Monday, October 09, 2017

The Case of the Curious Optical Effect...

Something strange happened on the way to experiencing "softness" nirvana.  I've stumbled across an optical effect that I can't explain.

In optical designs I'm familiar with, "depth of field" increases as an aperture size is decreased (stopped down, as it were).  That's the commonly expected effect and is partly why lenses often come with aperture controls.

After looking at Jim Galli's soft focus large format images and thinking about how I might achieve similar effects in APS-C digital without returning to large format film, I stumbled across a small Pentax 85mm f/2.2 Soft in a Nikon F mount.  I'd never seen this lens in Nikon F, so I picked it up for not a huge sum of money.

Today I performed a quick comparison between the Pentax and my Nikon Nikkor 85mm f/1.8 K pre-Ai.  I was looking forward to shooting the Soft wide open and controlling the depth of field by stopping the aperture down a bit.

Looking at the comparisons and after the noting that the spherical aberrations decrease with aperture size (as expected), I noticed that the circle of confusion (said another way, the out of focus rendition) did not seem to change with the aperture.

What's going on here?  I have a mystery on my hands.  Perhaps one of my readers can help explain the optical effect?

Pentax 85mm f/2.2 Soft, Nikkor 85mm f/1.8 K Comparison

Here is a Nikkor "control" image where the 85mm f/1.8 K was shot wide open.

Nikon Nikkor 85mm f/1.8 K

Here is the Pentax 85mm f/2.2 Soft at f/2.2.  As you scroll through the images, look at the "size" of the out of focus bright area situated between the green plant in the center and the red flower just to the right.  You can also look at the "size" of the leaves in the background (upper left region).

Pentax 85mm Soft at f/2.2

Here is the Pentax 85mm f/2.2 Soft at f/2.8.

Pentax 85mm Soft at f/2.8

Here is the Pentax 85mm f/2.2 Soft at f/4.

Pentax 85mm Soft at f4

Here is the Pentax 85mm f/2.2 Soft at f/5.6.

Pentax 85mm Soft at f5.6

No comments: