(Updated 1/30/12) Soft proofing is new in the Develop module of Lightroom 4 (the Beta at this point). It allows you to get a better idea of what your photo will look like in print when printing with profiles, and what it will look like when you export to sRGB or another color space. If your photo has richly saturated colors, for example, when you soft-proof you will see them become muted, since many printers and papers, as well as sRGB, can’t display them so saturated. When you print to paper, your photos will also lose contrast, as paper whites are never as bright as monitor whites. Soft proofing allows you to preview this, and then if necessary, make further adjustments to your photo to optimize it given the constraints of your chosen output method.
I will have much more to say for beginners on this topic, but for those already using soft proofing in the Lightroom 4 Beta, I wanted to show you two ways to display the original next to the proof as you work on the proof. (Thank you to Mike for the question that prompted my idea for this post.)
With Soft Proofing checked in the toolbar below your photo, as you make your first Develop adjustment, you will be asked to either make the original the proof, or to make a proof copy.
1. If you prefer to make your output / proof adjustments on the original, choose Make this a Proof. Then to see both versions side by side, type Y to view Before and After side by side. Next you will change the definition of “Before”: in the History panel, right-click on the History step that immediately proceeds your first proof adjustment (or the top step if you haven’t made any yet),and choose “Copy History Step Settings to Before”.
2. If you prefer to have a separate proof copy, you will choose Make a Proof Copy. Normally you can’t view two different photos (or a photo and it’s virtual copy) side by side with Before/After, but for soft-proofing, the Lightroom team has brilliantly built this in. (I didn’t know this at first so I had posted a work-around, but I have updated this post with the direct way.) Simply make sure you have your proof copy selected in the filmstrip, go to Before/After (Y), and in the toolbar below the photos, choose “Master Photo” from the Before dropdown.
With either method, only the proof will change as you work.
Finally, no need to go to Photoshop for soft proofing!