Picking the right resolution depends on what you want to do with your scans. To be safe, you can scan all your photos at 300 DPI. This will give you an exact copy of your original photo. And at 300 DPI you can view your digital photo on your HDTV at excellent quality.
If you're scanning more than 300 DPI you're just scanning the surface of your photo. What I mean is, since your photo was physically printed at 300 DPI, scanning more than 300 DPI won't scan anymore detail.
You'll only scan more artifacts off the photo-- such as the gloss off the photo, minute dust and debris, etc. You are not going "deeper" and scanning more pixels and image detail. That's because your physical photo has a limit of 300 DPI. It was printed using 300 dots per inch.
So, why would you scan higher than 300 dpi resolution? That's next...
Jason, a recent customer, came to me with 100 photos. It was his Mom and Dad's 60th Anniversary, and he wanted to make a slide show for the party. He told me that he was going to display the digital images on a large projection screen. I told him that a 300 DPI scans will display perfectly on any size screen. Even a 8' x 8' projection screen.
How can a 4" x 6" photo scanned at 300 DPI be displayed on a 8' x 8' projection screen, at excellent quality?
Let me show you...
Have a look at your standard 4" x 6" photo. Obviously it is 4 inches by 6 inches...
A standard 4" x 6" glossy photo.
When you setup your scanner at 300 DPI, what it does is it "squeezes" 300 dots (pixels) per inch. In other words your scanner multiplies the height and width by the amount of resolution you chose. Here's what happens to a 4" x 6" photo scanned at 300 DPI...
You scanner converts a 4" x 6" photo into a 1200 x 1800 digital photo when scanned at 300 DPI.
This number (1200 x 1800 ) is called the dimension. And it's the MOST important number you need to know. Forget about DPI, mega pixels, 4K. Dimension is most important. Why? Here is why...
To put this number in perspective, have a look at your 1080p HDTV. The dimension of your HDTV is 1080 x 1920. Now, your digital photo is 1200 x 1800. So if you put your digital photo on your HDTV it will be displayed in HD quality. That is because the dimension of your photo is pretty close to your HDTV resolution. Check it out...
Your HDTV is 1080 x 1920, and your digital photo is 1200 x 1800-- a snug fit.
Oh, and what if you have a projection screen? Well, if you look at the specs of your projection screen it is probably 1080 x 1920 as well. And even if you watched your digital photos on a 8' x 8' projection screen, it will still display at 1080 x 1920. It is the same with an HDTV. It does not matter if you have a 32" HDTV or 52" HDTV-- they both will display at 1080 x 1920.
Well, the same thing happens. Your scanner will squeeze (multiply) 600 dots for every inch. What happens is...
At 600 DPI you get a 2400 x 3600 digital photo.
Your 4" x 6" photo is converted into a 2400 x 3600 digital photo. Does this mean when you play this digital photo on your HDTV it will be twice the quality? Nope. Your HDTV has a display limit of 1080 x 1920. If you your digital photo is larger than that, two things can happen. One, your HDTV will crop your photo. Or two, it may re-size back down to 1080 x 1920.
Your HDTV may crop any digital photos that are larger than 1080 x 1920.
When you scan you photos at 600 DPI, you are not picking up any more detail off your photo. You are just making it bigger. Just like when you upgrade your 32" HDTV to a 52" HDTV, you are not getting twice the resolution, you just have a bigger picture. You're not getting any more detail. Yeah, the the TV is bigger, but the dimension (1080 x 1920) is still the same. But if you get a 4K TV, then you'd get more detail. A 4K has a dimension of around 3840 × 2160.
A 600 DPI scan is twice as big as your 300 DPI-- but it does NOT have twice as much detail, it's only bigger. What your scanner did was make sure to add enough extra pixels to maintain the same quality of the 300 DPI digital photo, not add more pixels for extra detail.
The same thing happens when you have a 32" HDTV vs. a 52" HDTV. You're not getting more picture quality. You just make sure you maintain the same quality when you have a larger screen size.
Just like when you go from a 32" HDTV to a 52" HDTV, you do not get more picture detail, you just have a bigger screen.
At 900 DPI you will get a digital photo with a dimension of 3600 x 5400. And I know at this resolution I am NOT getting more detail, but just making my image bigger. And I know that I will pickup a lot of artifacts like finger prints, etc. that I do not need. But, at 900 DPI I feel I'm pushing my scanner to its limits. And it's better to have too many pixels, than too little!
A 4" x 6" photo is converted into a 3600 x 5400 digital photo.
Remember my customer Jason? He wanted photo scans so he can display them on a 8' x 8' projection screen. I told him 300 DPI will do the job.
So naturally, Jason thought since the images can be displayed at excellent quality on a projection screen, he can also make poster size prints.
When he took one of his 300 DPI photo scans, and had the print shop make a 12" x 18" re-print, the poster looked horrible.
How can the same digital image look great on a 8' x 8' projection screen, but when printed out at 12" x 18", it comes out looking horrible?
Remember, your photo scanned at 300 DPI will be the exact same size as your original. And if you try to take that 300 DPI photo scan and re-print it at double the size, it will turn out pretty bad. That's because displaying your 300 DPI on your HDTV is VERY different from physical printing.
If you want to re-print your 4" x 6" photo at 8" x 12", you'll need to double your DPI to 600. If you want to print your 4" x 6" photo to 16" x 24", you'll need to use 900 DPI.
I just want to show you that at 300 DPI you'll be safe at displaying your scans on any digital screen. But if you want to make physical re-prints, you'll need to use more DPI.
And here's another reason to use more than 300 DPI. I rather have too many pixels than too little. You see, at 900 DPI I know I have "future-proofed" my photos. Maybe in five years they will come out with a 2160p HDTV! I'll be ready.
(2015 Update! And they did come out with a bigger TV... 4K. Like I said, 4K has a dimension of 3840 × 2160. And when I scan my photo at 900 DPI, I get a 5300 x 3500... bigger than 4K. )
Also, I can always make a copy of my 900 DPI photo scans, and re-size the copies to whatever I want to do. If my HDTV is cropping my 3600 x 5400 digital photos, I just make a copy and re-size the copies to 1200 x 1800.
The same math applies. Say you have a 2" x 2" photo. Here is what happens when you convert it to a 300 DPI digital photo...
Your 2" x 2" photo scanned at 300 DPI is converted to a 600 x 600 digital photo.
So now you have a digital photo that is 600 x 600-- way below your HDTV dimension. So in this case you probably would want to scan it at 600 DPI. Check out what happens...
At 600 DPI your 2" x 2" photo is converted into a 1200 x 1200 digital photo.
At 1200 x 1200 you can display your digital photo on you HDTV at excellent quality.
And that is the point here-- once you understand how resolution works, then you have more control. You can scan all your photos at 900 DPI and archive them. Or if you want just simple, quick scans, at HD quality, use 300 DPI and you will be safe.
Which of these scanning troubles do you want to overcome?
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