Eyes wide open

I’ve had my eyes opened, and I’ve seen what lens designers are up against.

A few days ago I went to the ophthalmologist. She needed to peer deep inside my eyes so she put some magic drops in them. My pupils dilated ... wide ... wide ... and wider.

Because my eyes could no longer select a smaller aperture, the light became painfully bright. The pain was not helped by the fact that the good doctor had thoughtfully decorated her surgery all in white. Furnishings too – white leather – so stylish.

But as I tried to fight off the white, there was something else that puzzled me. Everything had a wide fuzzy halo around it, something like this:

Before the magic drops ...

... and after ...

Then, when, finally, I set off home, car headlights, shop displays, and other points of light had become indistinct starbursts. (I was on my bike, and finding my way was an interesting exercise.)

I knew that, with pupils wide open and I wouldn’t be able to control the amount of light entering my eyes. Hence the painful brightness. But why the halo and starburst effects?

Then it dawned on me. My eyes were working at full aperture and their lenses were showing up all their optical shortcomings. They obviously hadn’t been created by a particularly intelligent designer. There were quite a few faults – reduced resolution and chromatic aberration for starters.

The Japanese are better at it with their lens design computers.

A couple of hours later the effects of the drops had worn off and my pupils could open and close as normal. My vision returned to its usual hawk-like acuity ... well, clear and halo-less, anyway ... and I could move about without feeling I was in some sort of weird, over-lit fog.

The moral of this story?

The performance of lenses gets worse at full aperture. Lens designers go to huge lengths to overcome these faults and, with a good camera lens they are hardly noticeable (it’s a different matter with cheaper lenses).

But, even so, whatever lens you use, try to avoid shooting with it full open. The only reasons to shoot at maximum apertures are if you deliberately want a shallow depth of field, or if the light is so dim it’s the only option.

Outside those situations, shoot at around f8. Almost all lenses – including those in your eyes – give their best performance at middle-range apertures.

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