Understanding Nikon's Flash Metering Mode

Metering is the technique, which is used to determine the optimal exposure for the subject and the overall scene based on the information camera gets from available light sources. Nikon uses the flash metering system to determine the flash output when the flash is set to auto (also called TTL or i-TTL) mode, and the camera metering system to determine the overall exposure of the image if the camera is set to A, S or P exposure mode. In manual (M) mode, you would take the reading from the camera metering system as a reference whether it is negative (underexposed) or positive (overexposed) and set the exposure values (Aperture, Shutter speed and ISO) manually to adjust the exposure. Nikon D90 and most of the other Nikon DSLRs support three types of Metering Modes, and they are Matrix Metering, Center-Weighted Metering and Spot Metering, which we discussed in previous blog.

In TTL mode, the built-in flash, as well as the external flash unit attached to the camera, adjusts the flash power to properly expose the scene by using the information from one of these three available metering modes. When the flash is set to TTL mode, choice of the camera metering mode determines whether the flash metering system, which shares the same metering sensor with the camera metering system, should consider the ambient light and balance the overall exposure or ignore the ambient light all together and only count the light from the main subject to determine the flash output power.

What is TTL mode?

Nikon’s latest DSLRs and the flash units use the TTL technique to determine the amount of lights reflecting from the scene and adjust the flash power accordingly. TTL stands for Through The Lens and it refers to the process of gathering information through the lens and passing that information to the metering system in order for the camera to deploy the appropriate flash power. Older camera systems either used manual calculations to determine the flash exposure or used a sensor in the flash itself rather than the metering sensor in the camera. And, i-TTL (intelligent-TTL) refers to the latest TTL flash system from Nikon. In latest Nikon cameras, TTL refers to i-TTL by default and I am using TTL and i-TTL interchangeably in this blog.

There are two types of i-TTL methods used in Nikon’s lighting system and the DSLRs, which are iTTL Balanced Fill-flash and Standard iTTL Fill-flash. Now, let’s talk about these two different TTL methods and discuss how they work differently.

1. iTTL Balanced Fill-flash (i-TTL/BL or TTL/BL)

Nikon's i-TTL (intelligent through-the-lens) Balanced Fill-Flash automatically balances the output of the Nikon Speedlight to match with the scene's ambient light. This flash mode is selected by the camera automatically when the flash is in TTL mode and the camera metering is in either Matrix Metering mode or Center-Weighted Metering mode. If the camera is set to A, S or P exposure mode and the flash is in i-TTL/BL mode, Nikon Speedlight fires the series of pre-flashes to determine the exposure value for the subject by taking the ambient light into account and send that information back to the flash metering system in the camera. The flash metering system then combines that information with the metering information from the camera metering system and the focal length information from the D or G lens to analyze and calculate the final flash output in order to balance the overall exposure. All of this complex processing happens in a fraction of a second, before each exposure, to provide unprecedented levels of flash precision and performance. This is a very powerful and easy to use technology from Nikon and probably one of the best in the industry.

iTTL Balanced Fill-in flash

In short, using this technology, Nikon DSLR measures the available light and then adjusts the flash output to produce a natural balance between the main subject and the background. It will reduce the harsh shadow as well as the highlights in the subject caused by the over powered flash.

2. Standard i-TTL Fill-flash

This mode is activated when the flash is in TTL mode and the camera metering is in Spot Metering Mode or if you are using the external flash and the flash is set to standard mode. In this mode, the flash output is adjusted only for the main subject in the frame, and the brightness of the background is not factored in while calculating the flash power. If the standard i-TTL Fill-flash is chosen by the camera by dialing the camera metering into the spot metering mode in A, S or P exposure mode, the background light is ignored and the camera will fire the flash to give you the correct exposure for the main subject only. If the overall exposure for the scene and the main subject is properly balanced without using the flash (shooing outside in a bright daylight for example), using the standard i-TTL Fill-flash may overexpose the subject by adding the extra light to already properly exposed main subject. You should use this mode only when the flash is main source of the light for the subject (shooting in a dark room for example) and you want to emphasize the main subject at the expense of the proper exposure for the background.

Standard i-TTL Fill-flash

In this picture above, the exposure on the main subject was metered by the flash metering system by firing the pre-flashes on the main subject and then analyzed the light reflected from the main subject along with other information that camera provided, including the metering mode and the focal length. The flash metering system then adjusted the flash power and fired the flash just to illuminate the main subject. If you notice the background, which is relatively darker, was metered by the camera metering system and exposed by the ambient light only. The flash power was calculated by the flash metering system to expose the main subject only which was weakened by the time it reached to the background, and hence flash didn’t contribute for the background exposure at all.

Nikon D90 Autofocus Area Mode

In my previous blog post, we discussed about different Autofocus Modes available in Nikon D90 and learned how to use them in different situations depending on what kind of subject we are shooting. In this blog, we are going to discuss about “Autofocus Area” which allows us to choose which of the 11 focus points should be used to evaluate and lock the focus. By default, Nikon D90 selects the focus area among available AF points itself, which works perfect most of the time. But if you want to select the focus point yourself, you have to change the default settings on the camera and that is what we are going to discuss next.

If you press the menu button on the back of the camera and go to the CSM (Custom Settings Menu) menu which looks like pencil icon, the first option you will see is AF-area mode and this is where we will need to change the focus area setting. When you are in the menu, press the AF-area mode and you will be presented with the four options to set the autofocus area mode. Let me explain these different modes in brief so that you can decide which mode to use in your shooting environment.

1. Single point

When this mode is selected, you can choose which one of the 11 focus points should be used to focus the subject and the camera sticks with that focus point. This mode works best for the stationary subjects but will also be helpful when you want to focus on particular one subject among many. One of our readers asked a question: "how to force her D90 to autofocus particular subject" while shooting in a jungle and the Single Point focus mode was the best option for her. Nikon D90 automatically uses this option in a Close-up scene mode. In this mode, you have to select the focus point manually by using the multi-selector button (thump pad) on the back of the camera.

2. Dynamic area

When you select this mode, you can select the focus point but the D90 can use other focus points as well if the subject moves. This mode is the best option for capturing moving subjects.

3. Auto-area

This is default mode for Nikon D90 where camera selects the focus point for you automatically. I use this mode most of the time and works great.

4. 3D-tracking (11 points)

In this mode, you select the focus point using the multi-selector button but if your subject moves, D90 tracks subject in 3D direction and re-frame it as it moves right, left, up, down, forward and back. This mode is the best option for sports photography.

Use Panning Technique To Shoot Fast Moving Subjects

Panning is the photography technique which is primarily used to show the motion of the fast moving subject. The basic idea behind the panning is to follow the moving subject with the camera as it crosses your field of view, resulting in the subject being sharp and the rest of the shot including the background image blurred. Relatively sharp subject and the blurry background gives the feeling of the movement and the speed. It’s particularly useful in capturing any fast moving subject like racing cars, running animals, cyclist etc. If you want to master the panning technique, first and most importantly, you have to follow few simple rules and then practice with a lot of patience. Do not get frustrated with some blurry subjects initially. Panning is a skill which can be learned over time but you need to experiment with many variables including your camera settings, your distance with the subject, shutter release timing etc., which we will discuss later. In order to get the good result with panning, the ideal flow would be - maintain a balanced distance with the subject, use a little slower shutter speed, follow the moving subject and pan your camera along with the subject on the same plane of the motion, take the shot and continue to pan in the same direction.

Here are few guidelines you can follow to properly apply panning technique.

1. Use the Shutter Priority Mode.

2. Start from a slower shutter speed (1/15s - 1/30s) and go up or down little bit as you need depending on the speed of the subject and the lighting conditions.

3. Shoot in a Continuous Autofocus Mode because your subject and the camera are constantly moving in proportion to each other. This way camera continuously track and focus the moving subject.

4. If your camera doesn't support autofocus, you have to judge the traveling path of the subject, determine the subject's final point for the shooting and pre-focus on that point and then take the shot.

5. Use a Continuous Shooting Mode or a Burst Mode so that you can have a multiple shots and will have the flexibility to choose the best among many shots.

6. Position yourself parallel in front of the moving subject. If you keep larger distance from the subject, it might decrease the speed of the subject in relation with you and your camera, but it also reduces the effect of panning, and the effect is often why we do the panning. So, you have to balance the distance with the subject and the speed of the panning.

7. Place your subject position fixed in your viewfinder and smoothly move your camera as the subject moves on it's track.

8. Continuously follow the track of the moving subject, take the shot and keep moving the camera as if you are taking a shot. This will create a soft transition and gives better result.

9. If you need an extra support for heavy camera and big lens, you can use a tripod which supports panning feature. It gives extra stability to minimize the motion blur on the main subject.

Panning Technique (Image Credit : AdoramaTV)

Exposure : 1/30 sec     Aperture : f/14.0     Focal Length : 200mm     ISO : 100

If you are new to this technique, do not expect to get a perfect shot in the beginning. You have to keep experimenting with different settings until you get the best result. Also using this technique, you might not get a super sharp result but panning allows you to shoot the moving subject smoothly and at the same time gives you the feeling of the motion. If you do it right, you will get the subject relatively sharper than the motion blur background, which is the main idea of this technique to give a sense of the moving subject. Here is something I tried during my recent visit to New York City.

Exposure : 1/15 sec     Aperture : f/22     Focal Length : 16mm     ISO : 200

It's not as perfect as I would like it to be but I will try next time and keep practicing until I get the better result.