nikonclspracticalguide.blogspot.com/2008/01/nikon-flash-two-separate-metering.html
Nikonians Speedlight Forum , I decided to consolodate my thoughts in this blog. Note: I use the term TTL to mean the same thing as iTTL throughout this blog. Notice that the first one appears as though the subject is in a cave. But the second one has noticeable detail in the background. This blog post is about being able to adjust between these two extremes at will. Flash photography is a pretty complex subject, and you have to study it a while to understand it. It is also a very rewarding subject and leads to much better pictures in many situations. The first concept to wrap your head around is that when you take a flash picture it is actually a combination of two exposures; one from available light (also called ambient) and the other from flash. Already you may be able to see intuitively that flash photography will be easier in the dark, because the contribution from ambient is zero! And, it is also probably intuitive that if the ambient light is bright, things can get a lot more difficult as you try to balance the two contributions to the image. So, Nikon made things much easier for us by developing their camera and flash metering systems - these are two completely separate systems that are used together or separately, depending on how we set up our camera/flash. like what you would find in a typical indoor home setting at night. If you take a picture in such a setting without flash, in one of the auto modes, say A mode, a typical aperture/shutter would be about f/4 and 1/4th sec at ISO 100. Now, put the camera in manual mode and increase the shutter to 1/80th sec and leave the aperture at f/4 and shoot another shot. The picture will be very very dark - to the point that the image is barely discernable. Now, turn on the flash in TTL mode and shoot that same shot, leaving the shutter at 1/80th and the aperture at f/4 (still in camera manual mode). You will see that whatever is in the center of the frame will be properly exposed by the flash. This is because the flash metering system handled the exposure and added just the right amount of flash power to get a proper exposure. It still may not be exactly the right exposure for other reasons, but I'll discuss that in a future blog entry. Now, place a subject (a person is perfect for this) relatively close to the camera (say 5 feet) with a background behind the subject (say about 15 feet away), still in typical home indoor lighting. Make sure the subject is in the center of the frame and shoot a flash shot in straight TTL (not TTL-BL) leaving the camera settings at f/4 and 1/80th sec. You will find that again the subject came out properly exposed, because the flash metering system handled the exposure. But the interesting thing is that the background behind the subject is back to being very dark, just like in the shot without flash. This is because the flash power decreases very quickly from the distance of the subject to the background and it barely brightens the background at all. Now, decrease the shutter speed to 1/10th sec and shoot again. The background will be brighter, but the subject will be the same brightness as before. Also, the subject will be sharp, but the background may also show some motion blur, because 1/10th sec is too slow to hold the camera perfectly still (unless you use a tripod). ie, the ambient was overpowered by the flash, and the flash duration is normally faster than 1/1000 sec, the subject will be sharp with no motion blur. This shows you that in a dim room, the flash exposure of the subject is controlled by the flash metering system and the background exposure is controlled by the settings on the camera. This is the key behind using the flash in TTL mode and the camera in Manual mode. Even when the ambient lighting is a little brighter than this, you can still use this technique if you stop down the camera aperture enough to make the flash the primary light on the subject, which takes the ambient light out of the equation. And if you try to brighten it by decreasing the shutter speed, then the bright ambient light will begin to affect the subject brightness too, and you begin to risk overexposure of the subject. In this situation, shutter, aperture, and flash power all affect the brightness of the subject. It's usually best to simply live with a dark background in this situation and make sure the flash is primary on the subject. Now, if you are in bright ambient light (like outdoor in daylight), it is simply too bright to be able to take the ambient out of the equation by stopping down the camera, so now you have to balance the flash and the ambient. If the flash contribution to the shot will be less than the ambient contribution, then it is called Fill Flash. When there is a lot of ambient, the background and subject are no longer able to be adjusted independently, and camera manual mode becomes difficult to use. This is when you want to switch the flash to TTL-BL and the camera to one of the automatic modes (S, A, or P). The camera will then measure the ambient light to set its aperture and shutter and send this information to the flash, after which the flash metering system will set the power of the flash to make the subject the same brightness as the background. If the ambient is extremely bright, like direct sunlight, then it's usually best to select P or S mode (and TTL-BL), so the aperture will automatically stop down to avoid overexposure. In A mode, the shutter will increase to reduce the exposure, but it will be limited by the flash sync speed of 1/250th (D200) and that is often not high enough for bright light if a wide aperture is selected. One caviat for TTL-BL: the subject must be darker than the background for TTL-BL to work properly. The flash can only brighten the subject to balance it with the background; If the subject is brighter than the background to begin with, then you probably are best off not using flash. However, there can sometimes be slight shadows on the subjects face that can be 'lifted' with a very slight amount of flash in TTL mode.
Russ, Your "Nikon CLS Practical Guide" write up for the Nikon SB series Speedlight is superb. If your write-up came in the box with my SB-600 or my D80, I would not have spent so much time on the web trying to clarify information contained in the Nikon User Manuals. Nikon did not take the time to explain the interaction of the camera and flash systems that make up CLS, i-TTL and i-TTL BL. Without the additional information you provided, one would need to spend hours experimenting, as you did, to fully understand the interaction of the various flash and camera settings. Your explanations are clear and make it easy for one to understand how the camera settings, flash setting and lighting situations are processed for "i_TTL" and "i-TTL BL" exposures and the impact they have on one another. Thanks for taking the time to consolidate this great paper.
Right now the wedding business is pretty slow, so I have lots of time, but Easter marks the start of the busy times, and I won't have much time for studying things Nikon from then until the end of the year.
I just recieved my sb800 a couple of days ago and was finding some of the concepts very confusing. You did a really excellent job of making a complex subject easy to understand.
I just read these comments of yours and they are very very clear and informative. Quick question - at the end you say it is better to shoot in P or S with TTL-BL because camera will revert to 1/250 of a second. I was under impression the whole idea of -BL is to match the ambient. Once again, thank you for this extremely informative blog.
Hi Michael, You are correct that the reason for TTL-BL is so that the flash will match the ambient. The flash metering system does this by first analyzing the brightness of the background from the metered data from the camera. Then it adjusts its power to make the subject the same brightness as the background. So, to say it another way, the camera metering system controls the background exposure, and the flash metering system controls the flash power to match the subject to the background. Now the reason I suggest using with P or S when shoot...
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