I recently prepared a huge order for a client.  Nineteen canvas prints in total, and seven of them are 40×60″ prints.  I couldn’t just resize in Photoshop, save as a JPG, and upload the file.  It’s a tough job that required at least 20 hours just preparing the files to send to the printer.  But that extra work is necessary to make sure the images look crisp and the colors and tones really pop.  With large prints, the enlarging and sharpening process is especially critical, as you’re stretching the limits of your digital files.  My process generally looks like the following:

1. Start the best possible photo.  There’s no point in enlarging a mediocre image.  Whether I’m using a 12 megapixel Nikon D700 or a 36 megapixel D800 camera, there is very little room for sloppy technique when I enlarge to 40×60″.  Focus error is obvious.  Blur from mirror slap or a shaky tripod is obvious.  Shadow noise created by correcting an underexposed image is obvious.  So I shoot with high quality lenses, on a tripod (weighted down for stability), with a bubble level (both to avoid loss of pixels from rotating an image that isn’t straight and to monitor camera vibration), with mirror lockup, with careful exposure settings at the lowest possible ISO, and save to a 14-bit RAW image.  You can take a lot of shortcuts to create an image that will look beautiful on a website… and terrible on your wall.
2. Get as much out of the RAW image as possible.  There are several great options, but I personally prefer to edit my RAW images with Adobe Lightroom.  The newest version (4) offers dramatic improvements, and I’ve been blown away at how much I’ve been able to improve images I took years ago.   To prepare for enlargement, be sure to pay extra attention to sharpening (don’t overdo it early), noise reduction (use the masking feature to minimize noise in skies or solid areas), and chromatic aberration.  All of those settings work primarily at the pixel level, and pixels start to really matter when you enlarge.  Additionally, do as much tone adjustment as possible here.  If you are going to dodge and burn, this might be the right time to do it.  You may choose to do some of this work later if you are going to manipulate the original file significantly before printing.
3. Retouch.  I use Photoshop to clean up sensor dust spots, chromatic aberration I couldn’t correct in Lightroom, portrait retouching, etc.  Sometimes I do this last if know I likely won’t use the image again at some other size (doing it during my final inspection is faster, but resizing twice becomes an issue, especially if the image has been sharpened).
   
4. Enlarge.  I use Resize (formerly known as Perfect Resize and as Genuine Fractals).  Many people swear by using Photoshop’s resize tool (enlarging 10% at a time until you reach your final size), but I love the simplicity and beautiful results I get from Perfect Resize.  I dial in a few settings – often just selecting 300 dpi and the default “landscape” settings, with no sharpening or other treatment.  One nice bonus of Perfect Resize is that it includes the ability to mirror the edges of your image so that you can print on a gallery wrapped canvas and keep the entire image on the front.  If I’m only doing a minor enlargement (50-100% maximum), Photoshop works great and saves time.
5. Sharpen.    I use both Nik Sharpener Pro and Photoshop’s Smart Sharpen.  Photoshop’s built in filter is an excellent for many jobs, but I like Nik for really large prints / canvas, as well as for jobs requiring selective sharpening.  It is designed to easily let you select the right settings for a given media (I select “continuous tone” when sending to my vendors).  And it has “U-point” technology, which lets you very easily select the sky to avoid sharpening grain, without having to do complex masking.  When I don’t need the extra features of Nik, I prefer Photoshop since it offers a much faster workflow.  When it’s done right, the sharpening should look a little too “crunchy” on screen.  Below is a sample.
6. Adjust for black point.  I’m amazed that many top professional labs don’t fully appreciate the value of this step.  Even with a calibrated monitor, I find that my prints are consistently darker than what I see on my screen.  It’s certainly possible that my monitor (which is at its minimum brightness) might simply still be too bright, but I have yet to find a vendor who can print detail down below about (8x8x8 RGB).  I send test targets to my vendors that have every shade of black from (0,0,0) to (20,20,20) to find out the darkest black that shows any difference from (0,0,0).  Based on that, I add a curves adjustment layer to my image that adjusts the image so that 0 on my monitor is set to the minimum value for my printer.  I find that adjusting to 8-10 works well for nearly all my labs, and my prints come out looking amazing.
7. Final inspection.  I zoom my image so that the ruler in Photoshop is to scale (ie, one inch on the ruler is approximately one real inch on the screen).  If I can see problems at this size, I’ll probably see them on the final canvas.  I start in the top left, and use the scroll bars to work across the image systematically and inspect everything.  I keep any eye out for grain in skies, dust spots, and anything else I might have missed.  I might have looked at the image 50 times on my computer and still have overlooked some small detail that is going to stand out in a large print.
8. Send to a pro lab.  You get what you pay for.

 

Here’s one of the 40×60″ gallery wrapped canvases (“A Hazy Glow Over the 3rd Ave Bridge”).

 

This was taken with a 36-megapixel D800, but is still only 118 dpi without enlarging.  Using Photoshop to enlarge and then sharpen (via Smart Sharpen), I get the following.

 

Photoshop has done a pretty decent job, and I could definitely print this as a large canvas.  If this had been a 12 megapixel image (such as from the D700), I would absolutely have to do better because I’d only be enlarging 4X (from 71dpi to 300dpi).  The sample below shows the results I got using Perfect Resize 7 and Nik Sharpener Pro 3.  The image is clearly sharper and more detailed, and is going to make the image look amazing when printed at 40×60″.