I have three optics that I've purchased over the years to see if I could use them as soft focus lenses on Full Frame and APS-C digital. One lens is an early 50mm plasmat formula f/1.4 that shows spherical aberration when shot wide open. One is an 85mm "meniscus" Soft Focus that has proven to be very difficult to control due in large part to its level of softness. And one lens is a first element focusing 55mm lens that is proving interesting to understand.
The lenses are ~
- Nikon Nikkor-S 50mm f/1.4
- Pentax 85mm f/2.2 SF
- Ricoh Riconar 55mm f/2.2
The Nikkor-S is well known. It's a classic plasmat design. As the lens was designed well before modern high-refractive index glass became available, the 50mm Nikkor shows obvious spherical aberration at f/1.4. The effect largely disappears at f/2. Stopped down the lens is indistinguishable from current 50/55mm lenses.
The Pentax SF is a beast of the lens to work with. There is an enormous amount of spherical aberration at all apertures. It's so strong that at f/2.2 and f/2.8 the underlying sharpness of a subject is heavily veiled and overall contrast is low. As the lens is stopped down the contrast and central resolution improves, the veiling spherical aberration decreases, but the edges begin to show weird coke-bottle-bottom smearing.
The Ricoh Riconar 55mm f/2.2 has a bad reputation on at least one of the Pentax discussion forums for being soft with inconsistant/incomprehensible behavior. The lens is a four element four group design and uses the front/first element to focus. The other three elements and aperture positions remain fixed. I've not encountered modern-ish SLR lenses with this configuration outside of this one Ricoh. The difficulty to control this optic comes from the fact aperture and distance change the character/rendering of the lens (though 'netizens don't fully discuss this fact).
Looking at these three different ways, I wanted to observe differences in how softness is achieved on a close subject. Softness behavior on distant subjects would be different, but I wanted to begin somewhere.
Wide Open ~
Wide open the Nikon and Ricoh show beautiful levels of softness. The Pentax is quite obviously soft and of lower contrast (I used the exact same RawTherapee image processing recipe in all cases).
f/2.8 ~
Stopping the lenses down to f/2.8 shows how the Nikon is becoming razor sharp. The Ricoh is cleaning up a little, too and the overall rendition is, to my eyes, rather pleasing. The Pentax continues to show strong softness, though contrast is slightly improved over f/2.2.
f/5.6 ~
At f/5.6 the lenses are cleaning up pretty well. The Nikkor-S and Ricoh look sharp. The Pentax is still a soft focus lens, but the subject is more clearly and cleaning revealed and the overall contrast is vastly improved over f/2.2 and f/2.8.
Closer Look ~
Trying to understand how each lens treats sharpness and out of focus areas reveals some interesting details.
I needed to stop the Pentax down to f/2.8 to show the subject better because at f/2.2 most of the details were lost. Contrast is still low at f/2.8, but I begin to see what I wanted to see. The left hand image is actually pretty sharp. This comes from the effects of spherical aberration (which this lens has in abundance) on perceived depth of field.
The Nikkor-S is sharp at the point of focus. The out of focus rendition is soft as one would expect. Due to the lens design the transition from sharp to soft is dramatic. Modern high speed optics consistently behave in this manner.
On the other hand, the Ricoh behaves rather differently from the other two lenses. The out of focus areas are extended due to aperture and spherical aberration (see the left hand image). The highlights glow just as with early "pictorialist" lenses where the Nikkor-S and Pentax do not. There's a sense of resolution that can be appealing even though the lens is not "bitingly" sharp. Contrast wide open is the best of the these three lenses.
I can see why I feel the need to boost image contrast when using the Pentax 85mm SF. The veiling spherical aberration is strong, but there's a potentially useful resolution under that veil. In practice, increasing overall as well as local contrast during image processing might have some uses. Back in the day I imagine photo-alchemists working to increase negative contrast as much as they could to try and overpower the contrast reducing veiling softness.
Nikon created a very decent lens. At f/2.8 I see the level of optical correction improving to the point I can make a perfectly usable image in the current sense of such things. The out of focus rendition remains soft and the contrast is nearly up to modern-lens levels. For "soft focus" work, however, the optic doesn't really sing to me. It's as if it's warming up but isn't quite ready for a full concert before an adoring "soft focus" audience.
Considering the overall rendition of the Ricoh at f/2.2 I see something I didn't think I'd ever find in a small format optic; controllable softness based on aperture and subject distance. To me this lens behaves a lot like an early "soft focus" portrait lens. While it's usable for landscape images (as I'll perhaps talk about another time), its specialty appears to be for portraiture and subjects closer to the camera. This might be an optic worth exploring for "soft focus" work on Full Frame and APS-C digital formats.
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