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 Flash And The Commander Mode

Nikon Commander Mode allows you to control the remote flash unit(s) from your camera wirelessly. If you use an external flash, this blog might be helpful to understand what is the commander mode, what is it used for and how does it work on your DSLR camera? Commander mode is one of the very powerful feature available in most of the advance Nikon DSLR cameras including Nikon D80, D90, D200, D7000, D700 and D300 that allows your camera's built-in flash to control the remote (off camera) flash by sending an infrared signal over the wireless media. Most of the today's flash unit like SB-600, SB-800, SB-900 and SB-R200 support commander mode except SB-400. Nikon SB-400 is not considered as an Advance Wireless Lighting strobe. It can only be used in the hot-shoe or with a sync cord connected to the hot-shoe. Let’s discuss few possible scenarios in which you can use SB-900, SB-800, SB-600 and SB-R200 speedlights.


Nikon’s SB-800 or SB-900 unit can also be used as a master unit (like the camera’s built-in flash in a commander mode) to control other flash units like SB-R200, SB-600 or even other SB-800 and SB-900 units wirelessly. But SB-R200 and SB-600 can't trigger other units and can only work in a slave mode (to be triggered by either camera’s built-in flash in a commander mode or other master unit like SB-800 or SB-900). So, based on their capabilities to act as a master unit or slave unit, we can use them in three different scenarios.

First scenario - If you have Nikon SB-900 or SB-800 flash unit, you can place it on the camera hot shoe and set it in "Master Mode" to trigger other master or slave units wirelessly.

Second scenario - If you have SU-800 Wireless Commander Unit, you can place it on the camera hot shoe and control any external flash unit (in slave mode) wirelessly.

Third scenario - And if you do not have SB-900 or SB-800 or SU-800 but only have slave flash units, SB-600 for example, you can trigger them using the built-in flash of your camera by configuring it in commander mode and that is what we are going to discuss in our next section.

Let’s go through the camera settings for Nikon DSLRs to set up a built-in flash unit into commander mode so that we can control the external flash units using camera's built-in flash.

How to set the commander mode in Nikon DSLR?

In this setup, I am taking Nikon D90 as an example but you can use this setup for any other Nikon DSLR that supports built-in flash to be in commander mode. Let's go right into the setup now.

1. Press the Menu button on the back of your camera.

2. Go to Custom Setting Menu option (pencil icon).

3. Select e, Bracketing/flash, menu and press OK.

4. Select e2, Flash cntrl for built-in flash, and press OK.

5. Choose the Commander Mode.


When you are inside the Commander mode, you can set different parameters to control the remote flash unit. You just have to be careful about which Group and Channel you select because you are going to use the same Group and the Channel settings in your remote flash units as well. Group is set to combine the multiple flash units in a single or different groups for the exposure settings and the Channel is used to avoid any interference with other camera's settings around you (used by different photographers if there are any).

Now, let's discuss about the different options your camera’s built-in flash unit can be set to.

  1. TTL - Through The Lens. If you select this option for the built-in flash, it will fire the pre-flash first, collect the exposure information from your subject Through The Lens (TTL) and send that information to the external flash unit(s). Then, your built-in flash also fires the main flash along with other external flash units to illuminate the subject.

  2. AA - Auto Aperture Mode. This is an older implementation of the TTL system and not used much these days.

  3. M – Manual Mode. This mode allows you to set the flash power manually rather than auto adjusted based on TTL information your camera gives.

  4. - - If you set the built-in flash into - - mode, your built-in flash will fire the pre-flash first, collect the exposure information from the subject and send that information to the external flash units so that the external units can set the flash power to illuminate the subject properly. In this setup, built-in flash doesn't fire the main flash to control the exposure but only send the exposure information to the external units. But if your subject is too close to the camera, pre-flash from the built-in flash might have some influence on the overall exposure of the image.

Important - After finish setting up your camera's built-in flash into the commander mode and an external flash unit to receive the signal, please do not forget to pop-up your camera's built-in flash. When you are ready to trigger an external flash units, it is important to pop-up built-in flash of your camera because this is how your camera sends the signal and the exposure data to an external flash units. It works great with my Nikon D90 and D700 using a SB-600 as an external unit and controlling it using camera's built-in flash. If you are having any problem working with this settings, please let me know and I will be happy to help you in any way possible.