Flash Exposure Problems

Flash exposure problems with the Coolpix cameras.

Version 1, last updated January 20, 2001

Introduction:

There is a constant stream of questions about flash (or speedlight as Nikon calls it) exposure problems with the Coolpix 950 and 990 cameras. Most of these questions are caused by using the flash in a "difficult situation" for flashes, and are not unique to the Coolpix camera series; any camera could have these problems. Some of the problems arise because of the way the Coolpix cameras meter flash. To better understand how these difficult situations arise, it is necessary to first understand exactly how the Coolpix meters its flash exposures and why this can be different than the other techniques that might be used by other cameras. Once you understand how the flash metering works, and what the difficult situations for the exposure system are, you will be able to anticipate problems and work around them.

How the Nikon digital cameras meter flash

The Coolpix series meters the flash exposure by using a small sensor to the lower right of the lens. During the actual exposure, that sensor looks at the scene and decides when there has been enough light and when to cut off the flash. Because the sensor is looking at a wide area, and because it is not looking at the actual scene, all it can do is to put an "average" amount of light on the scene. It is important to understand this averaging aspect, it is the cause of most of the exposure problems with the Coolpix cameras. This is called pseudo-TTL metering, because although it acts very similarly to checking the exposure through the lens, it is not really Through The Lens.

How some other cameras meter flash

Some cameras use a pre-flash. This is a short pop of the flash before the main exposure. The camera actually takes a picture during this time, and adjusts the amount of light for the main exposure correspondingly. This has the advantage of letting the camera see the actual scene for it's exposure making decision, and can result in a better exposure. The pre-flash technique has disadvantages too, and is usually found in more consumer oriented cameras. First, it takes time to pre-flash. If it takes too much time, your subjects have time to blink involuntarily; this is a big problem with may digital cameras that use pre-flash. Second, the pre-flash means that the camera cannot trigger standard "slave" flashes unless they have special adapters to work with cameras with pre-flash. Third, emitting the pre-flash uses some of the energy stored in the flash capacitor, reducing the power of the flash for the main flash a little bit. The Coolpix 700, 800 and 880 use the pre-flash technique.

Some cameras don't use any metering at all! They just flash the flash, and stop the camera down to a pretty good aperture, and hope for the best. This usually results in a narrower distance range for good exposures.

35mm film cameras can measure the flash in a real TTL fashon, by measuring the light bouncing off the film, sometimes even with a matrix arrangement. This isn't possible with CCD cameras, there is no film, so  one of the other techniques has to be used.

Some cameras rely on a sensor on the flash itself. This is usually called program mode, and is very similar to the pseudo TTL mode on the Coolpix 950 and 990.

Difficult flash situations

As you have probably guessed by now, most of the flash exposure problems with the Coolpix cameras involve "fooling' the flash metering system into making an incorrect exposure. Since the sensor is just a wide area light gathering sensor, it is easy to fool too.

Situation #1. Underexposure due to light background.

This is a very common problem. If the background is light, the camera will underexpose the whole shot to get the "average" exposure correct. A common example is a person standing against a white wall, or an object sitting on a light colored rug, with the background close to the object. Most of the light reflected to the sensor is actually from the background, and the camera tries to expose this to 18% gray. This means that in the example the wall comes out gray, and the person will be underexposed. The solution in this situation is to apply some + flash EV compensation on the CP990. With the CP950, there is no real solution other than going to manual aperture mode and trying to open up the aperture if possible; there is no EV flash compensation for the CP950 flash.

Situation #2. Overexposure of subject due to dark background

Well, this is the opposite of the first situation. A common example would be taking a picture of a person at medium range in a very large room that the flash cannot light up. This time, most of the light reflecting to the flash sensor is from the subject, and the camera pours on the light trying to get a good average for the whole scene. This overexposes the subject.

Situation #3. Underexposure due to overextending the flash.

A common example of this would be to try to use the flash at a sports event in a large rink, hall or stadium, or taking pictures of a large room. The flash simply does not have enough power to illuminate the scene, and it comes out dark. Keep in mind that the maximum range of the flash is about 10-12 feet. Keep in mind also that the maximum range will be shorter for dark colored subjects, and you might get a little more range for light colored objects.

Situation #4. Underexposure due to forced flash operation on the 990

The CP990 has a peculiarity when using forced flash. This is either a bug or a feature depending on how you look at it. The symptom is that forced or fill flash can result in underexposed pictures. The reason is that this mode is intended to be used not to force the flash on, but to for "fill in" of scenes lit by other lighting sources, for example to fill in backlit faces in an outdoor scene. What is happening on the 990, apparently, is that the camera does not use the flash sensor for fill in exposures, but uses the regular metering instead, cutting the exposure down by about 1EV in the assumption that the flash will fill in the light. This is a good choice in many cases, but not always, and it can result in underexposed shots in cases where not much light is actually returned from the flash. This trick is that you should think of the flash forced on feature as only for fill flash in situations where the subject is close enough to get significant lighting from the flash. Don't use flash forced on in any other situations.

Situation #5. Sensor fooled by light reflected from close object.

 This is the old finger in front of the sensor problem. What happens is that some light is reflected from the flash off the offending object right into the flash sensor, squelching the flash too early and giving you a dark shot. Common offending objects are your finger, the plam of your hand, table top holding the camera, camera strap, accessory lens, fence wire that you are shooting though, glass on a window that you are shooting through, the wire for an external flash. I have taken several shots like this myself by accident, usually it is my finger, or the wires of a fence if I am trying to shoot through one.

 

Executive summary!

* Compensate for close very light backgrounds

* Compensate for very dark backgrounds

* The flash has a limited range

* Only use flash force on on the 990 for actual fill in.

* Keep your fingers away from the sensor.