In my last post, I showed how individual optical aberrations affect image quality by presenting simulated point spread functions and Siemens star renderings with one aberration at a time. This approach makes it easy to isolate and understand the signature of each aberration. But lenses rarely suffer from a single imperfection. More often, multiple aberrations act together, and their interactions can produce effects that are not just the sum of their parts.
In this post, I’ll explore what happens when you combine aberrations, starting with the most familiar one: defocus. That raises a natural question. Is defocus really an aberration? From the perspective of optical engineering, it certainly is. It alters the phase of the wavefront in a way that degrades the image, just like spherical aberration or astigmatism. Of course, in practice, defocus is under the control of the photographer and is usually not a flaw in the lens itself. But when we simulate its effects and combine it with other aberrations, we gain a better understanding of how focus errors interact with more intrinsic imperfections in an optical system.
In the simulations that follow, I’ll show what happens when varying degrees of defocus are applied in combination with other aberrations. The results can help reveal subtle interactions and shed light on the importance of precise focusing, even with otherwise well-corrected optics.
First, the point spread functions, in the lower right corner of our three image grid. The zero-defocus images are marked with red outlines.
Compare the top and the second row. Notice that spherical aberration (SA) shifts the point of best focus. I will do some analysis of that in the next post. Also note that the PSF shape varies asymmetrically with defocus in the presence of SA, astigmatism, and LoCA. Another thing to notice is that you can identify astigmatism more easily when the image is slightly defocused.
Next, with a synthetic Siemens star. I’ve dialed the aberrations back so you can see the effect on the images better.
And finally, with a classic image engineering test image and the same aberration set as the Siemens star.
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