Image Quality for presentations (1 Viewer)

I'm not sure whether this should be in this section or under the photography threads. I'd be happy to be advised to change it.

I have a pretty poor understanding of resolution / ppi / dpi / image quality etc.

I deliver a number of presentations using a Macbook or iPad and a pretty cheap Acer projector. I use Keynote as I like to include good looking text and and transitions / magic move etc. I just allow them to connect and sort out their own resolution issues. I'm not really happy with the resolution of what appear on my iMac to be good quality images.

My question is: When I drag images into keynote they resize themselves to fit the slide format. (I never have the issue that I used to have with PowerPoint where the images were always too large or small and needed handles dragged to get them to fit.) What is happening to the resolution when I add pictures, is it reduced? Would I be better just making a slide show in Aperture or iPhoto? Does that provide better projected quality? Of course I'd have to give up my effects. I'm prepared to upgrade the projector if that might be the issue but I don't want to do that if it might just be the limitations of Keynote.

Hoping someone has the patience to explain this to me, thanks.

Roger

A few definitions:-

DPI - dots per inch is strictly a printing term, the number of physical ink dots per inch. But it is often used instead of...

PPI - pixels per inch, which is again is only really of issue in printing. The number of image pixels divided by the size you are printing at. Ideally the PPI of the image should match the DPI of the printer to get the best results. (PPI is also used to measure the pixel density of displays, often quoted in mobile phones)

If you are projecting images in a display then all these printing terms mean nothing! All that matters is the pixels dimensions. The projector you are using will have a set number of pixels it will use to construct the image this is often called the "native resolution", 1024x768, 1920x1200 etc. The maximum pixels of a projector is often less than a computer display and much less than a digital photo.

For the best results you should resize the images you are using to match that pixel size. If you don't do it then it will be done anyway either by the presentation software or by the graphics driver of the computer you are using, these might not give the same quality as your image editing software of choice or they might lead to a lag in the presentation if the resizing is done on-the-fly (as it often is if doing a slideshow of separate files).

I can't talk about Keynote as I don't have experience of that software but in PowerPoint they do a great job of confusing things by measuring everything in inches or centimetres even when you ask it for a onscreen presentation it then assumes a screen resolution of 96ppi and leaves you to do the maths! So if your projector is 1024x768 then you need a presentation 10.6" by 8" or 27cm by 20cm.

Hope this helps

Thanks Mono for that rapid and helpful reply.

Keynote 'seems' so easy to use. When I used PowerPoint I was used to having to drag the image handles to fit the template, sometimes this was a drastic reduction in size from the image onto the template. Was this process reducing the pixel size or the number of pixels?
Keynote seems to do this automatically; drag an image file onto the Keynopte template and it opens up to fit the dimensions as closely as possible allowing for the image shape.

Does this mean that the image is resampled in any way, does that process happen again when the projector displays the image? Would I avoid a stage of resizing if I used the photo programme to do the slide show rather than using Keynote (or PowerPoint) to display the images?

(I guess I may be using re-sizing / resampling and template in a non technical way; I'am happy to be corrected. ...If you don't ask you stay stupid for ever!)

I can't speak for Keynote but in PowerPoint the images are resized in three stages: When you bring in an image you resize with the handles, which can be a lot with a 20MP digital camera image. This only temporary and you can resize and crop right out to the original pixel dimensions of the image. when you save the presentation PowerPoint resizes the images to pixels that match the dimensions of the image on the slide at 220ppi (this can be changed or turned off in the options). So any editing you do on the saved file will be restricted to this new image size. This can be a problem if you have a rethink and decide to make images you have previously saved in PowerPoint much bigger. Then when you show the presentation PowerPoint will resize them to 96ppi for output to a projector. I'm not aware of any options to change this value.

I don't know what algorithm PowerPoint uses to resize images but I haven't noticed any problems with it shrinking images, making images bigger is a different kettle of fish!

Thanks again mono.