Stacking for Star Trails

Ever wonder how images like the above were made? There are two main ways: 1)one VERY long exposure, or 2) stacking several long exposures.  In this tutorial post, I'll go through the planing, strategy, and technique of the second option - stacking. 

First up is planning (see my post on that here). After you determine the location, the right night, ascertain whether or not you have do-able weather, and select where to place your camera, then comes the fun part - guessing the exposure length, aperture, and ISO, and subsequently figuring out how many you need to take. Looking at your sensor size + lens combo should give you a good idea of how many frames of a particular length you need to make if your goal is a star circle (facing N to Polaris). I have a  whole separate post on this here. In this post, I want to focus on the post-processing portion of making an image like the above. 

So you have your frames, and (hopefully) your "plate" exposure that you shot for the foreground. Now what?

Step 1: get organized by placing all frames in  your stack in a unique Lightroom Collection. 

Step2: Synchronize ALL develop settings across EACH star trail frame so they are exactly the same.

Step3: Separately develop your "plate" frame to taste. 

Step 4: Select all frames in your stack in Lightroom's filmstrip along the bottom of the screen. Right-click on one of the photos, and choose "Edit In / Open as Layers in Photoshop"

Step 5: In Photoshop, drag the plate layer to the bottom of your layers, if it's not there already. 

Step 6: In the layers panel, select ALL star trails layers, and change the Layer Blending Mode to either "Lighten" or "Screen." Choose one based on which looks better - if shot on a moonless night with dark sky and bring star trails, I find "Lighten" to work better. This should allow all the trails to add up and create your finished blend OF THE SKY ONLY. 

Step 7: Merge all the star trail layers together via the "Merge Layers" command

Step 8: Add a layer mask to the merged star rails layer. Use regular mask-painting techniques with different brushes, hardnesses, opacity, etc. to cover over the foreground of the star trails layer with black paint (remember: white reveals, black conceals). The black paint on the mask will "conceal" or cover up the part of that layer, allowing the part of the next-lowest layer (in this case your foreground plate) to come through to the top in your photograph. Take care to make the blend look realistic. 

If you find gaps in your trails, they will need to be dealt with. Stephen Christensen of the Star Cricle Academy has an excellent post on how to manage this scenario - here

That's it in a nutshell! Any questions, feel free to leave a comment or shoot me an email. 

 

Multiple Exposure Blending

My Landscape Astrophotography class at the 2016 Shooting the West Photography Symposium

My Landscape Astrophotography class at the 2016 Shooting the West Photography Symposium

In this post I'll explain the basics of one of the fundamental techniques in modern digital photography - shooting two different exposures (different length shutters, or different focsu positions) and blending them together. 

There are many scenarios in which a photographer might want to employ this technique. Firstly and perhaps the easiest to grasp is the case of a sunset when you don't have or want to use graduated ND filters. In this case, it's easy to imagine that the clouds above the horizon might be several stops brighter than the landscape in front of your camera, making it impossible to properly expose both the colorful clouds and the ground in the same exposure.

What do you do? Simple - take two different length exposures that each correctly exposes for the sky (exposure 1) and the ground at your feet (exposure 2). You must take EXTRA CARE to not move the camera AT ALL, not even the slightest amount, between exposures.

Another example might be the photo at the top of this post, where one image was exposed for the light painting, while the class was standing there in the scene, and then after they walked away, I made another, longer exposure at a higher ISO to suck out some of the starlight on the landscape. I could have done the same with a very, very long exposure if I had wanted the shy to be star trails. 

How do we bring these together into one image? With Lightroom + Photoshop, its never been easier.  

Step 1: Shoot two (or more) different exposures, one properly exposed for each part of your scene / frame

Step 2: Develop in Lightroom to taste, taking care to synchronize any cropping or lens corrections

Step 3: Bounce both frames into Photoshop as layers, with the "Edit In / Open as Layers In Photoshop" command. This should present you with both frames atop one another in the layers panel in Photoshop. 

Step 4: Add a layer mask to the top layer (HINT: Use the little mask button at bottom of the layers panel)

Step 5: Use the paint Brush (HINT: "B" key activates the brush, square brackets on the keyboard change the size bigger/smaller) to add black paint to the mask to cover up areas of the image with the mask. REMEMBER: "Black conceals, white reveals." You really should say that out loud now at least three times!

Step 6: Carefully, methodically, slowly use different hardness and opacity values, as well as brush sizes, to ad and remove layers of black and white paint to adjust the mask so that it's difficult to tell where the parts of the two different images are fitting together. This is the "meat and potatoes" of this technique, and where skills really comes into play as to it's successful application. Use selection tools like the magnetic lasso, in conjunction with the "Refine Mask" panel to help you save time. 

This image is the result of one frame made for the sky at ISO 6400, 20 seconds, f/1.4. The ground is another image, ISO 320, 15 minutes, f/1.4. To make sure everything was sharp, I  focused on the stars , and made sure the bottom of my frame was far enough away to be fast the " hyperfocal distance " of my lens. (Click the links to learn more)

This image is the result of one frame made for the sky at ISO 6400, 20 seconds, f/1.4. The ground is another image, ISO 320, 15 minutes, f/1.4. To make sure everything was sharp, I focused on the stars, and made sure the bottom of my frame was far enough away to be fast the "hyperfocal distance" of my lens. (Click the links to learn more)

This screenshot from Photoshop shows the two layers, and the mask used to block the bottom of the top layer (the dark sky) to allow the bottom of the bottom layer (the landscape) to come through. 

This screenshot from Photoshop shows the two layers, and the mask used to block the bottom of the top layer (the dark sky) to allow the bottom of the bottom layer (the landscape) to come through. 

That's it! Simple right? Go ahead and try it on your next shoot. Even if you aren't quite ready to dive into this full-on, I recommend to my beginning students to bracket for multiple exposures anyway, since storage is cheap and why not have the frames on your hard drive ready to go for you whenever you do decide to try it. 

Dark Frame Subtraction

What is this "dark frame subtraction" technique that I always hear about? Well in this post I aim to demistify this fantastic way to lower noise in your long-exposure photographs, whether they be landscape astro images or just plain old astrophotographs. 

Digital camera sensors suffer from an inherent amount of noise that bleeds into their images during very long exposures.  This noise is a result of heat in the electronics within the camera.

As the camera makes the long exposure, it is slowly gathering light through the lens, which constitutes the "signal" part of the image being created. Meanwhile, as long as the shutter is open and the sensor is active, the noise from the in-camera electronics is added to the "signal" (just the image data) which gives the overall end result of the photograph being "signal + noise."

If we want to remove just the noise, immediately after we are done with our long exposure, we can create an additional photograph with the lens cap on the camera lens which will provide a frame that is no signal, and 100% noise.

Then, by use of specialized techniques in Photoshop, we can subtract the "noise" from the "signal + noise" which will then give us just the clean "signal" or image without the long exposure noise. Voila: clean images!

If you have "Long Exposure Noise Reduction" (or "High ISO Noise Reduction") on in your camera, then this is exactly what it is doing, all by itself. Chances are you probably noticed your camera taking TWO images for every long (or high ISO) ONE you tell it to take. What is it doing? Dark Frame subtraction! In some cases, this is great and you wouldn't want to go through the trouble of doing it yourself in the way I am about to lay out. 

Personally, I like to retain control over 100% of the noise reduction in my workflow, so I have these options turned OFF in my cameras. I am frequently asked during classes which is the best approach - and the answer is the one I always hate to give - "it depends." Each camera is different, and my advice is to experiment in your backyard with long exposures in ALL possible combinations of Long Exposure / High ISO noise reduction settings (including OFF and doing the dark frame yourself) - and then carefully examine your images on a computer to see which is better. 

So you want to do your own Dark Frames! Great. The first and most important step is remembering to shoot them IMMEDIATELY AFTER you shoot your long exposures. Noise as I said above is a factor of temperature, so if you forget to shoot your Dark Frame right after your Long Exposure and think you can do it later before you got o bed after your camera has been in your bag/car etc., the temperature of your sensor will be slightly different, and the subtraction technique won't work. Here's a Step By Step:

Step 1: Make sure "Long Exposure Noise Reduction" is OFF if you plan to take dark frames. 

Step 2: Shoot your long exposure photo as normal. 

Step 3: As soon as your image is safely written to your card, place the lens cap on your camera and take an IDENTICAL image (same length, same ISO). Take extra care that your eyepiece viewfinder is covered, and that no headlamps or external light sources are shining at the lens of your camera. Even a few errant photons can negate this entire exercise. 

Step 4: Pull out your phone and record the file #s of the image and the dark frame, and make note of which is which. You'll obviously be able to tell later in Lightroom, but it's handy if you take many to know. 

Step 5: Post-Processing. Make sure to synchronize ANY AND ALL develop settings made to the Photo RAW file with your dark frame. 

Step 6: Open both files as layers in Photoshop with "Edit In / Open As Layers In Photoshop" command. 

Step 7: In Photoshop's Layers panel, place the dark frame ABOVE the image. 

Step 8: Change the blending mode of the dark frame to "SUBTRACT." At 100% zoom, it should be obvious now when toggling the "eyeball" of the Dark Frame layer on/off that the noise is being removed from your image. 

Step 9: SAVE!

That's it. Here is a >100% crop of the above image, on which I have used this technique. As you will see, it's not 100% perfect, but it does knock back a lot of the noise. 

One last thought on why one might choose this technique over the in-camera NR: witha  dark frame layer and layer masks, you can selectively apply the noise removal around your image; with in-camera NR you cannot.  

Favorite Photography Apps

I used both apps to plan this photo of the center of the galaxy rising above Squaw Valley

I used both apps to plan this photo of the center of the galaxy rising above Squaw Valley

Pre-visualizing your photographs is the single most important means of progressing your work from "good" to "great" by stepping up to the next level of creative expression. 

Before the advent of smart phones and planning apps, photographers either had to refer to tables published by the US military to calculate the location and position in the sky of celestial bodies such as the sun and moon, or they had to figure things out the old fashioned way - by going to a location many times through the year and paying attention to the sky. 

It's never been easier to utilize the power of smart phones + apps to create plans for your photographic imaginations. Want to shoot the full moon setting between the towers of the Golden Gate Bridge? Easy! How about the Milky Way arched in the sky over Delicate Arch in Arches National Park? No problem! Look no further than to either of my two favorite photography planning apps - Photo Pills and the Photographer's Ephemeris.

Below, I will go through both of them and discuss why they are fantastic tools for planning your photographs. 

Photo Pills:

This app is great for planning out just about any photograph you can possibly think of. For my photography, I mostly make use of the sunrise/sunset time planing features, the moon position and rise/set features, and the excellent Milky Way planner and Night Augmented Reality feature. Those are just the "tip of the iceberg" though in terms of what this app can do. It also includes all sorts of amazing other things such as a depth of field calculator, exposure tables, and even hyperfocal calculation functionality. 

Photographer's Ephemeris:

TPE was the first planning app I purchased and learned, so it's till my go to when I need to plan for the basics of sunrise/sunset and twilight time determinations. Also, it's still free as a desktop / web application - only the phone versions are $. 

In the most recent releases, the programmers have added many of the same functionalities (e.g. night mode) that heretofore were only available through more advanced apps (such as Photo Pills).   TPE still lacks the Augmented Reality mode that PP offers, which is a major drawback. 

Things to Beware Of: Not All Apps are Created Equal (on Android versus on iOS)

I recently taught a workshop where some of my students discovered that the Android version of TPE is lacking many of the same features as the iOS version. So beware and fully check the feature lists before you commit and buy. 

Which One Should I Buy?

Although similar, they both offer different functionality. Overall Photo Pills offers more features, but is more expensive. TPE is slightly easier to learn how to use, and you have the advantage of starting off with the free version on your personal computer. 

Download / Purchase Links:

TPE Desktop/ Web Application / TPE iOS / TPE Android