What resolution (DPI or PPI) to choose to scan photos?

Every time we look at the photos lying in a room inside a shelf, we are in the habit of taking a resolution that we will complete digitizing them the same year.

You can watch this video that we have created at ScanJunction about how important photo digitization is.

But wait a moment, this is not the type of resolution that we’re going to talk about here in this article.  

What we are trying to cover here is the resolution required for a photograph post scanning. While we are reading this article we are clarifying some myths regarding resolution.

To do that the first thing we should be clear about is the word resolution.

 So let’s first start by discussing what resolution is.

What is Resolution?

Resolution is the degree of detail visible in a photographic or television image.  

It is very important to know the connection between the photographic and television image to understand this article better.

So what is it that both a photograph and television have in common?  It is the picture.  But they both work in different fashions. Going forward in this article it is important to know this difference.

As per the definition of resolution, the degree of detail visible in a photograph is limited and cannot be changed because it is static unless we have a large print.

While the resolution of a television can be changed if we already have the best.  For example, if you have a 4K Television you can scale it down to an HD or high definition.

 In technical terms, the resolution of a photograph is measured in DPI which is dots per inch, since it is a physical copy. 

Difference between PPI and DPI

Resolution with respect to photography is the number of dots that are accumulated within an area inch. When you take a photograph from any printer for that matter, the printer prints by joining a lot of dots.    These dots In different colors form a picture. 

You can see the pixels when zoomed in

When it comes to Digital Photography resolution is not dots per inch or DPI,  instead, it is the Pixels per inch, PPI. We have already spoken about a television case study where the images are displayed in the form of pixels.

What we see in television is a collection of pixels together forming the image or video. 

What is a dot in the physical world is a pixel in the virtual world.

 In analog photography, a collection of different colored dots form a photograph, while in digital photography a number of pixels of different colors form a picture or a digital image.

Again going back to the definition of resolution, “the degree of detail”, is the key in increasing the resolution.  So, the more number of dots in a photograph within an inch, the better the resolution of the photograph, and the more the number of pixels within an inch in a digital image the more detailed the digital image is.

But without understanding the differences between these two types, that is 

DPI and PPI 

People often get mixed up with the concepts.  But now that we have understood this concept well, let us not interchange the usage.

Scanning is the process of converting analog materials to their digital counterparts.  So, while we scan the normal unit of measurement in terms of resolution is PPI which is pixels per inch and the increase in the pixels will make the digital copy of the image better. 

But this might not always be the case.  There could be exceptions where there are photos that aren’t sharp enough to be scanned at higher resolutions. In such cases, it is not worth scanning them at a higher resolution.   

Let us further in this article understand for what type of photographs it is best to increase the resolution depending on the purpose of scanning.

Types of photos

Before we determine when to increase the resolution, we have many other parameters to consider. The first parameter is a type of photograph. 

By type of photograph, we mean the material that the photograph is printed on.  Broadly there are three types of photographs.

  1. Glossy photos
  2. Matte photos
  3. Paper photos

By now I think most of you would have understood what I am referring to here.  The glossy photos are shiny in nature. They are by far the most commonly available photographs around.

The matte, on the other hand, is something that is not that popular.  Most of the matte photographs are not Shiny but smooth in nature, while there are some matte photographs that are rough, especially when the photographs are rough in nature they tend to get pixelated while we scan.

The last type of photograph is a paper photograph.  These are the photos that are printed on newspapers, pamphlets, and the like. 

The prints on these types of materials are of extremely low resolution.  if you are an expert you can see the separated dots looking at those prints. There is no point in scanning them at a higher resolution unless printed well.

The material used really plays an important role during the time of the scan. along with this. the print resolution also matters.  if the dots that are printed are far away from each other there is no question of scanning them at a higher resolution.

But don’t worry, the older photos are overall printed well.

Size of photos

Like how the material type has an impact on the resolution of the scanning,  the size of the photographs matters too.

Unlike the type of materials, there are a lot many sizes when it comes to photographs. The following are the sizes that are used on a frequent basis:

  1. 2.5 x 2 cm (Stamp Size)
  2. 2 x 2 inches (Passport Size)
  3. 2½ x 3½ inches (Old Photos)
  4. 4×6 inches (Postcard Size)
  5. 5×7 inches (Bigger Postcard Size)
  6. 8×10 inches (A4 Size)

As a thumb rule, it is wise to get a better resolution of smaller photos. It depends though on the way the photo is captured, which is the most important factor to decide upon the resolution.

Normally, we at ScanJunction get very few stamp size and passport size photos. It is either the old photos or the (bigger) postcard size photos that we mostly scan.

The small old photos are scanned at a relatively higher resolution than the (bigger) postcard size photos unless the old photos are not captured well or the clarity has deteriorated over a period of time.

Purpose of Digitization

This, according to us, is the most important parameter to consider, apart from the resolution of scanning. Each customer that comes to ScanJunction has a different purpose of getting their photos digitized.

Of course, the main purpose could be to preserve them, we measure this purpose in terms of the screen size of the medium they would want to view them in.

For eg., two people come to ScanJunction, let us call them PersonA and PersonB.

PersonA wants to preserve and has a screen as large as about 80 inches wide, while PersonB wants just preservation and watching the photos on their iPad. 

Now taking into consideration the size of the screen, we will have to calculate the resolution that is required, which is the very purpose of the individuals.

Normally, this is what we suggest to a common man who does not understand the mathematics behind photography. 

An image with 300 PPI resolution supports screen size of about 10.1 inches, which will typically be the size of an iPad. 

If they want to view the photos in a screen size of up to 32 inches, it is required to have a resolution of 600 PPI. A 300 PPI may get pixelated when viewed on 32 inches screen.

Similarly, scanning at a resolution size of 1200 PPI, an image can fit into a screen size up to 80 inches screen without getting pixelated. 

The above-mentioned calculation is a general thumb rule that we follow at ScanJunction based on our experience. 

To calculate the size of the image from resolution, in terms of megapixels, you need to multiply the size of the photograph with the resolution. 

The output of this will be in pixels that have to be converted to megapixels by dividing the value by a million.

For eg.:

If you want to scan a postcard size photo at 300 PPI resolution,

The size of a postcard size photo is 6 inches by 4 inches. Multiplying this by 300 will result in 

1800 x 1200 which will be 2160000 pixels.

Dividing 2160000 by 1000000 will result in 2.16 MP.

Screen Size vs Resolution

Let us now understand how the resolution of the photo helps us decide which one suits best for a particular device.

This is pretty easy, and by now you would have already guessed it right.

First, calculate the resolution like how we did last time. In our previous section let us take the same eg. of the 6 x 4 inches, but this time let us calculate for 600 PPI.

When we multiply 600 PPI with 6 x 4, we get 3600 x 2400. Look at how the equations increase the pixels.

Now we have a list of devices with their screen sizes and resolution. We can decide to which screen size a postcard size photograph at 600 PPI fits in.

Screen SizeResolution
85 inches4K Ultra HD (3840 x 2160)
75 inches4K Ultra HD (3840 x 2160)
65 inches4K Ultra HD (3840 x 2160)
55 inches4K Ultra HD (3840 x 2160)
50 inches4K Ultra HD (3840 x 2160)
43 inches4K Ultra HD (3840 x 2160)
40 inchesFull HD (1920 x 1080)
32 inchesHD Ready (1366×768)
24 inchesHD Ready (1366×768)
15.6 inchesHD Ready (1366×768)
14 inchesHD Ready (1366×768)
11 inchesHD Ready (1366×768)
11 inches (iPad)1668×2388 pixels
10.2 inches (iPad)1620 x 2160 pixels
Apple’s new iPad 9.7 inches2048 x 1536
Apple iPad / iPad 2 9.7 inches1024 x 768
Acer Iconia Tab 700 10.1 inches1920 x 1200
Samsung Galaxy Note 10.1 inches1280 x 800
Google/Asus1281 x 800
Toshiba Excite 7.7 Android 4.0 1280 x 8001282 x 800
Amazon Kindle Fire HD 7″ Android 4.0 1280 x 8001283 x 800

If you look at the table and the number that we have arrived at, which is 3600 x 2400, we can view the photograph decently on a smart tv of up to 40 inches.

Theory apart, there is one more way to do this easily, practically, and satisfactorily. All you need to do is scan the photo at a resolution, for eg. 600 PPI.

View the scanned photo on the largest screen that you would want to view in the future, for eg., 40 inches.

If what you are watching is sharp enough and is not getting pixelated, congratulations, you have just figured out your best resolution for your photograph.

If what you have scanned is getting pixelated in the screen size you are trying to open, for eg., you have an 80-inch screen.

In such cases, you might have to reconsider scanning at a higher resolution like 1200 PPI and repeat the process until you get to view an unpixelated image.

Off late, this is what we have been following at ScanJunction. We send general samples at different resolutions and ask our customers to view them on a large screen.

Based on their purpose and comfort, we convert their photos to the specified resolution.

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