I don't need google I take shots and I know how a camera works sorry
ETTR is about getting more light which is exactly why you need a fast lens
The EV of the 15mm lens at the shutter without blur is -1.8 Ev the Oly 17m is -2.8 Ev
So you are 1 stop less bright and you need to increase ISO accordingly
Increasing ISO decreases dynamic range and color response so your shots will be clipped and less colorful. This means the stars will be monochromatic at that point how sharp they are matters much less.
Considering the performance of MFT sensors you really want to avoid too high ISO values. I have compared shots from 1000 to 3200 and there is a drop of color perhaps go to the astro section of this forum to see how people actually do it.
You will see also some of my posts related to the colors and how many people come and say you need a lower ISO
I am sorry but knowledge is not based on 1st google result you need to go a bit deeper into topics. It happens always when the equipment you own does not come first to take it personal but don't i don't get any commission from anyone and those are just factual data point
I prefer to think of it more in terms of the clear aperture, hence light gathering, and FOV at a given speed to prevent trailing (assuming you're not using a tracker). Hence this chart has served me well in general:
https://www.lonelyspeck.com/lenses-for-milky-way-photography/
If you click the link therein below the chart you'll get a whole spreadsheet on Google Docs that mentions specific brands of lenses on different sensors. Spoiler alert, probably the best by a bit just for the light gathering mentioned therein is the Oly 17mm 1.8 and Oly 12mm 2.0. But there are lots of others, and that might help in your research.
I haven't done as much real astro with my MFT as I should, but sometimes the lowest ISOs don't work that well; it's getting a good balance. There's a good Milky Way calculator on that site as well. Using that 17mm I think 1600/3200 would work quite well, maybe 15-20 sec.
https://www.lonelyspeck.com/milky-way-exposure-calculator/ So the author there is basically on the same page you are.
I use the 24mm Rokinon btw, and it's great, and popular for a reason, but mostly on another body. Good luck!
The table on that site does a similar calculation but has only obsolete models
I think we need to make a distinction between exposure density and light gathering as here we are talking about Milky Way and not deep space which is what I used (Light Value 12 Ev)
The way I calculated the ISO value is as follows
Light Value of the Milky Way = 11 Ev
Light Value - ISO Value = Exposure Value
ISO Value - Light Value - Exposure Value
ISO Value = 11 - Exposure Value
So for the 12mm 1.4 ISO Value is 11-2.7= 8.4 Ev
Converting this value into Linear ISO gives a value of 984 rounded up to 1000 and a table that looks like this the ranking does not actually change
Table adjusted for Milky Way
Now as we know the ISO value is important as with ISO going up Dynamic Range, color and tones drop.
So ask yourself would you shoot your camera at ISO 4000? Maybe yes and then use denoise however once you are at ISO 4000 all the stars colors have disappeared already and the light is very faint
So it remains as per this table that in terms of exposure the Panasonic 12mm and the Olympus 17mm Pro are the best lenses. Obviously there is also a field of view consideration to be included.
With regards to the lens showing in red those will generate in most conditions monochromatic shots you will see the Milky Way but even if you brush it or stretch it there won't be much color in it.
Finally lenses like Laowa or Samyang that are not autocorrected need
1. Distortion correction
2. Vignetting correction
3. Chromatic aberration correction
All those can be done in a raw converter in my opinion you don't need to shoot flat frames for vignetting. Distortion can be corrected by some programs while CA is something you need to do yourself
I want to be clear if you want to do a single shot with the Milky Way and foreground you are likely to exposure mid way between the 11 Ev and the foreground that will require more light. At that point the stars will clip and the colours will be lost so the Milky Way will look monochromatic. If you look at most shots on the internet they have wrong color balance and all sorts of tricks to give some tones to the image that has lost it
With regards to clean aperture a longer lens will always have bigger aperture for the same f/number however practically you cannot shoot the Milky Way with a 25mm lens so this consideration is not that important but clearly a 17mm lens at f/1/4 gathers more light on the surface than a 12mm 1.4 but at this level we are still talking about exposure density not light hitting the lens. And again if you shoot the Milky Way with a 17mm lens it is not likely to fit completely I find that 12mm is the right focal
If you performed a calculation based on exposure factor you would have
1. Oly 17mm pro
2. Sigma 16mm
3. Panasonic 12mm
The rest would not change much but the laowa would drop way down as the lens has a very small focal and therefore a small physical aperture
In conclusion the Olympus 17mm 1.2 is the best lens on all accounts, the sigma 16mm is a good value alternative. Both lenses may be a bit narrow for a landscape shot or Milky Way in landscape but would work fine in portrait.
The panasonic 12mm has a great field of view for a single shot but on shots with stars only may not resolve faint stars. I have this lens and I can confirm it resolves easily 20000 stars on a Milky Way shot so would not loose my sleep too much
With regards to ETTR etc is only exposure value that determines exposure it is the lens that gathers the light according to the time and if you want to get more light you need a tracker this will mean your EV will increase your ISO will decrease and you will get a better image. If I take a shot with 64 seconds instead of 13 seconds I can drop the ISO to 200 maximising the performance of the sensor
--
instagram
http://instagram.com/interceptor121
My flickr sets
http://www.flickr.com/photos/interceptor121/
Youtube channel
http://www.youtube.com/interceptor121
Underwater Photo and Video Blog
http://interceptor121.com
