The first problem I could see is that my lens does not cover thirteen full f-stops, so it would not be possible for me to complete the test fully in this manner. The example given to calculate six stops darker didn’t seem helpful to me either – “if the average setting was 1/250 sec at f2.8, change the aperture to f16”. If the ‘average grey’ image was taken at f2.8, then there’s no way that six more images could be taken at wider apertures!
I turned to the example images on the CD for guidance. However, examining the EXIF data for the ‘as shot’ images showed that most of them had been taken at the same aperture. Instead, the shutter speed had been changed each time. I therefore concluded that perhaps the information in the course manual was a misprint, and that it was actually the shutter speed, rather than the aperture, that should be changed. In any case, this was the only way I’d be able to complete the test.
I put the camera in ‘aperture priority’ mode, and set the aperture to f7.1. The recommended shutter speed was 1/125 sec, so this would enable me to vary the shutter speed in order to take six darker and six lighter shots. The previous exercise had made me aware that my camera enabled incremental settings of shutter speed and aperture, rather than just complete stops, so I made sure that I changed the shutter speed by one full stop each time, rather than just turning the wheel one ‘click’.
The results:
It was apparent straight away that the images at the right-hand end were at, or extremely close to, 255,255,255. Measuring the values in Photoshop confirmed that the +4, +5 and +6 images were all pure white.
I found the values of each image to be as follows:
So, excluding those images that measure less than 5,5,5 or more than 250,250,250 (the unprintable extremes of dark and light) I concluded that my camera’s sensor has a range of eight stops, which the course manual says is ‘quite normal for good compact camera’. I suppose I should be happy with that, but I’ll admit to being a bit disappointed that it isn’t higher. The Nikon D300 is, after all, a low-end professional DSLR, so I suppose I was hoping for it to have an extended range as compared to a compact!
What have I learned from this exercise?
Firstly, things aren’t always as simple as they seem! Perhaps the guidance notes were just misprinted, or perhaps it was a covert exercise to make us find a creative solution! On a more serious note, I’ve learned that my camera would struggle to record detail in all areas of a high contrast scene. If the dynamic range of the scene is more than eight stops, there will be some inevitable burnt-out highlights or irretrievable shadows.
One possible solution I can see to this is to take bracketed exposures of high contrast scenes, in order to either blend the images together to create an image with an extended dynamic range, or to go one step further and create a High Dynamic Range (HDR) image. Both options have worked well for me when photographing subjects such as sunsets. Of the two, I prefer the more subtle exposure blending, but both options enable retention of detail in both the shadows and the highlights.
I've also learned that the mid-grey step lies closer to the right-hand end of the range than to the left. According to the course manual, this means that the D300's sensor is typical of many digital sensors, and that my images are more at risk of having burnt-out highlights, rather than blocked-up shadows. This would explain why I often find that images which the camera measures as being slightly underexposed actually have a better balance between light and dark than images that I take with no change to the exposure compensation.
Phew – Project 1 complete, and because of my confusion at the beginning, it’s taken me about four times longer than I anticipated! Let’s hope the next one’s an easier ride. :)
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