Digital imagery has wonderful potential for enlargement. Making that enlargement recommends a procedure that eliminates the color aliasing that can sometimes occur when one simply chooses Image>Image Size and resamples the data to a larger size in RGB or CMYK. Once again, we will make a mode change in Photoshop to the Lab color space. Lab is Photoshop’s native colorspace and it has advantages over color descriptions that couple the luminance and the chrominance together (see Colorspace: What is It?). Lab has the ability to describe color where the luminance is described separately from the chrominance. This means the density of a color can be described separately from the hue and its saturation. This color space has value when making enlargements; we can enlarge the density of the image after subtracting the color, sharpen the newly enlarged file, then reapply the color before converting to cmyk and saving the file.
Lab is very much like the Beta videotape format. Remember when the war went down between Beta and VHS? Everyone said Beta was better but since no one had tape decks or tapes, everyone opted for the VHS standard. When you could find tape decks they were more money. Video stores had very few Beta format tapes for rental so Beta became a bother and finally died altogether as a consumer format. Professional videographers have always used the Beta format. Why? Because of the wonderful ability to make changes to the luminance portion of the image without affecting the chrominance portion of the image and conversely, making changes to the chrominance without affecting the luminance. Here, we’re simply making changes to the luminance after subtracting the chrominance, we re-sample the file, sharpen if necessary, realign the chrominance, and save the file. This type of Lab interpolation has been standard procedure on high-end pre-press equipment for some time. Since Photoshop allows this type of advanced interpolation, you should take advantage of it. Start by saving the capture to disk. Open the file in Photoshop. Decide if the image needs unsharp masking. Use MegaSharp if you can, MegaSharp is available as a Photoshop Plug-In. Make the mode change, Image>Mode>Lab. This conversion will produce no change in the monitor display, unlike some mode changes to the cmyk color space. Once converted to Lab, hit Command+1, Command+2, and Command+3. Command+1 will change to a normal looking black and white image. Command+2 will look like a color negative without the orange mask and so will Command+3. Command+1 is the place where we’ll do our enlarging. Choose Command+1, then Image> Image Size. You will get a dialog box that looks like this:
Make sure the Resample Image option is checked and make sure Bicubic is the interpolation method selected. Type in the new size desired, note the new file size at the top of the dialog box and click OK. When the monitor updates, you’ll have a new, larger image. Carefully evaluate the image for additional unsharp masking. If the interpolation has softened edges to the point that some additional unsharp filtering can improve the image, apply it while you are still in the luminance channel (Photoshop calls this channel Lightness). Convert the file to color by choosing Command+~. Toggle the sharpening on and off using Command+Z. If the sharpening is acceptable, make the mode change to cmyk and save with a new name to avoid confusion with the raw RGB. It’s a good idea to keep the original RGB capture unchanged so that you can use it in a variety of different places. This alternate usage is sometimes called repurposing. How big can you enlarge? That depends on the quality of your capture. If you’ve done everything just right you can enlarge the file size about 10 times, up to 120 MB. Sometimes you can go more, it depends on the scene, kinda like shooting sand with a 35mm camera; the grain isn’t objectionable when you render something already grainy. Expect smooth reflectances to enlarge well and resolution intensive reflectances (like type on a page) to allow more moderate enlargement.
A new interpolation technology call fractals is something to keep your eye on. It uses a different mathematical algorithm than the usual bicubic type we’re use in Photoshop. Rumor has it, lots of R&D is going into this field. You can try one of the Photoshop Plug-in implementations free of charge, by downloading Genuine Fractals from their website www.altamira.com. It’s slow but may be worth wait; we have seen impressive results.