There are so many myths about PPI (Pixels Per Inch) perpetuated by both photographers and printing companies that just cause confusion and extra effort for no benefit!
Myth 1: You should use 72PPI for web images
Repeat after me:
- PPI does not affect file size
- PPI does not affect file size
- PPI does not affect file size
Ok, so I mentioned it three times. But, it’s important, it’s the one myth I see propagated more than any other! There are a few things that have an impact on the filesize of your image. The content, the compression and the size in pixels. A 3000 pixel wide image, set at 72PPI, has the exact same information as a 3000 pixel wide image set at 300PPI, or 10000PPI if you want! The only thing the PPI does is set the default print size if you were to not specify a size when you’re printing. So, a 3000 pixel wide image, set at 72PPI, would as a default print at 41.7″ wide.
Your PPI is actually set at the time of printing – if you print a 3000 pixel wide image at 10″, then the number of Pixels Per Inch is 300. Regardless of what you’ve set in the file.
Which obviously means, that as well as not saving filesize – it does not protect your images from being taken from your website and printed. If you want to stop people from being able to do that you have 3 real options:
- Upload low resolution files
- Use an intrusive watermark
- Don’t upload any images
The only time the PPI setting on Lightroom or Photoshop will do anything is when you specify a size in physical dimensions, as well as a PPI setting. But please be careful when using these settings, as increasing the size of an image, does not make it higher quality – see Myth 3.
Myth 2: PPI and DPI are the same thing
PPI is the number of pixels your image is wide, divided by the print size in inches.
Dots Per Inch, (DPI) is a physical property of whatever printer it’s being printed on, you can’t change it. But these terms tend to be used interchangeably.
OK, so this one doesn’t really matter.. But I thought I’d mention it anyway.
Myth 3: Everything should be printed at 300PPI
This is one that’s perpetuated by a few big printing companies which tell you if an image is “ok”, “good” or “excellent” when you upload it. Generally only telling it’s the highest quality when it’s set at 300PPI.
Well, 300 is great, and it’s what I always make sure my images are when printing in a normal size. But what about enlargements, big canvases and the like? Surely you want them to be 300PPI to?
Well, a 40″ wide canvas would need to be 12,000 pixels wide. Which would mean you’d need a 96 Megapixel camera. Know anyone with one of those? Nope, Me neither.
So you have two choices, you can either re-sample the image using Photoshop (if you don’t know how, feel free to ask), accept having a lower PPI, or a combination of both. A full, non-cropped image from my camera (Nikon D750) is 6016 pixels wide. So at 40″ this would be 150.4 Pixels Per Inch.
Sounds low quality? Well, if it was a small 6″ x 4″ print it possibly would be, because you’d be viewing it from close up. But the further away you stand from an image, the lower the Pixels Per Inch has to be. But it’s 40″ wide, so you’re going to view it from a difference.
Think about those flash new Quad HD / 4K HD Televisions that are now becoming the norm, a 40″ one of those only has a PPI of 96. So that 150 PPI canvas is actually a higher resolution than a 4K TV!
Bottom line – ignore the guides on those websites, trust your photographer if they tell you it’ll print fine as an enlargement 🙂
If you do want to re-size, just resize it in Photoshop and make sure you’re re-sampling it – but remember Photoshop is just interpolating, creating additional pixels based on the pixels around it, so you won’t gain any real quality. I doubt you could even tell the difference between a 40″ image printed at 150PPI and one upscaled from 150PPI – 300PPI and
Any questions? Or other tips? Comment below
Very informative, thanks for sharing!
Thanks for the info!
Good to know!
I think the idea that you “must” print at 300 PPI (Note: PPI – a property of the input file, not DPI a property of the printer and final print) is propagated mainly by people that have rarely, if ever made, a large print. I suspect they just copy and regurgitate whatever they have read, in the hope that they will be mistaken for experts.
In that way misinformation circulates and becomes the accepted “wisdom”. It is a problem that some of these clueless writers state their falsehoods with great authority. The widespread desire to have a clear and fixed answer for every conceivable situation also contributes to how easily myths are accepted and believed.
What is the truth? There are too many variables that affect the appearance of a print to specify a fixed PPI figure that always applies. Here re some of them:
The quality of your demosaicer
All the adjustments made in post-processing
The file format (e.g. a TIFF file has teh potential to store enormously more colour information than a JPEG)
If the image has been up-ressed then the up-ressing algorithm and quality of the up-ressing software
The engineering quality of the printer, and its present condition
The number of shades of ink available to the printer, and their precise shades
[basic printers have 4 basic colour: Cyan, Yellow, Magenta and Black. Photo printers have 8 or 12 different shades]
The properties of the inks … how they spread, how they are absorbed (if at all), how quickly they dry
The type of paper – textured or not, coated or not, &c. [Note: every ink/paper combination is different]
The algorithms by which the printer maps pixels in the input file to multiple dots in the output file
The amount of detail in the digital file (which is not necessarily the maximum that could be captured by that No. of pixels)
The size of the print
The intended viewing distance
… and even the temperature and humidity of the printing environment (they affect the absorbency of the paper and the viscosity of the inks)
Some images look great printed at as little as 100 PPI, whereas others need more than 300 PPI