I decided to test my Canon 5D Mk2 exposure accuracy as I know underexposing is an issue with digital cameras. See How accurate is your camera’s exposure? for more information.
As noted previously the early EOS range of digital cameras underexposed by about 2/3 of one stop. This may have been due to concerns of loosing image detail when the highlights burnt out. However, this also meant that a lot of noise was introduced in the darker tones of your images. And the problem was compounded further by photographers underexposing deliberately through fear of burning out the highlights.
The digital technology has improved in the last few years with having a great number of bits per colour channel and extending the dynamic range captured within RAW images files. This has changed the thinking of deliberately underexposing images. Now you’re more likely to hear about slightly overexposing to reduce noise in the dark tones.
My personal view is that the darker tones are best controlled with fill-light in the studio. And it is best to aim for correct exposure in ambient lighting conditions. This is very much a conclusion I’ve come to after following a series of tutorials by Dave Montizambert a world expert on lighting for digital. This is a vast topic and is way beyond what I’ll cover here.
The test
To determining the accuracy of the exposure meter, I used my pop up neutral 18% grey card and carried out a series of exposures. Although the card isn’t calibrated it is more than adequate to find the right exposure compensation required to correct the camera’s exposure. The camera’s exposure meter and compensation can be adjusted in 1/3 stop increments at it’s finest granularity.
The first shot is lit by a studio light and metered at f/2.8.0. The following shots are taken using the ambient light spot metered off the centre of my reference grey card. The white of the small target area will have minimal impact on the final exposure.
Studio reference shot
-1/3 Conpensation0 Compensation
0 Compensation
+1/3 Compensation
+2/3 Compensation
In Lightroom I measured the brightness levels for each colour channel to compare with the reference shot. 18% grey should be the mid point, so 50% is the target figure for each channel after colour correction.
| Exposure | Red | Green | Blue |
| Studio | 49.9 | 50.0 | 50.3 |
| -1/3 Compensation |
33.2 | 33.2 | 34.6 |
| 0 Compensation | 43.2 | 42.8 | 43.5 |
| +1/3 Compensation | 47.1 | 46.6 | 47.7 |
| +2/3 Compensation | 55.0 | 55.2 | 55.5 |
Conclusion
The accuracy of these camera’s meters is improving all the time and using your camera with zero exposure compensation will produce perfectly fine pictures. However for the most accurate exposure +1/3 stop is closest to the reference shot taken with a studio light. Once the camera is set to +2/3 stops compensation it is clearly overexposing the image.
Pat
Bookmark and promote!
{ 1 trackback }