MHWPC - Blog: Tuesday's Digital Training on Night Landscape photography

Karen Hamilton shared a lot of great info in the digital training session last night. Hopefully later this year we can have another club outing for night photography like we did pre-pandemic. Then it would be nice to have a follow-up digital training meeting for processing. We could also have a digital training night photo processing session before that too if people are interested.

I have web articles that cover some of the issues/questions that were raised during the session.

ISO: ISO does not change sensitivity of a camera and does not change how much light a camera collects. ISO is a post-sensor amplification. This article, What is ISO on a digital camera?
https://clarkvision.com/articles/iso/
explains what ISO is and why you may want to raise ISO. Raising ISO does not increase noise, it actually decreases it. In regular photography, when we raise ISO, we usually close the shutter and/or shorten the exposure time. It is the change in exposure and aperture that reduces light to the sensor and the lower light level is the reason for increased apparent noise. In night sky photography with a fixed tripod, exposure time is limited by rotation of the Earth to keep stars from trailing. So one can keep aperture and exposure time fixed and raise ISO the get better noise performance. But raising ISO reduces dynamic range, so there is a balance that needs to be found: high enough ISO to raise faint signal above camera electronics noise but not too much to reduce dynamic range and lose star color. I generally use ISO 800 on newer cameras when using fast lenses (f/1.4), ISO 1600 most of the time, and only occasionally ISO 3200. I never use above ISO 3200 unless I made a mistake in setting it.

Color. Blue night sky is a Hollywood invention from the beginning of color movies where they would film "night" scenes during the day with neutral density filters and/or underexposing. Blue Milky Way photos became popular circa 2008 when the new digital age kicked off and some professional photographers used white balance to "reduce" red-orange light pollution. White balance is a multiply, but light pollution is added light that needs to be subtracted. In using white balance, it just changed the color of the light pollution along with everything in the scene, shifting red to blue. It is like putting a thin veil of red cloth between your camera and the scene (portrait, or a landscape illuminated by the sun) then trying to color correct the image by changing the color of the red cloth to grey with low Kelvin white balance. The person will come out blue, and so will everything else. I have a series of articles on colors in the the night sky starting here:
https://clarkvision.com/articles/blue-lions-on-the-serengeti-and-natural-colors-of-the-night-sky/

In the club competition, blue Milky Way images would go into the creative category.
For natural color, use daylight (sunny) white balance and subtract light pollution as needed.

For beginning night sky photography, see: https://clarkvision.com/articles/beginning-astrophotography/
This article includes settings, focusing, exposure times to keep stars round, and more.

Processing night photos is similar to daytime photos with some additional steps in some conditions. The main one is learning to subtract light pollution. Here is my Basic Work Flow:
https://clarkvision.com/articles/astrophotography.image.processing.basics/

Stacking was mentioned. Like many terms we have today that originated with darkroom techniques (like dodge and burn), stacking is another such term. Stacking refers to physically stacking negatives, aligning them and then putting the stack in an enlarger to make a print. Yes, I have done it. It is tedious and difficult. Stacking averages film grain. With digital photography stacking is aligning a group of images and averaging them (or compute the median, but average is better). There are free tools to do that and it can even be done in photoshop. I think this is difficult for the beginner, and some of the alignment programs don't work well with a fixed landscape in the scene.

The main problem in night sky photography is collecting enough light, Contrary to popular belief, light collection for objects in the scene is controlled by lens aperture area (not f-ratio) and exposure time. This is true for all of photography, whether the Milky Way, a bird in a tree, or a landscape image of a mountain. Wide angle lenses have tiny apertures. For example a 12 mm f/2 lens has a 6 mm diameter aperture. A 24 mm f/2 lens has a 12 mm diameter aperture and collects 4 times the light from every object in the scene. This article gets technical, but pictures illustrate the problem:
Exposure Time, f/ratio, Aperture Area, Sensor Size, Quantum Efficiency: What Controls Light Collection?
https://clarkvision.com/articles/exposure-f-ratio-aperture-and-light-collection/

There are multiple strategies for night landscape (nightscape) photography. 1a) Popular is recommend a wide angle lens. The disadvantage is the small aperture collects little light, even with a fast lens. Then one needs to do multiple exposure and stack them if you want to reduce noise. 1b) wide angle lens with a tracker to increase exposure times. 2a) Longer focal length lens and do a mosaic (stitching) if you want to cover more area. One can get great lenses in the 35mm f/1.4 range (25 mm aperture diameter) which collects a lot of light. 2b) The longer focal length lens from 2A plus a tracker. A 35 mm f/1.4 lens collects 17 times more light from even object in a scene than a 12 mm f/2 in the same exposure time. A mosaic with a 35 mm lens to cover the same area results in a higher resolution image with less noise. This is important if you want to make big prints, but doesn't matter much if you just want web sizes images.

Here is a recent mosaic with a 35 mm f/1.4 lens, 22164 x 13724 pixels (304 megapixels, 73.9 x 45.7 inch print at 300 ppi), I did last May in Bryce Canyon National park. Natural color, natural light (except in the lower right some night photographers were using some low level lighting that I had to tone down).
https://clarkvision.com/galleries/gallery.nightscapes/web/bryce.canyon-n.p-rclark-.c05.07.2021-4C3A0180-264-h-c1-1500.html

I use 35 mm to 200 mm lenses for nightscapes and only use shorter focal lengths for unusual situations (e.g. fast moving aurora). I make mosaics with PTGui.

One other thing mentioned last night was dark frames. Most modern cameras from the last 8 years or so do not need dark frames, especially for the short exposure times in night landscape images (as opposed to deep sky photos of galaxies and nebulae which typically run tens of minutes to hours exposure time). Subtracting dark frames needs to be done on linear data from the sensor so can not be used with photoshop--you need special software and again not needed with newer cameras of the last 8 or so years. The technical reason for this is a new hardware design of the pixels that block the thermal dark current.
My technical article on this subject is:
On-Sensor Dark Current Suppression Technology, Dark Frames Are No Longer Necessary
https://clarkvision.com/articles/dark-current-suppression-technology/

Ignoring all the technical details, night sky photography can be rewarding and if you want to push into new forms of photography, this is certainly one. You can start simple with camera and tripod and existing lenses.

Roger

I thought last night's webinar was a great overview of astrophotography. Roger, thanks for explaining some of the esoteric nuances of astrophotography and even explaining the airglow/nightglow phenomenon. That was news to me. To dark skies! -Max

Thanks Roger.
I edited one of my photos based on what Roger said last night and posted it to the club's Facebook page. I'd appreciate any comments, especially from Roger.

For any looking for a tracker, I have two. One is the iOptron with external battery pack and wedge for $200.00. This handles 7.5 pounds of gear.

I also have the TT320-AT Astrotrac with the adjustable Astrotrac wedge, an extra polar scope, battery pack and home-made wedge. This one will handle over 30 pounds of gear. $850.00.

303-887-5666

Sorry! Sometimes things don't go as planned. My bad!

Scenic