When I play a slideshow in Photos on my Mac Book Pro, it is not full screen; there is wasted space all around the picture as illustrated below.
MacBook Pro (Retina, 13-inch, Late 2013), OS X Yosemite (10.10.3)
I have the same problem, black wasted border all around. When the computer uses screen saver, it projects the same images at full screen without the border. I have the same problem on my laptop, but the wasted border is smaller. I have the same problem at 1024x820 as at 1280x1024. It seems in both cases the horizontal extent WILL be allowed to fill the entire screen (a very wide picture fills the screen horizontally) but the vertical extent (a tall picture) will not. I only seem to have the options screen that rjlorenz has (OS10.11.6; Photos 1.5 (370.42.0)). The options you show look very appealing - set duration, transition, etc. as well as the fit to screen option which I do not see anywhere.
Thanks for your interest, but that's not a solution for me. I did find the menu that does it, the presentation has to be made into a project first, and opened as a slideshow from within the project, in order to see it. So that is a little less obvious than the "play slideshow" button in the menu, which works directly from an album, and does not show that option. But even unselecting the "fit to screen" option in that menu, the slides don't fill my screen. They fill some window, as displayed in your gif. But that window you see in the gif (in the preferences menu) does not fill my actual laptop or desktop screen. There is always wasted black space at the top and bottom. Moreover, that option works in a way that crops out large amounts of images, depending on the aspect ratio. Portrait-format images, mostly gone. So, problem not solved. Why are large amounts of viewing-screen space wasted and left black? Is there some way to force that window in the "option" menu to actually fill my computer screen as best as possible? To me, the optimum way for this to work is not to crop out any of the images - just let them fill the screen as I composed them either from top to bottom or left to right, depending on their aspect ratio, without cropping any of them. Clearly portrait-format slides will never fill anyone's screen, there will always be black space left and right. But they should fill the computer's screen top to bottom and they do not now. Appreciate any further guidance.
Which system version are you using? On macOS 10.13 High Sierra Photos is filling the screen top to bottom, when I play a slideshow using the Ken Burns or Classic theme, for the instant slideshow from an album as well as from saved slideshow project.
Léonie's suggestion hit the nail right on the head:
Try, if disabling the option "Scale to fit screen" will use more of the screen real estate for the theme you picked.
There will always be empty spaces, if the aspect ratio of your display is not the same as the aspect ratio of the photos, since Photos will not crop the photos for the slideshow.
I can understand that Photos should not crop pictures...although certain themes do so in order to zoom in. However, as you can see in the screen capture I provided, there is wasted space all around the photo so the problem isn't the system avoiding cropping.
The options in your screen capture do not appear in my version of Photos. At least I cannot find them...? The window that appears in my version is as follows...
Unselecting that button does not solve the problem. Even photos that are the exact screen resolution get cropped top and bottom.
How do I make slideshow full screen in Photos?
