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Slide Resolution

About Slide Resolution – 35mm Slide Scanning Services

Resolution allows you to see the details of an image and makes the image look sharp.  The two pictures of the dog above illustrate this.  The dog on the left looks out of focus, but he’s not, he’s just at a low resolution compared to the dog on the right.  This is how higher resolution scanning makes your digital images look sharp!

Slide Resolution Example - DPSDave Slide Scanning Services

Higher resolution also allows you to zoom in more.  People look at digital images about the same way they look at physical pictures.  Back in the day, when you were looking at your slides and wanted to zoom in, you got up and walked over to the screen.  With the digital versions of your slides, you will be zooming in as well, and a high resolution scan is necessary so you don’t lose small details in the original image.


The slide scanning process should capture the smallest detail on the film.  The finest film that exists has “dye clouds” which can be as small as 6 microns in diameter.  At 6,000 DPI, each dot that we image from the film is about 4 microns in diameter. Therefore, we can capture all the detail from a slide, but further increases in resolution would not make a difference.

So, to produce the best possible image fidelity, we scan slides at a whopping 6,000 DPI.

Slide Resolution for Sharpness and Focus:

When scanning slides to digital format, higher resolution makes the scanned picture look sharp and in focus. Knowing that computer screens at best have 150 pixels per inch (ppi) resolution, while most have 72 ppi, people are confused. It may seem that if you scan your slides at 2,000 ppi they should look great on your computer. However, this is not true, and here is why:

  • Your computer magnifies the images to put it on the screen.  A 35mm scanned slide gets magnified 100 times to fill the screen on a notebook computer.
  • In days gone by, people got up out of their chairs and got closer to the screen when they needed to see more detail when viewing projected slides.  Today they zoom in to meet the same need, increasing the slide scanning resolution requirements.
  • We humans look for the edges in a scene.  Sharp edges appear to be in focus, whereas fuzzy edges appear to be out of focus.  If you zoom in a lot on an edge, you will see “digital artifacts”, the checkerboard edges of all digital images.  If you zoom back out, it looks like they disappear, but they don’t really.  The checkerboard is still there, but we don’t see it as a checkerboard anymore, we see it as a fuzzy edge, which looks out of focus.  The finer the checkerboard, the more in focus the image appears.

For these reasons, digital image quality improves exponentially with more resolution.  A slide scanned at 6,000 DPI will look almost 10 times better than the same slide scanned at 2,000 DPI.

A Word About Metadata Resolution Numbers…

Metadata is “Data about Data”.  Every slide scanning service embeds metadata in the image file when they scan the slide.  This is where information about the picture (like name, date taken, etc.) is stored. You can view this information by right clicking on an image in Explorer and clicking on “Properties”, or choosing “View Metadata” in a photo editing program.

There are hundreds of data items available. Three data elements which are relevant to resolution are resolution, width and height. Resolution values range from 72 to 9,600. To understand the resolution of the scanned slide, a 35mm slide scanning service must also look at the height or width of the image. For example, I have a scanned image of a 35mm slide. The physical dimensions of the slide are 1.37 x 0.90 inch.

The metadata is as follows:

Resolution: 72 dpi (dots per inch)
Width: 3145 pixels
Height: 2072 pixels

Do the math, and the picture size works out to 43 inches wide by 29 inches high. Digital cameras assign resolution numbers depending what the manufacturers think you are going to do with the image. A resolution of 72 dpi corresponds to computer monitors. Other cameras assign a resolution of 300 dpi, as this is what a lot of printers use.

Now, back to the 35mm slide scan. The resolution that the image was made at can be calculated by dividing one of the image dimensions (in pixels) by the actual size of the 35mm slide. This reveals that the image was scanned at (2072 / 0.9) = 2300 dpi.

Resolution for your slides is an important consideration and one DpsDave takes into account!