprised with the results!
To find the sweet spot, there is a general rule you can start with. And it’s pretty simple. Find the maximum aperture of your lens (where it is at its widest). This will typically be written right onto the lens itself or put the lens on your camera and open the aperture as wide as it goes. Now simply increase your aperture by 2 or 3 stops of light. So, if your maximum aperture was f/4, you would want to shoot at between f/8 and f/11. Or if your aperture is f/2.8, you would want to shoot between f/5.6 and f/8. So, here is where it may get a bit tricky. Cameras allow you to increase your f-stops in increments. So every increase on your camera doesn’t represent a full stop of light. It’d be effortless if all you had to do was turn the aperture wheel two or three times, and there you go, right?
But that’s not how most cameras work. Most will move aperture values by about a third a stop of light every time you change it. So you will need to move the aperture wheel 6-8 times to get to where you need to be. If you get to know the full f-stop values, this will become easier. To help you with that, we will have a chart on f-stop values on the show notes at travelandadventurephotographyschool.com.
Now, what about zoom lenses? Because some zoom lenses don’t have one widest aperture, it’s variable across its zoom range. While the same math applies to each time the maximum aperture changes. So, if at your widest end the aperture value is f/3.5, then your sharpest image should be between f/7.1 and f/10. Then, as you zoom, if your aperture changes to f/5.6, your sharpest point becomes around f/11 to f/16 (although diffraction will limit sharpness at f/16, try to keep it around f/11).
Hopefully, all of that makes sense. Basically, you are looking to increase your aperture by 2 to 3 stops of light from its maximum to get the sharpest image.
Now, there is one last thing to take into consideration. On a prime lens, you only have one focal length to worry about. But on zoom lenses, the sweet spot may be in just one part of the zoom range. Or, it may change depending on the focal length you are using. This will vary depending on the lens, so there is no trick to creating this quickly. The best way is to test out your lens at various focal lengths to see which part of the zoom range offers the sharpest image.
One way to do this is to set up a test area for your camera and the lenses.
Grab an object that has tiny writing on it (I’ve used a vintage camera and lens for this before) and set it up somewhere where you can use the full zoom range of your lens without going out of focus. Now, put your camera and lens combination on a sturdy tripod and set the first focal length you want to test. For a lens like the 70-200, I try to test it in quarters. So I would test it at 70mm, 105mm, 135mm, 165mm, and 200mm.
Now, set your camera to a low ISO and your aperture at it’s minimum (the largest number) and then work backwards through aperture values until you get to your maximum. Make sure you take a photo at each value along the way.
A few things to watch as you do this. Use a remote trigger to make sure you don’t bump the camera. If you can, change the aperture and shutter speed values without touching the camera (something like the Alpine Labs Pulse will let you do this, but I’m sure there are others). Don’t change focal lengths at all. Complete the test throughout your aperture range on one focal length, then move to your next focal length and repeat the process. Try to take these images in a well-lit area outside in good light or using studio lights indoors.
Once you have completed this process for each of your lenses, upload them to your editing program (I use Lightroom for this). Now pull up the images for each focal length, zoom in, and compare the photos from each aperture rating. In Lightroom, I used a 3:1 scale to be able to get right up into the picture. Eventually, you will find the ones that are the sharpest. And you will also get to see where your lens turns out its worst images in terms of sharpness. Once you know this, jot down the results in a note on your phone, computer, or somewhere you will be able to reference them. That way, if you do forget, you don’t need to retest.
You can also read charts and lens reviews from companies like DxO Mark, but this will only give you a general idea of where a lens is sharp. Every lens is slightly different, so it is best to do a test on your own lens to know precisely which values work the best.
And that’s it. That is how you find your lens’ sweet spot. To start, and without testing, try increasing your f-stop from its max by 2 or 3 stops of light. After that, try setting up a test to find out the exact number for yourself. Now, of course, sharpness isn’t the be all end all of an image. You may want a shallow depth of field, or you may want a wider depth of field. You will need to decide what is essential to the image you are crafting.
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