Finding the Sweet Spot!
A while ago my dad bought some lenses off another photographer. On the lenses was a small piece of tape that had various f stop numbers written on them. I wondered about what it all meant. Dad graciously enlightened me. The f stops written on the tape were the apertures that the particular lens was sharpest at. Essentially, they were where the lens was at its best performance for sharpness and optimal image quality.
This can be referred to as “lens tuning” or “lens diagnostics” – whatever you want to call it, its the process of finding out where your lens shoots the sharpest images at. I went digging around the internet and came across some really great videos on how to sharpness test your lens. The people who provided the videos also provided a comprehensive list of Nikkor and Zeiss lens sharpness. When I tried their techniques in the video I came up with the same results they did and they were startling!
If you want to try this, watch the videos and follow them exactly.
I tried three of my lenses in these diagnostics: my “nifty-fifty” 50mm AF f1.8, my 105mm AF-S VR Macro and my 16-85mm AF-S VR zoom lens. The primes were quicker to test because you just sail through the f stops shooting at various apertures. But with the zoom lens, you have to do it at various focal lengths which took a little longer.
Anyways, we will look at the findings of my favorite lens for portraits, my trusty 50mm. In the past, I’ve almost always shot it at f1.8. Why? Because it can shoot f1.8!! It can open up super wide in junk light, making shots possible you might not have otherwise got. But, after doing this sharpness test today, I’m rethinking some things. According to the sharpness chart, the optimal apertures for this lens are:
DX: F/4.0, F/5.6, and F/8.0 (For FX & Film:F/4.0, F/5.6, F/8.0, F/11.0, & F/16.0)
When I compare my test images at f1.8 and 7.1 the difference in sharpness is truly incredible. The 7.1 image is far sharper.
Also, for the 105mm Macro at f2.8 and f8, the difference is wild. It confirms the chart which says DX (normal use/non 1:1 close up): F/4.0, F/5.6, and F/8.0 are sharpest.
And finally, for my 16-85 zoom, the chart’s findings were: DX: 24mm-50mm at F/8.0.
Which tells you that the sweet spot of this zoom is f8 in that range. I included a test shot of the 16-85 at 85mm f8 vs f32 for comparison. It’s striking!
What all this nerdy stuff tells us is two fold. 1) It can confirm for you that the lens you have is accurate and not a dud. If in your comparisons, you see soft images where they should be sharp, you may have a bum lens. 2) It confirms in your own mind where your particular lens should be shot to be at it’s very best (sharpest). Of course lighting conditions, situations and artistic intent play into this, but it’s safe to say if you know where your lens shoots sharpest, you’re more likely to get consistently sharp images if you know the sweet spots.