Mixing strobes with existing light seems to be one of those things that confuses many photographers. This is most likely due to the fact that two sets of photographic rules are at play when blending flash and ambient light. The first set is what natural light shooters tend to live and die by, the “BIG 3”. If you have read my earlier blog post you know that I tend to look at this set of rules as being very malleable. The second set of rules come out of studio lighting, where light is controlled by light, or by aperture if you are working with a set of strobes that are not adjustable (but even in this case you can control flash power by distance to the subject or the use of neutral density gels and not need to use on camera settings).
So where does the confusion come in? When a photographer is shooting using all natural light where they have very little or no control over the light, such as landscapes, their only recourse is to adjust camera settings. If the scene is already properly exposed and the photographer wishes to adjust shutter, they then have to adjust aperture or ISO to get an equivalent exposure. In the days of film they were really stuck with adjusting aperture or wasting the rest of a roll of film to change ISO/ASA.
When shooting under studio strobes shutter speed is almost meaningless. A simple way to demonstrate this is to set a camera up in a room on manual setting. Set the ISO, shutter aperture and flash for the exposure you want when the lights are turned off. Turn the lights off take a picture with flash and then adjust your shutter - just your shutter - and take a second image. As long as both shutter speeds are within your cameras sync speed there will be no difference in the two images. This is because flash photography has an effective shutter speed which is the burn time of the flash, in most cases somewhere around 1/1000th of a second. This effective shutter is much much faster then even the cameras fastest sync speed (for most Canons 1/250th) As long as you understand this you understand how to mix flash with ambient light.
The how too blend flash and ambient is actually quite simple. Set your scene, set your camera for an ISO you are comfortable with and set your aperture to create the Depth of Field that you wish to have and set the shutter to your maximum sync speed. Once this is done set your flash output to properly expose your subject (depending on your flash this can either be done with output, neutral density gels, or distance to subject). At this point when you release the shutter you should have a properly exposed image. If you do not adjust your flash power accordingly.
Once you have a properly exposed image you can then mix ambient light in simply by slowing your shutter speed. Remember from the lights turned off room exercise above that shutter speed does not effect flash exposure, so the Big 3 methodology does not apply to the exposure from the flash and you do not need to adjust aperture or ISO as you change the shutter(this is where people get confused). By slowing or “dragging” the shutter all you are doing is allowing more ambient light to balance the areas of the image that are not being exposed by the flash.
So why do we blend flash and ambient light? There are several situations which can call for this, the most common is when more light is needed then is available yet we do not wish to loose the ambience such as in a club. Another common place to use flash and ambient is when photographing a person at night and wishing to have a city scape behind them visible as well. This is similar to the first but the flash is needed because the person would not expose at all, they would simple be a shadow blotting out the city lights. Another case is to freeze action, such as bullet photography, the flash is used so the bullet can be frozen in the frame and the longer shutter allows the rest of the scene to also be exposed. Another reason to add flash is to create shadows, in the sequence below you can see that the room is very evenly lit and by adding flash I added shadows which creates depth in the images. In actuality blending flash will be used for multiply reasons in the same image.
So now we have covered the theory, lets look at the nuts and bolts of what I did for this exercise and how it affected the images. Photographed at the top you can see how I set the camera and flash in relation to the scene. I treated the piano as the subject of the image, I placed the 600EX-RT on 1/4 power, into a 24” softbox, 6 feet from the piano. The camera was the Canon 6D with the 24-105 f4 L set to 24mm. The ISO was 400 and the aperture was f4 for all of the images in the following sequence the only thing that changes is the shutter speed which is listed on the images.
These last two images below show the scene, the first with the camera set to take a flash exposure but no power to the flash, and the second with the camera set to take the image just with ambient light. The Canon 6D sync speed maxes at 1/180th which is why there is a discrepancy between the first image in the sequence above and the image here for flash with the flash turned off. In situations where you need a faster sync speed Hi-Speed Sync can be used to get speeds above your cameras sync speed. (This will be covered in a later blog.) For this example I allowed the camera to set the default shutter sync for the 1st image. I also did not correct for Color Balance, the camera was set to Flash which is why the total ambient light image has a yellow cast. I did this deliberately, in the sequence the competing colors make it a little easier to spot how much ambient light is effecting exposure. If you use AWB it will set to flash when a flash unit is attached to the hot shoe so the area exposed by the flash is closer to white. If you use gels to match your environment you will need to set your cameras white balance accordingly with the camera’s custom white balance settings. The exception to this is when using Canon CTO gels with the 600EX-RT. The 600 has a sensor which allows it to correct white balance to the Canon CTO gel being used if the camera is set to AWB. I have not explored this function yet so I am not sure how well it works. It also only works if the flash is mounted in the hot shoe.
So that is the basics of mixing flash with ambient, like color balance the only way to really learn this skill is through practice. Depending on your set up it is possible that if you are mixing a lot of ambient light you would need to step the flash power down slightly. Do not be afraid to experiment once you have a good grasp on how the toe exposure models work and how well they blend.