What Is The Guide Number And How Do I Use It?

This post is written by Guest Contributor, Alex B. Wright. If you are also interested in writing a guest blog, please reach out using the form in the Contact page.

This is a very common question that is asked by those who want to get a better flash than the one they already have. Put simply the Guide Number is the output of the flash that can be used to find out the f-stop to be dialed into the camera when the flash distance is known. The trick is to divide the Guide Number (GN) by the distance (in feet) and the answer is the f-stop you set your camera into. In simple math, if your Guide Number for a particular flash is 56 and you are 10 feet away from your subject, the f-stop you are going to use is f/5.6. The shutter speed can only be as fast as the camera manual states;with my camera it is 1/180th of a second. You can shoot less but if you go faster it will cut of part of the image and this part will be black (because of the flash sync speed). The flash fires at an extremely high speed but the sync speed of the flash can be lower so that you can get some of the ambient light in your exposure. This works well when you are taking mood images. One thing to remember is, the higher the Guide Number the more power the flashgun has. To be sure, you can check the flash output if you have a flash meter. One thing you have to make sure is that the Guide Number and the distance to the subject both are measured in the same unit. If it is an European device, then it usually is in Metric system. And, in that case, you can measure your distance to subject in Metric system (centimeters or meters) as well and divide GN by distance and you will get your f-stop number.

Safe-Sync Hot Shoe To Hot Shoe

Now one really important thing to remember is do NOT use a flash unit which trigger voltage is not known to you because if it is over 5 volts, it may fry the camera internal parts. If this is the flash that you used with your film camera then it is probably not a good choice. Most of the new digital cameras trigger voltage is 5 volts. If you want to go higher voltage, you will need an adapter that steps the voltage down to 5 volts. If you are in  doubt, have the flash voltage checked by a competent camera repair-person. Alternatively, you can get the product like Safe-Sync Hot Shoe To Hot Shoe (SSHSHS) made by Wein which is about $70.00 Canadian dollar. If you buy your stuff online, you can get it from Amazon for $49.95 USD right now. This device sits on top of the hot shoe and the flash fits on top of this device. This device works with AC powered strobe units. I have a set of Bowen’s studio units. There is a PC port on the Wein unit that the flash can plug into. This is a very handy thing to have in your camera bag. When you want to purchase a newer flash there are several third party manufacturers that make compatible units. You do not have to go with the manufacturer of the camera it is going to be used with. Most of the units have price going for them. You can get some of the flash unit as low as $150. Just check out the features you need and compare the price and then go to the camera store and make your choice.

What Is Meant By 18% Gray And How Is That Number Arrived And Why?

This post is written by Guest Contributor, Alex B. Wright. If you are also interested in writing a guest blog, please reach out using the form in the Contact page.

We know that there are two types of exposure meters for measuring light that the camera uses. One is reflected and the other is incident. What does this mean and how do you use both? The type that is in most cameras is a reflected kind. Incident is the meter that has a dome on it and this dome is pointed to where the camera is. The reflected meter is pointed at the subject to get proper exposure. Some meters also measure flash but that is another topic. You might ask why 18% gray? Where did they get that number? Well here is the math behind your question. If we use the number 3 for our starting point at the blackest image you can have. Double that number we get 6 and double it again we get 12, double it again we get 24, double it again we get 48, double it again we get 96 and this is the lightest that can be recorded. This as you may have already guessed is tied into f/stops; each time you go to a larger lens opening, you double the amount of light that hits the light receptor that records the image. Now write this number down. Between 12 and 24 you get the number of 18 and that makes it in the middle of the exposure. That is half way between the darkest black and the lightest white. That is why exposure meters are programmed to 18% gray.

Now let’s talk about the exposure meter in your camera, a Reflected Meter, regardless of camera model you have. It measures the light coming into the receptor, mixes it all up and gives you an 18% gray which is the half way from Black to White. The meter measures the light reflected back from the subject in front of the camera.

The second type of meter is the Incident Meter which is not built into the camera. It is a separate meter that has a dome. This dome is at the subject location and is aimed at where the camera is to be located. As an aside, this is the type that is used in most studios. With this type of meter you can get the ratio of the light that falls on the subject when you are in a studio set-up. This is not an explanation of lighting ratios and how to get them. Lighting and ratios is another topic for a later day. But you can use your camera even though it is a REFLECTED light meter and can be changed it into an INCIDENT light meter later. This is not that hard to do as it sounds. There is a simple way to accomplish this which we will discuss in next paragraph.

Only the secrete thing you need is a clean white Styrofoam coffee cup and then place the coffee cup over your lens. It is best to use a 50 mm lens. Now, aim the camera from the subject’s location towards where the photographer will be and get that exposure reading noted down. Manually set the camera to this exposure reading, remove the coffee cup, go to the camera location and take the photograph. This is what the dome on the incident meter is for and how it is used. Just try this out and your exposures will be more accurate.  Use the meta data information about the image you have just taken.