Flash Guide Numbers

Arthur "Weegie" Fellig

When we left our intrepid hero, he was struggling to make sense out of a non-automated flash that didn't provide a non-through the lens (TTL) or an  iTTL exposure control option. His mind was reeling with thoughts of diminished light intensity over unknown distances, the Inverse Square Law, ISO sensitivity settings, and the unfathomability of measuring a burst of light that would last less than a 1/1000th of a second.

Guide numbers to the rescue!

The guide number has been relegated to the status of "arcane", but once upon a time, it was the only way photographers could get reasonably close approximations of correct flash exposures. And until somebody got the idea to put an adjustable dial as a reminder, photographers had to tape this valuable information onto the backs of their flashes, or commit the data to memory. In fact, I have it on best authority that press photographers of yesteryear set their Speed Graphic shutters to 1/200 of a second, their apertures to F 16, and their distance scales to 10 feet. So adjusted, the photographer just had to stand about 10 feet from the subject, insert a flashbulb into the flashgun, and make a shot that would be properly exposed and in focus. Any modest variations in distance would be within the lens' depth of field, resulting in a reasonably sharp photo most of the time.

The Guide Number for a given ISO/ASA value brought together the two factors that governed flash power, flash to subject distance, and aperture setting, and combined them into a relatively simple formula:

Guide Number = Distance X Aperture

Establishing Guide Numbers: In come cases, flash and speedlight manufactures will suggest guide numbers for the products, providing one number for each popular ISO/ASA setting. However, most of these numbers assume that you'll be photographing indoors with a 10 foot ceiling and light colored walls. When working outdoors, you'll find that relying on these numbers will result in significant underexposure. This means that you'll need to determine your own guide numbers if you plan to work outside.

For these base-line exposures, I set a Nikon D70 camera in the manual mode to 1/500 of a second, ISO 200, and Flash White Balance. The Nikon SB-800 speedlight was set to 1/4 power with a beam spread of 35 mm. It was mounted on a light stand 10 feet from where I stood. I made these photos outdoors to simulate the how the SB-800 would behave outdoors where there were no walls or ceilings to reflect additional light onto the subject. These "selfies" were made with a Tokina 12-24 F 4.0 held at arm's length.

As you can see, this early morning sequence of photos was taken at one-stop increments.  Several things become apparent. First, there is an optimal aperture setting for photos taken with a flash-to-subject distance of 10 feet is probably F 8.0. (I chose this slightly underexposed image to prevent blocking of the highlights). Second, the direct lighting from the speedlight produces some very harsh shadows and lots of specular highlights (glare). Lastly, flash on the main subject (my face) does have an impact on objects in the background, as you can see by the pillars that are behind me. As I opened up my aperture, the sky would brighten accordingly.

Now For The Magic: Through some mathematical quirk*, you can calculate the guide number by multiplying the Distance (10 feet) by the best Aperture Setting (8.0) to get a Guide Number of 80for this specific combination of flash output, beam angle, and camera ISO. In use, you simply divide the guide number by the flash-to-subject distance to determine the proper aperture. For example: For a flash distance of 20 feet:

80(Guide Number)/20 (Distance) = 4.0 (Aperture Setting) 

Ambient Light: The existing, or ambient, light was not a factor in the initial series of photos. In fact, the 1/500 of a second exposure time was chosen to minimize any influence it would have on the foreground exposures. When when you start utilizing the background as part of the overall composition of your image, the background exposure becomes more important. Remember, the aperture controls the flash exposure, and must be set first and left alone. The exposure time controls the ambient light, which in most cases, will be the background. This you'll do with the shutter speed.

In this second series of photos, I left the aperture set to F 8.0 and lengthened the exposure time in one stop increments. Notice that the background gets lighter as the exposure time gets longer, exactly what you'd expect. However, when the exposure time reaches 1/30 of a second, you'll see that my face is starting to get lighter. At this point, the ambient light is now strong enough to add to the light provided by the flash, giving me a lighter foreground (face). This illustrates some of the problems you'll encounter whenever you try to match ambient with supplemental flash. A simple solution is to minimize the ambient's influence on the foreground in any way you can.

1/125 of a second, F 5.6, ISO 200, bounced flash at full power.

For this last shot, I stood in the alcove of my hotel's entrance, effectively blocking all of the natural ambient lighting. I bounced the flash off of the walls and ceiling and had to both increase my flash output to 100% and narrow the beam angle to 105mm to compensate for the light loss whenever bounce flash is used. I made the shot, called it good, and left for breakfast.

On viewing the image, I was bothered by how artificial it looked. Then it occurred to me: The soft and even bounce flash didn't look right when compared to the relatively contrasty morning light. The prominent background shadows don't match those in the foreground, something I hadn't thought about at the time.

Determining you own custom guide number can be an interesting exercise in understanding how flash works. But in researching this post, I learning something more profound when I examined that last image. Obtaining a properly exposed image is only half the battle. The continuity of lighting, or put another way its believability, is the finishing touch that can make, or break, the photo.

*It has something to do with the square root of two.

Out! Out D#&% Spot!

This is a photograph of the LCD display on my Nikon CoolPix A camera. You can see, near the top edge of the image, the shadow of an enormous dust spot. It is so big (and bad) that it is clearly visibly on the screen. What makes this unusual is the CoolPix A is a fixed lens super Point-And-Shoot, and one would have assumed that the camera was assembled in a clean-room environment. The camera is currently at the repair depot, patiently waiting for a technician to exorcize this unwanted intruder.

Dust on the camera's sensor (or in most cases, the low-pass filter) is a nuisance as inescapable as death and taxes. This situation is exacerbated by the very nature of digital photography. First, the decreased size of the APS sized sensor found in most amateur and prosumer digital single lens reflex (DSLR) bodies makes any tiny bit of airborne flotsam so much larger in proportion, just as dust became an issue when photographers went from large format roll film to the 35mm format. Also, the electronic nature of the sensor itself tends to attract dust due its normally electronically charged state.

www.delkin.com

There is a bit of a conundrum when it comes to finding dust. If you locate some dust, you can decide to send it our for cleaning, or simply ignore it. Dust particles become more noticeable when your lens shooting aperture gets smaller, which explains why dust can sometimes go unnoticed if you shoot wide open. Manufacturers tend to discourage owners from making sensor cleaning a do-it-yourself endeavor, and I for one follow that path. I normally use a Delkin Sensor Scope to periodically check my cameras, and when dust is discovered, attempted to blow it/them off the sensor with a Giottos Rocket Blaster. If that isn't enough to dislodge the little pest, I'll take it out for cleaning. Currently, I make the drive to Keebel and Shuchat Photography in Palo Alto and have them take care of it. For more information go to

I've adopted a quick and accessory-free way to detect sensor dust. While it's easy to do, it only works on the really bothersome (large) particles. Here's how I do it:

• Set your camera's ISO to 200, the lowest most cameras go.
• Set the camera to Aperture Priority.
• Set your lens aperture to F16, or the lowest setting (smallest aperture) you can. 
• Set your focus to Manual. You see why in a moment.
• Point your camera skyward and locate a clear area of blue sky. Be careful that there are no flying birds or airplanes within the frame, as you might mistake them for dust that isn't there. 
• Take a shot. If you didn't set the focus to Manual, the camera will try, in vain, to find something to focus on. Don't worry about the lengthy exposure times - the shadows of the dust particles are on the sensor and not affected by movements made during the exposure.
• Enter the Preview Mode, and zoom in on the image as far/close as you can.
• Move the viewing frame to the upper left hand corner, and using the mulch-directional control, scan the image from left to right, top to bottom. If there is any dust on the sensor, you see it move past as you scroll.

In this photo, you can see the spot and its approximate location near the edge of the sensor. At this point, you can use your Rocket Blaster to attempt to blow the particle away. Do NOT use compressed air, as the air stream may be strong enough to damage the lo-pass filter or the sensor, depending on your camera's design.

Dealing with dust is just a fact of life, but if you're an available light shooter working with more open apertures, you may never notice the presence of these tiny intruders. You may also tell yourself that cloning out a single speck of dust is not a big issue, but if you ever have to make two hundred identical photographs on a clean white background, that little retouching exercise will get pretty tiring after the first dozen.

Incidentally this specimen on my Coolpix A is the biggest dust spot I can remember. I'll be happy when it's removed.

"See" Trials - The Fujifilm X-E1 Outdoors

Photo #1

Pancake Breakfast: I decided to shoot this Journal assignment with my 2 Fujifilm X-cameras: My X-100s for those situations where I might need to supplement the ambient lighting with a flash, and my X-E1 with a 12 mm 2.8 and 35 1.4 lens. I figured that the fixed lens X-100s with its 35mm equivalent lens would be the happy medium lens, allowing me to go wide or slightly longer when the appropriate lens was fitted to the X-E1 body. I also brought a dedicated Fuji EF-42 in case I needed the assistance of a flash with dedicated TTL capabilities. The kit, incidentally, weighed a fraction of my digital single lens reflex (DSLR) kit. Photo #1 is the shot I submitted.

Unlike my earlier posts, this is not a Fuji love-fest. When photographing plants and other inanimate objects in my office, it was easy to be mesmerized by the sharpness of the lenses and the surprisingly rapid focus acquisition. My field experience would prove otherwise. 

Photo #2

The first thing I noticed was the difficulty of making exposure adjustments. While the ability to see both the shutter speed and aperture setting when viewing the camera from above, it wasn't as convenient (for me, at least) as viewing the control panel located atop most DSLR cameras. Pressing the clearly marked ISO or the White Balance buttons on the body would display the current settings in the control panel. Checking these settings in an X-camera requires you to look for the shutter and aperture settings in two different locations, and then switch to the viewfinder/LCD display to access menus to change the ISO and white balance. After years of working with digital Nikon bodies, this difference was more than a little inconvenient. Granted, there are programmable keys on the X bodies, but one would need to remember what these programmed functions were, something I haven’t really considered until now. But when used under even lighting conditions (open shade in the case of Photo #2), the X-E1 proved to be an excellent camera. This shot, made with the 35mm 1.4 shot wide open, is just about perfect.

Flash! Fuji's EF-42 Flash is definitely not up to the X camera’s professional potential. First off, this dedicated flash was extremely difficult to adjust on the fly. 

• The flash has only two adjustment buttons: one to select the mode, and one to make the adjustments. 
• The adjustments are changed in an “around the world” mode: You can go from full power to half power, but returning to full power requires a trip through quarter, eighth, sixteenth, and thirty-second power before you can get back on top. 
• Plus and minus flash Exposure Compensation, along with zoom (beam spread) adjustments must be done by navigating to a specific menu dedicated to each adjustment. 
• Exposure Compensation for the flash was only +/- 1.5 stops in 1/2 stop increments, compared to Nikon's +/- 3 stops in 1/3 stop increments. This is too narrow a range to be of much use beyond the liberal requirements of a "party flash", and can make it difficult to fill shadows at short shooting distances. 
• When the flash is mounted in the hot shoe, you can no longer see the shutter speed dial.

Photo #3

The X-E1's Flash Synchronization: The X body itself has limits. The maximum flash synchronization speed of 1/180th of a second translates into relatively small apertures when shooting in daylight. To get this shot (Photo #3), I switched to my X100s set to 1/400 of a second, 5.6, ISO 200, and the flash set to manual, full power. You can see the flash improves the detail in the shadow side of the children's faces , even at a distance of over 20 feet. The image was cropped severely.

http://www.kenrockwell.com/leica/m3.htm

It Loves Me, It Loves Me Not: If Love and Hate can exist together in photography, I found it with the Fuji X-E1 and the X100s. After some thought, I concluded that the very things that made them such a retro "hit" are what made them difficult to work with. In the film based Leica, the shutter speed dial was located near the mechanism that governed the shutter. In other words, it had to be placed where it was placed. In this top view of the venerable Leica M3 (left, above), you can see that the shutter speed dial is placed on the top plate, next to the accessory shoe and in line with the shutter mechanism deep inside the body.

http://www.kenrockwell.com/fuji/x-e1.htm

This photo of the Fuji X-E1 body (right) clearly shows the similarity of control placement. Aside from the electrical contacts located in the hot shoe and the obvious digital references (ON/OFF, Fn, Auto settings), you probably couldn't tell this was a digital body. These two images are not to scale, and there really is more room on the Leica than there is on the Fuji. Incidentally, Ken Rockwell, a popular photo blogger, provides the beginning photographer with mountains of practical information.Check out is web site: http://www.kenrockwell.com.

http://www.kenrockwell.com/nikon/d300/

When "fly by wire" digital cameras were designed, the locations of the controls could be anywhere a small control module could be placed. The designers started with a traditional camera body forms and infused those digital controls in locations dictated by convenience, not by mechanics. In this photo of a Nikon D300 (left), you can see that the controls and the display are clustered together for convenient access by the user's thumb and forefinger. The LCD Control Panel displays nearly all of the information a photographer could possibly need to see, and it's all in one place.

The Nugget: After some thought, I felt I could summarize the Fuji X-camera experience by saying this:  

• You set the exposure on an X-camera; you control it on a DSLR.

• X-cameras are marginally more responsive when it comes to handling (weight, eye placement), while DSLRs are more responsive when it comes adjustments. 

I would concede that if I were working in a small indoor room and needed to be as inconspicuous as possible, I would definitely choose an X-camera because of its nearly silent operation and my ability to "chimp" an image without taking it from my eye. If weight were also an issue, the X-cameras would be a better choice. But when it comes to outdoor shots where longer distances and fast action are expected, the DSLR would win on all counts. And if I needed flash to supplement a wide variety of lighting situations, the Fuji X-100 with its sync-at-all-speeds leaf shutter might save the day, if I could get by with a 35mm equivalent lens.

http://life.time.com

I won't be "retiring" my newly acquired X-cameras any time soon. I can be as much of a sentimentalist as the next photographer, and the nearly silent operation of the X-cameras gives the digital shooter  a chance to re-visit the fundamental operational differences between the Nikon F and the Leica M3 cameras. In fact, many photographers carried two Leica M3s equipped with wide angle lenses and two Nikon Fs with telephotos, each camera body playing the advantages of the lenses attached to them. At the right is Larry Burrows, (1926-1971), a Life magazine photographer shown preparing his cameras for what would become "One Ride With Yankee Papa 13", an essay published in Life Magazine on April 16, 1965. He typically carried two Leicas in the front, with two Nikons hanging from his shoulders. I know there were few (if any)  zoom lenses available to the working photojournalist during Burrows' time, so multiple cameras were necessary to avoid changing lenses in the field.

Both the Leica M3 and the Nikon F had their adherents when Kodak was king, and with the introduction of the Fuji X-cameras we have a way to experience two different approaches to photography in a digital venue. Obviously Burrows knew the circumstances when one camera platform would be superior to the other, and in his choice of both rangefinders and SLRs, appears to have been well prepared for whatever came his way.