You Say You Want a Resolution: How Much DPI/PPI is Too Much?

Migrant Mother” by Dorothea Lange. Courtesy of the Library of Congress.

Preserving digital stuff for the future is a weighty responsibility. With digital photos, for instance, would it be possible someday to generate perfectly sharp high-density, high-resolution photos from blurry or low-resolution digital originals? Probably not but who knows? The technological future is unpredictable.

The possibility invites the question: shouldn’t we save our digital photos at the highest resolution possible just in case?

In our Library of Congress digital preservation resources we recommend 300 dpi/ppi for 4×8, 5×7 and 8×10 photos but why not 1000 dpi/ppi? 2,000 dpi/ppi? 10,000 dpi/ppi? Is there a threshold beyond which the pixel density is of little or no additional value to us? Isn’t “more” better?

Recently we received a comment at the Signal in response to a blog post in which the commenter expressed concerns about our ppi/dpi resolution recommendation. The commenter raised some intriguing issues and I asked two digital photo experts to respond to his concerns.

Barry Wheeler, one of the experts who responded, is a photographer, staff member of the Library of Congress and one of the digital photograph preservation researchers for the Federal Agencies Digitization Guidelines Initiative. Wheeler has also written several blog posts for the Signal about scanning and photo digitization.

David Riecks, the other expert, is a photographer, co-founder of Controlled Vocabulary and PhotoMetadata.org. Riecks has written several blog posts for the Signal about photometadata and about processing digital photos.

Below are the comments from all three people. Please read them through and decide for your self what the best digital photo resolution for archiving is.

Mark S. Middleton wrote:
I am concerned that advising people to save at 300 dpi will result in lots of regrets for future generations. The quality of printing, computer monitors and televisions will continue to improve (and thus the ability to see details in higher quality imagery). Also, a person may want to zoom in and view just a portion of a scan or even cut out a piece (just their grandmother from a school group photo) all of which will suffer from 300 dpi.

I believe that 600 dpi is a better recommended minimum size. It’s better to build the quality into the original scan (saving as a TIFF), then saving JPEGs from that for sharing with relatives or posting online (for smaller file sizes). I recommend looking at the “use cases” of scanned photography and as well as better future proofing recommendations. 600 dpi does cause larger files, but with hard drive prices coming down I believe the value is worth it.

David Riecks responded:
I think the answer really revolves around what you are scanning. For “photos” (i.e. a photographic gelatin silver print, or chromogenic dye print like RA4 process), you can scan at a higher resolution. However, in most cases, all you will see are the defects.

If the original you have to work with is a 4 x 6 inch print, and you scan it at 600 or 1200 pixels per inch, you could then make the equivalent of an 8 x 12 inch print, but it’s not likely to give you better quality. It will…take up much more space on your hard drive.

If you have a high-quality 8 x 10 inch glossy print, in which the image is sharp (no motion blur from the camera moving), it might be worth going to a higher sampling setting. But I would recommend that you do some tests first to make sure it’s worth it.
In my experience, higher scanning resolutions usually just give me more dust to spot out later and the enlarged images never look as good as the small original.

If you are scanning a b&w or color negative or a color slide, then you certainly want to scan at higher resolutions. Which is best has much to do with your intentions (now and in the future), the quality of the original and the type of hardware you are using to make the scan.

Many scanners advertise an interpolated sampling rate in their “marketing speak” though you will often get better results scanning at the maximum “optical resolution” of the scanner.

Barry Wheeler responded:
First, begin with how much detail is there actually in the original. This amount of detail varies widely. A halftone screen for an old newspaper may result in less than 200 dpi actual. A modern lens on a quality black 7 white emulsion may be 2800 dpi.

In the old days, (the 1990s) when scanning became widely available, 300 dpi was a good starting point because many, many books and documents did not contain more detail than that, and even today, 300 dpi is a good starting point.

For example, at the Library of Congress we currently print our digital photographs using high quality pigment printers that may claim a resolution of 1200 or 2400 or much, much more. But those are microdots of different color merged to produce the variety of shades of gray or color. Usually the printer driver produces a finished resolution between 240 dpi and 360 dpi.

Second, we need to sort out the term “resolution.” Scanners and cameras contain pixels and “sample” the image at a “sampling rate” depending on the distance between the camera and the image. So when people talk about “resolution” using 300 ppi or 600 ppi or 3000 ppi they are actually using the “sampling rate” of the device. But few devices are 100% efficient.

Common scanners may be only 50% efficient; cameras may be 80 – 95% efficient. Thus the actual resolution achieved at 300 ppi may only be about 200 ppi – higher ppi rates are the result of image processing which may give the appearance of sharper lines but which does not produce additional detail. Many scanners will claim 1200 ppi and produce less than 600 ppi true optical resolution. Federal Agencies Digitization Guidelines Initiative standards (http://www.digitizationguidelines.gov/) are currently at 80% efficiency for a 2 star, 90% for a 3-star, and 95% for a 4-star outcome. Many of our projects for prints and photographs and rare books are 400 ppi at 3-star levels, although some are much higher.

Third, many people want to enlarge an image. We often try to scan film – particularly 35mm film – at a resolution necessary to provide a final print at 300 dpi. So if you want a common 4″ x 6″ print you need a true resolution of 1200 ppi. Specialized film scanners and high quality camera setups can achieve this. Commonly available consumer flatbed scanners cannot. (If you read the fine print specifications, they will often say something like “true 2400 ISO sampling rate” not ISO “resolution.”)

But once you reach the limits of the device resolution and the detail in the original, then additional enlargement doesn’t help. I think I have a couple of illustrations of this in my most recent blog article about enlargement (http://go.usa.gov/j2q4). I don’t believe you can magnify a newspaper image and find additional detail in a scan with a true resolution above 300 ppi.

Finally, Apple claims that human vision is only capable of resolving 326 ppi (search online for their “Retina display” marketing materials). There is a lot of quibbling about that number but most still claim not more than 450 ppi.

In the end, I doubt that you will see any significant improvement in an image of reflective materials beyond an ISO standard resolution of 400 ppi. I doubt you will find any improved image quality on consumer scanners above an ISO standard resolution beyond 1200 ppi unless you scan 35mm film in a specialized, high quality film scanner.

Two final notes. I believe the costs of higher resolution are vastly underestimated. Scan time will increase significantly with increased resolution. Transfer times increase, processing times increase. The expertise needed increases to get better quality. Storage and multiple backups increase. Consumer hard disk drives are not archival devices. Your children and grandchildren may not be able to retrieve images from a hard disk even 15 years from now. Increased image size means greatly increased cost.

And I believe 300 ppi / 400 ppi is future-proof. At least for reflective materials, I don’t believe we will see greater detail in a 1200ppi scan no matter how improved future equipment is.

14 Comments

  1. SWJenn
    July 16, 2013 at 2:02 pm

    Great blog post, so good to hear back from the experts.

    But this all underscores for me that digitizing is not always the best means of preserving an image – a good print can have a longevity that is difficult to match digitally because of changing technology and the frailty of digital media. I advise my photography students to print good quality prints and store them properly as another layer of ‘backup’. I have prints that have withstood 150 years that are remarkably well preserved, and digital images lost to the ages locked away in a zip drive I am no longer able to access. So the dpi/ppi doesn’t matter if you can’t read the file. High quality silver halide prints are important, and digital files, while allowing uses of the image not possible with a print, are no substitute.

  2. Carl Fleischauer
    July 16, 2013 at 2:29 pm

    Great to read about this and hear from Barry Wheeler on the interesting and little-known matter of “sampling efficiency.” While we are at it, we ought to reminder readers of Barry’s earlier postings on this general topic — here’s the link to part 3 of a three part blog: http://1.usa.gov/11UemG7. (Find parts one and two on your own!) I will also suggest taking a look at a pair of blogs I wrote even further back on determining the inherent level of picture information in photo negatives: http://1.usa.gov/15j9Pu3 and http://1.usa.gov/18jngww. But my blogs are not as clear as Barry’s regarding sampling efficiency.

  3. Ken Hopson
    July 16, 2013 at 2:59 pm

    Mr. Riecks and Wheeler –

    I completely understand what you say about not being able to improve upon the quality of an image just by scanning it at a higher resolution (like capturing a VHS tape to DVD doesn’t improve the quality either).

    However, as Mr. Middleton noted, the quality of printing, computer monitors and televisions will continue to improve. Given that, would not a much higher res scan than 300 dpi, with higher pixel dimensions, project better on really large viewing areas (like, for example, a display wall or an Ultra HD TV with an 8MP monitor)?

    Just wondering, thanks – Ken Hopson

  4. John Black
    July 16, 2013 at 3:10 pm

    My question is: what strategy should be used when scanning old family photographs (on paper) assuming the image will be viewed on a next-gen TV.

    I believe we are in the midst of an evolution in the way photos are viewed – and that this should be a factor in how we digitize our existing material.

    There have always been two ways to look at a photo: as a print, or a projection. Right now, very few photos are viewed as prints, yet this conversation seems limited to discussions about the effect of dpi on subsequent same-size prints (or enlargements) with reference to scanner and printer technologies. I think we should be scanning for the way images will be viewed in the future.

    And how is that?

    Right now, photos are hardly seen at all. They get snapped by the dozen, glanced at via the camera’s 3 inch screen, and deleted when the memory card is full. Some are emailed – and then squinted at on the other guy’s phone, or seen on a tablet. A few are appreciated once or twice on a computer monitor – but not exceeding 20 inches – and then lost. Today, the average person’s photos are stored in so many places and are so poorly labeled they have become ephemera – enjoyed for the moment and then forgotten.

    I think this is about to change. People are starting to ask – why am I taking all these photos if I’m never going to look at them again? When Grandma dies and nobody can put their hands on a decent recent photo, much less find Uncle Jimmy’s baby pics, people will re-think their habits.

    I predict we are headed right back to 1865. Or 1965. Remember lantern slides and Kodachrome?

    The distance between a tiff (or RAW) and a high-quality, hand-held print is so arduous a trip I don’t believe prints will recover. Their function – images to be easily seen, cheaply duplicated, and readily shared – has been usurped by digital media. The newer technology is so much better, I don’t see prints coming back.

    What I see is a resurgence of the old-fashioned slide show. People will select their best images, put them in a certain order, burn them to a DVD, and view them on the TV in the living room. I say “old fashioned” because I don’t mean the kind of production with a synchronized music track, frozen voice-overs, and multiple KenBurns effects. People won’t make that effort, as easy as it can be. But, when they start losing precious keepsakes to digital clutter and failed hard drives, they will begin making the effort to get all the best of last summer’s vacation pics in story-order, in one safe place, which can be easily viewed to maximum advantage – a DVD, plopped in a player, watched on a 42” (or 72”) 1080 LED TV, in front of a comfortable sofa – just one image after another with a simple dissolve, advanced by remote control, accompanied by a spontaneous narration (probably by the photographer) geared to the time and the specific audience present.

    Compared to the days when Dad had to darken the room, find the projector, check for a burned-out bulb, unfold and stretch-out the 3 foot 3 legged screen, and hope the slide would drop thru rather than jam up the tray – today’s slide show technology is a dream.

    So, in deciding what resolution to use in scanning old family photographs (on paper) my question is: what strategy will be best assuming the image will be viewed on a next-gen TV.

  5. Toke Eskildsen
    July 17, 2013 at 8:32 am

    John Black: I do not think that you should consider the display size when scanning, assuming you scan for preservation. Whatever your guess for a display size is, it it likely to change over the years. Go for the largest size that makes sense information-wise from the original material and the scanner (just as the article suggests for paper originals).

    Unfortunately, the article is right about consumer level scanners being inadequate for scanning negatives. I checked my own cheap scanner (Epson V500) and even though it is advertised as 9600DPI, it really only resolves around 1200DPI, just as the article suggests. I have a bunch of negatives that I would like to scan for the first & final time and I am considering renting equipment that does them justice.

  6. Richard Saunders
    July 18, 2013 at 11:49 am

    As an archivist, I think the question of digital resolution is somewhat misplaced, a bit like arguing about the best manner for holding a handful of sand. Digital imaging is not a means of preservation (in the archival sense) but rather a convenient means of simplifying distribution and advertising. I sincerely hope we have learned something from the Great Microfilmed Newspaper Debacle of bygone years–where newspapers were chopped up for photographing and then simply discarded: reformatting at any resolution is never an adequate substitution for the original object. Digital technology offers some great benefits, but for routine use electronic forms are expensive conveniences at any resolution. No digital file is ever nearly as “permanent” in the way a physical original is.

    Am I arguing for no longer digitising? No, just ensuring that we don’t let the inherently fugitive format become the practical standard of preservation. An individual or institution would be well advised to pour their limited funds first into preserving original material, and only secondarily into digitisation–with the explicit understanding that maintaining and migrating digital forms is far more expensive in the long run than costs involved in preserving the negatives or prints.

    Beyond that, maintaining a general-use digital image at 300/400dpi is an adequate practical step. If a higher resolution is needed, rescan the object and let whomever requests it foot the bill.

  7. Charles Bailey
    July 18, 2013 at 1:21 pm

    The LC’s Digital Scan Service scan specs are here: http://1.usa.gov/18qlAEQ

  8. Toke Eskildsen
    July 19, 2013 at 4:57 pm

    Richard Saunders: What is the state of negatives after 100 years? After 300? 500? Your time perspective seems limited to the near future. I would argue that digital is exactly the definition of permanent as it is unchanging (assuming using best practise of having multiple copies with checksums, migrating losslessly and all that). Opposed to that are the originals, be they negatives or paper, that are doomed to a – hopefully – slow deterioration.

  9. Sarah Dobbin
    July 22, 2013 at 9:52 am

    At the museum where I work, we use images in two ways. The first is reproducing the image as artifact. In this case, we do little to no retouching or recolorizing, and show images at a 1:1 scale, or slightly larger if it will benefit the visitor experience. The second way is to apply some treatment, scaling, etc., to provide a different experience. This may be applying a color to images, making them a monotone image in the background, or making the image very large for impact. In the case of making images very large (e.g. 8-10 feet tall), I have found that we are often able to scan-in original images at sufficient resolution to scale them. If the image has already been scanned in at 300 dpi and the original is not available, the image is rarely big enough to print with good quality. Although this may seem obvious, it is important to note that depending on your final image use, you must allot potentially a significant amount in your budget and timeline to process images to render them usable at whatever size you are printing them at.

  10. Jeremy Rowe
    July 24, 2013 at 9:42 am

    This analysis assumes the end result is a print roughly the same size as the original for reproduction or display, though scaling for large prints is mentioned in a comment. It fails to address the benefit of higher resolution scanning for analysis of the photographs or for use in video, both of which require higher resolution and lower resolution scans limit both potential uses.

    Again the intent and possible future use are deciding factors, but given scanner capabilities and low storage costs, why not recommend higher resolution scans, particularly for content such as interior and exterior views that contain clues that can be uncovered by enlarging the image?

  11. Michael Horsley
    July 24, 2013 at 10:46 am

    There is no perfect answer. However there are informed choices to be made.
    Factor #1: is the resolvable or perceivable information content in the original and that varies with media and within media. Newspaper halftone vs Kodachrome 5×7 transparencies have very different levels of information.

    Factor #2: The true or actual sampling efficiency of the capture equipment. 300, 400, 600 ppi are just numbers that you set your equipment to in the software they are not indicators of the end result. And no two scanners scan the same.

    Factor #3: Total lack of a standard viewing interface. And most prints or display devices can not render the fine information content found in the original.

    Factor 4#: Scanning approaches that aim to maximize quality and anticipate future outcomes have not been proven to be an efficient means to “preserve” anything. The historical record favors chance survival of data regardless of the substrate the data was contained within. Ubiquity and Utility greatly increase the chance of “preserving” information. Also there must be some informatic maxim that states: Expect Loss

    Factor #5 Are we getting more or less visually literate to demand greater quality? History of Photography more or less has been to sacrifice quality for the sake of ease of use. I tried Wet Plate it was fun, but…

    So, the 300/400/600 ppi numbers serve as an informed comprimise that factors in sample rate/image quality/and utility of the file.

  12. Marek Zielinski
    July 31, 2013 at 9:10 am

    A couple of comments

    A common confusion with ppi: it only applies to conversion of a flat object, not to digital photo. The statement “human vision is only capable of resolving 326 ppi” is misleading, because the resolution clearly depends on a distance. For example we can see with bare eyes the spider web: 0.00012 in = 8333 ppi minimum.

    In general the scan resolution should be such, that one can distinguish the smallest details visible on the picture, whatever they are. It may mean 300 or 600 or more depending on image. Good magnifying glass may help..

    Future-proof means that we do know, what a future researcher will look for. Do we? If you scan a smallest newspaper text (say ruby) at 300 dpi, you will get a barely readable text, but difficult to determine the details. A researcher was looking for tiny holes made in the picture by insects – hi resolution scan allowed to see the shape of the hole. This is especially important if the image is of archival value, and it is expected, that the original will not be accessible (or accessible with difficulty)

  13. Sherwood
    August 12, 2013 at 9:12 am

    There is little point in scanning for information that isn’t there. The resolution of most 35 mm camera lenses is about 2000 x 3000 pixels. You can increase this to 3000 x 4500 pixels if:

    * Using very good lenses
    * Using very CLEAN lenses
    * The image lighting has minimal light on the lens to produce internal reflections.
    * You are using a very fine grain film.

    One way you can test the effect of increased scanning resolution:

    1. Make a series of scans at various resolutions.
    2. Now, using a package such as ImageMagick or NetPBM, do some processing:

    a. Try expanding the 300 dpi image to 600 dpi using appropriate intermpolaton (usually bi-cubic)

    b. Now, subtract one image from the other. You want to get the absolute value of the differences.

    c. The resulting image will be VERY low contrast. Normalize the contrast.

    d. Look at the image. If there is additional information in the 600 dpi image, you will see ghostly, shadowy detail in the the difference image. If, OTOH, all you see is speckles, then the additional scanning density was just meauring noise.

    Working with prints is even worse: A print can only resolve a luminance range of about 100:1 — About 7 stops. Many films can do 1000:1 — 3 stops more. Good printers take this into account. Plus the enlarger lens multiplies the original lens’s limitations by it’s own.

    From 35 mm or smaller film I would be surprised if 600 dpi reveals any information not present at 300 dpi even for high quality 8×10 prints.

    If the original was shot in a larger format, however, it may be worth doing.

    In either case, going back to the negative has merit. And all negatives should be scanned at 2-3 thousand dpi.

  14. Phil von Stade
    March 5, 2014 at 6:06 pm

    Sorry, I’m late to this party, but thought I throw in some thoughts, incase anyone has new ideas…

    I am working on a one year history project for a small community in Maine. I have to scan hundreds of postcards, photos, documents and maps, some going back to the 1700s!

    1. PRESERVATION: I want my stuff to be safe. (LOCKSS – Lots of Copies Keeps Stuff Safe). I return original material to owners, or store in our library. I use Cloud Backup(DropBox), off-site hardrive, and local harddrive(flash).

    I remember paper prints and DVDs. Both were useful in their day. I find it amazing that anyone worries about printing an 8×10. The VAST majority of photos will be looked at and zoomed on digital devices. DVDs are quickly becoming obsolete. (Why, Apple hasn’t used them in years :)

    2. PRESENTATION: Every person under 20 in America (and most the rest us) looks at photos on an iPhone, iPad, HD TV or other digital device – and the results are phenomenal! My 83 year old mother and my mother in law (in South Africa) LOVE their iPad PhotoAlbums.
    ERGO: I want a backlit HD (‘retina’) digital display.

    3. DETAILS: Most people know to pinch to zoom. And they love to zoom. They HAVE to zoom because smartphones are so small, but they make it so easy to zoom.
    ERGO: I need the BEST RESOLUTION possible, because I’m going to look very closely at my mother’s face in the 1942 group photo.

    And I agree fully that some photos need 2400dpi, and others (e.g. half-tone postcards) max out at 200dpi.

    TIME: I want to SCAN as quickly as possible, and I want displays that will DISPLAY large files as quickly as possible. (e.g. A high quality image of a MAP can be over 100mb) I used to scan all photos to end up with about a 1meg file, because that was all most PCs and Macs could deal with in a fast slide show.

    SPACE: Getting cheaper all the time – almost not an issue.

    Help?: Anyone know of a great, FAST flat-bed or other scanner? (For maps I guess I still have to use a good copy-stand.

    Cheers!
    Phil

Add a Comment

This blog is governed by the general rules of respectful civil discourse. You are fully responsible for everything that you post. The content of all comments is released into the public domain unless clearly stated otherwise. The Library of Congress does not control the content posted. Nevertheless, the Library of Congress may monitor any user-generated content as it chooses and reserves the right to remove content for any reason whatever, without consent. Gratuitous links to sites are viewed as spam and may result in removed comments. We further reserve the right, in our sole discretion, to remove a user's privilege to post content on the Library site. Read our Comment and Posting Policy.

Required fields are indicated with an * asterisk.