Why we deal in pixels not DPI

Here at Stock Passport we try very hard to be a little different.  One of the things we decided right off the bat was that we weren’t going to blindly follow our competitors.  Sure, we looked at other Stock Photo websites but always with a critical eye.  If we saw something we didn’t understand or something that was confusing, we asked ourselves, would our customers want that feature? If so, how can we improve on what others have done? How can we empower our customers with a clearer understanding?  What can we do, to cut through the confusion and make things simple?

One of the trickiest questions for us was;

“What resolution images should we offer.”

Naturally we knew what resolutions were going to be easy for us to work with.  We knew what resolutions our contributors would be submitting.  Hell, we even knew what cameras they were shooting with.  Most of the technical bases were covered.  What we didn’t fully understand was what resolution and size information our clients needed in order to help them select the perfect travel image.

When we looked to the industry for guidance we found only misleading information and confusion.

We were determined to do something different.

Why we don’t offer images for sale using DPI and PPI

One of the most bemusing things we saw on other Stock Photography websites was the prolific use of DPI (Dots per Inch) when offering images for sale.  DPI is everywhere in the world of digital photographic media and for the life of us we can’t understand why.

Let’s be clear.

Just like thousands of other Stock Photography websites we deal in pixel resolution. Unlike most of the rest however, we choose not to use DPI when we refer to the resolution of our images.  Why?  Because it’s dumb and very misleading for our customers.

DPI has no bearing on image quality when you deal in pixel resolution.

DPI is nothing, pixels are everything

Let’s get our terminology clear.  DPI stands for Dots per Inch.  It’s a measurement that printers use.  It’s only relevant to printers and printed media. (More on printing later).

PPI or Pixels per Inch is the digital version of DPI. When talking about digital media we should refer to PPI and not DPI.

Now, I know what you are thinking.

“For printed images I need to be concerned with DPI and for digital images I need to think about PPI, right?”

“…Err, no.”

Whether you are printing or rendering images online you don’t need to think in terms of DPI or PPI.

But higher DPI and PPI means better quality, obviously.  How is that not important?

Image quality is important but DPI and PPI are not important when we have a set number of pixels to work with.

DPI and PPI are not starting points when talking about image resolution.  They are endpoints.

Think of it like a recipe from a cookbook.  Pixels are your ingredients.  DPI and PPI are just serving suggestions.

A sensible image workflow

This is how we approach working with images for advertising and branding.

We begin with how many pixels we have.  (Width x Height).
Then we decide how big we want to print or render the image.  (In inches or centimetres).
Those combined attributes determine our PPI.
If our resulting PPI is too low for the level of detail we require, we look for an image with more pixels or we reduce our size dimensions.
Finally, if you want to print the image, we set a high DPI at the printer.

That really is all anyone ever needs to know about PPI and DPI.

Still not sure?  Let’s do some math.

The math

Let’s say you find the perfect travel image and you download Stock Passports XLarge resolution version of that image file.  Then you decide you want to use it for an advertising campaign for your travel magazine. What PPI will you get?

Example 1 (Printing the image on a double page magazine advertisement).

First take the pixel resolution and work out the diagonal number of pixels. (Remember how to find this from school? Here is a handy web calculator in case you are not sure, Link)

XLarge image 4961 x 3508 pixels
Diagonal number of pixels 6075.9843

Now take the printed size and work out the diagonal number of inches.

Size 11.69 x 16.53 inches
Diagonal number of inches 20.2459

Finally to work out the resulting PPI we simply divide the diagonal number of pixels by the diagonal measurement in inches.

Diagonal pixels 6075.9843
Diagonal inches 20.2459
Resulting PPI = 300.073

Example 2 (Printing the image on a large poster)

The source image and math are the same but the poster is obviously much larger than a magazine advertisement.

XLarge image 4961 x 3508 pixels
Diagonal number of pixels 6075.9843

Poster size 23.4 x 31.1 inches
Diagonal number of inches 38.92

So this time we divide;

Diagonal pixels 6075.9843
Diagonal inches 38.92
Resulting PPI = 156.115

See what happened?  We increased the final image size but kept the same number of pixels so the DPI went down.  Seems obvious huh?

Why is DPI/PPI misleading for customers?

DPI becomes an issue for customers when it’s advertised as an attribute of an image at point of sale. If Stock Passport offered an image for sale like this;

XLarge 4961 x 3508 @ 300DPI

We would be making a false assumption.  The assumption being that all our customers will print or render this image at exactly 11.69 x 16.53 inches. How can we possibly know that?

Furthermore, we would be making a false promise.  We would be saying to our customers;

“Buy this image and you’ll have 300 DPI.”

This is simply not true.  Sadly however, the majority of digital photographic websites continue this practice.

We do not mislead our customers

We sell pixels.
You decide the size.
The DPI and PPI decides itself.

Printing and DPI/PPI

You may have heard that you need to print at 300DPI for photo quality resolution.  That’s a bit more accurate.  If you print an image on A4 at 300 DPI and then print the same image at 150DPI a sharp eye may see a difference.

“Ah, so is that why a print shop may ask for 300 PPI digital images?”

“Yes.”  They ask for 300 PPI digital images because they understand it to be a measure of image detail and they understand how printers work.  They have however misunderstood how pixel resolution works.

When they ask for 300PPI what they are actually asking without realising it is;

“Please provide me with a digital image of sufficient pixel dimensions to be able to meet my 300 pixels per inch count when printed at your specified size dimensions.”

Here is an example.

No more math after this, I promise

Example 1

I need an 8 x 4 inch print. The print shop ask me to provide a 300 PPI digital image. All I have to do is make sure my image dimensions are 2400 x 1200 pixels or more. (2400/300 = 8 x 1200/300 = 4). If I have more pixels, great!  If I have less pixels, it’s not going to look as sharp and detailed as it could when printed.

Example 2

Now let’s say I want an 8 x 4 inch print but I only have a digital image with 1000 x 500 pixels. Will everything be fine if I save this image file at 300 PPI?  Of course not. But why?

The print shop is asking for 300PPI.  I’m giving them 300PPI.  What’s the problem?

The problem is I don’t have enough pixels now to print a sharp detailed image at 8 x 4 inches, even though I have a 300PPI digital image, just like the print shop asked me to provide.

What’s the largest nice image I can print at 300DPI with 1000 x 500 pixels? (1000/300 = 3.3 x 500/300 = 1.6) Answer: 3.3 x 1.6 inches.

You see, even when we talk about printing a digital image it still makes no sense to deal in PPI and DPI as an image standard.  Pixels are all that really matters.

Conclusion

This is a tough subject to get your head around no doubt. We see terminology like DPI and PPI being misused on the internet every day. Many digital photography websites have stubbornly stuck with this standard and print shops continue to ask clients for images measured in DPI and PPI.

Much like camera specifications, we think there is a tendency on the internet to over complicate, to exaggerate the benefits or drawbacks of this vs that.  The truth is much simpler.

When it comes to buying great digital travel images, pixels are what matters.

The only question a customers need to ask is

“Will I have enough pixels for a sharp and detailed image?”

The answer here at Stock Passport is

“Yes you will”.

 

Take the horrible pixel quiz?

Question 1

Which image has more fine detail?

a) A digital image with a resolution of 1920 x 1080 pixels saved at @300 PPI
b) A digital image with a resolution of 1920 x 1080 pixels saved at @72 PPI
c) Both the same

Question 2

Which digital image will result in a larger file size when saved?

a) 1732 x 1155 @600 PPI
b) 1732 x 1155 @300 PPI
c) Both the same

Question 3

Which image will produce a sharper and more detailed 8 x 4 inch print when printed at 300 DPI?

a) A digital image file with a resolution of 2400 x 1200 pixels saved at @ 300 PPI
b) A digital image file with a resolution of 2400 x 1200 pixels saved at @ 150 PPI
c) Both the same

Question 4

Which image will produce a sharper and more detailed 8 x 4 inch print when printed at 300 DPI?

b) A digital image file with a resolution of 2400 x 1200 pixels saved at @ 300 PPI
c) A digital image file with a resolution of 4800 x 2400 pixels saved at @ 300 PPI
c) Both the same

Question 5

Which image will produce a sharper and more detailed 8 x 4 inch print when printed at 600 DPI?

a) A digital image file with a resolution of 2400 x 1200 pixels saved at @ 300 PPI
b) A digital image file with a resolution of 4800 x 2400 pixels saved at @ 300 PPI
c) Both the same

>

>

>

>

>

>

Answers :

Question 1:   C.  They are the same.  PPI means nothing remember.
Question 2:   C.  They are the same size.  Test it yourself.
Question 3:   C.  They are the same.  It’s the same image. Remember question 1?
Question 4:   C.  An image with a resolution of 4800 x 2400 pixels printed at 8 x 4 inches certainly has more pixels to play with. These extra pixels are ignored by the printer thought at 300 DPI. (Sorry, trick question)
Question 5:   B.  As above, but without any tricks.

 

 

 

Comments are closed.
error: Content is protected !!