Manual Flash

Fire scenes are terrible places to take pictures. Fires often occur at night making for dark, shadow-filled scenes. If the lights in the building still work, it’s the happy exception rather than the norm. Even when fires happen during the day, fire scenes are still dark and sooty. They have boarded up windows, allowing us little natural light. This is why a powerful external flash is essential equipment for the fire photog. 

External flashes can seem intimidating. The settings and the interfaces aren't very intuitive. Navigating through the mode settings you may wonder, “What is TTL?" "Should I use that or Balanced TTL?” “How are TTL modes different from Automatic mode?” "How does Manual mode work and why would I ever use that instead of one of the automatic settings?" In the next few articles I’m going to do my best to answer those questions and also offer some helpful flash tips and tricks that will help you make better flash photos.

First though, I want to back up from the technology and get down to the basics. I often find that the best way to learn what tech is doing is to understand the principles behind its operation. In this case, that means understanding Manual flash. Manual flash isn’t something you're going to need very often, but in cases where you want consistent subject exposure for a series of images, Manual flash is the way to go. Once you understand it, it's pretty straightforward. Just as important, understanding manual flash goes a long way towards understanding how your flash works. Once you understand that, all the other settings, the bells, and the whistles cease to be intimidating.

What About Pop-up Flash?

Why do I recommend an external flash over the one that is built into your camera? The biggest reason is power. Flash power is often described in terms of the flash's Guide Number (GN). The GN tells you the effective distance of the flash unit at a given ISO.  A powerful external flash will have a GN of 100 - 200 feet. The popup flashes built into camera bodies typically have a GN between 20 to 40 feet. Pop-up flashes can be handy for close-up work. They can also be used as a trigger for a compatible off-camera flash. But for dark scenes and for large scenes you'll be much better off with a powerful external flash.

How About Raising the ISO? 

I’ve seen a lot of folks on Reddit and photography forums writing about how great the high ISO performance is on their new mirrorless cameras. Some of them will even tell you that their camera can see in the dark and that their camera's magic sensor makes flash photography a thing of the past. This is B.S. Don’t fall for it. Raising the ISO will increase noise and decrease image quality. Fire scenes are too dark with too much dynamic range to rely on ambient light, regardless of how great your camera is. 

Pop-up Flash vs External Flash

Here are two shots made with the same camera and the same camera settings. The first shot was taken with my external flash which has a guide number of 125' at ISO 100. In the second photo, I used my camera's built-in pop-up flash which has a guide number around 30. 

High Dynamic Range Example

I asked the captain at my firehouse if he would model for some pictures for this post. He said no. Fortunately, my son's friend, Captain Scallywag, was available. 

Here Scallywag is helping to demonstrate a common problem at fire scenes. With very little interior lighting but bright daylight outside, we have a scene with a huge dynamic range. Dynamic range refers to the difference between the brightest part of the photo and the darkest. In some cases, the dynamic range is too great for a camera to capture detail in the highlights and the shadows in a single shot. Below is a good example. If we expose for the daylight outside, the exterior is well exposed but our subject is a silhouette. If we meter for the subject, as in the second image below, we get better subject exposure but the exterior is blown out. The backlighting also leads to fringing and poor subject detail. We need more light on the subject for a good exposure. That's where flash really shines.

In the third shot I metered for the ambient light on the exterior, ensuring that I had good detail at the window. Then I set my flash output to properly expose for the subject (more on that later).  The result is a much better exposure. In photography, and in forensic photography especially, light equals information. Adding an external flash to your camera is a great way to capture more information!

Flash as the Main Light Source

Flash photos can be broadly categorized into two types — 1) photos in which the flash is the primary light source and 2) photos in which the flash is used in conjunction with ambient light. We’ll address flash as a primary light source first. 

There are a couple scenarios when you would use a flash as the main light source. The most common situation is shooting a dark scene that has too little ambient light for a good exposure. This describes most interior fire scenes. As forensic photographers, we need to maintain adequate depth of field in our images. As a result, we are limited to smaller apertures. Because we want good image quality, we are somewhat limited in our choice of ISO. We also need to keep our shutter speeds reasonably fast so that we don't get blur from camera shake. With all this in mind, when I shoot an interior fire scene I'll often set up my camera at f/11 or f/16, 1/60 second, ISO 400 or 800, depending on conditions. Shooting these settings with ambient light alone will usually result in underexposed fire scene images. In these cases, flash supplies the main source of light for the photos.

Just like we do with an ambient light photo, we need to balance three sides of an exposure triangle to achieve a proper exposure. The Flash Exposure Triangle, however, is a little bit different than the natural light exposure triangle I have described in previous posts. The difference is that  when you take a flash photo in low light conditions, the shutter speed has very little effect on the overall exposure. This is because the main source of light is the flash and the burst of light from the flash is practically instantaneous.  It doesn’t matter if the shutter stays open 1/250th of a second or 1/60th of a second if the light is only present for 1/5,000th of a second! 

Controlling Flash Output

Manual flash controls allow the user to adjust the power of the flash output, usually in ⅓-stop increments. Using these manual controls, the photog can balance the light from the flash with the aperture and the ISO to achieve proper exposure. Let’s say for example we are shooting with a flash at full power, and we find that ISO 400, and an aperture of f/11 gives us a proper exposure for our subject. We can also achieve proper exposure using the reciprocal settings: ISO 400, f/8, with the flash at ½ power. Because f/8 is a full stop larger than f/11, it allows twice as much light into the camera. That means we would need to reduce the flash output by 50% to achieve the same exposure.

The Complication

So far so good, right? If we know the flash’s GN rating and one of the other factors, we can use simple division to figure out the third. Now for the monkey wrench. The above examples have assumed that we are shooting at ISO 100 and that the flash is firing at full power. In these cases, and in any case where we have a constant ISO and flash power setting, exposure is based on the aperture and the flash to subject distance. But we can also control flash exposure by increasing the ISO (thereby artificially brightening the image) or by reducing the flash brightness. Doing either changes the effective guide number for the flash exposure calculation. Unfortunately for those of us who are not adept at calculating square roots, the relationship between these factors and the guide number isn't linear. Due to the inverse square law and the nature of light falloff, if we set a GN 125 flash to ½ power the effective guide number doesn’t become 62.5 as we might expect, but 88.4. Likewise, increasing the ISO one stop won’t double the capable flash distance. It will increase about 41%. 

The Solution - Your flash is its own calculator

If all the talk of square roots and such is getting to be too much, you can relax. You won't need a calculator to make a manual flash exposure. That's because your flash probably has a distance calculator built in. If your flash has one of these, while in manual mode, the flash reads your ISO and your aperture and will display the correct flash-to-subject for those settings. If you adjust the flash power setting, the flash will adjust the distance scale accordingly. All you have to do is estimate your distance to your subject and adjust the flash until the display matches that distance.  In the example below, my subject was about 6 feet away. The distance scale let me know that a full power flash would be too powerful. As I dialed down the power, I watched the distance scale until I saw that 1/8 power got me to a distance of 5.8 feet — just about right for this shot.

If your external flash doesn't have this handy distance scale then you can just guess and check! One of the best things about shooting in the digital era is the ability to instantly see our results. In the days of film, a photographer needed to be certain that they had their flash exposure settings dialed in. If they made a mistake, they would only find out after the film came back from the developers. Today, we have instant feedback. If you want to shoot in manual mode for the sake of consistency but your flash doesn’t have a built in scale, take a test shot at ½ power. You can check the playback image, the blinkies, the histogram, etc. and adjust the flash exposure as needed. If your first shot comes out too dark, bump it up to ¾ or 1/1 power. If it’s too bright, dial it back to ¼ power. Once the exposure is dialed in, you can shoot over and over from that distance and achieve a consistent result with every shot.

Using the Manual Flash Distance Scale

Thankfully, we don't need to do math every time we take a flash photo. But I wanted to include the guide number calculations for a couple of reasons. I included them partly because I want everyone to thoroughly appreciate how fortunate we are to live in the days of modern flashes with TTL and Manual flash distance scales! Mostly though, I included them because the calculations help to illustrate a couple of important concepts. The first is that calculating a flash exposure for a given ISO and power output is all about flash to subject distance and aperture. Shutter speed doesn’t come into the equation. Literally. The second concept is that for any flash setting, the flash can only provide optimum exposure at a certain distance. Items that are closer than the subject will be brighter. The scene behind the subject will be darker. This is one of the important  limitations of on-camera flash. Whether or not that is a big deal for your exposure depends largely on the depth of the scene you are trying to illuminate. 

Using Flash to Exclude Ambient Light

In the example above, we see how a flash, when set to expose for a subject, can underexpose the background. A photographer can use this effect intentionally to isolate their subject with flash. The effect is a photo of a well exposed subject against a dark background. While this isn't a technique you will want to use in forensic work, it can create a cool, artistic effect and it's a fun technique to try. 

Stuffed animal portraits and ridiculous Christmas cards aside, knowing how to use flash to exclude ambient light can be useful at the fire scene too. In the example below, light from an emergency exit sign is casting red light on the evidence. The red hue is very distracting and hides the actual colors in the scene. We can use our flash to illuminate the evidence and to kill the problematic ambient light. The first step is to set the camera to underexpose the ambient light photo by three or more stops. Then we can add the correct amount of manual flash for the flash-to-subject distance. (TTL would work here too.)

Ambient Light with Flash

So far we've covered how to use manual flash as the main light source for a photo. This includes cases where there is too little ambient light and also cases where there is ambient light but we choose to exclude it and rely on flash exposure alone. We can also use flash in combination with ambient light if we choose. The scenario pictured at the begining of this article is a great example. In this scenario we have plenty of light in the background for an ambient light exposure. For this part of the photo we use the normal exposure triangle — aperture, shutter speed, and ISO. Metering off the vegetation outside the window, I decided on F/11, 1/125 sec, and ISO 400. This gave me a good background exposure but left the foreground and my subject way too dark. With the flash set to Manual, I dialed in the flash power until the distance scale matched my flash to subject distance, which was about 6 feet. The combination of ambient light and flash resulted in a good overall exposure.

Conclusion

Manual flash is a handy tool for your photographic toolbox. It gives the photog complete control over the flash exposure instead of relying on the camera's auto-flash algorithms. Manual flash is also the best way to achieve consistent results across a series of flash exposures. Whether or not you choose to use manual flash in your work, understanding how manual flash exposures work and understanding the physics behind flash illumination is important. The principles that govern Manual flash are the same principles that govern all the other flash modes. If you understand these principles, you understand flash photography.

In the next article we'll discuss two more important flash techniques... Fill Flash and Bounce Flash.