Properly sizing an image for on-screen shows

[PSA DC-9-30]

Over the course of my years as a graduate student and postdoc in Biology, I've had to give numerous research seminars using Powerpoint. The bulk of these presentations consist of light and electron micrographs. The former comprise both 8-bit color .jpg and .tif files ranging in size from roughly 1200 x 1600 to 1800 x 2400 or so. The electron micrographs are all 8-bit grayscale .tif images ranging in size from 1024 x 1184 (Hey, what do you want for only $800,000!?) to 1920 x 2560.

After a few encounters with some horrible quality digital projectors, I gave up on spending much time tweaking my images. There really is no point spending hours in Photoshop on your images, if you're going to get stuck with some piece of crap video projector that is going to give you horrible results. Even in some recent presentations, I will admit to using Powerpoint's native tools for adjusting contrast, brightness, and even resampling. Really, given the variability in projectors, there really is not much point in even using Photoshop for these basic adjustments. Even using these crude techniques, some of my recent presentations have looked really good, as I've been lucky enough to be able to reserve a projector that I like...

But aside from making basic image adjustments such as these, I've always wondered about how the images should be sized for on-screen shows. I know about printing at 300 dpi, or close, but what should I be doing for an on-screen show? I usually convert tiffs to .jpgs to cut down on filesize; I don't think the difference is really noticeable on screen provided one uses high quality .jpg settings. But what should my target size be in pixel dimensions? Should I leave resolution at 72 dpi? Do these settings differ between Powerpoint and just showing photos using Windows Picture and Fax viewer? I would guess the size of the screen matters, but often I don't know how large my venue is until I get there!

Any help would be greatly appreciated.

Thanks,
Kevin

[Gordon Buck]

I've made many PowerPoint presentations that incorporate digital photos.  Although most images in my presentations probably do not require the resolution that you might want, sometimes my presentations do show small details such as cracks and surface patterns.  Here's what I do; you can easily test and compare it to your own needs.

1.  The "resolution" doesn't matter.  That is, there is no need to change the pixels per inch from 300 to 72 ppi -- whatever.

2.  Downsize the image to something like 800x800 pixels maximum.  Apply specific purpose sharpening (I use Pixel Genius Sharpener settings for screen display).  

3.  Save the downsized image in jpg format at a Photoshop compression setting of 6 to 8.

4.  Insert the downsized, compressed image file into your presentation.  Even at 800x800 max, you may still have to resize the image to get it to fit on the PowerPoint slide (depending on your slide template and details).

Although projection resolution is increasing, most digital projectors that I have to work with are only 1024x768 and sometimes an old 800x600 projector still shows up.  This is why there isn't much point in working with the details.  This is also why sharpening for apparent detail is important.

If your PowerPoint file gets really big, here's a useful tip for reducing the size.  Select one picture, right click and then select "format", there is an option to compress that picture or all pictures.  Indicate to compress all pictures for optimum screen viewing and the resulting file will be much smaller -- sometimes about 1/10th the size of the original file.   Be careful not to overwrite the original PowerPoint file when you save the compressed file!  Now view and project the uncompressed and compressed versions of the PowerPoint presentation.  You'll probably not be able to tell the difference between them.  

... and if the above is wrong or you find a better method, please let me know!

[PSA DC-9-30]

...

2.  Downsize the image to something like 800x800 pixels maximum.  Apply specific purpose sharpening (I use Pixel Genius Sharpener settings for screen display). 


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Thanks.

Is Pixel Genius Sharpener a feature of CS2? When downsampling, I usually select the "bicubic sharper" option--will this work as well?

Also, on a few occasions I have been stuck with a projector that renders images uniformly too dark or too contrasty. Is there any last ditch correction that can be applied (i.e., to all the slides) very quickly to compensate?

[Gordon Buck]

Pixel Genius makes plugins for Photoshop, including their sharpener kit.  There are other companies making sharpening plugins as well.  Just search and browse a bit on this forum and you'll find lots of information and recommendations about sharpening.

Although some projectors have more than one brightness setting, I'm not aware of any particular "last ditch" corrections to an already prepared PowerPoint presentation.

[sloow]

I use Iview media pro to downsample my images, number them so they show up in the right order etc.

I make and read a lot of submissions to arts juries, and the general guideline is jpegs sized to 1024x768 pixels, medium quality. File sizes remain small, and if you're lucky enough to get a high end projector, it will utilize the full resolution of the machine. Even if the projector is 800x600, it's not serious overkill or anything to have files that are a little bigger. In that instance you just want to be sure that the images are fit to screen, rather than 100 percent. I don't think that's an issue with powerpoint, only if you're presenting the work image by image without a software program.

[dalethorn]

If you have to embed the images in a host document, ignore the rest of this reply.  I always display my images in a separate window or windows from the cover doc, using IrfanView, which is free, has many autosize options, and is perfect for A/B toggle comparisons among many other great features.  If there's an issue with projector brightness, I would simply have a backup set of images in two or more brightness ranges, generated automatically from one of the robotic options in the photo editing software.