As yours is the first website that gets near to the information I’m looking for, I’m sorry, but you’re it for my question.

Problem is that (a)I’m old and (b) I don’t have PowerPoint but this can be amended.  I’m working with Google Slides at the moment as that was the first slideshow software I could find where I could use animated layers.

I’ve been asked by a local group to create a slideshow for an 8 m x 4 m screen as a background for an Alice in Wonderland themed garden expo.

The next problem has been finding Tenniel images online that are good enough to enlarge.

Found a little old book in the shed and we’re scanning the images at 800dpi – they’re lovely!

Using Photoshop CS6 the image size ends up at 5 596 x 3 997 px. 17.77cm x 12.69cm So, please could you tell me what dpi I should be scanning in.

I’m sure the answer is blindingly obvious.

Thank you


Nice to be ranking first for your needs😊.

So you want to know what dpi you should be scanning optimally, without too much and too less. Let me explain. Everything starts with your output device. You have mentioned that you are presenting to an 8m x 4m screen. I guess that this is a projector then since I don’t know of a screen with those dimensions (everything is expressed in inches).  So I assume a projector where you want to project on a screen or wall with approximately 8 x 4m.

In case I’m right about the projector, then look for the specs of the projector and look up the resolution. Current projects mostly support Wide XGA (1280 x 800) or Full HD (1920 x 1080). Let’s assume Full HD or 1920 pixels in width and 1080 pixels in height. Most people over-estimate the importance of dpi settings, so good that you ask. And you understand that the higher your scan dpi is, the bigger the file and presentation are.

Let me explain. You have to understand 2 things:

  1. Basically, a dot in dpi is the same as a pixel in the resolution of your display.
  2. You cannot display more pixels of your source (the scanned picture) on a single pixel of your screen, so the computer is merging and taking ‘averages’ of your pixel information beneath one screen picture-pixel.

If you scan your lovely pictures at 800 dpi and you display it in full, then this means that you cannot display more than your first 3 inches or 7.6 centimeters on your projector or screen.

Why not? Because 1920/800 = 2.4. So only the first 2.4 centimeters can be displayed (at full, when displaying 1 on 1).

So the calculation for your dpi to scan it is easy. But, there is another factor. You need to take into account how large your source is. Let’s assume a normal picture size of 15 cm x 10 cm.

Now we can calculate. Let’s take the width (1920 of our Full HD project).

1920 px / 15 cm = 128 pixels on 1 centimeter.

DPI is calculated in inches, so we have to multiply by 2.54.

128 x 2.54 = 325 dpi.

So when you scan your picture at 325 dpi, then you will have a perfect marriage between a scan-pixel and a display-pixel. When you go lower, the computer will drop some information/pixels. When you higher, the computer will merge some pixels. Personally, I would suggest scanning a bit higher than 325 when 325 is not on your list to scan it. Probably you have to choose between 300 dpi and 400 dpi, so take 400 dpi so that you don’t lose information (although the loss of 25 pixels is not dramatic).

All clear? Let me know and good luck with your project(or).

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