How to compare two or more photos side to side with Photo for OS-X

It's January 2018 as I write. The macOS is now "High Sierra" (v10.13) and Photos is up to v3. I realize I'm replying to a question asked over two years ago, but perhaps this might help someone else. Couple of points.

1. Apple Photos (v3.0) still lacks a side-by-side compare function. This is deeply disappointing as Photos has in many other respects gotten MUCH better. It's a sign to me that the folks at Apple who are working on Photos are not actually photographers and don't use the app themselves. They apparently are engineers. They worry about lots of photos as a storage problem — but don't worry about evaluating photos to pick which are better or worse.
2. That said, the latest version of Lightroom (the new online CC version) doesn't have a compare feature either (yet). Neither does the wonderful and relatively new DxO PhotoLab. I have a fair amount of confidence though that Lightroom will add it soon. No confidence this feature is ever coming in Apple Photos — in or DxO PhotoLab. (DxO Photo Lab is really a one-image editing app, not an asset management tool).
3. ON1 Photo RAW 2018 does have a side-by-side compare mode that is terrific. (ON1 Photo RAW 2018 is terrific in many ways, although somewhat more challenging than Apple Photos.)

Okay, now, there is a way to compare two images in Apple Photos. It's not quite side by side, but the best you can do. First, in one of the Library views where you can select individual photos ("Photos" or "Memories" or "Imports), click the first photo you want to compare, then command-click to also select the second photo. Then hit the spacebar. This will open one of the images in full-size view, but keep the other image selected. You can now toggle back and forth from one image to the other using the arrow keys.

I'm beginning to think that I made a horrible mistake by importing my entire photo library into Photos, and I'm starting to rethink what I've done.

Will

This will open one of the images in full-size view, but keep the other image selected. You can now toggle back and forth from one image to the other using the arrow keys.

You will need a very good visual short term memory to compare photos by toggling back and forth. To compare photos, we need to see them at the same time. I can understand that the option to compare two photos side by side is missing from Photos on the iPhone with its small screen, but why use the same code on a Mac with a 27" Retina display?

In Photos 3.0 I am now using the method to open one of the photos in Preview enlarged (Image > Edit with Preview) and I double click the other Photo in Photos to open it enlarged. Now I can see both photos at the same time.

Léonie, I have done the same thing as you — open one image in preview, the other in Photos. I've used other word arounds, too. I've exported the two photos to my desktop and opened 'em BOTH in Preview. Or I've opened 'em in ON1 Photo Raw 2018 which does have a side-by-side compare mode.

The reason I tend to use the method I described (select two images in Photos, hit spacebar to view one, then arrow keys to toggle between the two) is that the workarounds are a pain in the neck. If I'm trying to evaluate twenty different shots that are very similar, opening one after another in another app is too much trouble even without the additional step of exporting that I do sometimes. It's not quite as good as side-by-side, but you can toggle back and forth very quickly, so your memory doesn't have to be THAT good.

(What I really miss here is the ability to rate the images (1, 2, 3 etc). A rating feature would make it easier for me to evaluate a bunch of images.)

Anyway, we agree that a side-by-side compare feature should be there, absolutely. Without it, Photos continues to be just a consumer photo management app. I made the mistake of thinking that Photos 3 really was the promised replacement for Aperture, that is, an app for serious photographers. I fear I was wrong.

Will

I am using the google method to compare adjustments, but if I want to compare the composition of two images and to decide, which to keep, I need to be able to see them at the same time and to compare them, like on Apertures Lighttable.

No, Photos is no replacement for Aperture. we can make it a decent photo editor by adding photo editing extensions (with Luminar 2018 we finally have the editing brushes back!) , but there are extensions to make the asset management more powerful. I rarely needed to fall back on Apple script, while I have been using Aperture, but since I switched to Photos, I have spent more time writing Apple Scripts than organizing my photo libraries!

(What I really miss here is the ability to rate the images (1, 2, 3 etc).

I am missing the star ratings too. I add them as keywords - 1*, 2**, 3***, etc, but we cannot see them as a badge overlay. We would have to prefix the titles by stars to see the ratings.

(How to Simulate Star Ratings in Photos)

Seconding all the comments here. The functions on iPhoto have become increasingly superfluous to the point of being maddening. The compare fxn, (at one time you could compare 6 horizontal, 8 vertical) was great. Also, the image numbers were visible.

That is not supported in Photos.

The best you could do would to add both photos to an album, so you can drag them side-by-side and then use the zoom slider to enlarge the thumbnails. That will however not enlarge fully.

Or add the two photos to a page of a photo book with placeholders for two photos side by side.

None is ideal.

You can also export both photos an open them in Preview.

You may want to compare Lightroom to Capture One Pro. Capture One is closer to Aperture or iPhoto in many ways, but more expensive than Lightroom (http://www.pcmag.com/article2/0,2817,2459853,00.asp).

Photos will change with the release of the next MacOS X version El Capitan.See this page: http://www.apple.com/osx/elcapitan-preview/

New editing extensions let you go further with your photos.

OS X El Capitan supports third-party tools that will be available from the Mac App Store and accessible right in the Photos app. Use multiple editing extensions from your favorite developers on a single photo, or use a mix of extensions and the editing tools built into Photos. From adding subtle filters to using beautiful texture effects, you can take your photo editing to a whole new level.

Everything in its place.

Photos has been fine-tuned to make it even easier to manage your library. Now you can add a location to a single image or to an entire Moment. Naming your favorite people in Faces is faster with a streamlined workflow. You can also sort your albums — and the contents inside them — by date, title, and more.

It seems ridiculous to me that this can't be done in Photos?!

Very much a backwards step from iPhoto. being able to compare several photos in full screen and/or side by side is incredibly useful and I believe should have been built on.

Now that we are on El Capitan is there an extension that can help with this task?

After spending considerable time looking for the "compare photos" function in the Photo app, I share others deep disappointment that this very useful function has been eliminated. I used "compare" all the time to pick the best of multiple exposures, angles, etc. I'm no professional photographer and I'm all for simplifying tasks, but this new app seems limited to all but the most casual photo takers. I wonder how hard or if its possible to convert back to iPhoto?

Tell Apple - http://www.apple.com/feedback/photos.html

LN

I wonder how hard or if its possible to convert back to iPhoto?

If you did not delete the iPhoto Library or if you can restore the iphoto Library from a backup, try to update to iPhoto 9.6.1.

That is possible, if your AppleID is associated with iPhoto at the App Store, see Barney's user tip:

Get iPhoto 9.6.1 if you didn't update before OS X 10.10.3