Grayscale, lighter digital black and white
In Photoshop, file sizes increase when layers are added. In black and white, keeping an image’s three RGB layers is not necessary. Converting it to grayscale reduces the image size.
Digital cameras record images in RGB mode (red, green, and blue) to reproduce colors. A Leica Monochrom, which delivers black and white images, does the same. When opened in Photoshop, the three layers of the images can be observed, with one monochrome layer per RGB color.
Cameras record images in RGB mode (red, green, and blue) to reproduce colors. When opened in software like Photoshop, they are composed of three layers, one layer per color.
Sampling
The most common image recording formats, JPEG, TIFF, and PSD, are commonly used with a sampling depth of 8 or 16 bits. A bit is composed of two values, 0 or 1. A digital image with a depth of 1 bit only has black (0) or white (1). The pixels of an image must have multiple bits to reproduce nuanced gradients without tone breaks. With a depth of 8 bits, the image has 256 values; at 16 bits, it reaches 65,536. In a color image, when each RGB layer has a depth of 8 bits, we reach 24 bits: 8 for red, 8 for green, and 8 for blue. Thanks to the combination of each layer, a 24-bit image delivers 16.7 million different colors. When each layer is sampled at 16 bits, we obtain 48 bits (3 x 16 bits), or 281,475 billion tones.
The most common image formats, JPEG, TIFF, and PSD, are commonly used with a sampling depth of 8 or 16 bits. 16 bits are recommended when making changes to the image to preserve the nuances of tones.
File Size
The size of an image is expressed in bytes, kilobytes (KB), or megabytes (MB). A pixel sampled at 8 bits weighs 1 byte. 1 million pixels at 8 bits, 1 million bytes, or 0.95367431640625 MB (to convert bytes to MB, they must be divided by 1024×1024). In RGB, we have 3 x 8 bits, so 24 bits, or 2.86102294921875 MB for 1 MP.
The size of an image is expressed in megabytes (MB). In RGB, each million pixels weighs approximately 2.86 MB with a depth of 8 bits, double in 16 bits. An RGB file of 45.4 MP weighs 130 MB in 8 bits. If it is saved in 16 bits, its size reaches 260 MB.
Grayscale
Lightroom is primarily compatible with RGB images. However, Photoshop can save them in grayscale, CMYK, Lab, etc. Black-and-white enthusiasts who work on large files in Photoshop with multiple layers sometimes experience computer slowdowns. If an RGB image is converted to grayscale, its size is divided by 3. Post-production is smoother. A 24 MP image goes from 68.7 to 22.9 MB with 8 bits and from 137.3 MB to 45.8 MB with 16 bits.
The 45.4 MP RGB file’s size goes from 260 MB in 16 bits to 86.7 MB in grayscale, which is one-third of the initial size. Such a reduction lightens the computer’s workload, especially when using multiple adjustment layers.
The three identical layers of the RGB image are transformed into a single layer without losing information quality. However, it is impossible to add color to the image unless the file is converted back to RGB.
Grayscale Conversion
How and when should the grayscale conversion be performed? When working with Lightroom and Photoshop, it is best to convert the color image to black-and-white in either of these applications, depending on which one you feel more comfortable with. When the grayscale values seem satisfactory, you can switch to grayscale in Photoshop by selecting Edit > Convert to Profile. Photoshop has several grayscale spaces. We recommend Gray Gamma 2.2. The conversion mode from one working space to another (for example, from ProPhoto or Adobe RGB to Gray Gamma 2.2) is Relative Colorimetric; if another mode is selected, Relative Colorimetric will be applied.
A B&W image in RGB has three identical layers. It is better to convert it to grayscale using the Edit > Convert to Profile… menu. Photoshop has several grayscale spaces. We recommend Gray Gamma 2.2.
Adjustments and Filters
In grayscale, all the essential adjustment layers for image processing are available, including those for curves and levels and most filters. Soft proofing with RGB ICC profiles works. When printing from Photoshop with color management controlled by the application, the image can be converted from the Gray Gamma 2.2 space to the RGB printing space. For printing with online labs, convert a copy of your grayscale files to the Adobe RGB space (Edit > Convert to Profile…). Finally, if you need to add a colored tint to the image (sepia, etc.), convert it to RGB. We recommend Adobe RGB for black and white images. When going from Gray Gamma 2.2 to Adobe RGB, the file is not altered because the tone response curve of the two spaces is a Gamma 2.2.
The essential adjustment layers for image processing, such as curves and levels, as well as most filters, are available in grayscale. We recommend making adjustments with the 16-bit depth.
Discover the photography courses at Spéos
Spéos offers various training courses ranging from simple one-week photography workshops (initiation and advanced level) to 3-year courses. The long courses to become professional photographers allow you not only to master all the photographic techniques and its vocabulary (blurs, hyperfocus, sharpness zone, depth of field, backlighting, focal length, shutter release, autofocus, wide-angle, rule of thirds, etc.), but also all the stages of shooting and image processing.
Visiting the school allows you to discover the premises, the studios and the equipment, and is undoubtedly the best way to familiarize yourself with your future way of working. This is why, in addition to the open days, Spéos offers throughout the year personalized visits by appointment to come and discover the school with a member of the team.
Text and photos: Philippe Bachelier, teacher of Printing techniques at Spéos
