Active D-Lighting, be careful!

I am going to upset some people, because I will be the first to point out that the Emperor has no clothes. There are some things about Active D-Lighting that are counter intuitive and no one has pointed this out, yet.

I am going to point out that there are only two instances in which you ought to use ADL. Both require that you use matrix metering and ADL set to Auto. The first is where you have to include a fully sun illuminated scene AND a shaded or interior scene like a room with good window lighting or the shade of a large building. The second is when using stage lighting where there is shadow area that needs to be included.

In most situations, there are about five zones, or stops of light, such as the still life shown in the header. Black and white objects illuminated by the same light source require only five zones to hold all the detail. The sunny sixteen rule says that an ISO 100 setting requires 1/100th shutter speed and f16. It further states that a shady eight day or the shade side of a building or a large tree requires f8. The f11 perfect day is part sun and part shade. Armed with this knowledge, you can take a lot of pictures in manual exposure mode.

Knowing that the difference between the light on the North and South sides of any building in any hemisphere is two stops and you are going to take a picture where the average light is two to three stops different from the shade scene to the sun scene. This means we have to find two or three extra stops to include the details of the typical ADL scene.

Before it rolls off the top of the page, I need to point out where the stops are that are on the histogram above. The frame is delineated into  four segments in each axis. At this point I wanted to define what you see in the histogram as it relates to exposure. I made a series of exposures of test targets to prove what must be obviously true. It didn't happen. The only thing you can say is that the center vertical line represents 128 or the supposed middle gray value of a gray card. You would expect that line to be the lower boundary of the first stop of white, but it is not, because the sensor has non linear output and the camera has applied an S curve of its own and the x axis is supposed to represent input values, but in fact the histogram along the x axis represents the scaled output values. The height of the spikes on the histogram represent the count of the number of occurrances of the value on the  low axis. The numbers 0 and 255 on  the vertical axis have nothing to do with the scale value of the histogram.

Left of center represents many more stops of exposure than the right. How many? It depends on the contrast, but for all practical purposes it is infinite. Each stop moving towards black has half as many units as the current stop. The right side although having some compression is finite and when 255 is reached, that is the limit and information is lost.

The two right hand segments constitute the two first bright stops or zones. You can see that there is 1/2 stop on the right that is not used. The tall peak of the histogram is the white wall. The shorter peak in the same segment is the gray cards on the table and behind the objects. The center line represents 127 or 7F in hex. The line at the left is the next stop boundary and represents 63 decimal or 3F in hex and is the second stop. If we draw an imaginary vertical line midway between the 63 line and zero we have the third stop down. We then create the fourth stop or zone which has half as many steps and so forth until the steps are very small.  Theoretically this can go on forever.

Remember the riddle that a man can walk half the distance to his destination on the first day and then half the distance on each day thereafter? When does he reach his destination? The answer is, never. This is true of our light problem. The black end of the spectrum is like a black hole of data especially when you have a raw file which has 16 times the steps as a JPEG file at 255. So what happens on the bright end of the histogram? Any value that exceeds 255 stays 255. No new data is recorded beyond 255.

It is the job of the camera and photographer to prevent data from being lost and that is what the magic is in D-lighting. Nikon knows that eight out of ten photographers would rather poke a stick in their eye than to set the camera to low contrast. Everybody knows what low contrast does. It make a flat lifeless print. The solution? Give it a new name. Call it something positive like "Active", "D-Lighting", but don't call it low contrast.

Matrix metering knows when the  scene itself has higher than normal contrast and so it can anticipate that first in camera contrast and lower the exposure by 1/3 of a stop for each step to accommodate for the extra space needed by the too bright scene. It would be foolish to try to anticipate how many steps to take in such circumstances, so let matrix metering compute the settings automatically by setting ADL to auto. In auto, if you don't need ADL then contrast is not lowered and exposure is not decreased.

Some people have reported that flash photography in dark rooms is enhanced such that several people in the same photo at different distances from the camera seem to show a better balance of light. I don't do that kind of picture very often and I use large diffusers when I do, but you might consider that suggestion if that is the kind of pictures you are taking. The photo at the right is the situation that might make Active D-Lighting work. Because of the distance from the flash, the third person back should be in deep shadow, but is not because of low contrast being imposed and an aggressive boost in the low values. The best time to do these adjustments are very early in the process when there are 4000 raw units to work with instead of 255 for JPG in order to have smooth texture of the shadows.

In review of the methods used to control exposure I would like to point out some facts we have learned.

We must by all means possible prevent over exposure, because overexposure causes data loss in the highlights. If you must err in exposure, then underexpose. Data is never lost in the shadows. You must get the exposure right in order to center the data in the histogram so it can be manipulated by the conversion from raw to something useful such as print making or web publishing. In my experience that means setting the exposure bias to -1/3 EV on my particular camera in the Standard Picture Control.

I have become accepting of seeing the left side of the histogram pile up instead of gently sloping to zero. One of the things I have begun to see is that slope to zero on the left of an ADL exposure. I think many photographers including me have tended towards high contrast when lower contrast would have been more appropriate.

Auto Adaptive D-Lighting will additionally reduce exposure by up to one stop as needed to protect the highlights as required by measuring the range of values between black and white. Remember the exposure needs be reduced to either compensate for the overly contrasty subject or the camera's addition of contrast by a flat subject. Applying anything other than Auto ADL leaves you at risk of making a fatal result. Off or Auto is the only choice for me.

None of the other choices for exposure control adjust the exposure relationships of Speed, Aperture or Auto ISO. Any modifications of tone happen after the raw data is captured. Auto Contrast within the Picture Control will give about +/- 1/2 stop of adjustment and can only protect the highlights by that range. Any of the brightness settings only effect the middle range of tone, not the outer points on the histogram.

A curve can be designed to minimize the chances of overexposure but cannot help in the case of gross overexposure. The curve you design is geometrically added to the base curve of the picture control and can result in a curve with a step in it. Note on the curve to the right that the  value of 255 is translated to something less than 255 on the output.

D50 and D70 owners are fortunate that they can shoot flash in the bright noonday sun to remove face shadows because of the fast 1/500 sync speed of the electronic shutter on those cameras. The mistake I make most since getting the D90 is that if I leave any part of the camera in auto mode and if my aperture is f8 or lower the auto ISO will cause my picture to be overexposed because it will not go down below ISO 200. Using the rule of thumb for exposure my exposure should not be brighter than f11 at 1/200 at ISO 200. The D90 will not set shutter speed above 1/200 with the flash on and the default shutter speed for auto operation is limited to 1/60. I am forced to set the camera in manual at f11, 1/200 auto TTL flash at -.3 EV or less and auto ISO off and set to L1 to 200. That does not leave much wiggle room and if I forget to set auto ISO off will result in overexposed pictures.

In the next chapter we are going to consider what can be done after the picture is taken.

Continue to Chapter 6  

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Back Home to Exposed

1  History of curves and programmable contrast and gamma

2  Picture Control and Picture Control Utility

3  Creating and installing Custom curves in the camera

4  How to design a curve for your needs

5  Active D-Lighting

6  D-Lighting applied after the shot

7  Where to go for more information

8  The DOWNLOAD page


© Leon Goodman 2009

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