How Can A 300 DPI Photo Scan Be The Same As A 1500 DPI Slide Scan

Learn More About Resolution And You Will Have More Control Over What You Do With Your Scans 

All you need to know are three numbers-- and the rest will be easy.  Lets begin.

1) The Physical Size Of Your Slide, Negative, And Photo

Take out a 35mm slide or negative.  Write this number down, it is important.  The actual size of your slide or negative is, 0.85” x 1.30”-- give or take a few points.

And the size of your physical photo is, well, 4” x 6”.

That is pretty easy.  But these numbers are important because what happens later.  But first, lets talk about DPI.

2) What Is DPI And How Many Do You Need To Scan Slides, Negatives And Photos

DPI or Dots Per Inch comes from back in the day when you had your photos printed at a photo lab.  The photo lab printed 300 colored dots on a 4” x 6” glossy sheet.  In other words, you got a 4” x 6” photo with 300 dots per inch.

Now move over to digital.  Instead of colored dots, we now use pixels.  Technically it should be PPI, but we still use DPI.  Anyway, you will see a setting when you load your scanner asking you what DPI, PPI, or resolution you want to scan your photo.

If you scan your photo at 300 DPI, you will produce the EXACT image as your physical photo.  That is because your physical photo is also 300 DPI.  If you double your DPI to 600, you will NOT double your detail or quality, you will just make your digital image twice as big.  Your digital image will be twice the size of your original, but maintain the SAME quality as your original.

3) How Do These Two Numbers Work Together And Give You A Digital Image

So far you know two numbers: the size of your original (slide, negative, photo) and DPI. What happens when you multiply these two numbers?  Here is what your scanner does with these two numbers.

Your photo is 4” x 6” and you scan it at 300 DPI.  Here is what you get...

4” x 300 DPI = 1200
6” x 300 DPI = 1800

Or 1200 x 1800.  This number is called your Pixel Dimension.  It is the most important number-- I will show you later why.

Now look at your 35mm slide-- it is about 0.85” x 1.30”, and say you scan it at 1500 DPI.  Here is what happens when you convert a 35mm slide to digital...

0.85” x 1500 DPI = 1275
1.30” x 1500 DPI = 1950

This number, 1275 x 1950, again is your Pixel Dimension.  

Can you see that your photo scan and slide scan have pretty much the same Pixel Dimension?  Have a look...

4” x 6” Photo scanned at 300 DP = 1200 x 1800 pixels
35mm Slide scanned at 1500 DPI = 1275 1950 pixels

Pixel Dimension is the most important number, and next I will show you why.

Conclusion: Forget About DPI / Resolution, Pixel Dimension Is The Most Important Number

Take a look at your computer monitor, HDTV, iPad, etc.  Every screen uses pixel dimension to display digital images.  For instance, my monitor is 1366 x 768.  My 52”, 1080p HDTV is 1080 x 1920.  An iPad is 1024 x 768.  Quick note: pixel dimension is also called screen resolution-- this is why resolution is so confusing, marketing uses the different names for the same thing.

So, what does this have to do with your scans?

Well, remember a 4”x6” photo scan at 300 DPI give you a 1200 x 1800 digital image.  That means even if you put it on your 1080p HDTV, it will display at HD.  That is because your HDTV is 1080 x 1920 and your digital photo is 1200 x 1800.

Digital Photo = 1200 x 1800
1080p HDTV = 1080 x 1920

This is why pixel dimension is the most important number.  Who cares about DPI-- that is not the true measure of resolution.  And you saw this where you have a photo scanned at 300 DPI and a slide scanned at 1500 DPI, and you get a pixel dimension that is pretty close to each other.

So, What Is The Best Resolution To Scan Slides, Negatives, And Photos?

That, my friend, is a different post.  Stay tuned and I will show you when you should use 300 DPI / 1500 DPI, and when you should go higher.

Thanks for reading, and good luck with your scanning project.

Konrad M.
Owner of ScanCanada.ca and HowToScan.ca