Photography

Dots per inch (DPI) and pixels per inch (PPI) in scanning negatives and printing images

This is the first guest post ever on Down the Road. Reader P wrote such an excellent comment on how to understand dots per inch and pixels per inch in scanning and printing that, with his permission and a little editing, I’ve turned it into this post. Thanks, P, for demystifying PPI and DPI!

By P

Pixels per inch (PPI) and dots per inch (DPI) are challenging to understand in large part because the Internet is littered with outright wrong information about what these things are. Also, plenty of people use the terms improperly.

For the sake of brevity, parts of the following discussion are oversimplified. The purpose is not to explain everything, but rather to eliminate a lot of the confusion surrounding these terms by establishing a foundational understanding what PPI and DPI are, and what they are not. Doing a deep dive into every technical aspect, of every technology, of every possible situation where these terms might be used, in order to be 100% accurate and technically correct in every possible way, would defeat the purpose entirely as it would no doubt add to the confusion instead of alleviating it.

Understanding PPI

PPI has to do with screens: monitors, televisions, cell phones, tablets, and so on. It is merely a measurement of how densely packed the physical pixels are on a display — the pixel density. This in turn tells you how much physical screen real estate a given digital image will take up when viewed at 100%. PPI is simply the ratio of the screen’s native resolution, a×b pixels, to the screen’s linear physical dimensions, x×y inches.

Polka dots
A 1 DPI pattern on this cup

Horizontal PPI is a pixels/x inches = a/x PPI. Vertical PPI is b pixels/y inches = b/y PPI. These days most screens use square pixels — pixel height and width are the same, so there’s no need for separate horizontal and vertical PPI values. We just say a monitor or screen is such-and-such PPI, a single value, because it’s the same horizontally and vertically.

The closer you are to a screen, the greater the PPI needs to be to provide an image of acceptable quality. For example, a big-screen LCD TV offers far fewer PPI than your cell phone.

Understanding DPI in print

Screen PPI and print DPI are similar in concept, but they are not the same thing. However, people use them interchangeably and it causes confusion. Instead of being the density of pixels on a display, DPI is the density of dots laid down on a physical medium such as paper to form a physical image.

The closer you are to a print, the greater the print DPI needs to be to provide an image of acceptable quality. For example, the DPI of a billboard advertisement is far less than that of your 4×6 vacation photos.

Understanding DPI in optical scanning

In scanning, DPI measures the scanner’s resolution. Look at it as the number of dots per inch the scanner can allegedly resolve when scanning a given piece of film. Let’s say a scanner has a maximum optical scanning resolution rating of 3600 DPI. This means that for each linear inch of film, the scanner is capable of resolving 3600 dots of information — allegedly, as the true, effective resolution will be less, a topic outside the scope of this discussion. These individual dots of information captured become individual pixels in the output digital image, the “scan.”

For square medium format negatives (1:1 aspect ratio), which are 56mm square, the calculation is:

3600 dots/inch × 56 mm × (1 inch / 25.4 mm) = 7937 dots

In other words, you get a scan of 7937×7937 pixels.

For 35mm negatives (3:2 aspect ratio), which are 36x24mm, the calculations are:

Horizontal: 3600 dots/inch × 36 mm × (1 inch / 25.4 mm) = 5102 dots
Vertical: 3600 dots/inch × 24 mm × (1 inch / 25.4 mm) = 3402 dots

That’s a scan of 5102×3402 pixels.

Going from scanner DPI, to screen PPI, to print DPI

Polka-dotted chair
Dots per yard (DPY)?

In print, dot density matters, combined with the pixel resolution of the digital image that’s being printed, along with the image’s overall quality or the amount of information and detail it contains. Pixel density, which is not the same thing as the pixel resolution of a digital image, applies only to screens. The term DPI as it relates to the optical resolution of a scanner and the density of dots on a physical print are not the same thing. The former is a measurement of how much information the scanner can resolve while digitizing a piece of film, while the latter is a measurement of how densely packed the dots are that form the image in a print.

That said, when printing from a digital image the number of dots per inch (DPI) the printer lays down is related to the pixel resolution (a×b pixels) of that digital image. In simplified terms, due to differences in various printer technologies and how each one lays down dots on a physical substrate such as paper (which is also beyond the scope of this discussion), if there aren’t enough pixels in the original image to match at least 1:1 the number of dots that need to be printed at a given DPI and physical print size combination, then the digital image will have to be upscaled to a higher pixel resolution to match the printer’s DPI and print dimensions. This is a problem, as it means interpolation will occur, artifacts are likely to present themselves, and image quality will be greatly diminished. If an image is already lacking in pixel resolution or resolved image detail, you can’t do anything to salvage it. You can’t create detail that didn’t exist in the first place.

Advice for scanning

As stated previously, a scanner’s actual effective optical resolution is less than the rated value. For flatbeds, it’s much, much less.

For a typical flatbed scanner, I’d scan everything at a DPI that provides scans that are at least 4 times the pixel resolution (total area — 2 times the length and width = 4 times total area) of what I want my final output resolution to be.

For square, medium-format images, if I want my final files to be 2500×2500 pixels, I’d scan them at no less than 2400 DPI as 16-bits-per-color/channel TIFFs (i.e. 48-bits-per-pixel for color images, and 16-bit-per-pixel for greyscale). I would then do all of my levels editing, dust/scratch spotting, cropping, and so on at that original large resolution, but not yet sharpen anything. I’d save these as my “master” images, again as 16-bpc TIFFs. Then, I’d resize them to 2500×2500 pixels using the Lanczos-3 method, and finally use unsharp masking to sharpen them to my liking. That would be my final image for output, for sharing online, which I’d save as JPEGs.

I’d follow the same practice for 35mm. So, if I want my final 35mm images to be 3000×2000 pixels, I’d scan at no less than 4400 DPI, and then follow the same procedure as I did for medium format, cropping 3:2 instead of 1:1.

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Film Photography

A new (old) scanner

I’ve been unhappy with the 35mm scans my Canon CanoScan 9000F Mark II produces. They lack sharpness and shadow detail. I’ve done everything I can figure out in VueScan to make them better.

I’ve complained about this before, and reader P paid sharp attention. He contacted me recently to recommend a dedicated 35mm scanner he found used for a good price, refurbished, at KEH. I bought it straightaway.

It’s the Minolta DiMAGE Scan Dual II, which was manufactured in about 2003. This scanner’s maximum output is 2,820 DPI, yielding images of roughly 3680×2580 pixels. That’s nearly 10 megapixels, which is enough for anything I do with my images.

When it arrived, I quickly scanned a negative strip from a roll of Ilford Delta 400 I shot in my Olympus XA in December to make sure the scanner functioned. It did, but my scans weren’t sharp. So I tried again later with the same strip, digging into the manual and into VueScan’s settings to get focus right. I got very good sharpness that time.

I’m going to show you all four frames from both scanners. In each pair, the Scan Dual II scans are first and the CanoScan 9000F scans are second. I’ve tweaked both in Photoshop to my liking, within the limits of the scan — but the ScanDual scans didn’t need very much help. They are far better than the CanoScan scans, especially in contrast and sharpness. The contrast is apparent right off, but you need to see these scans at full size to appreciate the sharpness difference. To do that, click to see them on Flickr and then click them there to see them larger.

At McCormick's Creek State Park
At McCormick's Creek State Park
At McCormick's Creek State Park
At McCormick's Creek State Park
At McCormick's Creek State Park
At McCormick's Creek State Park
At McCormick's Creek State Park
At McCormick's Creek State Park

Even though the Minolta is 17 years old and relies on a USB 1.0 interface, I got scans faster than I ever do from the Canon. This is in part because VueScan was able to accurately detect frames in the Minolta, and it can’t in the Canon for some reason. I have to painstakingly select each frame before scanning.

The Minolta scans are far sharper than the Canon scans straight off the scanner. No amount of Photoshopping can make the Canon scans look sharp, while a tiny bit of unsharp masking makes the Minolta scans look great.

This scanner’s native software doesn’t work with Windows 10. Fortunately, VueScan recognized this scanner instantly and was ready in seconds to make scans from it.

I kept going, this time with a strip of color film. This is Fujifilm Superia X-tra 400, shot in my Olympus OM-2n using the 50mm f/3.5 Olympus Zuiko MC Auto-Macro lens. I sent this film to Fulltone Photo for developing and scanning. My scans from the Scan Dual II are first, and Fulltone’s scans are second. I adjusted VueScan’s settings as best I could but still got rather cool scans. So I adjusted white balance and a few other settings on them in Photoshop.

The Scan Dual II scans are not far better than the Fulltone scans. I rather prefer the color Fulltone delivered — but it could be that after all these years I’m just used to the color a lab’s Noritsu scanner delivers. Now that I’m looking at these again, the ScanDual scans might have a slight magenta cast, and removing it might help. Yet these scans are acceptable for the day I might choose to develop color film at home, or wish to rescan an old color negative.

Tree tunnel
Tree tunnel in autumn
Harvested
Harvested
Harvested by the barn
Barn in the harvested field
Abby and Amherst
Abby & Amherst

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Film Photography

Scanning color slides at home

I enjoy color slide film for the bold, beautiful color it delivers. But I shoot it seldom because the film, developing, and scanning are expensive.

The Velvia I shot recently was a gift, but it costs about $10 a roll in 120 and $18 in 35mm. Other slide films cost about the same — there are no bargain slide films! It cost me $30 plus tax to have those two rolls developed and scanned. Fulltone Photo did the work for by far the lowest price of any of the labs I normally use.

I knew I could cut costs even more by scanning the slides myself, but could I get scans as sharp and colorful as Fulltone’s? Also, Fulltone’s scans are smallish at 1024 pixels square. I can easily get more pixels from my Canon CanoScan 9000F Mark II.

I tried it on a couple frames. I thought I’d show you my scans and the Fulltone scans to see what you think. My scans are at least 5100 pixels square — I select each frame by hand in VueScan, so the pixel dimensions vary slightly frame to frame. I shrank them to 1200×1200 for this comparison. WordPress shrinks them further to fit the blog template. I edited them all in Photoshop to my liking — nothing too invasive, mostly stuff like color temperature and exposure.

My scans are first, Fulltone’s are second.

Red flowers
On Talbott Street

Fulltone managed to bring out far better shadow detail than I could get from the CanoScan, VueScan, and Photoshop flow I use. Their scans look slightly sharper than mine.

But the Fulltone scans have a green cast that I couldn’t entirely erase, a cast that isn’t present on the slide. My scanner captured color that looks a little truer to the actual slide. Also, I was able to capture more of the frame than Fulltone did.

I don’t think there’s a clear winner here. Both Fulltone’s and my scans are fine. It’s a roll-by-roll judgment call whether saving $5 in scanning charges is worth the couple hours I’d spend scanning the roll myself. But when I want scans with large pixel dimensions, it’s very good to know that my existing scanning setup produces good results.

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Film Photography

The mysteries of exposure and film exposure latitude

I thought it was a shame I hadn’t shot my Nikon F2AS in a long time, so I put some film through it recently. The meter led me to shutter speeds that seemed slow for the full-sun conditions, out of line with Sunny 16.

I shot four subjects twice, once using the F2’s meter and once using the my phone’s light meter app. The app consistently had me expose two additional stops!

I shot Ilford FP4 Plus through my 35-105mm f/3.5-4.5 Zoom Nikkor lens (which I like less and less the more I use it). I developed the film in Rodinal 1+50 and scanned the negatives on my Canon CanoScan 9000F Mark II using VueScan. I brought the scans into Photoshop where I unsharp masked them all, corrected perspective if that was necessary, and on one shot toned down the highlights, but otherwise left them alone.

There are so many possibilities in any scene, from how you expose it to how you develop it to what you do with the negative in printing or scanning and post-processing. These pairs show it well. The F2 metered shot is first in each pair. In this first pair, I like the second shot more for its better definition in the houses, and the more silvery reflection in the pond.

Reflected houses, overexposed
Reflected houses, underexposed

In this pair, I prefer the second shot again for its rich, smooth tone in the tennis court surface and the better definition in the houses.

Tennis net, overexposed
Tennis net, underexposed

In this pair, I like the first shot better for its slightly better shadow detail. The first photo is the one where I toned the highlights down slightly in Photoshop. The path was a little washed out in the original scan.

Path, overexposed
Path, underexposed

In this pair, I like the first shot better for its slightly better shadow detail and its better definition in the sky.

Lowe's, overexposed
Lowe's, underexposed

What do you see in these photos? In each pair, which do you like better?

I think to some extent what we’re seeing here is the good exposure latitude of FP4 Plus — these are all technically decent photographs. Also, what we all like in a photograph is subjective.

After I finished this roll I checked my F2’s meter under a bunch of lighting conditions and couldn’t reproduce the odd meter readings I was getting. Soon I’ll mount a lens I know and like better, probably my 35mm f/2.8, and shoot this F2 again to validate the meter’s functioning.

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Photography

Still more 35mm color scans from ScanGear on the Canon CanoScan 9000F Mark II

Lab scans of 35mm color negatives are miracles. Any lab I routinely use reliably sends me crackerjack digital images.

Getting usable scans from my CanoScan 9000F Mark II via its ScanGear software, on the other hand, is a lot of work involving a number of subjective choices in scanning and post-processing.

I used to think that the colors I got back from the lab were the film’s true colors. I see now how much of that is in the scanner settings, and that I don’t actually know how any film I typically use renders color.

The improvements I made this time were to scan to lossless TIFF files, and to turn off ScanGear’s Image Adjustment setting (which I had overlooked when turning off all the other image-enhancement settings). It helped? I think?

Here’s my scan of a photo I made on Kodak Gold 200 with my Olympus OM-1 and a 50mm f/3.5 Zuiko Auto Macro lens. There’s a little of that mottling in the blue sky that I keep trying to prevent. But it’s not as bad as in previous scans.

Roberts Camera scanned this film when I had them process it. It’s a touch brighter than my scan. The sky has a slight turquoise tint and lacks any mottling. Otherwise, either scan is fine.

North and Maple

Here’s my scan of a butterfly pausing over this flower. Notice how purple the flowers in the background are.

Roberts made those same flowers quite pink, but brought out the detail lurking in the butterfly’s wings.

Butterfly

I also tried scanning some Kodak Ektar 100 I shot in my Pentax Spotmatic F with a 35mm f/3.5 SMC Takumar lens.

Here’s Robert’s Camera’s scan. They got richer colors than I did, although I’d say the sky in mine looks more realistic. The green tint on the right edge of my scan is clearly an artifact of the negative that Robert’s somehow edited out.

Around Zionsville

I walked over to the building to make this close shot. My scan:

Roberts Camera’s scan got a richer red, but my scan offers better highlight detail.

Around Zionsville

It was so much easier when I accepted whatever color I got from my lab scans, as if they were the final word on film and lens. Now I’m suspicious of every scan, because of all the choices it represents. Is it possible that the only way to truly know what colors are in a negative is to make a darkroom print?

This, by the way, is the last in this series of experiments. I’ve learned what I need to. I get good enough black-and-white scans now to start processing and scanning black-and-white film, which was my goal. Now that I work Downtown in Indianapolis, eight blocks from Roberts Camera and their C41 lab, I’m likely to have them process and scan my 35mm color negative film. They charge just $10.

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Film Photography

Scanning 120 black-and-white negatives with the Canon CanoScan 9000F Mark II and ScanGear

I made one more experiment scanning negatives on my CanoScan 9000F Mk II and its bundled ScanGear software. This time I tried scanning black-and-white medium-format negatives.

I hope to start processing my own black-and-white film, especially in medium format, this year. I don’t shoot as much medium format as I’d like because processing and scanning costs about $17. That’s a buck and a half to two bucks per frame! Processing and scanning my own will manage medium format’s costs better.

I went back to 2016 to find some images I made with my Yashica TLRs, a Yashica-12 and a Yashica-D. These cameras have wonderful lenses that make the most of whatever film I put behind them. Here is a scan I made of a scene on the square in Lebanon, Indiana, on Kodak Tri-X.

Here’s a crop of my scan at 100%. I scanned at 2400 dpi, by the way, and applied unsharp masking and other tweaks in Photoshop until the image was to my liking. That’s some pretty good detail right there.

Here’s the scan Old School Photo Lab delivered, after I tweaked it in Photoshop to my liking. Both my scan and Old School’s scan are crops of the original image to the interesting part of the scene. My scans are about 5200 pixels square, give or take, while Old School’s are slightly off square at 4832×4760 pixels.

Please be seated

Here’s my scan of the Boone County courthouse in Lebanon’s square.

And here’s Old School Photo Lab’s scan. Either scan is acceptable. I like the tonality in my scan a little better as it feels more realistic to me. The Old School scan looks to be a bit sharper.

Boone County Courthouse

The next two images are from my Yashica-D on Fujifilm Neopan 100 Acros. I could have done a better job of cleaning minor dust marks off my scan, which is below. It’s otherwise a perfectly usable scan.

Here’s Old School Photo Lab’s scan. At blog sizes, they’re hard to tell apart. Both images are crops of the original frame, by the way.

Moore Road

These two images are from the far-northwest corner of Indianapolis, which is quite rural. Here’s my scan of a cemetery that lies along the road above.

Old School Photo Lab’s scan appears sharper — compare the grass in both scans. But either scan is eminently usable for my purposes.

Pleasant View Cemetery

I am pleased with my scans. I would use them for any of my usual purposes.

These experiments, and your comments on them, have taught me some key techniques. First, thanks to your advice I’ve turned off all the built-in image improvements in ScanGear and scan at as close to 4800 dpi as I can. Second, I’ve learned enough about the Amount, Radius, and Threshold settings in Photoshop’s Unsharp Mask filter to sharpen my images acceptably.

I see I’ve still much to learn about how to look at a photograph and see its details. In these experiments I’ve studied my scans in far more detail than I’ve ever studied a photograph, and compared them in depth to the lab scans, and thought about what I like in a scan. I realize I need to study far more photographs to learn how to see their details and decide what I like.

I also now realize just how much the quality of a lab scan might have affected my views of various cameras, lenses, and films, and how excellent scans might have enabled the praises I heaped on particular gear or films.

I feel like a man who thought he’d climbed a mountain, only to find that he had scaled but a foothill to see the real mountain emerge from the mist.

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