Upsampling is a means of increasing the resolution of an image. Upsampling is different to stretching an image to make it bigger. Upsampling doesn’t stretch an image but allows you to increase the resolution of the image while keeping the number of pixels in the image the same.
Resolution Controls Size
For this post I am going to talking a lot about PPI (Pixels Per Inch) which is the resolution used to measure a digital file. A resolution of 200PPI tells us that for every line of pixels in an inch there are 200 pixels. If the image has 1000 pixels on each side making the image 1megapixel in size but this doesn’t tell use the physical size of an image, this is dictated by the resolution. If a 1MP image has a resolution of 200PPI the output size would be (1000/200=5) 5” by 5”, at 300PPI it would be 3.33“ by 3.33” inches.
Imagine you have come across an amazing image opportunity, but you have left your camera at home and you only have your smartphone camera on you. You don’t want to walk away from the image and decide to snap it on your phone. You upload it to your computer work on it in Photoshop and want to make a large print. This is when you may need to upsample an image.
In this hyperthetical I am going to claim a recent Instagram shot as my example. This image has a resolution of 72PPI with 2448 pixels tall and wide. With this information I can calculate that the physical image size is 34” by 34”. Rule of thumb for good quality images is that you need to have an image resolution more than 200PPI. I want to have an image with good fidelity as it is a present for my mother-in-law. This means that I need to at least double the resolution of the file.
How to upsample
1) In Photoshop I open , Image- Image size.
2) I uncheck the option at the bottom named Resample Image.
This means that the number of pixels in the image stays the same but the resolution can be changed. By changing the resolution to 300PPI the output size is now 8 inches by 8inches. The image has been up sampled and I can print a high quality image.
The Dark Art of Upsampling
There is a way to enlarge an image, though it needs to be done with care an only in situations when you really have to. It is always better to use an image of the correct or larger size and resolution.
In my scenario I am happy that I have created a high quality 8 x8 image with good resolution but I need a larger image double the size to print for my mother-in-law’s present.
1) Still using image size (Image- Image size), I check resample image and choose Bicubic Smoother from the menu at the bottom. I also change the document dimension options to percentage. To enlarge the image I change the option from 100% to 105%-110% depending on the image.
2) I repeat this process until the image is at the desired size.
This can easily be done by creating an action and just click and repeat (slowing enlarging the image is the best process as you are creating new information to be added to the image, doing this too quickly will result in undesired effects).
There is no answer for how big you can enlarge an image in this way, it depends from image to image as well as the amount of detail in the image.
3) As a finishing touch I duplicate the background layer and add a small gaussen blur to remove some small artefacts created.
4) Next I repeated duplicating of the background layer, placing it above the blur layer and applying a small high pass filter. I then set the blending mode to overlay to add some clarity to the image.
In comparing the two images there is no big difference in quality that won’t be lost from the viewing distance.
Viewing distance (using your legs to upsample)
Sites like Smug Mug and Photobox give a minimum resolution for print sizes which are a lot less than 200PPI and 300PPi that I upsampled my image to. This is because of the relationship between viewing distance image size and resolution. A way to understand this is that the bigger the image the further away you have to be to view it clearly. A basic 4”x6” image has a viewing distance of about 11 inches (about 30cms) compared to a 40”x60” image which needs a viewing distance of 9 feet ( nearly 3 meters). With a longer viewing distance the lower the resolution can be. A slightly pixelated image close up becomes less pixelated becoming clearer the further away you are. This is an optical illusion that benefits photographers.
|Print Size||Diagonal||Viewing Distance(1.5 x Diagonal)||PPINeeded|
|4 x 6″||7.21″||11″||313|
|8 x 10″||12.81″||19″||181|
|8 x 12″||14.42″||22″||156|
|11 x 14″||17.80″||27″||156|
|16 x 20″||25.61″||38″ (3.17′)||89|
|16 x 24″||28.84″||43″||80|
|20 x 30″||36.06″||54″ (4.5′)||64|
|40 x 60″||72.11″||108″ (9′)||32|
This means that instead of up sampling an image you could down sample the image (reducing the resolution), this would create less pixels for each inch making the image larger.
Going back to the original resolution of my example image, it had a resolution of 72PPI with an output size of 34”x34”. This is above the recommended minimum resolution for the viewing distance.
Should I Upsample?
If you are looking to print an image that has a low resolution at a high quality and don’t mind sacrificing the output size then my answer is yes, upsampling is what you should do.
The dark art of resizing is when I would add a word of caution as this is not a quick fix for your image. It is an option in certain situations to get more out of the image. When it comes to printing big take into consideration the viewing distance of the image, there are minimum resolutions but the higher the resolution of your image the better. You will also need to consider that the required image resolution will be linked to the printer’s half tone screen resolution.
More information about resolutions can be found in my post Pixels, Dots and Megawotsits.
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