Today’s Question: How do you determine the right f-stop and shutter speed for a photo captured during blue hour?
Tim’s Quick Answer: As a good starting point for exposure during “blue hour” you can use a lens aperture of f/8 with an 8-second shutter speed at ISO 100. You can then fine-tune or refine from there based on the conditions and your artistic preference for the photo.
More Detail: “Blue hour” is the time shortly after sunset (or before sunrise) when the color of the sunset starts to fade (or before the color of sunrise appears) and a clear sky will take on a deep and rich blue color. This will gradually fade to a very dark sky that appears nearly black of night. Despite the name, blue hour doesn’t generally last for a full hour, depending on how you choose to define the span of time with a nice blue color in the sky.
The optimal exposure will vary throughout the duration of blue hour, as the amount of light in the sky (and on the foreground scene) will change. This is especially true if you are photographing a scene such as a city at night, where the artificial lights of the city will add light that you may need to compensate for with your exposure.
However, a good starting point is to use the “8 and 8” rule of thumb, using a lens aperture of f/8 and a shutter speed of 8 seconds, with an ISO setting of 100.
Of course, you can then fine-tune the overall exposure settings based on the actual results you’re achieving, as well as your preferences for the image. For example, one of my favorite blue hour shots is of the Duomo in Florence, Italy, photographed from Piazzale Michelangelo. Instead of f/8, I stopped down two stops to f/16 to get a starburst effect for the lights of Florence. I kept the shutter speed at 8 seconds (using a sturdy tripod, of course), and increased the ISO setting by two stops from 100 to 400, to compensate for the change in lens aperture.
So, with exposure settings equivalent to the “8 and 8” rule of thumb for blue hour photography, I got a good exposure and a blue hour photo I’m happy with, which you can see on my Instagram feed here: