Understanding Flash's Guide Number (GN)

Guide Number, usually abbreviated GN, determines power rating of flash unit that describes how powerful flash unit is and how far it can shoot. In another word, GN specifies the power of an electronic flash in a way that it can be used to determine the right f-stop to use at a particular shooting distance and ISO setting. GN is mainly used to calculate how your camera’s aperture should be set to get proper flash light. When there were no automatic flash units available for photographers, they used to do a manual calculation using the GN and the distance of the subject from the flash unit. GN is usually given in reference of ISO value 100. For example, Nikon D90's built-in flash has a Guide Number of 12 meters or 39 feet at ISO 100 in auto mode (i-TTL). And in Manual mode, it has a Guide Number of 13 meters or 43 feet. GN is slightly less in i-TTL mode than in Manual mode because it spends energy to fire pre-flashes before the main flash goes off (to meter correct amount of the light for the subject). Basically, to determine the right exposure at any given ISO settings, you would divide the guide number by the distance to get appropriate f-stop. We can write the mathematical formula as,

Aperture = GN / Distance

Using D90’s built-in flash as an example, at ISO 100 with its GN of 43 feet in Manual mode, if you wanted to shoot a subject at a distance of 10 feet, you would use f/4.3 (43 divided by 10), or close to this value in practice. Similarly if subject is at 5 feet, an f-stop value f/8 would be used to get proper exposure. But in practical, we may need to increase the ISO few stops to adjust with f-stop and the distance of the subject.

When we are talking about the Guide Number, we didn’t mention about the Shutter speed and we only talked about the Aperture. It is very important to remember that Shutter speed controls the brightness of the ambient light and Aperture controls the amount of light from the flash. We use Guide Number calculation to help control the flash exposure. Today, GN is mostly used for comparing the power of various flash units, rather than actually calculating the exposure value to use. You don’t need to be a math genius to see that an electronic flash with a GN in feet of, say 98 at ISO 100 (like the SB-600) would be a lot more powerful than the Nikon D90’s built-in flash. Using that GN as a reference, Nikon SB-600 allows you to shoot up to 22 feet using f/4.5 at ISO 100 easily.

Please refer to your flash user manual to get the exact GN value.


Nikon SB-700 Speedlight Review

I am a big fan of Nikon CLS technology and Nikon impressed me once again with it’s newly announced small and compact but powerful lighting system, SB-700. This flash unit lives up to the Nikon legacy. I always repeat the fact that photography is all about adjusting the composition and the lights. To support this fact, most of the DSLRs come with a built-in flash but the features they provide is very limited. That's why it is almost necessary to have an external flash unit which can provide creative control over any given lighting condition. If you are using a Nikon DSLR, you have many options to choose from; SB-R200, SB-400, SB-600, SB-700, SB-800 or SB-900. The Nikon Speedlight SB-700 is the newest addition to the Nikon flash lineup and can be considered as a replacement for SB-600. I was using a SB-600, which worked fine, but the SB-700 was just a great upgrade. Let’s talk briefly about this new gadget from the Nikon.

Nikon’s SB-700 is a compact flash unit but filled with features that SB-600 lacks and some of the features are inherited from more professional flash unit like SB-900. The built and the design of the unit looks professional, supports commander mode, capable of multi-step auto zoom up to 120 mm. It is designed with high-speed recycling time of 2.5 seconds with the NiMH batteries and 3 seconds with the Alkaline batteries. The built size of Nikon SB-700 (2.8 × 5.0 × 4.1 in) is a bit larger than SB-600 but smaller than SB-900 unit. However, it resembles more with SB-900 and shares some of the features and the control layout with SB-900 as well. The interface on the SB-700 is much more straight forward than the SB-600 which requires you to push two buttons at the same time to get into the menu items and configure the flash unit. The SB-700 is a much easier and faster unit to setup and has more features than the SB-600.

SB-700 is redesigned with the interactive control and very easy to setup and use. As I mentioned before, some of it's features and even controls are borrowed from SB-900. It has a nice LCD screen on the back and includes two mode switches on the left and the right side of the LCD screen which allow user to change modes without navigating through all the menu items. The SB-700 comes with the incandescent and the fluorescent filters for the color balance when shooting indoors. The SB-700 automatically detects the type of filters in use and adjusts the settings accordingly. It also comes with a built-in bounce card that can be used for the bounce flash effects and the diffusor to soften the intensity of lights.

Nikon Speedlight SB-700

Nikon Speedlight SB-700

Features & Performance

1. Automatically senses the FX or the DX-Format camera in use and optimizes the light distribution.

2. Automatically identifies the mounted hard type color filters and adjusts the camera white-balance.

3. Works with the Nikon’s i-TTL (intelligent through-the-lens) metering system. This system works by firing a series of flash bursts to assist the camera in analyzing the scene. The camera’s metering system then processes this information and sends it back to the flash unit, balancing the flash output with the ambient light.

4. Using the simple slider switch on the back of the SB-700, you can optimize the light quality by selecting the Standard for general illumination, Center-weighted for portraits or Even, for groups or interiors.

5. Most of the newer entry level Nikon DSLRs do not have a commander mode to remotely control the flash Off Camera. And in that case, SB-700 can be used as a commander flash unit as well to control multiple wireless flash units.

6. SB-700 has a built-in thermal cut-out detection feature to prevent it getting damage from overheating. It works by extending the recycling time when SB-700 detects the heat build-up.

7. Flash Value (FV) Lock feature locks in a specific flash output on the main subject, regardless of the aperture, composition or the lens’ zoom position.

All in all, I highly recommend this flash over the SB-600. I wanted to get the SB-900 but its just too big and heavy for my needs and also couldn't justify the additional cost as well.

Nikon Wireless Speedlight Commander Unit, SU-800

SU-800 is a Nikon speedlight commander unit to control other Nikon wireless remote units. It is a very powerful device, easy to use and supports up to 4 Channels and 3 Groups. There is a little switch inside the battery chamber that allows you to switch between the close-up mode and the commander mode. Close-up mode is used for controlling the SB-R200's units and the commander mode is use to control one or more of the Nikon SB-600, SB-800 or SB-900 units wirelessly. Previously, we discussed about how to configure the SB-600, SB-800 and SB-900 units as wireless remote units. In this blog, we will briefly discuss on how to set the SU-800 to control these remote wireless units.

SU-800 Unit

SU-800 Unit

When you turn the SU-800 unit on, it will give you the information about the available Channel groups, their settings (TTL mode and the exposure for example) and the Channel number associated with that group. When you change any of the settings, those settings will be applied to the remote units which are under the same group and the channel as this master unit is. You can use the SEL button to change the values for the Group and the Channel number. Left and right arrow of the SEL button can be used to increase or decrease the exposure compensation value (EV) or the Channel number depending up on which settings you are in. The MODE button will allow you to program the mode of the remote flash unit. Flash can be programmed in TTL (Through The Lens) mode, AA (Auto Aperture) mode, M (Manual) mode and - - (don't trigger the flash) mode. If you want to reset all the setting at once, press the MODE and the ON/OFF together for a while and it will reset the unit to factory default settings.

The Nikon SU-800 unit can only be used as a master unit unlike other speedlight units, SB-800 or SB-900 for example. Nikon SB-800 and the SB-900 can act as a both master flash unit and the remote flash unit which I already discussed in my previous blogs. The SU-800 can control the multiple flash units in four different channels and three different groups and the combination of the SU-800 and the SB-R200 units are popular for a closeup photography or also called macro photography.

How To Setup Nikon SB-900 As a "Wireless Remote Unit"

Last time we discussed about Setting up the SB-900 as a Master Flash Unit and Setting up the SB-800 as a Wireless Remote Unit. Now, we will see how to configure a SB-900 speedlight unit as a wireless remote unit or also called a slave unit. Compared to the Nikon SB-800 and SB-600, it’s fairly easy to setup SB-900 into the slave mode. We can set the SB-900 for a remote operation by using the power ON-OFF switch or also called the wireless setting switch.

Here are the steps to follow,


1. Turn the switch while holding down the button at the center.

2. Align the index on the power ON-OFF switch to REMOTE.

This is all you need to do to configure a SB-900 as a remote unit. The next step would be setting up a group and the channel number on the remote flash units.

Setting up the remote unit was pretty easy and straight forward. Now let’s setup the proper group and channel number as you have set in the camera or the Master Flash Unit or SU-800 unit whichever is controlling this remote unit.

1. Press the Function button 1, then turn the selector dial to choose a desired group name, and press the OK button.

Note : Left button to the lock sign is the function button 1 and the right one is the function button 2.


Group name and the channel number being set appears to be larger than the others. For the remote flash units where the flash mode and the flash output level compensation values are to be set identically, place these flash units into the same group.

2. Press the Function button 2, then turn the selector dial to choose a desired channel number, and press the OK button.

Be sure to choose the same channel number as set on the Master flash unit or in the Commander mode on the camera otherwise they do not communicate with each other. Setting up the SB-900 is much more easier and straight forward than the SB-800 or the SB-600.

Source: A Hands-on Guide to Creative Lighting by Nikon School