Brian M writes:
1) I want to consolidate my lenses, and
2) I have started doing IR photography.
I have the Zeiss ZE 25mm f/2 Distagon and the Zeiss ZE 18mm f/3.5 Distagon, and I was considering selling both to buy the 21mm 2.8 to replace them (thinking a "happy median")... Either in the classic or the Milvus version.
I take photos of landscapes, cityscapes, and occasionally Milky Way out in my area. From your experience, would you have any recommendations?
DIGLLOYD: the new Milvus designs are all optically excellent. The Milvus 21/2.8 is not a new design but it has improved lens coatings.
Illumination over the frame (vignetting) is a major consideration for astrophotography.
Zeiss Milvus 25mm f/1.4 illumination at corners vs center should be about 50% (excellent for a 25mm at f/2.8).
Zeiss Milvus 21mm f/2.8 illumination at corners vs center is only 22%—more than a stop darker. Zeiss Milvus 18mm f/2.8 illumination at corners vs center is also 22%.
Losing a stop can mean a lot more noise in a night sky., more than one might think because exposure needs to rise out of the baseline noise. A good example of how f/2.8 varies with an f/1.4, f/2, f/2.8 lens is found in Shaded Boulder Field, North-East Escarpment.
But it’s actually about a lot more than the illumination/vignetting considerations: the actual light transmission is not given by f-stop. It is given by T-stop and is generally 1/3 to 1/2 stop greater at f/2.8 for an f/1.4 lens than an f/2.8 lens. No big deal for regular photography, but for astrophotography it is a very big deal.
All of these Milvus lenses have enough mid-zone field curvature that care should be taken with the balance of focus across the frame—center vs mid zones and edges. And all of them offer superior performance to the older Zeiss 18mm f/3.5 Distagon.
Of course, 25mm is not 18mm, and the choice might be drive by field of view first.
The Zeiss Milvus 25/1.4 should be right around 50% illuminance at f/2.8.
The Zeiss Milvus 21/2.8 is around 20% illuminance at the corners at f/2.8.
The Zeiss Milvus 18/2.8 is just under 20% illuminance at the corners at f/2.8.