Today’s Question: You said that you retain all layers for images you work in in Photoshop in part to maximize image quality. How does using layers improve image quality?
Tim’s Quick Answer: Using layers in Photoshop primarily provides for a more flexible workflow. However, using adjustment layers rather than adjusting pixel layers directly can also benefit image quality by virtue of not compounding the effect of multiple adjustments.
More Detail: When you apply an adjustment to an image you are obviously altering the pixel values. Each time you apply an additional adjustment you’re compounding the effect, and this can have a negative impact on image detail and overall quality.
For example, increasing contrast can cause a loss of some detail in the dark shadows and bright highlights. That loss is generally very modest, and the improved appearance from the enhanced contrast is likely worth the very slight loss of detail.
However, if you enhance contrast in multiple steps rather than a single step, there can be a more significant loss of detail than if you achieved the same result with a single adjustment. For example, if you gradually increase contrast in ten steps that can cause more degradation than if you achieved the same degree of increased contrast in one step.
When using an adjustment layer in Photoshop, that adjustment layer provides the same effect as if you had only applied one adjustment, no matter how many times you fine-tune the settings for the adjustment layer. Therefore, when using an adjustment layer there is no compounding effect cause by altering the same adjustment multiple times.
To be sure, with normal adjustments for a photo the benefit in terms of image quality when it comes to using adjustment layers is very modest. However, I strongly recommend using layers when optimizing photos to take advantage of the greater flexibility and the potential benefit in terms of image quality, minor though it may be.