Enlarge your images without losing quality!*

*Enlarging your digital images beyond a certain point will affect quality. Your home is at risk if you do not keep up repayments on a loan or mortgage secured on it. Caution: contents may be hot. May contain nuts. Do not bend. Insert generic anti-litigious statement here.

OK… so the title of this post is a statement that comes with strings attached, but nevertheless I will show you how to squeeze some serious wall estate out of your images without noticeable drop in quality.

Firstly, I come at this nugget of advice from a serious dislike of post processing. I hate it. I much prefer trying to get the shot right in the first place, using polarising filters to get the intense colours (talked about here), underexposing to set a gloomy feel, selecting lenses and focal lengths carefully to create compression/distortion as necessary. As much of a techno-geek as I am, I’m happier with a camera in front of my eye than a computer trackpad under my fingers, and so applying any kind of post-process to my images makes me grumble and want to procrastinate. Don’t even get me started on the horrors of keywording and cataloguing…

But my business model is all centred around physicalisng the digital photograph. Putting it bluntly, I sell big images for folks’ walls. I sell little ones too, but some people (myself included) want a huge-arse image right in full view to really dominate a space. So how huge-arse (it’s a technical term, means ‘big’) can I get?

What size is my camera sensor, and what size prints will it do out of the box?

Now, I mostly capture my photos with a 12 mega-pixel (MP) sensor. To be precise, if I don’t crop the image in any way, I have 4,288 pixels on one axis, and 2,848 on the other (multiply them together to get the total number of pixels – a little over 12 million). Bear with me on the number facts and maths – it’s not complicated, won’t take long, but is necessary. The “optimum” resolution for viewing a printed image is, very arguably, between 260 and 340 pixels per inch (ppi), though this gets complicated very quickly with other stuff that I’ll brush over later. Let’s call this optimum resolution 300ppi – one: it sits as a nice average in between the two ppi figures above, and two; it’s easier to do the maths. So, with my 12MP camera sensor, in order to print an “optimum” resolution of 300ppi, I divide the total number of pixels I have by the 300ppi to give me my maximum print dimensions. I hope you’re still with me, but if you’re beginning to glaze over let me rescue you with the answer: 4,288 pixels one axis A divided by 300 = 14.29 inches. 2,848 pixels on axis B divided by 300 = 9.49 inches. So the largest print I can produce at 300ppi is roughly a 14″ x 10″ – twice the size of a 7 x 5, and a little smaller than a sheet of A3 paper. For those of you who don’t know what huge-arse actually means, this is not it. Now, we can cheat a little by reducing our 300ppi to something more like 250, 200 or even 150ppi – but the lower this number gets, the more ‘pixelly’ your image will look. You might get away with this if the viewer is going to be far away from the image, but certainly not in someone’s living room.

Interpolation, dear!

So, how do I enlarge my potential print size (we need huge-arse, remember?) without dropping the ppi and making it look pixellated? I will show you one method of many for interpolating your image. Interpolation is the process of making something bigger by making up some more pixels to squeeze in between the existing ones. Think of it as a, ummm…. a petri dish of fungus. Yeah, that’ll work. At first, you look at the dish and you have to look really hard to see the individual spores, and you can’t make out colour or shape very easily. Add more fungus to it to fill in the gaps between the existing fungus, and soon you can see a rich, colourful shape of fungus that could be examined closely without seeing gaps between individual fungi spores. And that, children, is how we make stilton cheese. But I digress…

The Aperture to Photoshop Enlargement Workflow

Interpolation on Photoshop is a doddle. But it’s important to get that doddle to work seamlessly in your workflow otherwise you’ll get bored and stop bothering to do it. Now, workflow is for another post, but suffice to say here and now, I use Aperture. I did use Lightroom for a while, but it was a while ago and I can’t quite remember how this process works from there, but I seem to remember it being similarly straightforward. So, you have your image in Aperture, and you want to upsize the resolution through interpolation in Photoshop. Here we go…

1. In Aperture Preferences, choose Photoshop as your External Editor in the Export tab (one time only).
2. Go to your image, apply any exposure tweaks, crops etc, but not sharpening. No, we do that later now!
3. Right-click the image and click ‘Edit with Photoshop CS6’ or whatever version you’re on.
4. Photoshop will open, with your image there and waiting. Click on the Image menu at the top.
5. Click on ‘Image Size’.
6. In the dialogue box, you will see the current pixel dimensions in the top section (for me, this will be the 4,288 x 2,848 we used before). The next section will be document size. It will display the desired resolution in ppi or ppcm if you prefer, and will calculate the print width and length for you based on that. Don’t do anything yet.
7. Below, are three tick boxes and a drop down. Leave the tick boxes ticked (scale styles, constrain proportions and resample image) and choose Bicubic Automatic from the drop down.
8. Now that you are sure your proportions won’t be jiggered unnecessarily, go ahead and type in the desired ppi resolution (300 ppi is best – not 300 pixels per cm!).
9. Now, you will see what print size that will get you with your current pixel count. Go ahead and type in the sizes you desire (e.g. if you desire a 30″ print on the longest dimension, replace the larger inch number with 30). You will see the other dimension change automatically to keep your image looking normal, and you’ll see the pixel dimensions at the top change.
10. Click OK, let the program do its thing, and then zoom into 100% to get check it at nose-to-paper distance. Bear in mind that your viewers will probably not be examining it this closely.
11. If you’re happy, close down the Photoshop window and it will ask you if you want to save it. Click yes, and when you return to Aperture, your grossly overweight image is there, stacked in with the original.
12. Now you’re back in Aperture with your overweight image, you can sharpen!

A screencast video of this step by step guide is here: http://youtu.be/OCVQJr_aV3w

So how much can I enlarge by using this method?

This question gets us into some of the murky confusion I mentioned earlier. The answer is… it depends. No, really. It depends on how far away your viewer will be – if they’re going to be looking at it across your cavernous reception hall and spiral staircase, then you can afford to reduce that 300ppi some. If you’re printing embarrassing pictures of your ex on the mahoosive (note: this is significantly bigger than huge-arse) billboard down the High Street, you can reduce the resolution a lot. Maybe even sub-100ppi. If your image is of something wispy, fluffy or generally not needing to be pin sharp and edge-specific, you can afford to cheat a little on the ppi resolution. If you’re viewers suffer from pretty poor vision, you can bring down the ppi. If your image is of a style that would benefit from more noticeable noise, grain or pixellation, you can afford to bring down the ppi more than normal. Do you get the picture? It is entirely subjective. So here is my subjective opinion. I go for 300ppi and no more than double the dimensions as a rule of thumb. For long distance prints, like theatre sets, I will drop down as low as 150ppi – but no lower. If the image is sharp and is being printed on glossy paper, I won’t even go as much as double the print dimension, but will limit to double the MP count (Photoshop calculates this too in that little dialog box).

I don’t like the sound of this here Interpolation…

Then the other options are: to simply stretch the existing pixels without adding new ones; use a bigger camera sensor, or; put up with prints smaller than A3 and stop whining.

All joking aside, they are your options. There’s nothing wrong with any of them, so long as you understand the limits of each. My next preferred option after interpolation is to buy a camera with a bigger sensor. That would either be a very pricey but amazing medium format camera, or the more likely and awesome-looking D800 from Nikon. The latter offers over 36MP over a full-frame sensor, giving the ability to print huge-arse without interpolation. Or another way of looking at it – the ability to print mahoosively with acceptable interpolation. I’ve just got to convince the wife now…