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To see if there were notable differences between the RAW and JPEG formats when processing underexposed and overexposed images, I conducted the following test. In a series of images, I underexposed the test subject by 1 exposure value (EV), 2 EV, and 3 EV, and then overexposed it by 1 EV, in both the RAW (.NEF) and JPEG file formats.
In processing, I made the requisite correction (+/- EV) necessary to bring each image back to the same exposure. For example, if I underexposed an image by 1 EV during capture, I processed it at +1 EV. I processed the RAW files in Adobe Camera RAW (ACR), and the JPEG images in Photoshop CS.
On all shots, I used a Nikon D70 six megapixel camera and a Nikkor AF 20-35mm EDIF f/2.8 lens at 35mm. The aperture was f/16, and the ISO rating was 400. I made the images in Aperture Priority mode with Matrix metering. I mounted the camera and lens on a Gitzo tripod, and the lighting on the test subject was natural directional lighting from the left side. I took the JPEGs at the largest file size (the same size as the. NEFs) with the detail setting at “normal.”
The only variable in the compared images at any given exposure value was the format: one was RAW (.NEF) and the other was JPEG. After processing the test images, I viewed them in Photoshop at 100%. I sampled and compared four different areas of the subject. The table below, Figure 1, reveals the amount of increase or decrease in exposure value that each sample received during capture and processing.
The test subject was the Quaker Oatmeal box in Figure 2. I selected the samples labeled +1, 2, and 3 EV to compare mid-tone details. I made the samples labeled +1, 2, and 3 EV S for shadow comparison. Finally, I made the samples labeled -1 EV A and -1 EV B for highlight comparison.
Samples from Test Images at 100%. Exposure Value (EV) During Processing Noted
+1 EV RAW (Sample 1)
+1 EV JPEG (Sample 2)
+1 EV S RAW (Sample 3)
+1 EV S JPEG (Sample 4)
+2 EV RAW (Sample 5)
+2 EV JPEG (Sample 6)
+2 EV S RAW (Sample 7)
+2 EV S JPEG (Sample 8)
+3 EV RAW (Sample 9)
+3 EV JPEG (Sample 10)
+3 EV S RAW (Sample 11)
+3 EV S JPEG (Sample 12)
-1 EV A RAW (Sample 13)
-1 EV A JPEG (Sample 14)
-1 EV B RAW (Sample 15)
-1 EV B JPEG (Sample 16)
Observations
- The underexposed RAW and JPEG files, which I corrected during processing, each reveal different levels of noise directly proportional to the degree of their respective underexposure. However, at any given level of underexposure, the noise was more noticeable and pronounced in the 8-bit JPEG files than it was in the 12-bit RAW files.
- In both formats, at any given level of underexposure, noise was more noticeable and pronounced in the shadows than it was elsewhere in the image.
- When the 8-bit JPEG test image was overexposed 1 EV, it lost highlights that could not be recovered during processing. See Samples 14 and 16. When the 12-bit RAW test image was overexposed 1 EV, at least some of the highlights were recovered during RAW conversion. See Samples 13 and 15.
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