The length of a light trail, created by a moving light source, is dependent on the time of the exposure.
The width or thickness of the light trail is determined by the size of the aperture. The wider the aperture opening, the wider the streak of light recorded by the camera.
We will address the effects of aperture size on stationary light sources in the next post.
HOW TO : Aperture Optimization
In my earlier post exposure guidelines, I recommended shooting with an aperture setting of f5.6 at ISO 400. This was based on a relationship between your aperture and ISO sensitivity settings for recording desirable light trails.
For optimal moving light source treatment in an image, set your aperture according to the following equation and examples:
f-stop = SquareRoot ( 0.08 * ISO )
f4.0 ⇐ ISO 200
f5.6 ⇐ ISO 400
f8.0 ⇐ ISO 800
This is a rule, and you should not break it … just kidding.
It is safe to vary this by + or – one f-stop, but any aperture setting outside of this range may be less pleasing, and may render light trails that are overly thick or underly thin.
I encourage you to treat this equation as a starting point for your own experimentation, in search of your own visual aesthetics.
IMAGE : Rail Bridge, Engine Lights & River, Carlsbad, NM
This railroad bridge crosses the Pecos River. Every morning a cancellation of train engines travels north across the bridge to the railyard, where freight cars are on hold.
Yes, “a cancellation of trains” is the correct collective noun.
Five mornings I was up before dawn to capture the engine lights streaming through the frame of the bridge toward me.
I opened the shutter just as the front engine began illuminating the trusses of the bridge. My biggest challenge was to close the shutter at the right time so the streaking lights complemented the shape of the bridge.
This was my favorite of five exposures, all shot with the recommended f-stop for my film speed setting, ISO 800. The length and width of the light trails are aesthetically proportional to the bridge, matching what I envisioned in my mind’s eye.
This was a 45 to 50 second exposure taken at f8 with TMAX 3200 film shot at ISO 800. The engine lights were crossing the bridge in the image for only 5 to 10 seconds.
IMAGE TINT : GallerySteelGray
The cool steely gray toning was chosen to evoke the look of the bridge in the cold damp air. The same toning was selected for the image in my previous post.
This B&W image was toned in Adobe Photoshop with an ICC Profile I generated from my Mac App SuiteProfiler. The Profile was derived from the “GallerySteelGray” Color Map created in SuiteProfiler.
Click these buttons to download the ICC Profile and SuiteProfiler Color Map:
EXERCISE : Aperture Settings
Seek out one or more moving light sources to photography. Take a series of bracketed exposures using the recommended ISO and aperture combination.
Perform more bracketing sessions with different apertures without changing your ISO.
Compare your results to see the difference in light trail widths. Decide what aperture to ISO ratio looks the best to you. Start using that to capture your expanded moments.
Some examples of moving light sources are headlights, flashlights and stars.
Be sure to review the Safety & Precautions Page.
NEXT TIME : “Aperture Starring”
Don’t miss my future posts!