The Core Revisited

It was above one year ago that I did my first tracked close-up imaging of the milky way core. Using drift alignment I managed to achieve a pretty solid polar alignment using apps on my mobile phone. I’m still really happy with the image I was able to produce using my small Micro Four Third sensor Panasonic GX85 and Olympus 75mm f/1.8 lens, however having just treated myself to the new Sigma 85mm f/1.4 DG DN Art Lens for Sony-E Mount I have certainly had my perception of what is possible shifted.

Sigma has long had an 85mm f/1.4 lens available on most full frame mounts, including Sony E-Mount, for a long while now. Even though it is optically excellent, at 1130g it just didn’t fit with my ethos of keeping my kit as compact and lightweight as possible. Then Sigma did something I did not expect, they updated the lens for Sony E-Mount and Lecia L-Mount. These mounts being built from the ground up as mirrorless systems have much shorter flange distances than their SLR counterparts. The impact of this shorter flange distance on lens design resulting in the lens itself being far smaller and much, much lighter at just 625g. Initial reviews also reported the optics being just as, or perhaps even a touch sharper than the original SLR design.

I’d been on the fence on which fast 85mm lens to get for which there is plenty of choice. Should I risk optical imperfection to get the super fast f/1.2 options of the Mitakon Zhongyi or Canon EF adapted Samyang XP, then there is Sony’s own GM f/1.4 as well as various generation Samyang lenses at this f/1.4 aperture as well. It’s not often a 3rd party lens manufacturer beats an OEM at their own game, especially when we are talking the Sony G-Master line. However I believe Sigma have done it solidly in this case as not only is this new Sigma 200g lighter than the G-Master, it has an RRP two thirds that of the GM with coma and astigmatism performance that also far outstrips the GM. Enter an opportunity to purchase at 15% off RRP and I finally made my move.

10×60 second stacked exposures on a Panasonic GX85 at ISO200 using an Olympus 75mm f/1.8 lens @ f/1.8

Above is the tracked, stacked and processed final image I was able to get with my Panasonic GX85 M43 camera. Below is a single tracked and processed image I was able to get with my Sony a7R.

Single 15 second exposure on a Sony a7R at ISO640 using a Sigma 85mm f/1.4 lens @ f/2.2

Although a wider field of view on the Sony, their composition is very similar as marked by the inclusion of the Lagoon and Trifid nebulae which appear pinkish in colour. Without a doubt, the improvement in colour fidelity, fine detail and contrast is immense here. There is just so much more data to work with. Should it be though?

If we step back and look at the basic premise that it is the clear aperture of a lens that determines a camera systems sensitivity to light. To determine light gathering capability of these two lenses we must keep in mind that the ratio of light passing through two apertures is proportional to the squares of the diameters. In this case the Olympus was shot wide open (75/1.8)² = 1736.11 versus the Sigma stopped down to f/2.2 (85/2.2)² = 1,492.76. So in this case the Olympus actually should be gathering 1.16 times more light (1736.11/1,492.76) than the Sigma given the same exposure time. That’s not all either as the total exposure via the Olympus was very significantly more than the Sigma as it is a stack of 10 x 60 second exposures Vs the single 15 second exposure of the Sigma.

The biggest factor I think that explains this difference is the images shot with my GX85 with the Olympus lens where taken from my deck in a Bortle 5 urban area where as the shot with my a7R using the Sigma was taken in a rural Bortle 2 area. Fine detail and contrast would have especially suffered here. Really very interesting as it makes me wonder what my Panasonic GX85 with the Olympus 75mm (which is 15mm Full Frame equivalent btw) would be able to achieve with good polar alignment in a similarly dark location.

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