Best dpi for projection

Question

Hello - First time doing a keynote presentation of paintings for my husband's work. I started with scanning his transparencies at 1200 DPI. I wanted them to look their best (detail) when being projected. Now I am thinking this might be over doing it but, I don't know. Can anyone give me some advice on this?
Thanks.

Answer 1

While it is important to scan at a high DPI setting for printing quality purposes, it is not what you need to concern yourself with for projection purposes. It is most important for you to determine what the pixel resolution capabilities are for your projection system and create your presentation size (and saved image sizes) accordingly.
Unless you are digitally archiving these paintings you can probably save yourself some image resizing work down the line and scan at a lower DPI setting. However if you are scanning these for archival purposes you can save copies of the images at a lower resolution for presentation purposes. I'll put some general guidelines on this below.
If you are not using your own projection system, or if you will be presenting in many different venues where the projector systems will vary, then chances are it will be safest for you to use the standard size of 1024x768 when creating your Keynote presentation. (I'll use that size as the example for this response).
Then it really depends on how you will be displaying the images within the presentation when you select what image size (width/height in pixels) to save the individual images at. I'm guessing that the paintings don't all fit neatly into a 4:3 aspect ratio frame. But unless you will be zooming into the details of the images, chances are you won't need to save the images at any higher than either 1024 pixels in width or 768 in height, especially if you will be scaling down the images so that the entire painting fits within the 1024x768 presentation window.
BUT, if you will be zooming into details of the pictures, or if you will be masking/cropping out portions of the pictures and then scaling the masked/cropped picture up to fill the presentation window more completely, then it is a good idea to save the images at a higher resolution. How much higher depends on how much you will be scaling the portion of the image up -- you may want to save at twice or three times the presentation window size just to be safe, or even more if you are zooming way into certain details.
Keep in mind, the higher the resolution of the image, the larger the actual file size in MB, so a presentation can become quite large if your images are much larger than they need to be. Large images can also cause problems when animating certain effects. So it can be a trade off depending on how much disk space you have available, or how smoothly you need certain animation effects to happen during the presentation, etc.
If you don't have access to image processing software (such as Photoshop) you can use the Mac Preview tool to resize your scanned images. Simply open the image in Preview, go to the Tools menu and select "Adjust Size". Change the width/height unit display (pull down menu to the right of these settings) to Pixels and make sure "Scale proportionally" and "Resample image" are both selected. Then change either the width or the height value as needed, but do not worry about the pixels/inch Resolution setting (again, this is only important for printing, not display/projection) and save the image.