Night Sky Shooting / Astrophotography on X-T2 : ETTR? ISO 1600?

tradesmith45 wrote:

shiftyonthemic wrote:

Hi All - I had my first attempt at doing some dark sky territory shooting of the Milky Way with the X-T2 and everything came back underexposed from Death Valley National Park.

Q1) Should I have cranked up the EC to ETTR to get more information? I've read conflicting information.

Q2) Does the forum agree that ISO invariance starts around ISO1600 and up? or is it ISO 800 and up?

Q3) Foolishly I didn't use the histogram and just went by what I saw on the screen. What are some settings to use it properly?

-Turn Pic Preview Effect OFF

-DR200

-Highlight -2, Shadow -2

-Daylight White Balance

Anything else?

I'm going to Acadia National Park to try one more time this weekend and want to make sure I come back with better night sky (landscape) photos.

I have a few examples I can share from Death Valley, but here's just one of them:

Have read through this thread & some basic info is missing.

Let's deal w/ faint parts of astro images first. Go over to Photons to Photos Shadow Improvement chart and you'll see the XT2 yields its max shadow improvement @ ISO 1600. (The XPro2 is better at ISO 800 BTW.) So the faint detail in an astro image will not brighten much further as you increase ISO. All you get is more bloated colorless stars due to reduced dynamic range.

Now the bright parts. A star field is a very high dynamic range subject. You can't see it on the cam histogram but even modest exposures like 20 sec, f2.8, ISO 1600 are way past ETTR for the stars. They are just to small to show on the histogram & are past the right edge. You'll ALWAYS need to do heavy highlight recovery in these images if you want to retain star color. LRCC Whites slider does a limited job of highlight recovery. You'll still have bloated stars. Rawtherapee seems to do better. Using 3200 or 6400 ISO looses 1 or 2 EV dynamic range without gaining detail in faint parts of the image. (BTW the histogram in LR is really nice because you can see the star pixels crawling in from the right as you turn the Whites slider down. W/ RT, you have to use highlight warnings.)

Both these problems are worsened by aberrations in nearly all large aperture lenses. Aberrations put photons where the don't belong. For example the XF23 f1.4 has over 20% astigmatism wide open so stars will turn into arrow heads even w/ the correct camera settings & processing. Bloated, colorless stars are a common problem & distracting in most astro images you'll see here & other places.

UWA f2.8 lenses do not collect many photons so IMHO you always need to stack when using one. Signal to noise increases w/ the square of the change of the number of images. Stacking 9 is twice as good as stacking 3. You'll need 81 image to double again.

Conclusions for the XT2: always stack w/ wide f2.8 lenses, use no more than ISO 1600, use strong highlight recovery, use the exposure slider or better increased contrast w/ curves last to get the image brightness you need. W/ the XPro2 use ISO 800.

Nice writeup there Tradesmith. One minor point I would say is that using a lower ISO may preserve some features but it can also shift colour. ISO1600 may be safe but if you go too low and then boost it can shift colours.

Also the XT2 and XT3 would be using Sony sensors with the Aptina DR PIX technology which gives a point in the ISO range where the read noise drops significantly. With Sony A7r2 and 3 there are 2 points where this happens. As I recall with XT2 its around ISO800.

So if you want to follow the above approach perhaps try using ISO800 and boosting. You will need to take some test shots at ISO6400 just to check framing and focus before you commit to taking a series of exposures (always take a test image and zoom in to check if the stars are in focus before taking a series - I am talking from annoying experience here!).

Slightly longer and faster lenses that have low aberrations in the corners like Sigma Art 35 1.4 can go deeper as well but need shorter exposures as the stars will elongate in the image more quickly. Its about aperture - focal length divided by F stop. You want the biggest number you can get. 85mm 1.8 is great but it shoots a relatively narrow portion of the sky so either frame artistically or do a mosaic and you will really need a portable tracker to get long enough round star images.

Greg.