Recently I was able to make a beautiful enlargement to 40×60 inches from an 8.3 MP capture from a Canon 30D using the following method. (Actually, I cropped the image in Lightroom, so I started with a file with an even smaller native resolution of 3128×2085.)
1. Process the DNG file with no sharpening. Using the controls in the Develop module, get the file as “perfect” as possible with regard to cropping, color and contrast. Apply very slight noise reduction if necessary, and make sure to correct any chromatic abberation, which dramatically affects sharpness.
2. Export from LR as 16-bit ProPhoto PSD (or TIF, if you prefer) at 240 ppi with no resizing.
3. Before doing anything else, apply PhotoKit Capture Sharpen Expert > Digital Hi-Res Sharpen.
4. Make any necessary Adjustments that were not taken care of in Lightroom, such as localized dodging and burning or selective color adjustments.
5. Apply PhotoKit Creative Sharpener > Sharpening Effects > Luminance Sharpen 3 (this selection will depend on the characteristics of the image you’re working with; it’s a good idea to experiment to learn the best kind of sharpening for different types of images).
6. Save out the layered Master PSD file, then flatten it. (And don’t save again just yet.)
7. In Image Size, upsample the image 200% using Bicubic Smoother.
8. Zoom in to see if there was any apparent loss of detail from the resampling. If so, apply another round of sharpening using either PhotoKit, Unsharp Mask or Smart Sharpen, depending on the needs of the image.
9. Upsample the image to its final size, again using Bicubic Smoother. Apply another light round of sharpening if necessary.
10. Flatten again and Save a copy of the resized file for printing.
11. Before printing, apply PhotoKit Output Sharpener > Inkjet > 240 ppi glossy
In summary: making good enlargements from digital captures requires several rounds of gentle sharpening interspersed with several incremental rounds of upsampling. You will get much better results – and have more control over the final image – if you don’t try to do all your sharpening or resampling in one step. And of course, some images will have more potential for huge enlargements than others, and no amount of sharpening will make up for an out-of-focus image. (5D owners properly using these techniques may be amazed by the possibilities for enlargement.)
I strongly recommend that you work on your Master File at native resolution, as far through the pipeline as possible. Do your capture sharpen, creative sharpening and all adjustments on the Master File and when you are done with all your edits, do your upsampling on a flattened copy of the file. This way, you can always go back to your Master file and make new derivatives for specific print sizes without having to do your primary work on huge files. And keep in mind that the ideal settings for any sharpening depend on the pixel resolution of the file.