One of the most important decisions you face when scanning anything with your scanner is choosing what dpi (“dots per inch”) to scan with. And specifically for this post, what is the best dpi to use when scanning and archiving your 8×10″ and smaller paper photographic prints – which for most people, make up the majority of our pre-digital collection.
Making this decision was very challenging for me and certainly a huge part of my 8 year delay. The reason for this is that dpi is the critical variable in a fairly simple mathematical equation that will determine several important outcomes for your digital images:
- Detail – how much image detail you will extract from your photograph
- Image Size – how much resolution in pixels you will have to work with (e.g. 2400 x 3000 pixels)
- File Size – how large the file size will become (e.g. 64.9 MB or 64,878,462 bytes)
It's definitely a decision you want to make before you complete your very first scan. Trust me, you don't want to get halfway through your collection and realize you could have been extracting more image detail from your prints if you had just chosen a slightly higher dpi to begin with.
If you study the routine of a professional photo restorer, you will learn they tend to see each photograph as a separate challenge – like a doctor attending to an ailing patient for the first time. It's a laborious investigative process for them. They may even scan each print several times with varying dpi's, carefully comparing each image until they find the most appropriate dpi for the photo's personalized workflow.
Because you probably have anywhere from hundreds to a few thousand photos on average in your family's collection, it's not practical or even reasonable for me to suggest we would want to attempt such perfectionism. So instead what I decided to do was focus my effort on finding a single dpi setting we could scan with for each print dimension in our collections – like a 3×3″, 4×6″ or a 5×7″ picture. This way, not only is it one less decision you will have to make when you're sitting in front of a large stack of photographs to scan, but it will also create a pleasant consistency across your entire digital collection.
I know, so far this all sounds great doesn't it?
Well the trouble with trying to settle on just one dpi setting per dimension is that it's going to be used to produce (in most cases) just one “master” digital file that we will be archiving. So this one dpi is burdened with producing a satisfactory result for each of the three important outcomes I listed above.
To make it easier for us to wrap our heads around this problem, I have summarized all of my concerns into this one question:
What dpi should we scan our paper photographs with that will capture as much detail stored in them as we possibly can, will create a manageable file size, but will also produce enough image resolution should we choose to do some radical cropping or print out an average-sized enlargement from them someday?
Before we go any further, I feel it necessary to mention the term “dpi” really is incorrect when we're talking about scanning images. Because of laziness in the industry, we have been saddled with this acronym carried over from “halftone dots per inch” used by press reproduction with screened plates used for magazines and newspapers.
Instead, what we are dealing with are actually pixels and not dots. It's really referred to as ppi or “pixels per inch.” The resolution of an image on your computer monitor is measured by the amount of pixels tall and pixels wide they are composed of (e.g. 2162 × 2194 pixels). However, to avoid any unneeded confusion here and because we are talking about the setting in your scanner software that almost always refers to ppi as dpi, I will use dpi instead. Just understand that as far as the setting on your scanner, they mean the same thing and can be used interchangeably.
Alright then. Now that we have that taken care of, let's figure this out.
Choosing DPI Based on Time Required
One of the biggest mistakes I find people make when choosing the dpi is allowing the length of time it takes to complete the scan influence their decision. Those new to scanning may be surprised to learn that the higher the dpi, the longer it takes for the scanner to make its capturing pass and for the computer the process the information. I've recorded for you the time it takes my current scanner, the Epson Perfection v600, to complete scans with various dpi's.
|Scanning Times for a 3.5×3.5″ Print on an Epson Perfection V600|
|Time Elapsed (mins:sec)||:11||:12||:14||:18||:31||1:02||2:34||3:53||6:37|
You can see there is in fact a vast difference in the amount of time required to scan at one of its lowest settings (150 dpi) and scanning with one of its highest (4800 dpi). But more importantly, I want you to notice the time it takes to scan at 300 dpi through 800 dpi is almost the same length of time. On this model, we are only talking about a difference of 19 seconds. And specifically between 300 and 600 dpi, it's only a measly 6 seconds.
All scanners scan at different speeds so your scanner may or may not be slower than this model. But if it is, chances are the percentage of time between each of the dpi settings will be very similar. Regardless, if you think you are going to be one of the ones waiting impatiently for your scanner to work its magic on a higher dpi, I would like you to remember this. Scanning and archiving your family's photo collection is an investment. It's going to be an investment of your time and energy, and for that reason, you probably will only want to do this one time.
I believe it's worth a few extra moments – as agonizing as you may find them to be – to insure your image quality isn't being compromised by rushing the process. Open wounds need time to heal, baking dough needs time to rise, and photo scanners need time to scan. Got it?
To fill the extra time, consider taking an extra long sip of your nonfat mocha latte, play solitaire, watch some television, or better yet – lightly dust off your next photo to be scanned with a lint free cloth.
So what happens when you scan with the dpi set too low?
Paper prints were never intended to replace the original slides and negatives they were made from – they simply can't hold that much detail. Instead they were only made so it would be easy to share them. We could print them out, stick them in a fun colorful album and bring them around town to show our family and friends.
If you are fortunate to have the original film negatives or slides, you definitely want to use them instead of your prints to create the highest quality digital images possible.
Sadly, many of us have accidentally misplaced our negatives or threw them in the trash throughout the years and are only left with these fair to moderate paper representations. But don't get me wrong – we are lucky to have these. Just talk for a few minutes with someone who lost their entire collection to a house fire and you will want to cut them off mid-story and rush home to your shoebox of Polaroids and give them a big ol' hug! I'm not kidding. Really hug them. Hug them now.
Paper prints can only hold up to 200 or 300 dpi worth of image detail. If you're really blessed, you might have some prints with detail up to 600 dpi.
It's really difficult to look at a photographic print and know how many “dots per inch” of detail it holds. Professionals that have been doing this for many years can probably tell based on a lot of criteria such as the year it was printed, the type of film negative it was exposed with, the type of paper it was printed on etc. But for the rest of us, I think it's fair to say we can only estimate after some trial and error.
If you're really motivated, take one of your prints and scan it at different resolutions such as 100, 300, 600 and 1200 dpi. Then put each one of your images on your computer monitor (zooming in when necessary) and compare the level of detail. Try and determine at which ppi setting you no longer are getting more picture detail from the setting below it. This dpi setting is then the one you may want to use for this photo because it gives you all the detail at a smaller filesize than that of a higher dpi setting.
So if you scan with too low of a dpi – for example 75, 100 or 150 dpi – you run an incredible risk of not capturing all of the detail that your paper prints are holding.
In order for us to settle on one dpi setting per photo dimension, we will need to choose one that, in many cases, will be too “high” – attempting to capture more detail than is realistically there – just to insure that we are able to capture the detail when it is available in a given print.
So can you scan with too high of a DPI?
When you buy a dedicated photo scanner, or even an all-in-one printer/scanner, you are led to believe from the specifications on the box it came in there is some real value in scanning with these extremely high dpi scanner settings.
I mean if the scanner is rated for “6400 x 9600 dpi” and that's why I paid so much for this new fancy model, why not just go for it and set it to that 9600 dpi?
That's a fair question to ask, especially considering all my talk about making sure that when you scan a photograph, you understand this may be the last you will ever get to scan it.
In most cases, you won't do any harm scanning at unnecessarily high resolutions. Just don't fool yourself into thinking the higher up you go, the more detail you will extract – because it probably isn't there to begin with. If you had this much detail, it would be in your original film negative or slide – not in the print.
Also keep in mind the higher up you go, the exponentially greater amount of hard drive space you will need to save the file and the faster computer you will need to process this information in a photo manager. For example, the 3.5 x 3.5″ print at 4800 dpi (48-bit) I did for the chart above took up a massive 2.22 GB (2,216,808,780 bytes) of disk space. Those interested in archiving their masters on today's single-sided recordable DVD's will be unhappy to learn you will only get 2 of these to fit on a single-sided (4.7gb) recordable dvd.
I would agree that a 2.22 gigabyte image is rather large and way too excessive to be the average file size for your collection. But you really shouldn't concern yourself if you start to see file sizes between 40 megabytes and 200 megabytes – especially when you choose to work with file size intense 48-bit uncompressed (TIFF) settings. And on occasion they could even be larger than that for your special photos.
We are finally past the time when hard drive space and processing power is in short supply for digital images. Except for some of the ultra-portable computers, an average personal computer bought today is more than competent at handling 100MB images, and enormous multi-terabyte (thousands of gigabytes) hard drives can be found under $100 if you shop around.
Keep in mind the ultimate goal for these scans are for their archival purposes, and will find their greatest reward years and years from now when computing power and storage space won't even be a consideration anymore.
Optical Resolutions vs. Digital
The really high resolutions however – 2400, 3200 and 4800 dpi – are really intended for capturing really small and highly detailed sources like film negatives and slides.
If you want to experiment with these resolutions, just make sure you stay away from the “digital” ones. On the box your scanner or printer/scanner combo came in, you will see a rating with two numbers. The Epson Perfection v600 for example is rated for 6400 x 9600 dpi of resolution. The first number is the highest “optical” resolution your scanner is capable of, and therefore the highest dpi you should ever scan with. The second number, is often the highest resolution it's capable of scanning digitally – faking the results by interpolating the data. Thankfully, some scanning software now won't even let you select the digital dpi's from the list.
In the case of the Epson Perfection v600, the maximum “digital” resolution is actually 12,800 dpi, much higher than the second number given. So you may need to consult your specifications printed on the box or find it in the printed or .pdf manual if you are curious about your scanner's digital capabilities.
The Advantage of Having All This Archived Image Resolution
In the end, there won't be any benefit to having our entire collection in a digital format if we aren't able to make paper prints from them like we can with our film negatives. Just like we need a certain amount of “dpi” to capture images into the computer, we need a certain amount to print them back out to paper. The larger the piece of paper you want to print on, the more image resolution you are going to need in your digital files.
Printers today need on average between 200 and 300 ppi (dpi) of image resolution information to print a high quality image on high quality paper. And I am going to make the assumption that most of us seldom print out a photograph larger than 8×10 inches. Which is good, because almost all of the printers out there won't even print larger than 8×10 inches!
I know you probably hate math as much as the next person, so don't worry – I'll do it for you. What this means is that in order to print out a photo on an 8×10″ piece of paper, we need up to a 2400 x 3000 ppi (dpi) image. Here's my work:
(8 inches x 300 ppi) x (10 inches x 300 ppi) = 2400 x 3000 pixels per inch
Given a choice, without a doubt in my mind, it's better to scan too high than to scan too low.
And if you don't want to take just my word for it (grin), when I started learning all about scanning years ago, I found great comfort in this quote from Wayne Fulton of scantips.com and kept going back to it:
It is true that if the image might be resized after the scan, it's always much better quality to resize to reduce the image size rather than to resize to increase the image size. If you aren't sure what your future intentions for the image might be, and won't be able to scan it again, then it's probably best to err on the large side (if storage space allows, up to reasonable amounts anyway). Resizing to be smaller discards excess pixels. But resizing to be larger must create (or fake) new interpolated pixels which were not in the original scan. There is no additional detail possible in interpolated pixels, even if the image is larger. The results are not at all the same as scanning at the higher resolution.
Okay so really, what DPI is the best for each print dimension?
We are finally to the point where I can tell you what dpi I use on average for each print dimension. It was just a matter of crunching the numbers to insure each sized print receives a high enough dpi to not only capture all of the detail possible but will also have enough image resolution to safely print out an 8×10″ photograph from it.
So here's what you have been waiting for. Here are the dpi settings I came up with to fulfill all of my concerns yet still produces file sizes that are manageable on today's hardware.
|DPI Scanner Settings for Archiving Paper Prints|
Again, these dpi's are formulated to be used across the board for lots and lots of prints. You might decide certain prints in your collection are way too soft and lack the detail to warrant the suggested dpi so if you want to scan these few at 300 dpi for instance, be my guest. And for example, you may find some black and white prints seem to have a tremendous amount of detail, so you may decide it's worth scanning them at 1200 dpi. Or maybe there are a few prints here and there you know you will want to make really big enlargements of someday – your prized few. Whatever reason, just use the above settings as the default.
And a little note here – the pulldown menu in your scanning software where you choose the dpi may not have every conceivable value to choose from – in this case 900 or 1000 dpi. But chances are you can choose another dpi such as 800, hit the delete key to clear that amount and then type in the value you wish. If this doesn't work, you may have to choose the preset value below or above my suggestion. Of course you know now to choose higher dpi right?
Be content in knowing that what these default settings have done for you is create a digital collection with consistent image dimensions throughout that will be extremely beneficial to you as you begin to work and print with them in your image managers. You won't have to worry whether you have enough detail and resolution for future tasks because you have already planned ahead for them.
What dpi do you use to scan your paper prints with? Has this made you change your mind one way or the other? Tell me what you think about all of this in the comment section below.
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