I notice that these lenses have hugely different element counts and very different amounts of special glas. Nevertheless, higher element count and more lavish use of special glas does not necessarily lead to better performance, or does it?
What are the reasons?
Sigma 90/2.8: 11 elements, 10 groups. 5 SLD elements, one aspherical element
Voigtländer 90/2.8: 7 elements. 5 APD elements.
Loxia 90/2.4: 7 elements. 3 APD elements.
What is it, that higher lens count does not appear to lead to better resolution, or does it?
And higher APD/SLD count: Does it lead to better correction of LOCA and lateral chromatic correction?
In which way does lens perfection and perfect spacing (Zeiss quality) compensate for higher lens counts that other lenses may need to achieve similar performance?
Is close up performance a differentiator between these lenses and a reason higher lens counts are needed?
There are just so many things I do not understand here.
Is that an apples to apples comparison? If the design goals were the same, then maybe, but Sigma's need for the more complex lens may have been determined by the relatively tiny size of the lens.
Knut. wrote:
What is it, that higher lens count does not appear to lead to better resolution, or does it?
And higher APD/SLD count: Does it lead to better correction of LOCA and lateral chromatic correction?
I am not a lens designer, but I have researched this recently due to curiocity. Think of the optical elements or exotic glass as inputs into an optical formula. More inputs = more possibilities. And a lens designer deals with a huge number of possible trade-offs across several conflicting dimensions:
- resolution
- focusing method
- minimal focus distance
- optimal focus distance
- field curvature
- manufacturing tolerances
- several types of aberrations
- target sensor stack
- distortions
... and probably many more I'm not even remembering. Basically, each formula is a set of trade-offs. For example, the Voigtlander is a unit-focus lens which is a choice not available for AF lenses.
freaklikeme wrote:
Is that an apples to apples comparison? If the design goals were the same, then maybe, but Sigma's need for the more complex lens may have been determined by the relatively tiny size of the lens.
The Voigt VM 90/2.8 Apo Skopar is the same lens as the Voigt 90/2.8 Apo Skopar SL II s with Nikon AI mount. The SL II s version is a very tiny lens for DSLR: 66x41 mm, 260 g. The VM version is a couple of cm longer in that it has to be "adapted" to MILC/rangefinder cameras (the barrel has built-in adapter ). The (full-metal) VM version is even smaller and lighter than the Sigma 90/2.8.
I guess the more complex optical scheme of the Sigma [11 elements in 10 groups (4x SLD, 1x hybrid)] might arise from AF group, which adds complexity to the design.
I remember an interview to a Cosina manager, or lens designer, who stated that focusing entirely on MF lenses grants them immense structural and optical flexibility; by eliminating AF constraints, they bypass the compromises that limit modern mainstream lens design.
Sigma is 250g and can focus to 0.5m. Voigtländer VM is also 250g and can focus to 0.9m. Loxia is 594g and can focus to 0.8m.
I used to have Sigma and Voigtländer (VM) but in the end I kept Sigma since it focuses so close and since I appreciate AF more in tele lenses (even if I generally prefer MF). I like to use these type of lenses for close range shooting as much or even more than for long distance shots. Sigma is also native to E-mount while I used VM adapted.
I didn't notice big enough differences in IQ that they would have impacted my choice in the end.
I never got the Loxia as it was quite heavy and quite long (the hood it comes with makes it even much longer)... I once tried it out a bit at CP+ though.
That is interesting! So I guess some of elements in the Sigma are needed for focussing.
I also note, that the Sigma is the only lens with an aspheric element. Usually these are needed to get the corners sharp in wide angle lenses. They are rare in normal lenses and and only occasionally utilized in short tele lenses.
). The (full-metal) VM version is even smaller and lighter than the Sigma 90/2.8.