Posted by: In: Lightroom, Printing 05 Mar 2017 0 comments Tags: , , , ,

I teach now – hence a long break from Shuttercount, and one of the questions I often get on my Lightroom course, is “what resolution do I need to have to be able to print at xxx size?” or “what size can I print with my camera?”

Some people say you need 300dpi for a print, others say “72 dpi for web”. Both myths.

The Lightroom export module allows you to choose many different options when outputting a file for single use. But before we look at resolutions, lets look at what’s going to be looking at your images –

 

The Human Eye.

Assuming 20/20 vision, the average eye can resolve 876dpi at about 4 inches. This is the closest detail resolution you can see if you really scrutinise an image up close. As we move back further away from an image, that number goes down – and it goes down quite dramatically. For example:

At a standard reading distance (for example 12″) the eye can only resolve about 300 dpi.
At a standard monitor distance (for example 30″) the eye can resolve about 115dpi.
At a TV viewing distance (for example 6 feet) the eye can resolve no better than 50dpi.
Looking at a cinema screen distance (about 40 feet) the eye can’t do any better than 7dpi.
(Source)

So viewing distance is a huge factor in determining how big you can go. Let’s look at the print itself.

 

Physical Limits

An image is (very basically) made up of pixels – the number of which you can work out from your camera specs. My Fuji X-T1 has a resolution of 4896×3264.
A printer can only print at a certain resolution. My Epson can print at a max resolution of 720dpi so I can’t go higher than that. In reality, my eyesight renders every image I print acceptably sharp at close scrutiny at 360dpi, so 360dpi is the absolute highest quality I will ever need.

Dividing the longest edge of the sensor (4896) by 360 gives me 13.6″ on the longest edge as the biggest I can print with superb – up close – quality.

Most of my prints go on walls though, so with a viewing distance of 30″ assumed, my 4896 sensor will now print to (4896/115) a whopping 42″ on the longest side (115 dpi).

So without any sizing, it’s quite easy to determine what resolution is “best” for you.

Resolution (dots per inch) = sensor size (pixels on longest edge) / size (inches)

So if you want to print to 8 feet with your X-T1 the resolution will be 4896 / 96 or 51ppi – perfectly acceptable for viewing at 6 feet and beyond.
If you wanted a billboard that could be viewed at 40 feet without loss of detail, you could print to a huge (7 = 4896/x) 58 feet along the longest side!

 

Software Limits

We can go even bigger though using resizing algorithms. The method above will result in your software neither adding or subtracting pixels from your image. Going smaller is quite straightforward as Lightroom is excellent at downsizing. Using the examples above, when I print an A4 image from my X-T1, LR tells me that my dpi is 463 (10.5″ image size with small border divided into 4896 pixels). The printer only “does” 360, so some bits have to be lost & I can’t visibly determine what they are.

Going up though is a bit more hit and miss. Lets say I wanted to make a 20″ print. 4896/20 = 244dpi, but if I wanted better resolution (say 300dpi), I’d need to “add” 56dpi in order to get that. The software would need to artificially create pixels to compensate. This isn’t something I want to do and in this case, I’d feel that 244dpi is an acceptable print resolution for an image that big.

 

Screen Use

Monitors, TVs and tablets (and phones) are slightly different. They already have a “size” in pixels which may be independent of the size of the screen. My 38″ TV has a 1024×768 resolution whereas my 24″ monitor has 2560×1440. So it’s possible to have more pixels in a smaller space (the iPhone 7 has a 760 x 1334 display in a 4″ screen which is better than my TV!) Because of that…

For screen display, “inches” are irrelevant.

No need to set resolution when exporting for screen use. Untick the resolution box

No need to set resolution when exporting for screen use

As an example, a 1000px image would take up my entire TV screen (30-odd inches) but that same image would only take up half of my PC monitor (12″). When exporting an image for use on a screen, consider only the number of pixels in it. In the Lightroom export module, that means ignore the DPI box.
Most browsers will resize an image to fit, so it can be tempting to upload full resolution images. However these can easily be downloaded and printed out by someone else so what resolution should you upload to avoid people being able to print?

Well, if we assume that people want a decent image quality of at least 200ppi we can do some more maths.

If you export your images at 500px they will look tiny on a high resolution monitor (they’ll fill a quarter of my screen) but they will only print out to (500/200) 2.5″ on the longest side. That’s pretty small and useless.

If you export your images at 1000px they will look ok on a high resolution monitor (they’ll fill a half of my screen) but they will only print out to (1000/200) 5″ on the longest side. That’s quite a big difference.

If you export your images at 2000px they will look great on a high resolution monitor (they’ll fill almost all of my screen) but they will print out to (2000/200) 10″ on the longest side. That’s a decent free print for someone. Even at magazine quality 300dpi, that’s a 6.6″ print!

So the upshot of this is that there’s no definitive answer. It depends on what you’re trying to do with your images.

 

Summary

In summary, most modern cameras today are capable of excellent quality images printable to any appropriate size for the viewing distance. Even my monitor (2560×1440) is even capable of getting game screenshots that will print out brilliantly to A4 and excellent (won’t stand close scrutiny) to A3. And that’s effectively only a 3.6MP camera!

Game screenshots from a decent monitor will have enough resolution to physically print.

My Spitfire being chased down in War Thunder