Today’s Question: Regarding image posterization in the Library module that does not appear in the Development module [in Lightroom], isn’t it possible the image may not have enough resolution to display at the magnification the user has set in the Library Module?
Tim’s Quick Answer: It is true that the preview image available in Lightroom may not have adequate resolution to present a good quality preview, especially when you zoom in on the photo. However, that would present a different visual effect than the posterization that can occur with the preview images in the Library module in Lightroom.
More Detail: The previews for your photos presented in Lightroom are created differently for the Library module versus the Develop module. In the Library module you are essentially viewing a JPEG preview for each image, while in the Develop module you are viewing a full-resolution preview based on the underlying image file and the adjustments you’ve applied in the Develop module. This is why, for example, when the source image files are not available (such as when you’ve disconnected an external hard drive) you can still view your photos in the Library module but not in the Develop module.
Because the Develop module is creating a real-time preview based on the underlying image file and the adjustments you’ve applied, the source image must be available to view the image in the Develop module. That also means you’ll see the most accurate (and highest quality) view of your image by viewing it in the Develop module.
In the Library module you are viewing a JPEG preview of your photo. In general that means the overall quality may not be as high as what you see in the Develop module. Among other things, that can cause a degree of posterization (loss of smooth gradations of tone and color) as noted in a previous edition of the Ask Tim Grey eNewsletter.
If the resolution is low (such as when zooming in on an image for which a 1:1 preview has not been generated yet) the quality will suffer, but not in a way that produces posterization. Instead, the image will take on something of a “jagged” appearance. For example, curved lines within the image will look rough rather than smooth, with more of a “stair-step” pattern than a smooth curve.
Both of these issues can obviously impact your perception of the overall image, especially when taking a close look as specific details in a photo. Therefore, it is always best to use the Develop module in Lightroom when you want to critically evaluate the quality and details of a photo.