Our clients often need to print images on very large display formats. From billboards to airport murals (we supplied images for the AKL and CHC airport murals for instance), from office walls and trade show banners, we regularly discuss with our clients the technical aspects of digital files and how clients can make sure that the image they’ve chosen “will go the distance”.
So today, we’re excited to share this handy comprehensive guide you can refer to next time you need to blow up an image on a large display.
Image Size 101
A digital image is an electronic file which is captured on a device of some sort (a high-end, medium-format DSLR, a cellphone etc.).
The devices used to capture an image all have a lens which carries light onto a sensor or ‘chip’. This sensor records that light and converts it into digital information which is stored on a memory card in the format of a file. When displayed on a viewing device (a computer monitor for instance) this file forms into square picture elements also called “pixels”. Each displayed image is composed of thousands or millions of pixels which have their own address, size, color representation.
While all devices follow the same process to capture an image, they’re not all able to do it with the same degree of quality. And this is where it becomes interesting…
Not all pixels are created equal
There are a tons of cameras and devices which salespeople boast can create umpteen megapixel files – your cellphone even may be able to take a 21 megapixel file!
The number of pixels is not really what matters though as the quality issues often come with the LENS quality and the CHIP quality.
Like pretty much all things in this world, there are high, middle and low quality options when it comes to cameras and devices. In order to keep size, weight and costs down, compromises must be made and the loss is invariably that often overlooked ‘feature’ …. quality! Generally, the smaller the device, the lower the image quality.
So, if you want to blow things up, 21 million pixels are worth “squat” if they are low quality pixels to start with. As the saying goes… ‘Garbage in, garbage out’.
But the quality of the pixel is not necessarily the only issue when it comes to image technical quality.
Quality of the lens and chromatic aberration
Some lenses struggle when shooting back-lit subjects. The result is an ugly, often purple, halo effect, called Chromatic Aberration, which we see most often in ridgelines on hills and mountains and in tree branches (see example above).
While on screen it is often not particularly noticeable, when you blow the image up really big it stands out like a sore thumb.
Chromatic Aberrations can easily be fixed and every photographer should know how to use software which can help solve these issues (the Lens Correction tool in Lightroom for instance). And thankfully, our rigorous quality control procedures weed these images out before they are accepted into our One Shot collections.
But if chromatic aberrations can easily be fixed, another key element should never be forgotten… Read on.
Not everyone is a good photographer.
Even when a top quality camera and lens are used, there is still no guarantee that the resulting images will go the distance. This is particularly true when you’re trying to decorate a 10 metre long wall at an airport or other public space (a lot of those images you see at NZ airports come from One Shot).
The reason? Inexperienced photographers…
Many ‘photographers’ are self taught and they often have limited knowledge of some of the more technical abilities and limitations of their camera and computer equipment.
For instance, many capture images in JPEG format, letting the camera make many decisions on image quality when it is actually far better to capture an image in RAW mode as it gives a lot more latitude to customise how the final image will look.
Then comes the post-processing phase – and here again, the file captured on the camera is only really just the start. What is done with that file is the critical part.
Unfortunately, many photographers are horribly under-skilled when it comes to post-processing a file in Photoshop or Lightroom. When faced with ‘slider controls’ to adjust saturation, brightness, contrast (etc.), many photographers take it way too far with unintended and often disastrous consequences (as per example above).
These consequences are usually only revealed when looking at a digital file at 100%… and when printing the image on a very large display. And you’d be surprised how often we see images which have been destroyed through over-zealous slider use.
This is the reason why at One Shot, we manually review each file submitted to our collections and accept only photographers who not only know how to use their cameras, but also know how to edit their images.
So, how can you know if an image will go the distance? Well… it’s just maths!
DPI – also known as print output resolution – refers to the number of dots per inch which will be printed on your selected media.
Stating that you want “a file at 300DPI” will never be sufficient information to ensure that the image you’ve chosen will go the distance. And saying that you need an image to be printed on a wall that measures 3m x 2m without knowing the print output resolution is also pointless.
When looking for an image to print on a large display format, you must not only know which output DPI will be used by your printer but also how big the image will be printed (the surface of the wall, the size of the billboard or banner etc…).
Once you have this information, then you can then look at the physical pixels available in a file and see if the image will go the distance.
Here are the formulas you can use to convert pixels to inches or centimeters, taking into account the print output resolution:
px = in * dpi
in = px / dpi
dpi = px / in
px = cm * dpi / 2.54
cm = px / dpi * 2.54
dpi = px / cm * 2.54
Each of the images below is exactly the same file size and pixel dimensions (6000px by 4500px). By lowering the DPIs, you can see how it will print much bigger if the print output resolution is lower.
File still too small for your project… what can you do?
You’ve licensed an image, got the hi-res file but the image isn’t big enough for how you need to print it? Thankfully, you haven’t run out of options … yet!
Some software will allow you to physically increase the number of pixels in a file so that it can be reproduced even bigger. And since this is qualitative process, the resulting quality will depend on the technical abilities of the software used and the skills of the person using that software.
There are limitations to what can be done with satisfactory results but often someone experienced can get some great results as long as the original file was taken on a half-decent camera to start with.
Not sure how to do this yourself? Fear not! At One Shot we offer this service so do not hesitate to contact us for help.
The One Shot Team