As you probably know by now, no lens is perfect. The key to making the most out of the lenses you have is to find their sweet spots. Joel Grimes explains what that means in this video:
Every lens suffers from optical imperfections. Not imperfections like damage or mechanical defects, but imperfections caused by aberration and diffraction.
What is Aberration?
Aberration is color fringing that occurs mostly when you use the lens at its widest aperture. Why does it occur? Because you’re using the whole glass elements to focus your image on your sensor. Since the glass is thicker toward the edge, the light passing through bends differently for different wavelengths. That is why you get aberration in terms of magenta and blue or yellow and green. The more glass the light needs to travel through, the bigger the speed difference between wavelengths. This usually occurs more around the edges, and it is significantly less of a problem toward the center of the frame.
Aberration is avoided with better quality glass. The bigger light transmission the glass has, the less aberration it will have. It can be minimized with glass coatings that reduce aberration when effectively combined.
What is Diffraction?
Diffraction, on the other hand, occurs when light is bent. It occurs mostly when you stop down the lens, because the light hits the edges of the aperture ring, causing it to bend. This results in softer details.
Diffraction affects the overall image, but it’s difficult to avoid due to the nature of the lens.
What is a Lens’s Sweet Spot?
A lens’s sweet spot refers to the point where you have the least amount of aberration and diffraction. That’s the point where the lens produces the sharpest image. This usually means stopping down the lens until aberration is minimal and diffraction doesn’t occur on a visible scale. For most lenses, the sweet spot is around f/8. But it’s not always f/8; every lens has a different f-stop value for its sharpest point. Grimes’ general rule is that the sweet spot is two to three stops from wide open. But it’s best to find (or test) the particular sweet spot for all of your lenses.
“Understanding the basics of optics is important when it comes to photography, so I encourage you to maybe explore a little more.”
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