JPEG or Raw Images?
With many cameras you can choose to take photographs as a JPEG (Joint Photographic Expert Group) or as a raw image. Which to choose?
As well as the camera shutter speed, the ISO and the lens aperture, which together determine the exposure, other factors which affect the resulting photograph include contrast, colour saturation and colour hue as well as the degree of sharpening and the white balance (eg daylight, cloudy, shade, flash etc). The choice, JPEG or raw, will determine how these variables are managed.
Digital cameras record the image on the camera sensor. The resulting digital file is a raw file and it contains all of the data captured by the camera during the exposure. It is a greyscale image file which has embedded colour information to enable raw converter software to change the file to a colour image. The initial raw image is dark, lacks contrast and it is not sharp when compared to film. It is a digital negative which needs to be developed. If you choose JPEG, the digital raw file is developed internally by the camera’s own raw converter software. The JPEG file is then copied to your camera’s image card. Alternatively, if you choose raw, the file is copied to the image card as the basic raw file to be developed by you subsequently in a computer: in addition, a small JPEG file is also presented on your camera screen to enable you to see an example of the image.
Choosing JPEG
JPEG is a good choice if you require your images to have small file sizes.
JPEG images are automatically compressed by the camera’s computer to reduce the overall file size. With JPEG images all of the choices outlined above (contrast, colour etc) are locked into the image when the shutter is pressed and it is difficult subsequently to change them. You choose the degree of compression and it is not uncommon for the majority of the raw image data to be lost through this process. As the degree of compression is increased the quality of the file decreases but even at five per cent of its original size a JPEG file may still be of sufficient quality for its required purpose.
What is JPEG compression? It is the removal of data which are deemed to be surplus to requirements. For example, if a significant area of the image has a similar colour, such as in a blue sky, then instead of recording the full range of RGB colour data for each of the relevant pixels, that data will be reduced to a single value. It is just this compressed information that is then transferred from the camera sensor to the camera card and the rest of the data are discarded. This compression is “lossy” and the data in question cannot subsequently be recovered.
It is worth noting that with some file formats, for example with TIFF images (Tagged Image File Format), file compression is “lossless” in that the whole file is shrunk but no data are discarded and the compressed data can subsequently be recovered by restoring the file to its original size.
The fact that the JPEG file is reduced in size has significant advantages:
a limiting factor when using high speed shooting, with many frames per second, is that the camera’s buffer is rapidly filled and the sensor then stops recording images while the buffer offloads its data to the camera card. However, with small JPEG files the camera is able to accommodate an increased number of images before the buffer is filled
an increased number of images can be recorded on to the camera’s image card
the small size of a compressed JPEG file enables the image to be transmitted rapidly and easily over the internet. This is often important, for example in sending sports pictures to editors for publication in daily newspapers. Decisions have already been made in camera about colour, sharpness etc so the images are immediately available for editorial use without the need for any further development
JPEG images are readable by a wide range of hardware devices such as printers and scanners.
Disadvantages of JPEG include:
permanent loss of data
JPEG is restricted to files with 8-bit data (2 to the power of 8 which gives 256 levels of variation) but modern cameras record data at 14-bit (2 to the power of 14 which gives 16,384 levels of variation) and this extra information is discarded, significantly reducing quality
unlike TIFFs, JPEG format is not compatible with the development of layered images and, in the computer, layers must be compressed before saving an image as a JPEG.
Choosing raw
Raw is a good choice if you want the highest possible quality from the data captured by your camera, especially if you intend to print the photograph. All of the data recorded on the camera’s sensor are copied to the camera’s card. None of the information is discarded. However, high quality data files are accompanied by large file sizes so that hard drives are filled up rapidly!
Once the raw file has been downloaded from the camera card to the hard drive, all of the parameters associated with the image, such as contrast, colour saturation and hue, sharpening and noise, picture style and white balance, as well as the image exposure, can be adjusted on the computer. This is highly significant when quality is paramount.
With raw files:
the photographer has extensive creative control. The photographer develops the image rather than the image being developed by the camera’s in-built algorithms
14-bit data can be used to determine parameters such as the image illumination levels and colour spectrum
white balance, which makes a substantial difference to the image, may be altered / fine-tuned in the computer
colour space may be selected variously depending on the image output, for example sRGB for screen output and Adobe RGB for print
changes made to the raw file during development are saved as additional information to the raw data so that development of the image does not include any data loss from the raw file
the raw file remains intact and is available for later redevelopment, for example with new software.
Disadvantages of raw include:
file sizes are substantially larger than the size of compressed JPEG files
large files quickly fill the camera buffer and card as well as computer hard drives
the photographs are not immediately ready and need to be developed via a computer using a raw converter programme such as Capture One, Lightroom or Photoshop
skills to use raw converter software need to be acquired
whereas JPEG files are generic, raw files are proprietary with respect to the camera manufacturer. To make raw files generic they need first to be converted to DNG format (Adobe: digital negative).
Both Raw and JPEG
Some cameras provide two card slots and it is then possible to record photographs as both a raw file and a JPEG file. This enables the immediate file transfer of the JPEG over the internet along with the opportunity subsequently to develop the raw file to personal taste,