Today’s Question: I generally make basic adjustments in Lightroom, such as Whites, Blacks, Highlights and Clarity. I then take the image into Photoshop where I make additional adjustments such as Tonal Contrast using Nik plug-ins, which I’ve used for years and love. Is making adjustments like that in both Lightroom and Photoshop (using 16-bit images) likely to have an adverse effect on the image quality? To my eye they look better and I haven’t noticed any gapping in the histogram.
Tim’s Quick Answer: The approach you describe will not cause any significant degradation in image quality. There is a theoretical disadvantage to applying multiple passes of adjustments to an image, but as long as those adjustments are relatively modest and you are working with a 16-bit per channel image, there won’t be a visible degradation in image quality.
More Detail: The core issue here is that adjustments can cause a certain degree of image degradation. Obviously the adjustment is aimed at improving the overall appearance of the image, but some degradation will occur. For example, many adjustments will reduce the smoothness of transitions of tone and color in an image. Increasing contrast or saturation will tend to have the strongest impact in this regard.
When you use multiple adjustment steps rather than a single step, there can be a compounding effect, where the final image has degraded more than if the final result had been achieved with fewer adjustments. So, for example, if you can produce the same final appearance in the photo with one adjustment rather than three, the image will exhibit better quality.
It is important to keep in mind that the differences here are generally going to be extremely minor, unless the adjustments are especially strong. In other words, with typical adjustments you wouldn’t be able to see a visual difference between two versions of an image processed with more versus fewer adjustments. It would require very detailed analysis to find any variation in pixel values under typical circumstances.
In addition, keep in mind that in many cases when you are applying adjustments in multiple steps, you aren’t actually having the same cumulative degradation in image quality. For example, in Lightroom all of the adjustments you apply don’t really alter pixel values until you export or otherwise share the photo. In other words, no matter how many times you move an individual slider in Lightroom’s Develop module, the result is as though you only moved the slider once to its final decision.
Especially when you are making use of specialized tools (such as a plug-in as described in today’s question), I wouldn’t hesitate at all to employ multiple adjustment tools in my workflow. When using a 16-bit per channel workflow, you don’t need to have any real concern about image degradation using a workflow such as this. That said, for 8-bit per channel images, these concerns can be very real indeed, especially for black and white photos.