Here is a comparison of extreme crops of two 60-second exposures of M22 in Sagittarius at ISO 1600 using an SMC Pentax-M 200mm f/4 lens on a Pentax K-3II DSLR (Astrotracer mode). The camera was pointing to the same place for both shots, the only difference being the f-stop setting. This shows the highly beneficial effect of stopping this lens down to f/5.6 from wide open at f/4. The chromatic aberration (CA) is drastically reduced and stars tighten up a bit, which is much more obvious in the corners than near the center of the frame where these crops came from. However, the lens still shows some CA with even brighter stars.
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