Many of us are challenged determining what a good exposure is for fireworks. The real challenge is to understand what an exposure is for the camera. We have options. Do we want colors? Do we want broad flowing star streams? Do we want or need to freeze the star streams? If we have a tripod there are no time restraints. If we are shooting hand held we need to use a shutter speed of 1/30 or faster. If we want to create artsy pictures then hand held with unlimited time is an option.
What are the Options?
A camera's pixels responds to a fixed amount of light. Too much light will drive a pixel to saturation losing color value. For an 8-bit jpeg image the saturation value is 255. If all three colors are saturated the color appears white. The color of a saturated pixel can not be recovered. The information is lost.
Too little light will keep all the pixels near 0 creating a dark, black image. We need to find the sweet spot.
Exposure Value (EV) will illustrate the concept. A full discussion of EV is in the Wikipedia EV link.
This discussion is geared for fireworks but the concepts are applicable to all photography. For Fireworks scenes the optimum value is between 3.5 and 5.5. For other scenes the optimum EV value will be higher or lower.
Cameras use three variables to arrive the exposure: the ISO equivalent, the F-Stop lens opening, and the camera shutter speed. Each has it's advantages and disadvantages.
- An increase in the ISO increasing the sensor sensitivity will increase the sensor noise. But at the same time it will reduce the shutter speed and / or reduce the aperture.
- Enlarging the lens aperture. will reduce the depth of field. But by making more light available to the sensor the ISO can be reduced and / or the shutter speed can be increased.
- Reducing the shutter speed will increasingly blur moving objects. But the longer shutter time will make more light available to the sensor reducing the ISO and / or the aperture.
Clearly there is a trade off between these three variables.
- Is freezing motion important? Reduce the shutter speed and adjust the ISO and aperture.
- Is a large depth of field important? Reduce the aperture. (raise the F / stop) and adjust the ISO and shutter speed.
- Is high sensor noise important? Never! Always keep the ISO as low as possible to support the two options above.
The ISO speeds determines the pixel's sensitivity to light. A definition of 0 EV is an exposure of 1 second at ISO 100 with a F / 1.0. The table below illustrates there are many ways to obtain a 0 EV exposure.
Scan the mouse across the chart below to see the effect.
ISO f / stop Speed
100 1.0 1 sec
Scrolling across the table shows similar tables created by holding the F / Stop or the ISO constant and varying the Shutter Speed.
The table below may be even more instructive. After reviewing all my fireworks pictures I found that the pictures with the best color were between 3.5 to 5.5 EV. A spreadsheet was created on Google Docs. The seven tables below were created by changing just the ISO.
Scan the mouse across the chart below to see the effect of ISO on exposure.
The table shows that at ISO 100 and a F / 5.6 lens would require about a 1 second exposure. But by using a F / 1.4 lens would require an exposure of only 1/16 seconds. The 4 stop improvement in the lens creates a 4 stop improvement in shutter speed.
The motivation to explore faster lenses was the desire to photograph fireworks from a boat. A tripod was not an option. the shutter speed must be 1/30 second or faster. The table shows that this is readily possible with today's camera ISO capabilities and a fast lens. B&H shows nine f / 1.8 or faster lenses for less than $400. A fast, low cost Sigma lens is the choice I used. The old Canon 20D and 40D had poor noise performance. It worked.
My library has firework pictures all over the map. With the old 20D and 40D cameras, high ISO's were not much of an option. A fast lens was the only option. I got some good pictures. But the keepers were few. The lack of live view was a major impediment.
Modern cameras add higher ISOs to our options. Check your sensor capability at DXO Mark. You may be pleasantly surprised.
August 2014 was the first time I tried the Ron ISO 100, F / 11, Bulb mode scheme. I was not happy with the pictures. While the pictures were sharp, too many of the sparks were blown out. The brilliant colors I saw and that were shown in the reflections were not there. I used a 95 mm focal length cutting off some of the higher fireworks.
At the same time, the reflections were about 3 stops too dim. Next year I'll try a 3 stop hard grad filter and open the aperture. to F / 5.6. That will open up the reflections 2 stops and dim the fireworks by 1 stop to an F / 16 equivalent. Alternatively I could keep the aperture. at F / 11 and raise the ISO 2 stops to ISO 400. I'll use a flocal length closer to 75 mm.
There are going to be some tough decisions next year.