Im­age Pro­cess­ing

Com­bin­ing land­scapes with dark skies.

Sky at Night Magazine - - CONTENTS - CHRIS MUR­PHY is an award-win­ning, New Zealand-based astropho­tog­ra­pher

How to put to­gether a stun­ning night scene by blend­ing two sep­a­rately shot im­ages

S ky­fall is an im­age I had in mind be­fore shoot­ing it. Hav­ing shot a sim­i­lar im­age on the op­po­site side of the God­ley Val­ley in New Zealand, I knew that this moun­tain wa­ter­fall would make a strik­ing fore­ground for the Ga­lac­tic core. The pre­vi­ous im­age was fin­ished in black and white and I al­ways had a mono­chrome im­age in mind when shoot­ing this one.

So how would you go about pro­duc­ing an im­age like this? Be­cause it was be­ing shot in a dark-sky re­serve only starlight il­lu­mi­nates the land­scape. This means that in the darker land­scape ar­eas our sig­nal-to-noise (SNR) ra­tio (pho­tons hit­ting the sen­sor ver­sus var­i­ous elec­tronic noise) is low and will pro­duce a noisy, sin­gle im­age. Stack­ing im­ages is a way to solve this prob­lem. This means you need to com­mit to blend­ing a fi­nal im­age of the sky with one of the land­scape to pre­vent the Earth’s ro­ta­tion caus­ing star trails.

Land­scape first

This im­age was com­posed of two land­scape-ori­en­tated frames from an 85mm lens, cropped to por­trait ori­en­ta­tion. The land­scape re­quires a much longer ex­po­sure to gather enough sig­nal to ex­tract de­tail. So 13 two-minute ex­po­sure im­ages at f/3.2 were stacked to let in a rea­son­able amount of light and re­tain sharp­ness. The ISO was set at 3200. If you’re shoot­ing with a lens that you’re con­fi­dent can be opened wider while re­tain­ing sharp­ness, it’s al­ways good to let more light in and raise the SNR. Gen­er­ally, a longer ex­po­sure at lower ISOs is likely to be bet­ter than the equiv­a­lent to­tal ex­po­sure at high ISOs.

Shoot­ing the land­scape gave time for

the core to move into po­si­tion. The sky was shot us­ing a tracker for longer ex­po­sures with­out trail­ing and a smaller aper­ture for sharper stars. Ex­po­sure-wise, you don’t need to use as wide an aper­ture or as much time to get a clean im­age of the sky, so this was five im­ages stacked at f/4, 60 sec­onds at ISO 6400.

Pro­cess­ing was in Light­room and Pho­to­shop. Sep­a­rate im­ages were stacked us­ing the me­dian fil­ter. In Light­room this is done by se­lect­ing im­ages to stack, right-click­ing and se­lect­ing ‘Edit’ in ‘Open as lay­ers in Pho­to­shop’. You’ll want to check that your lay­ers are aligned.

For the land­scape shots, if you’re con­fi­dent they’re aligned you can be­gin stack­ing. For tracked shots it’s best to make sure. This can be done au­to­mat­i­cally by se­lect­ing all lay­ers and ‘Edit > Auto align lay­ers’. If you have a lot of land­scape in the frame you may need to mask it out and some lenses may need man­ual align­ment. Once aligned, select all lay­ers

and right-click ‘> Con­vert to smart ob­ject’. This com­bines all lay­ers be­fore stack­ing. Select ‘Layer > Smart Ob­jects > Stack Mode > Me­dian’. This will fil­ter out noise and leave a much cleaner im­age.

Com­bin­ing the im­ages

When stack­ing is com­plete the two im­ages need to be blended via mask­ing. This can be te­dious but if you want the high­est qual­ity for a cer­tain im­age, there is no bet­ter way. For this im­age, the tops of the moun­tains were easy to blend. It was done by past­ing the land­scape im­age on top of the tracked sky im­age and adding a layer mask. The layer mask icon is at the bot­tom of the ‘Lay­ers’ panel in Pho­to­shop or un­der the ‘Lay­ers’ menu.

Se­lect­ing the mask, choose a black brush and erase the blurry sky to re­veal the tracked sky. You can use broad strokes for the most part but you need to zoom in and care­fully trace around the edges where the land­scape meets the sky. For moun­tains or

straight struc­tures this is quite easy; for trees, not so much. Once fin­ished, you can flat­ten the im­age and save it in a for­mat that re­tains all the in­for­ma­tion.

For the fi­nal look of this im­age, I used Light­room. Af­ter us­ing the sat­u­ra­tion slider to con­vert to mono­chrome, I made con­trast and clar­ity ad­just­ments.

Fi­nally, with de­sat­u­ra­tion leav­ing a warmer tone, ad­just­ments were made to cool it us­ing Split ton­ing, adding cooler blue in both the shad­ows and high­lights.

I think this im­age ben­e­fit­ted from some nat­u­ral el­e­ments which en­hanced the over­all look, such as some light fog above the moun­tains which ap­pears to en­large the brighter stars and make them stand out. For me, this gives the ef­fect of the dusty ga­lac­tic core re­gion be­ing like snow from an ap­proach­ing bliz­zard, or churn­ing wa­ter from a larger wa­ter­fall above.

The ma­jes­tic moun­tains of New Zealand’s Ao­raki Mackenzie Dark Sky Re­serve on South Is­land pro­vide an im­pres­sive fore­ground for the Ga­lac­tic core

The ini­tial stacked land­scape im­age. You can see the star trails, which is one of the rea­sons why the sky has to be shot sep­a­rately This is the ini­tial shot of the sky. As well as us­ing a tracker to elim­i­nate star trails, a smaller aper­ture also makes the stars sharper

In Pho­to­shop, the land­scape im­age is pasted over the sky im­age, then the land­scape’s blurry sky is re­moved us­ing a layer mask

For its fi­nal tweaks (in­clud­ing go­ing mono­chrome), the im­age was taken into Light­room for colour, sat­u­ra­tion and clar­ity ad­just­ments

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