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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Photography

Printing my best photographs

Yesterday I said I seldom print my work, because I overwhelmingly show my work online and that’s how I like it. But seldom doesn’t mean never.

I’ve framed a handful of 8×10 prints of my work. Here are some of them hanging over my desk at my old house. I’d like to print and frame more, but we have only so much wall space and I own a lot of other art. Even the three prints in the photo above aren’t hanging in my home at the moment.

I plan to print all of what I consider to be my very best work. I suppose those prints can serve as a portfolio, but the big reason I’ll print them is so my sons can have them when I’m gone. To them, photography is my main hobby. I frequently had an old film camera along when I was with them. My older son and I have gone photographing together with our film cameras.

I don’t want them to even think about sifting through images on my hard drive or squinting at all of my negatives. I’ve already made an overwhelming number of images — the Photos folder on my hard drive contains about 70,000 images today. I’m only 53. Barring catastrophe I have a lot of life before me yet and will make tens, maybe even hundreds of thousands more images. Most of my images aren’t valuable or important anyway; a good number of them are outright crap. Would my sons even want to look through them all? I wouldn’t if I were them. Instead, I’ll provide them a tiny, carefully chosen subset, as physical objects they can hold and enjoy.

My plan is to print on 8×10 paper, but leave the images in their native aspect ratio. That’ll give me wide borders in most cases, but the prints will all fit cleanly into these archival storage boxes. They’ll probably all fit into one box. That feels right — enough to enjoy and remember me by, but not so many as to be hard to store.

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Film Photography, Photography

Family Christmas photographs

I shot my family’s 2020 Christmas celebration on film. I decided to do it when I stumbled across a roll of Kodak T-Max P3200 I forgot I had. I shot it in my Nikon N90s with the 28-80mm f/3.3-5.6G AF Nikkor lens attached. I developed it in LegacyPro L110, Dilution B, but I misread the Massive Dev Chart and developed it for a few minutes less time than specified. The negatives looked plenty dense, but when I scanned them on my flatbed, the grain was pronounced.

I decided to print them. I don’t have a darkroom; I just sent the scans to my nearby CVS pharmacy’s photo department. The paper they use in their machines is thin, nowhere near as sturdy as the stuff they used as recently as 10 years ago. But the prints looked all right. I laid them on the dining table with the Christmas tablecloth still on and photographed a few of them with my Canon PowerShot S95. Even rendered this way, you can see the huge, ugly grain in these photos.

These scans are straight off the scanner. No amount of Photoshopping made them look any better, so I quickly gave up. I did tweak VueScan’s settings to bring out shadow detail, however.

When that roll was done I wanted to keep going, but I was out of P3200. Then it hit me: I develop my own film now and can easily push process it. I had some Ilford Delta 400 in the freezer, so I thawed a roll, loaded it into the N90s, mounted my 50mm f/1.8 AF Nikkor lens, and set the camera to ISO 1600. I knew this stuff would push well because fellow photoblogger Alyssa Chiarello did it recently and got great results.

Ilford still prints developing instructions inside their film boxes. They listed a developing time in Ilfotec HC (their HC-110 equivalent, also equivalent to the L110 I use) for the film at 1600! I followed their instructions and got gorgeous negatives and the best scans my flatbed can deliver (which still aren’t great). They look better than the P3200 photos — the grain is smaller and much more pleasing. Delta 400 is a darn sight less expensive than T-Max P3200, too. I think I need never buy P3200 again — I’ll push an ISO 400 black-and-white film to 1600 instead. I had CVS print these scans, too.

This was fun, but I don’t see this experience leading me to print my work more often. I get it that a photograph is meant to be printed, a physical object. But I’m an online kind of guy and that’s the way I show 99% of my work. My wife prints family photographs all the time, and I figured she’d like to add these to her collection, so I gave them to her.

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Essay, Photography

The lifespan of a family photograph

It always makes me very sad to find old family photographs for sale in an antique store or at a flea market. I want to rescue them all. Space is tight in our home, so I refrain. I’m not sure what I’d do with them anyway. But it is a shame that those family memories lost their connection with the family that made them.

Me and my brother on Grandpa’s lawn tractor, 1971

When I was a kid, mom made all of our family photographs. She had a big 126 camera with a built-in battery-powered flash (quite unusual in the early 1970s). I think she bought it with green stamps! She used it through the late 1980s.

I’ve never seen most of those photographs, except for an album she made and gave me of my preschool years. She took the pictures, had the film developed and printed, put the prints in boxes, stored the boxes in the closet, and that was that. I asked about them a couple years ago and she said she had been working through them to get rid of duplicates and bad photographs. She also said that she was throwing away the negatives as she went. Ack! I asked her to cut that out, as I want to be able to scan those negatives someday.

My sons on my lawn tractor, 2001

But what of those images after my brother I are gone? Will my children care? They won’t even know most of the people in those photographs — cousins and aunts and uncles and grandparents they’ve never met. One day I will show them those photographs, tell them my memories of the days they record, tell them about the family members in them. I hope they will appreciate those stories, and through them gain some feeling of connection to the family.

It’s the same for me with the few photographs my mother has of her family. I sometimes recognize my grandparents in them as young adults. A few other people in those photographs lived into my early childhood, and I’m told I met them, but I don’t remember them. The people in Mom’s family photographs are strangers to me. Mom knew them and loved them, and has sometimes told me her stories of them. I’m glad to know those stories, and I can try to remember those memories to share them with my children. But will they feel any connection to these distant ancestors who lived in another place and time? A tenuous connection at best, I feel certain. So, what of those photographs after my mother is gone?

But then there are moments like this. Not long ago my cousin Barbara shared a photograph of my great grandfather, John Eugene Grey, when he was a young man. I never knew him, my father said little about him, and I’ve seen few photographs of him. But when I saw this one, I was struck by how much my son Damion resembles him. That’s John Eugene on the left, and Damion on the right. Damion was 19 in his photograph, and I’d guess John Eugene was within a year or two of that age in his photograph.

Remarkable, isn’t it? Finding something like this is why it’s valuable to keep family photographs. It reminds us of where we came from, and gives us a full sense of our families through the generations.

But how do you keep generation after generation of family images — hundreds, perhaps thousands of them? In enough generations they’d snowball into a genuine storage problem.

I think they key is editing. Mom and I can sort through our family’s photos and keep a small but carefully chosen subset of them — ones that best show family memories and family members. We should write on each photograph what it depicts, where it was taken, and who’s in it and how they are related to us. This should deliver a manageable number of photographs for following generations to keep, and hopefully appreciate.

It will also be a big project, the kind that many families talk about but never get to. It’s why so many family photographs show up in flea markets and antique stores.

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Big old house

Big old house
Canon PowerShot S95
2021

Where Margaret and I will live next is a frequent topic of our conversation. We agree that it’s time to move on from this house. We’d stay in Zionsville if we could afford a house in the original town. It’s lovely and charming there, and a small but vibrant downtown is within walking distance.

Trouble is, homes here are among the most expensive in the state. The median list price for a home here is about $450,000. My neighborhood is the least expensive way to get a Zionsville address, but you can’t move in here for less than $200,000. I know that these prices may not shock you if you live on the coasts or in a major population center, but here in Indiana these prices are ridiculous. In Indianapolis, the median house list price is only $179,000. Outside of Indianapolis, it’s even lower than that!

We’d like to have a large home so we can host our seven kids, their partners, and their children. And our parents, while they’re still with us. This one would be a grand-slam home run for us with five bedrooms and four bathrooms. Built in 1870, it oozes character.

Unfortunately, it’s listed at three quarters of a million dollars. A similar house in Indianapolis, even as well cared for as this one, couldn’t command anywhere near that. If it were in a desirable neighborhood, I’d say half a million tops. In an average neighborhood, even less.

I’m not willing to pay a half million, either. But man, this house would be a lovely place to live.

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Personal, Photography

single frame: Big old house

A grand old house in Zionsville.

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It’s time for another look at the spectacular sunsets we get here on the western edge of Zionsville, Indiana. Our back yard overlooks a retention pond, a Toyota dealer, and I-65. It gives us a pretty good view of the setting sun. I made all of these images except the last one with my Canon PowerShot S95. A couple of the sunsets pushed the S95 slightly beyond its limits, but the sunsets were interesting enough that I’m showing the images to you anyway.

Sunset over the Toyota dealer
Sunset over the Toyota dealer
Sunset over the Toyota dealer
Sunset over the Toyota dealer
Sunset over the Toyota dealer
Sunset over the Toyota dealer
Sunset over the Toyota dealer
Sunset over the Toyota dealer

I found this final sunset when I looked through the images on my iPhone 6s before I traded it in. I made this in 2016, before the Toyota dealership was built. We aren’t thrilled to have a Toyota dealership in our view, but looking at this sunset it’s clear: the towering sign provides a strong anchor to these images.

Sunset before the Toyota dealer was built

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Photography

Still more sunsets over the Toyota dealer

Another look at the incredible sunsets we get here.

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