Aperture and bokeh tests with a Soligor 35 mm f/3.5 lens

[Stefano]

I hope this is the right section to post this into.

 

Following discussions about my f/22 photos and the "bubbly" bokeh in the foreground (link), I decided to thest both aspects of my lens.

 

I first tried bokeh. I took some images at f/3.5, using an orange LED light as a point source of light, trying both the "background" (focusing closer) and the "foreground" (focusing further) bokehs. I tried both focusing the lens with the focus ring or with the helicoid, and the results were the same.

 

Background:

 

Foreground:

 

The difference in brightness is mostly due to the different exposures, but you can easily see that this lens does not have a bubbly foreground bokeh.

 

Then I tested the apertures, in UV, focusing at infinity. I cut 500*500 crops of the images in the exact centers in Paint.

 

f/3.5:

 

f/4:

 

f/5.6:

 

f/8:

 

f/11:

 

f/16:

 

f/22:

 

I do see some softening at f/22. I am puzzled by the big difference between f/3.5 and f/4. I hope the tree didn't move too much because of the slight wind. I used ISO 800 and the shutter speed for the f/22 image was 1 second.

[Andy Perrin]

The higher F-numbers behave as I would have expected. I would go with F/11 to F/16 or so. The big difference between F/3.5 and F/4 is also normal — lenses can change a lot wide open. But in this case you may be seeing the effects of not having a good lens shade. Off-axis light matters more wide open.

[Stefano]

I may try this test later indoors with a UV torch shining on a tinfoil ball, that way there will be no movements and diffraction should show up better.

 

I too think now that f/8-f/16 is the sweet spot. F/3.5 is still acceptable at the center if you don't zoom in too much, but the edges are quite bad. F/22 is not so bad either, but you can start to see diffraction. F/11 to me seems to have a bit more contrast than f/16.

[bvf]

 

I too think now that f/8-f/16 is the sweet spot.

 

 

Whenever I have tested a lens or found lens resolution data, f/8 is always the sweet spot - in visible light. It doesn't matter what the folcal length, max. aperture, or age of the lens is. But as the effect of diffraction increases with wavelength, something smaller than f/8 is probably better for UV, and something larger is probably better for IR.

[Stefano]

I read that in general as a rule of thumb you can stop down one stop more in UV, and one less in IR. So if f/8 is the sharpest in visible light, you can go down to f/11 in UV and only f/5.6 in IR.

[Andrea B.]

I am puzzled by the big difference between f/3.5 and f/4.

 

Many lenses at their widest aperture have what is called "veiling flare". This produces a kind of overall haze which obscures some detail and supresses some contrast. Some photographers prize that veiling flare because they think it gives a kind of artistic glow.

 

What causes veiling flare is that is a bit too much light is being passed at the widest aperture which does not contribute directly to forming the image. Uncoated older lenses can be prone to veiling flare. Changing the angle of shooting might help a bit. A lens hood can help also. But stopping down will be the best cure. You have to experiment to determine how many fractions of a stop are needed to quell the veiling flare.

 

It looks as though your Soligor 35/3.5 still has a small amount of veiling flare at f/4, but by f/5.6 it is gone.

 

 

....corrections or enhancements welcomed if I missed something.....

[OlDoinyo]

In the digital era, veiling can be easily edited out of an image. In the old days, it was not so simple, and this issue was more critical.