Using Off Camera Flash for Astrophotography 

I’ve started using off camera flash more and more in my night sky images, so I thought I would share this technique with all of you. Some folks will simply not care for the results of this technique, and I completely understand that perspective - it's subjective. There are other ways to capture dynamic night sky images involving photoshop, light painting, and other techniques. My goal in photography is to try and capture each image in a single exposure, and this method allows me to do so. Below I’m going to tell you why I prefer this method and provide a quick how-to on setting up your flash for this purpose. If you sign up for a workshop, I usually have an extra flash or two in my bag for experimentation, and I can help show you how to use off camera flash in astrohptography. 

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Repeatable Results 

Compared to other methods, such as light painting with a flashlight, using a flash can yield more consistent results. This is not to say that light painting cannot be mastered and controlled to a precise degree, it’s just that a flash takes away some of the guesswork. Painting light with a flashlight can definitely be fun, and it's a great technique to have in your toolbox.

What I enjoy about using a flash is the ability to move quickly once I'm all setup for my astrophotography session. As soon as I have my camera’s exposure settings and the flash power output settings adjusted to my liking, I can easily move from composition to composition. Every time I press the button on the flash, the power output is the same, so I have a good idea of what my result will be. 

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Cleaner Image for Printing 

This is the chief reason I began regularly implementing off camera flash into my process. I've been printing a lot lately and the problem I'm running into time and time again is print quality issues with night sky images. When you're used to achieving a certain print quality with other images, such as sunset scenes, it makes anything of lesser quality stick out like a sore thumb. 

By lighting the foreground with an external light source, you are less reliant on high ISO settings to capture those details. I can photograph the same scene at ISO 3200 instead of ISO 6400 and have a cleaner file with less digital noise, while still capturing the necessary detail in the Milky Way. 

For extremely dark sky locations, such as Death Valley, this method will prove quite useful, as ISO 6400 paired with some lenses will provide nowhere near a proper exposure and the resulting file will suffer greatly with shadow noise when attempting to brighten the image in post-production. 

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Using Your Flash in Manual Mode

Even if you have never worked with off-camera lighting, that shouldn’t stop you from trying this for yourself. Follow the steps below to get started. 

1. Set your camera’s exposure settings as desired to capture the night sky. I recommend starting at ISO 3200, aperture wide open, and shutter speed set using the 500 rule. 
2. Turn on your flash unit (I'm currently using a Yongnuo model) and make sure it is set to manual and not TTL or other automatic modes. If your flash does not have a “test” button, make sure you have some other means for firing the flash, such as a remote trigger. 
3. Start the exposure on your camera and fire your flash during the exposure. 
4. I recommend holding the flash unit above your head and pointing the flash at a down angle on your foreground, while holding it slightly above and to the side of your camera. Holding your flash lower to the ground can help accentuate textures, such as grass, rock, etc, if desired.
5. After the exposure is complete, check your shot and study the histogram to ensure your image is well exposed and not “blown out.” If your camera has a highlight alert feature, I strongly recommend using this function. This will cause all blown highlights to flash when playing back the image. 
6. If you have blown highlights, adjust the flash power down and repeat steps 3-5. 
7. If your image foreground still seems too dark, adjust the flash power to a higher setting and repeat steps 3-5. Repeat this process until your image is bright to taste, and you also have no blown out highlights on the foreground.
8. Once you have set the flash power to the appropriate setting you can now move about capturing scenes in one attempt. 
9. Please note that changing the distance of your flash to the foreground will affect its brightness, so try to remain consistent with your flash placement in relation to the foreground/subject.

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Bonus Tip: Because the shutter speed setting does not have any affect on flash power, you can perform the above procedure while using a shorter shutter speed. I usually set my shutter to 2 seconds, while leaving my ISO and aperture at their normal night sky settings. This way, I can calibrate the flash settings as desired, but take up much less time in doing so. Remember typical night sky shutter speeds will be upwards of 30 seconds, so this tip could save you some valuable time. 

What are you waiting for?

If you have a flash, any flash, head out during the next new moon and give this a try. Maybe you will like the results and maybe you won’t, but having this method in your toolbox might just save your bacon. Mmm, bacon…

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