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DPI: (and when to use it)

From Ben King

Ok guys and gals

Many people still get very confused about DPI or Dots Per Inch and its use in the world of video.

This is a long FAQ but I feel it's necessary to understand completely.

Firstly a couple of real world examples

• A 720x480 image displayed in Photoshop at 72dpi

• Has a 1012.5K Raw filesize

• It would print out as a 10" x 6.667" image on paper.

• The same 720x480 Graphic displayed in Photoshop at 300dpi

• Has a 1012.5K Raw filesize

• It would print out as a 2.4" x 1.6" image on paper.

• Both have exactly the same impact on FCP, After Effects & Motion, irrespective of the DPI setting.

What often happens is that people get mixed up between DPI, PPI and Pixel Dimensions.

To reiterate: The DPI does NOT matter for video if the Pixel Dimensions are 720x480 that is all FCP cares about (and what Aspect Ratio to display it).

• The file IS exactly 720x480

• It is exactly the same 345,600 pixels

• It is the same 1012.5KB and does not increase processing requirements.

For some bizarre reason people imagine that a 300 dpi image is this huge massive file consisting of thousand of pixels by thousands of pixels.

Wrong.

DPI is a refererence as to:

"How many pixels of the image are printed out per one inch" on (for example) paper.

OR

"How many pixels are scanned in for 1 inch of source picture."

Some more examples and some very important rules for you all to follow:

Example 1:

• If you scan a 2"x 2" Graphic in at 72dpi

• Then the resulting file will be 144x144px

A tiny files size and not really good enough for video.

Example 2:

• The same 2"x 2" Graphic scanned in at 400dpi

• Gives a 800 x 800 pixel image

Which you COULD use for Standard Definition Video.

Example 3:

• Lets say we take a 5"x4" Photo

• Scan at 300 dpi

• You will get a 1500x1200 pixel image

This is fine for Standard Definition Video and 720 High Deinition Video plus you can pan around it on SD.

The filesize will be relatively small too - around 5MB uncompressed and FCP can handle much bigger than this.

Example 4:

• Now if you scanned in a 27"x 20" at 300dpi

• The pixel size would be something like 8000 x 6000 pixels

It would be about 130MB file - way too big for FCP or After Effects - in fact you would have to use Shake if I am not mistaken.

This is the monster that "300dpi" creates in your head isn't it!!!

Fear not I have 3 rules for you so you can sleep sound at night whilst your suite is a rendering...

Rule Number 1:

• DPI should only be taken into account on acquiring images via scanners or when you are outputting a print.

Rule Number 2:

• When scanning images always work in Square pixels then convert to the video pixel aspect ratio you need - eg: D1

Rule Number 3:

• For talking about video sizes you should only work with the pixel dimensions (horizontal x vertical number of pixels) and aspect ratio.

A couple of tips for scanning in images for use in video:

Scan in at a resolution good enough to get a file with the pixel dimensions you require.

I usually ask for/scan a minimum picture size as follows:

• 800 x 600 px for Standard Def 4:3

• 1100 x 600 px for SD Widescreen 16:9

• 1400 x 780 px for 720 HD

• 2000 x 1200 px for 1080 HD

This way you have a small leeway to move the pictures around in FCP - but if you want to do major moves then you will have to get them in larger pixel dimensions.

If you feel the need to - you can then crop and scale these images to the exact size of the Video in the FCP project you are using them in.

CALCULATIONS

To work out your scan size in pixels:

Image size (width in inches) x dpi setting = horizontal pixel size

Image size (height in inches) x dpi setting = vertical pixel size

To work out what dpi setting you need for a particular image size:

Horizontal pixel size you require (Square pixels) / Image size (width in inches) = dpi setting

Vertical pixel size require (Square pixels) / Image size (width in inches) = dpi setting

Aaaaaaand relax....

I do hope this is comprehensive enough for you all!

From Loren Miller:

Here's a related approach worked into a system:

Intelligent photo scanning

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