Slide, Film, Photo Scanning Resolution: Use This DPI To Get HD Quality Digital Images

Read this first if you are unsure what resolution you need for HD slide, photo, and film scans.
Understanding resolution is easy to grasp if you know these 3 basic numbers. And once you know the basics of image resolution, the easier your scanning project will be. Let's get started.
1. Know The Original Size Of Your Slide, Negative And Photo
The first number you need to know is the actual size of your negative, slide, or photo. Knowing this number is important because it is the first thing your scanner looks at.
Take out a 35mm negative or 35mm slide, and if you actually measure it, the physical size is about 0.80" x 1.30". The photos in your family albums are most likely 4" x 6" or 8" x 10". Older photos can range from 2" x 2" to 3" x 5".
When your scanner software starts up, look for something called "Original Size". This is where your scanner will determine the size of your original. So your 4" x 6" photo may show as 3.9" x 5.9". Do not worry if it is not exactly 4"x6", there will be some cropping.
2. Dots Per Inch -- What Is DPI And How Much Do You Need To Scan Slides, Negatives, And Photos
Dots per inch is a term that comes from the days of photo labs. Remember how you had to go into a photo lab to develop your photos? What the lab did was they printed 300 colored dots onto photo paper. So your 4" x 6" photo had 300 dots per inch that made up your image. Slides and negatives are a bit different. Instead of colored dots, emulsion is used to coat transparent film.
When I scan my photos I use 300 DPI, and for slides and negatives I use 1500 DPI. At this DPI you will be safe because you will produce a digital image that is EXACTLY the same quality and detail as your original.
So when you open your scanner's software you will see two numbers: 1. Original Size: again, this is the actual size of your original, give or take a few points. And number 2. is your DPI: this is where you choose what resolution you want to scan your slide, negative, or photo.
Now, these two numbers, put together, will make up your digital image. Here is how:
3. How Original Size And DPI Work Together And Give You A Digital Image
Say you are scanning a 35mm slide. Your scanner will show you an Original Size is 0.85" x 1.30" (give or take). Then look for your DPI or Resolution, and input 1500. What your scanner does with these numbers is simply multiply them. So your 0.85" x 1.30" slide is multiplied by 1500 DPI, which gives you 1275 x 1950. Here is another look: 0.85" x 1500 DPI = 1275, and 1.35" x 1500 DPI = 1950.
Now have a look at your photo. Remember it is 4" x 6", and you want to use 300 DPI. Here is what your scanner does: 4" x 300 DPI = 1200, and 6" x 300 DPI = 1800.
This number you get (1275 x 1950) is called Pixel Dimension. Do you see some similarities with a slide scan and a photo scan? You scanned your slide at 1500 DPI and your photo at 300 DPI, but you still have the same Pixel Dimension. Here is another look: 35mm slide scanned using 1500 DPI = 1275 x 1950 Pixel Dimension; 4" x 6" photo scanned using 300 DPI = 1200 x 1800 Pixel Dimension.
This is why the actual size of your original is important, and DPI is not. You used two different DPI's, but still got the same result-- a digital image that is about 1200 x 1800. But is 300 DPI / 1500 DPI enough to give you a quality digital image?
Conclusion: 1200 x 1800 Pixel Dimension Gives You True HD Quality
To put everything in perspective, check out the Pixel Dimension of your computer monitor (it is actually called Screen Resolution). The dimension of my computer monitor is 1366 x 768. Now take a look at an iPad. The dimension of that is 1024 x 768. Lets move onto your 52", 1080p HDTV. The actual dimension of your HDTV is only 1080 x 1920.
Do you see that even your HDTV has a pixel dimension that is somewhat lower than your 1200 x 1800 digital photo? So your 1500 DPI slide scan, and your 300 DPI photo scan will be displayed at full HD quality. That is because the pixel dimension of your scans are bigger than your HDTV.
But here is the thing: I scan my slides at 4000 DPI and photos at 900 DPI. Why? Even though 1200 x 1800 is a safe resolution, you never know what future technology will come out with. And it is always better to have too many pixels than too little.

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