Bulk photo scanning is an economical method of photo digitisation for everyday use, such as online images, standard printing, and simple archiving. Photos must be in good condition.

Manual photo scanning is of maximum quality for photo enlargement, fine detail and professional archiving. Photos can be in any condition.

Interested to know more? Continue reading ...

Let's keep this simple for now (we will go into the details later). A picture speaks 1000 words ...

It's that simple. You need to weigh up your PPI needs versus how much money you want to spend. I'll explain PPI later, but basically, it's a measure of scan quality.

Let's look at typical PPIs, their usage, and the services Scan My Photo provides.

72 PPI

• Online images, such as social media posts, web banners, and email graphics (small file size is important).
• Not provided by Scan My Photo. Special request only.

300 PPI

• Industry-standard print quality for photographs, brochures, posters, and magazines.
• Not provided by Scan My Photo. Special request only.

600 PPI

• High-quality scanning for fine art prints, high-resolution documents.
• Offered as standard bulk photo scanning.

1200 PPI

• Maximum detail for archival and photo enlargement.
• Offered as advanced bulk and standard manual photo scanning. Note that bulk photo scanning can do 1200 PPI, but it is a digital manipulation (a good one at that) called interpolation, not a "true" 1200 PPI. See below for more information.

Above 1200 PPI

• Extreme detail for professional archival, large format printing and "zoom and crop" (you only want a section of the photo.
• Offered as advanced manual photo scanning.

In summary, choose:

• Bulk photo scanning for everyday use, such as online images, industry-standard printing, and simple archiving.
• Manual photo scanning for photo enlargement, maximum detail and professional archiving.

Now the detail ...

I am going to take you on a journey, so please stay with me 😄.

Here is a photograph I took using a Nikon D5300 digital camera (notice "digital" is in bold; the difference between digital and film cameras is essential when scanning your film-based photographs, but more about this later).

It's from a hike I took in the Bluntautal Valley near Salzburg, Austria.

The original of this image has dimensions of 6000 x 4000. I can't put the original here as the file size is about 18000KB! So, I reduced the dimensions by ten to 600x400, and now the file size is a manageable 50KB.

What does "dimensions of 6000 x 4000" mean? Let's open the original image in Photoshop and look at the properties.

As you can see, this image has a width of 6000 Pixels (usually abbreviated to "px") and a height of 4000px. Ok then, what is a pixel? Let me explain by zooming in on the park bench in the original photo. What park bench?

Can you see the park bench now? It isn't clear, but you can just see it sitting behind the path.

Let's zoom in more ...

and more.

It's starting to look a bit "pixelated", isn't it? That is, you can begin to see squares of individual colour. Let's zoom in even more.

A digital photograph comprises many pixels, that is, squares of a single colour. These pixels are so small in the original image that your eye cannot distinguish them, and instead, you see a clean image.

So, how big is a pixel? ... It's as big as you want it to be 😄. A pixel has no defined size. So, how big is a pixel in the Austrian photo? Looking back at the image properties, the photo has a resolution of 72 Pixels/Inch, or 72 pixels per inch, that is, 72 PPI.

72 PPI? That isn't that good. 72 PPI is only for "Online images, such as social media posts, web banners, and email graphics (small file size is important)"". Why go all the way to Austria and then take a picture with such an insufficient resolution?

Let's do some maths.

Length of photo = 6000px ÷ 72px/inch = 83.3 inches ≈ 7 feet or 2.12 meters!

Height of photo = 4000px ÷ 72px/inch = 55.6 inches ≈ 4.6 feet or 1.41 meters!

Ok then, that is a colossal image, close to the size of a queen-sized bed! The park bench, which you couldn't see in the original photo above, is actually 2cm wide.

The pixels in this image are 2.12 m ÷ 6000 = 0.35mm, about the diameter of a medical needle or a few strands of hair. You could see the pixels in this image with the naked eye if you stood close enough.

72 PPI is a low resolution and unsuitable for printing as you can see the pixels with the naked eye. What if I wanted to print at the industry standard of 300 PPI? Well, I can manipulate this in the image properties.

The image is 300px/inch, but the overall size is now 50.8cm x 33.87cm. Much smaller than the original, exactly 72px ÷ 300px = 0.24 times smaller (2.1m × 0.24 = 50.8cm). The number of pixels is constant. The overall size must get smaller if I want more pixels per inch.

More pixels in a smaller area must mean the pixels are smaller. The pixels in the 300px/inch image are 50.8cm ÷ 6000 = 0.085mm or 85µm! Thinner than human hair (which can vary from 17 to 181µm). If I print this image, the pixels are so small that I wouldn't be able to see them with the naked eye (a human eye can't see below 0.1mm). That's why 300PPI is the industry standard.

If I manipulate the image properties and make the photograph the standard 6x4 inch format, the resolution becomes 1000px/inch, and each pixel is only 25µm. Remember this number (1000 px/inch) as it will give some perspective when we discuss scanning of film-based photographs. A modern DLSR camera, worth hundreds of dollars, only provides a PPI of 1000px/inch, less than 1200 PPI used for "Maximum detail for archival and photo enlargement".

The moral of the story is that PPI for a digital photograph (say, from your smartphone) doesn't matter. It is the total number of pixels which is important. Pixels means data. The more data, the better. This is why digital smartphone cameras stipulate the Megapixels, or MP, the total number of pixels divided by 1 million.

My Nikon D5300 digital camera has a resolution of 6000 × 4000 ÷ 1,000,000 = 24MP

The iPhone 4 (2010) had a camera resolution of 8MP, while the latest iPhone 16 (2024) has a 48MP camera. The resolution of digital cameras has come a long way.

With all this information, let'’'s summarise the standard measures of digital images.

Resolution: Refers to the amount of detail that an image holds. Higher resolution means more image detail, which results in sharper and clearer photographs. Ignoring lens effects, I wouldn't have been able to see the park bench if I had taken the photograph in Austria with an 8MP iPhone 4. If I took the photograph with a 48MP iPhone, I would get more detail of the park bench (i.e. it wouldn't be as pixelated) than I did with my 24MP Nikon D5300.

PPI: Pixels per inch, a measure of resolution

DPI: Dots per inch is similar to PPI but is used in the printing industry where physical ink dots can be applied. PPI and DPI are equivalent; they are used in different industries.

Pixel Density: The number of pixels in a unit of area, or PPI.

Megapixels (MP): The total number of pixels in an image divided by 1 million. The higher the MP, the better the resolution.

Phew! Thanks for staying with me. Now that we have these concepts out of the way let's move on to what's really important ... scanning your film-based photographs.

So far, I have only discussed digital images, not the physical film photographs that need scanning to a digitised form.

Let's take this standard 6x4 inch photograph.

This photograph of my dog Tessa and me was taken in 1986 using a 35mm film camera. My mum would have taken the finished 35mm film roll to the Big W photo developing section, and a few days later, an envelope full of paper 6x4 inch photographs would be ready. Magic! But what happened behind the scenes? The photographic prints were produced by placing the 35mm negative between a light source and photosensitive paper. When the light hits the stack of microscopic light-sensitive chemical grains in the photosensitive paper, the light-sensitive crystals react, forming an image.

What is the resolution of a physical photograph formed by this photo-developing process? The above measure of PPI for digital images isn't applicable as there are no squares of individual colour (pixels) that form the image. Instead, the image on a physical photograph is formed from a bed of stacked microscopic (less than 0.001mm!) grains layered on paper; There are no physical pixels. What makes resolution measurement even more difficult is that the resolution of the original negative will also impact the physical photograph. Different films have different resolutions 😖.

The best we can do is determine an equivalent PPI for physical photographs. This is too difficult for me, but luckily, Ken Rockwell, a photographic expert, has already done it (Film Resolution (Pixel Count)).

Ken equates a typical 35mm film to 175MP! That's 3.6 times more resolution than the latest iPhone camera! Let's say that full 175MP transfers to the 6x4 inch photograph. It's maths time ...

175MP = 175 × 1,000,000 pixels

Algebra (Hey, my high school maths teacher was right. I will use algebra one day 😊)

Let,

Y = number of pixels along the length of the photo

Z = number of pixels along the height of the photo

So,

Y x Z = 175,000,000 (1)

As this is a 6x4 photo

Y = 6/4 × Z (2)

Substituting equation 2 into equation 1 and solving for Z

6/4 × Z × Z = 175,000,000

Z = SQRT(175,000,000 ÷ 6/4)

Z = 10801px

Y = 16202px

The dimensions are 16202 x 10801! Four times larger than my Austria photo! Converting to PPI.

PPI = 16202px ÷ 6 inches = 2700px/inch!

Despite the inconvenience of processing, film photography does have a significant resolution advantage over digital photography.

2700 PPI is a high resolution, and it is possible via manual scanning of physical film-based photographs. But is it necessary? Let's look at the photo of Tessa and me at the different scan resolutions that Scan My Photo offers. Each picture below shows the magnification of the top screw on the door handle at differing scan types and PPI.

Bulk scanning at 600 PPI

Bulk scanning at 1200 PPI (interpolated)

Manual scanning at 1200 PPI

Manual scanning at 1600 PPI

Manual scanning at 2400 PPI

Manual scanning at 3200 PPI

The Phillips Head screw on the door handle becomes significantly sharper with bulk scanning from 600 PPI to 1200 PPI (interpolated, I'll get to this soon) and from bulk to manual scanning at 1200 PPI. The manually scanned image does improve from 1200 PPI to 3200 PPI, but not as significantly. There aren't any more details, but the pixels are less evident with increased PPI.

The pixels aren't as apparent with higher PPI as the pixels become microscopic.

At 3200 PPI, the pixel size is 11µm, about the size of a white blood cell! Good luck seeing these pixels without magnification 😆.

So, why does a 1200 PPI scan improve from bulk to manual? The most obvious reason is that different equipment is used, but more importantly, the 1200 PPI for bulk scanning is interpolated from a 600 PPI scan. Interpolation is a method used to estimate new pixels within the range of a set of known pixels. The 600 PPI scan is expanded, and a software algorithm fills in the gaps based on the actual pixels. Many think interpolation is a big no-no, but from the images above, it does a relatively good job. In the end, bulk scanning isn't really for enlargement (that's what manual scanning is for), but if you want an economical method for thousands of photos, bulk scanning with an interpolated 1200 PPI is good value for money.

Now that we know we get more detail and less pixelation with higher PPI, what can we do with the improvements? Well, enlargement for printing is the most obvious. Printing at 300 PPI is more than enough, so based on the scanning PPI, you can theoretically enlarge your photos by the following multiplication factors.

It would be extremely rare for someone to enlarge a 6x4 inch photograph to over one meter. Therefore, there is little reason to scan an image beyond 3200 PPI.

Great! Higher scanning PPI can enlarge a photograph without getting a pixelated look. However, all of these very high PPIs need to be put into perspective. Remember when I said that my DSLR camera (the one that took the photograph in Austria) had a resolution of 1000 PPI for a 6x4 inch photograph? From my perspective, if a fancy DSLR camera only needs 1000 PPI, why would you need to scan a 6x4 inch photograph any higher than 1200 PPI?

A final piece of advice. The higher the PPI, the larger the file size. For the photo of Tessa and me, the increase in PPI significantly increased the file size.

Please remember this when you transfer the scans to someone else. I wouldn't try emailing the 3200 PPI file at 85MB 😆. It would be best to bring a physical hard drive (and USB Flash Drive, for example) or share via a cloud-based storage provider like Dropbox. Storage of large PPI files isn't that much of a concern. If you had 1000 scans at 3200 PPI, you would need 85,000MB or 85GB of storage.

Thank you for sticking around and reading until the end. I hope this made the whole resolution, PPI, DPI, pixel, etc. mess easier for you. If you have any questions, please do not hesitate to contact me via the contact form.

Marc