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In The Wonderful World Of Scanning, What Is DPI?

What does DPI stand for? 

DPI stands for Dots Per Inch.  Some individuals think DPI is a misnomer – and should be actually called PPI or Pixels Per Inch.  Keep things easy, think Dots Per Inch.

 

What is DPI?

The easiest way to think about DPI - is it being unit of measure.  When you go to Walgreens and obtain a photo print of whatever size, the physical picture that you are holding is a DPI of about 300.  Whether you have a photo that is a 4x6, 5x7 or 8x10, the photo print from Walgreens is about 300 dpi.

 

So how does DPI relate to scanning?

Let’s say we are going to scan a 35mm slide; your scanner will request you to select a DPI.  Answer this incorrectly, and you can waste a ton of time.  The level of DPI that you scan at will dictate the sharpness of the scan and the size of print outs that you can obtain.

 

The “scanable” area of a 35mm slide is about 1 3/8 inches X 7/8 inch.  If we scanned this slide at a DPI of 300, we could run to Walgreens and obtain a print of the scanned image….

In a size of 1 3/8 X 7/8….the print would look just fine. 

In a size of 8x10 …the print will look pixilated. 

Each of the two photos that Walgreens produces would be at about 300 dpi.

 

If we had used a much higher DPI, our 8x10 Photo would have looked just fine.  Use too high of a DPI – Yes, the print will look just fine – but the time to scan the image will increase.

 

So how do you know what DPI level to use?

Whatever physical size of your actual media, scanning it at 300 DPI will enable you to obtain a print out of the original media size that will look great.  If you know that you are going to want a print out that will be twice as big as the original physical size, scan the media at 600 DPI.  If you want a print out of that will be 3 times as large of your original media, use a DPI of 900.

 

So what is the formula?

We like the calculations to be quick – your DPI Calculation does not have to be perfect – but your calculation should enable you to fulfill your end in mind.  So this is the formula that we use – If you understand it, you can do it in your head:

 

(L/S X 300) = DPI To Use

 

L = The longest side of a potential print that you may want to obtain later.

S = The shortest side of your original media (round up to the nearest ½ inch).

If you are not going to obtain prints but will keep things digital, run your DPI calculation like you will be obtaining an 8X10.

 

So if we are going to want 8x10 prints and are scanning a 35mm negatives ( 1 3/8 X 7/8), this is how are calculation would look: 

  1.  First we rounded the shortest side of the original media up to the nearest ½ inch.  So the 7/8s becomes 1 inch.
  2. The longest side of our end in mind is 10 inches (Remember, we want an 8x10)
  3. (L/S X 300) becomes à (( 10/1) x 300 )
  4. 10/1 = 10…..so our formula now looks like (10 x 300)
  5. The DPI that we should use at a minimum is 3000 – hint, 10x300 from step 4.

 

 

Let’s try another example, we are going to do 11x17 prints from 110 negatives that are ½ inches X  7/8 inches in size.

  1. First we rounded the shortest side of the original media up to the nearest ½ inch.  So the ½ stays at a ½ inch.  To make things a little easier to communicate, we will be using the decimal form of a ½ which is .5.
  2. The longest side of our end in mind is 17  (Remember, we want an 11x17)
  3. (L/S X 300) becomes à (( 17/.5) x 300 )
  4. 17/.5 = 34…..so our formula now looks like (34 x 300)
  5. The DPI that we should use at a minimum is 10,200 – hint, 34x300 from step 4.

 

Final example, we are going to do 8x10 prints from 110 negatives that are ½ inches X  7/8 inches in size.

  1. First we rounded the shortest side of the original media up to the nearest ½ inch.  So the ½ stays at a ½ inch.  To make things a little easier to communicate, we will be using the decimal form of a ½ which is .5.
  2. The longest side of our end in mind is 10  (Remember, we want an 8x10)
  3. (L/S X 300) becomes à (( 10/.5) x 300 )
  4. 10/.5 = 20…..so our formula now looks like (20 x 300)
  5. The DPI that we should use at a minimum is 6,000 – hint, 20x300 from step 4.

 

When you are running your calculations, think of them as a rule of thumb.  Doing a simply increase of a few hundred DPI is not a big deal – yes, your scan may take a little longer and your file size may be a little larger.

 

So why not just use a DPI of 12,000, that will work right?

Sure, you can – many scanners go to this level.  However, you will find that higher the DPI scans take longer!  Also, many photo viewers may not be able to handle a photo at this magnitude; so when you go to open this image scanned at 12,000 dpi, you may receive a message that the photo can’t be opened.  Bigger is not always better.

 

What do we use when we scan?

We suggest that when scanning color slides, use 3200 DPI.  110 negatives, 6500 DPI.  120s, 220s and 620s, use a DPI of 2400.  For 4x5s, use a DPI of 1200.  If the media is black and white, increase the DPI by about 800 to 1500 depending on the media type.  Using a slightly higher DPI for black and white will help to reduce the graininess of the final product.

 

Do we offer scanning services?

Parts-Distribution.com is a distribution company and does not offer scanning services. 

 

Conclusion

There is so much on the internet about DPI.  I have no doubt that one could find websites that declare themselves as “the expert” – to only find other sites stating opinions that are different.  This is a technical subject and this white paper is only meant to be a 101 class on the topic. 

 

With DPI, this is the key:  Have the end in mind BEFORE you start.  Doing test scans is a good idea.  Putting your scanned images in the format of your “end in mind” can’t hurt.  There is nothing more aggravating than to spend weeks scanning your precious memories to find that you used too low or too high of a DPI to enjoy your final product.