Interpolation

"Size really does matter..."

...especially when it comes to satisfying the demands of many of the stock libraries. For instance, Alamy demand a minimum file size of 48mb. Ironically if you are still using film this is not so much of an issue as a professional level scan of even a 35mm slide should achieve this. Do a scan of a medium or large format slide and you are home free. I used to use an Epson 4870 flat bed scanner (because it could deal with both 35mm and 6x4.5 transparencies) and with everything set to maximum a 6x4.5 slide would create a tiff file of just over 100mb. No worries then. Well not until I got rid of my film gear and went totally digital anyway. Then the head scratching began.

My first foray into digital was the Nikon D100. A perfectly good camera, but 'only' 6.1 megapixels (mp). I am now using a Nikon D200. A huge leap to 10.2mp but even with this degree of firepower I am only producing tiff files in the region of 27mb from the original RAW file. Way short of the 48mb bar. Based on my detailed and extensive research, which involved some dedicated flicking through magazines in the newsagents, I believe I would need to find a minimum of £4500, at today's prices, for a 16.7mp Canon EOS-1DS (MkII) just to squeak into the acceptable file size area without the aid of artificial stimulants. My current financial circumstances would require acts of a lewd and illegal nature to provide funds for such a purchase, and I certainly don't possess enough saleable body parts to be able to consider one of the 'medium format' digital options such as the Hasselblad H1. So what was a young, impoverished, photographer to do? The answer was obvious. Cheat. And so I entered the world of the Alchemist, and set about trying to turn photographic lead into gold by means of the dark magic known as 'interpolation'.

Imagine my disappointment when I discovered that interpolation is neither a secret, nor illegal. In fact it is a well-known process practised by many upstanding pillars of society and even encouraged by some stock libraries, as long as it is done in a controlled environment.

So, what is interpolation and, more importantly, how do you do it?

First things first. Interpolation is merely the creation of additional image data that was not present in the original image. More pixels in effect, and as we all know 'pixels mean prizes!'. Why is this important? Well it all depends on what you want to do with the image. I don't intend to get into the debate over the difference between 'ppi' (pixels per inch) and 'dpi' (dots per inch) in this article. For the purposes of illustration let's just assume the standard print resolution of 300ppi is what we are working to. Brace yourselves because here comes the maths. Our original file of roughly 3900px by 2600px (based on my Nikon D200) would give a print of about 13 inches by 8.5 inches, (3900 divided by 300 and 2600 divided by 300) or a shade bigger than a sheet of A4. But let's say I want an A3 print. That's about 16.5 inches by 12inches. I have two choices. I can print at a lower resolution, at this size I would be looking at just over 200ppi, or I can increase the size of the image file to suit the print size. Printing at 200ppi is going to reduce the quality of my print. It will be softer and the detail resolution will be poorer. More pixels it is then.

In this case, for an A3 print at 300ppi, I will need an image file of about 4950px by 3600px but how do I achieve this? There are many software packages and methods out there that will allow you to interpolate a file, but for the purposes of this article I will concentrate on just two. The 'image size' function in Photoshop, because it is readily available to most of us, and a dedicated interpolation software called 'Genuine Fractals', because I use it so I can comment from experience, and it is the only one recommended by Alamy.

First, the pure Photoshop method. Nice and easy this one. Just go to the 'image' menu, select 'image size' and type in the required pixel numbers in the height and width boxes. If you tick the 'constrain proportions' box you only need to enter one value and Photoshop will work out the other for you. That wasn't so painful and sure enough you now have a file of the required pixel numbers.

Second, the Genuine Fractals method. This is a plug-in to Photoshop, so you will need that installed first, but to the best of my knowledge it will work fine with later versions of 'Elements' so you don't need to fork out the eye-watering sums of cash for the full version of Photoshop to use it. Once installed you open your base image, choose 'file', then 'automate' then 'Genuine Fractals' and you should find a new window opens up. You are effectively doing the same thing as you did with the pure Photoshop method in that you are just telling the software how big you want the image to be but you have a couple of additional options to play with.

So, if both systems do exactly the same thing why would you want to spend extra cash on the plug-in? The answer lies not in what they do but how they do it. I mentioned earlier that interpolation was simply the addition of pixels that were not present in the original image. True enough, but the fun starts when the software has to decide where these new pixels should go and what colour they should be. This is done by the use of insanely complicated algorithms that the software uses to analyse the base image information and do very clever calculations to work out what new information is required. This is where the differences start. Different applications use subtly different algorithms and, therefore, produce subtly different results. Which one you prefer is down to personal opinion but the more sophisticated the software the 'cleaner' and more accurate the end result is likely to be. For this reason the dedicated interpolation software, more often than not, has the edge over Photoshop's internal option. I should make it clear that I believe Photoshop itself is a highly competent piece of kit capable of remarkable things. It's just that the image size function is a very minor part of its functionality, whereas it is the raison d'etre of Genuine Fractals and its dedicated interpolation stable mates.

Which one do I recommend then? It does kind of depend on what you want to do with the end product. If you are submitting work to stock libraries then its Genuine Fractals all the way. You may want to consider one of the alternative packages and be my guest, but whichever you choose, go with the dedicated software option. If you are enlarging for your own purposes, and are happy with the output, then by all means stick with Photoshop.

Just a couple of things I should mention to finish up. Whichever tool you use you have to give the software something to work with. If the base image is poor the end result will be just as poor, but bigger. You are not going to be able to take a snap from a camera phone and blow it up to a billboard size poster. You will be surprised at just how far you can take interpolation but everything has a limit so be realistic. Second, I would recommend that you do the bulk of any manipulation first, if only to give your computer a chance. Making changes to a 20mb file is no problem. Start manipulating a 100mb file and you will see your processing times slow significantly. The exception to this is sharpening. Of course you are all familiar with the use and application of 'un-sharp mask' so I won't go into that here, it's probably an article in it's own right, but sharpening can often create minor, though acceptable, distortions within an image. If you do this first and then enlarge the file you enlarge these distortions. Not good. I suggest you do all your work on levels, contrast, white balance etc first, then enlarge, then do your un-sharp mask. You should get a much smoother result.

So, now I can produce images with my D200 that keep the stock libraries happy and allow 30" x 20" prints to grace the walls of my humble abode. I am a happy bunny. But all is not well in paradise. If I can do all this with my current gear, how am I going to convince my better half that I simply cannot manage without that 12mp Nikon D2Xs??





About the author: David Stanley is a freelance photographer concentrating on landscape and travel images. He has growing portfolios of royalty free images with Alamy and istockphoto. For more articles and reviews, along with a selection of his work available as open and limited edition prints, please visit his website at www.davidstanleyphotography.com.

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