Scanning Resolution | The quick guide to choose the perfect DPI value

Have you ever had to scan something and wondered what scanning resolution you should choose?

Scanning resolution is calculated in DPI or dots per inch. In simple terms, the bigger the value the sharper your image will be. Practically, when you have more dots per inch, this will allow you to have better sharpness and color accuracy.

Your images will be clearer, they will have much finer details. Because of this, you can enlarge them much easier and with better printing results.

But let’s try and go into each aspect of this subject, and understand what is the scanning resolution and which one you should choose.

What does DPI mean or scanner resolution explained

As we said in the prior paragraph, the image resolution is calculated in DPI or dots per inch. If you don’t know this already, each digital image consists of a number of dots or as we call it, pixels.

For example, a 1024 pixels x 680 pixels image consists of 1024 dots on the long side and 680 on the short side. This is one side of the story.

The other relates to DPI. The more pixels or dots per inch, the sharper the image. For example, if an image is 87mm by 58mm, which is roughly the size of a business card, and the same image has 1024 pixels by 680 pixels, the DPI value will be of 300. Practically, this image has 300DPI.

To translate this better into human language, the optimal print size for that digital image is 87mm by 58mm. If you increase the print size, the elements will be less sharp and color accuracy will definitely decrease.

Optical Resolution

The example we gave you refers to optical resolution. This means that the digital image, either created digitally or if it comes from a scanning, has a dpi optical value of 300DPI.

You should only focus on optical resolution. Due to this, it does not matter how you work with your digital files. The sharpness and color accuracy always remains constant.

You will benefit from optimal printing quality if you print the document. When you work with the digital file, you will always have constant values and the overall quality of the image will be good.

The 3rd thing is OCR. As you probably know, OCR stands for optical character recognition.

We use OCR to capture data from scans and create digital text automatically. OCR software works by analyzing each pixel and trying to match them with characters.

You can immediately see that the better and sharper the image, the better your OCR results will be.

Interpolated resolution

Interpolated resolution or interpolation, is the process in which you artificially increase the number of DPI of a digital image.

To better understand, let’s take the example above. We have a 300DPI image of 87mm by 58mm. Based on these values, the image should be 1024 by 680 pixels.

Through interpolation, you can actually increase the number of pixels for this image. Practically, when you interpolate it to 400 dpi, the same image will now have 1370 by 913 pixels.

I am sure you think this sounds great and you think that using a higher optical resolution from the scanner is not really worth it. However, there is more to the story than this.

It does not really matter how much you increase the number of pixels, the quality of the image will be roughly the same.

Interpolation in itself is good, as it allows you to increase the number of pixels and to start editing an image. Yet, doing just the interpolation won’t increase the overall quality of the image.

This is a trick some scanner manufacturers do. A scanner needs to work at a lower speed to capture more pixels in a single pass.

Some manufacturers don’t like this, therefore they market their products with same speed, even at higher resolutions. They don’t tell you that they actually interpolate the original image. While the optical DPI is captured by the scanner through its optical parts, the interpolation is done by software.

Therefore, the overall quality of the image is more or less the same one as it would be for a lower resolution. A bigger problem is that you will also carry a bigger file for no reason.

What DPI should I use?

While the logical choice is always the bigger the better, in this case, it’s not as easy as it seems.

Let’s take 3 aspects into account :

  • The type of documents you have to scan.
  • What are you using the scans for?
  • Are there any size restrictions?

When you take into account the type of documents please consider a few things. First of all, what is the condition of the documents? Are they poorly written or have bad quality? Is the writing itself very small? If it is, then you might require a higher resolution.

What about the use of the scans? Will they be uploaded in a CMS and be used very rarely? Or are you scanning photos or heritage documents?

Do a bit of testing and see if any issues might appear. Select good and bad documents for the testing batch.

The best resolution for scanning documents

In general, when scanning documents, we tend to use 200dpi or 300dpi. I don’t think we’ve had a lot of cases when we had to go below or above.

  • The 200dpi resolution provides sharp details for general reading or even printing.
  • It’s well suited for office documents, not older than 15 years ( although it may work for older documents also).
  • It may even work well for handwritten documents, but you have to test first.
  • For really well-printed documents, it may even yield good OCR results. Although for high accuracy we suggest using 300dpi or even 400dpi in some cases.
  • Going above 400dpi does not make sense from our point of view. Maybe if you have really small characters, but that does not happen often.

For documents, we highly recommend to take into account the following aspects:

  • Standard documents of normal quality – 200dpi resolution and you can also try OCR – It will give you an average quality and a very low file size. You can also use 300dpi, but it won’t give you huge benefits.
  • Poor quality documents should be scanned at 300dpi minimum. If that is not enough, sometimes you should go above 300dpi and try to scan at 400dpi. In cases when the characters are very small, you can try to scan at 600dpi. We don’t recommend you use this resolution too much, as it will give you huge files and the process is very long.
  • If you have to reprint the documents at a larger size, choose the 600dpi option. It will give you the best printing results.

The best DPI for scanning photos

Once again, the most important thing you have to figure out is what you want to do with the photos.

Also, consider the time you invest in the scanning. If you scan at 300dpi, you will get a good quality, a decent file size and also a better scanning productivity.

If you only view them on your phone or your computer, 300dpi is enough to maintain the quality of the original photos.

Reprinting is a bit different. If you are reprinting them at their original size, then 300dpi is more than enough. When you want to enlarge them, then it get’s a bit tricky.

If you want to double the original size, then 400dpi might be ok, but we always recommend using 600dpi. This resolution should be safe for your print to look ok. 

I won’t go into technical details why 300, 400 or 600dpi. As a general rule, 300dpi is ok for viewing, 400dpi is ok for normal reprinting and 600dpi is good for enlarging the image. 

Deciding on the scanning resolution TO CHOOSE

As mentioned before, deciding on the best scanning resolution for your scans is not difficult but some things must be taken into account. 

  • Check the condition of your documents and if there are any file size restrictions. If there are none, go for 200dpi scanning. 
  • If you need to OCR the documents, start at 300dpi or even more. The smaller the characters the higher the resolution should be. Yes, sometimes even 200dpi might be enough for OCR, but 300dpi is recommended. 
  • Old documents, photos or documents that have small characters should be scanned at higher resolutions. You can try before you start the actual work, and see for yourself which resolution will yield the best results. 

As you can see, choosing the scanning resolution is not always straightforward. Testing your documents is very important, so you don’t run into trouble further down the line. 

 

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