- Normal indoor ambient lighting
- Subject five feet away is a small statue of a boy on a horse placed in the center of the frame to be sure the flash metering will measure it properly
- Background is a bookcase 10 feet away
- Camera in Matrix metering
- Flash in TTL mode
Image 1: Flash OFF, Camera A mode, 1/25th, f/2.8, Camera ev=0
I shot Image 1 with the flash Off using camera A mode with f/2.8 selected. I could see in the viewfinder that the camera automatically selected 1/25 shutter. Notice that the image is properly exposed with pixels in the histogram extending all the way from black shadows on the left to bright highlights on the right and there are some shadows on the face and is approximately the same brightness as the background. The yellow color is due to the incandescent lighting and the poor Auto White Balance function on the D200. I will not try to correct or balance the color during this study.
Image 2: Flash On in TTL mode, FEC=0, Camera A mode, 1/60, f/2.8, Camera ev=0 Note: FEC means Flash Exposure CompensationIn Image 2 the flash was used, and as soon as the flash was turned ON, the shutter changed to 1/60th, which is the lowest available flash shutter setting as determined by CSM e2. Note that the faster shutter darkened the background a bit compared to Image 1, which is to be expected. You can see how the dark pixels are bunched up against the left edge of the histogram, meaning that detail is being lost in the shadows. The hump of pixels on the right come from the subject. Note that the histogram shows that there are no blown out pixels (ie, climbing up the right edge), but the subject still looks too bright for the scene. This happened because the flash metering system measures a little too 'hot' on my D200/SB800 combination. I think this is a very common thing on the D200/SB800 combinations. That's why I normally dial in -.7 ev on the flash for indoor shots (but this is a sidebar and not the main subject of this study).
Notice also that the color of the bookcase in the background is still yellow since it is lighted mainly by the ambient incandescent lights while the subject is much whiter since it is lighted mainly by the flash.
Image 3: Flash ON in TTL mode, FEC=0, Camera A mode, 1/100th, f/2.8, Camera ev = -2 In Image 3 I decreased the camera EV to -2.0. You can see that both the background and the subject got darker (just like the SB800 book says they should). You can also see that the hump of pixels on the right moved left just like the darker pixels on the left, piling more of them up on the left edge. The question I am studying is exactly how did the camera & flash achieve this left shift of the pixels?
Well, I observed that the shutter automatically increased to 1/100, which darkened the background. You would have expected that when starting at 1/60th it would have increased to 1/240th to achieve two stops darker. However, I have discovered that the camera knew the ambient was actually at 1/25, and 1/100 is exactly two stops darker than 1/25. Pretty smart system.
Remember that changing shutter speed does not affect the flash portion of the exposure, because the flash occurs is so short (less than 1/1000th second).
And you can see that changing the camera ev definitely darkens the subject as well, which means it has to decrease the flash power. That indicates that the camera compensation information is sent back to the flash metering system when shooting in TTL mode.
So, when you change the camera ev, it changes the camera f/ stop or shutter speed settings to adjust the background, and it sends the ev change to the flash to change the flash ev by that same amount to adjust the subject.
Image 4: Flash OFF, Camera M mode, 1/25th, f/2.8, Camera ev = 0
To further verify exactly what the camera ev settings tell the flash, I first put the camera in Manual mode and set 1/25 and f/2.8 and shot Image 4 above without flash as a reference. Note that Image 4 and its histogram looks essentially identical to Image 1, as it should, since the same camera settings were used.
Image 5. Flash ON in TTL mode, FEC=0, Camera M mode, 1/60th, f/2.8, Camera ev=0
Then in Image 5 I increased the shutter to 1/60th and fired the flash, and this image looks just Image 2, as it should.
Image 6: Flash ON in TTL mode, FEC=0, Camera M mode, 1/60th, f/2.8, Camera ev = -2
Now, in Image 6, with the camera in Manual mode, 1/60th, f/2.8, you can see the effect of setting camera ev to -2. Comparing with Image 5, the subject in Image 6 definitely got darker, and the histogram hump on the right moved slightly left, but the background didn't change. This proves that ev changes on the camera are sent to the flash to the change its power, and the reason the background didn't change is because ev changes to the camera will not affect the f/ stop and shutter when in camera Manual mode. In the previous example, the camera was in A mode, and camera ev changes will change the shutter speed.
Image 7: Flash OFF, Camera M mode, 1/250th, f/2.8, Camera ev= 0
I took this shot without flash as a reference image to show just how little ambient there is at 1/250th. 1/250th virtually takes the ambient light out of the exposure in normal indoor lighting.
Image 8: Flash ON in TTL mode, FEC=0, Camera M mode, 1/250th, Camera ev=0
In Image 8 I turned on the flash and took the shot. Notice the dark background, because the flash exposes the subject correctly, but only a small amount reaches the background five feet behind the subject (due to the square law - power decreases by the square of the distance).
Image 9: Flash ON in TTL mode, FEC=0, Camera M mode, 1/250th, f/2.8, Camera ev = -2
Then, in Image 9 camera ev was reduced by 2 ev and the brightness of the subject (power of the flash) definitely decreased compared to Image 8. The brightness of the background barely changed, though because not much of the flash power is getting back there. This is showing that you can adjust the brightness of the subject without changing the background by changing flash power.
Image 10: Flash On in TTL mode, FEC = -2, Camera M mode, 1/250th, f/2.8, Camera ev = 0
Lastly, in Image 10 I set the camera at 1/250, f/2.8, and ev 0, and I used the Flash ev on the back of the flash to -2 ev. As you can see, the image looks exactly like Image 9, and the hump of pixels on the right (representing the subject) is in the same spot on the histogram.
This study proves conclusively that camera ev is sent to the flash in TTL mode and can be used to adjust flash power, just like the FEC button on the front of the camera, and the FEC button on the flash itself.
Next: Sequence of Events for TTL Flash