Originally posted by FastPhotography ...
Also, why do you need a 41mm when there's a 50mm? Or a 50mm when there's a 55mm? The difference is so minimal...
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Only in field of view. Different optical formulas produce different characteristics in the rendering of the image. Its like asking why Lambourghini bothers to make so many models of sports cars when one Ford will drive you all the way to Albuquerque.
But to give you a specific answer from one who has (and carries) several near overlapping lenses - here's a tight focal length cross-over section that I regularly carry and why:
clump 1 (note, most shots pertain to current weather conditions)
Voigtlander Nokton
58/1.4 --- very smooth oof, very soft diffusion around focal plane can blend subject into the background, painterly bokeh, very artistic. Also very sharp in focal plane.
Cosina
55/1.2 --- totally different oof rendering to Nokton or Takumar, larger, clipped bokeh highlights can give a radial blur effect in some situations and is susceptible to the highlight rings that lead to this. Not particularly sharp wide open, is quite gentle for portrait type shots.
Takumar
50/1.4 --- most intelligible focal plane, not much diffusion, best for strong contrast between subject and background. I use this lens also for obviously polygonal bokeh highlighting (hex in this case, a personal favorite)
clump 2
Zeiss Jena
75/1.5 Biotar --- quite sharp at is focal point but creates a very strong radial swirling in oof areas. Also has an older, vintage look (hard to describe).
Schneider Componon
80/5.6 --- used in a homemade tilt housing, very sharp, no fringing at all - technically a very strong lens.
Zeiss T*
85/1.4 Planar --- Mottled, oily bokeh totally resistant to bokeh highlighting. Bokeh always plays nice and supports the subject, quite predicatable lens and very sharp from wide open (albiet in a razor thin focal plane). Does diffuse around the focal plane to give a softened look to subjects at wide aperture, great portrait lens.
Voigtlander Lanthar
90/3.5 --- Similar to the Takumar in its low diffusion, this (indeed all) Lanthars are critically sharp. Probably the sharpest lens I've used. Good for very literal shooting, less artistic. If your subject wants to be the only thing you notice, this is the transparent master.
Meyer-Gorlitz
100/2.8 Trioplan --- early three element lens design leading to poor corrections for optical problems tackled by all modern surviving lens formulas... which lends it desirability to me. It doesn't look like everything else. Severe susceptibility to bokeh highlighting and with overpowering donut rings (spherical aberration). Lots of diffusion wide open, sharpness takes a hit at 2.8 as a result, but thats not why I put this lens on my camera. Interestingly, things were different back in the 50's. It was more common it seems to apochromatically correct glass and have more complicated (read: expensive) aperture mechanisms. As such, for all its failings this lens does not fringe in the least and has 15 blades in its aperture for those particular about round highlights at all apertures.
I suppose some will understand this, others will not - but this is why I do it.
Kelly.