Apple yesterday released a preview of its upcoming all-new Photos app for Mac, which replaces iPhoto and Aperture with a simpler all-in-one photo editor and library manager. Most of the discussion of Photos focused on the huge number of changes from iPhoto and Aperture, burying one very important detail: Apple is changing the way it handles cloud-based photo storage.

Before Photos, Apple offered free storage of photos with limitations in a feature called Photo Stream, which didn’t count against iCloud storage. But the new Photos app uses Apple’s beta iCloud Photo Library feature, which was recently added in iOS 8.1. iCloud Photo Library promises to let you synchronize your entire photo collection including edits and albums across all of your devices… but you have to share your iCloud storage with photos, and album syncing and edits don’t apply to the free 1,000 – 25,000 image storage of Photo Stream.

As most long-time iOS users know, the free 5GB of iCloud storage Apple offers is often not enough to store much more than a single device backup, and for many that will mean no spare room for a photo collection. Consequently, Apple is suggesting that users should buy additional iCloud storage, paying monthly fees to store and sync their photos. As the Photos app is rolling out, Apple is allowing users to stick with the old Photo Stream feature and continue using the new Photos app without turning on the iCloud Photo Library. But it remains to be seen if that will be an option long-term once Photos is released publicly and how users will respond when they find out their free 5GB iCloud storage isn’t cutting it for their photo collection…

Today, every iOS and Mac user gets a free iCloud account with 5GB of storage. The space currently holds device backups, your files in iCloud Drive, your iCloud Mail, and settings data from iCloud-enabled apps. If you use both iCloud backups and an iCloud mail account, there’s a good chance you have already seen an “iCloud storage limit reached” warning that you need to delete one or the other to make space.

Your iCloud account will get more crowded when you enable the new iCloud Photo Library feature. When you move from just using Photo Stream, your stored photos will now count against your iCloud storage and appear in a new “All Photos” album, but they’ll also be accessible across all devices and iCloud.com. Photos taken with recent iPhones require around 2.5MB of space each, which means that just your last 1,000 photos from Photo Stream, for example, will consume around 2.5GB of your 5GB of iCloud storage.

The number gets higher if you include photos taken with better cameras: each 22MP photo can easily require 6MB as a JPEG or 30MB as a RAW file. Do the math and even your last 1,000 photos (let alone your entire photo collection) won’t fit within that free 5GB of iCloud storage, assuming that it was empty. The new Photos app will prompt you to upgrade if your existing photos collection is too big for your current available storage when upgrading to iCloud Photo library.

That’s why upgrading to the iCloud Photo Library might also mean upgrading to a paid iCloud plan with more storage. This will depend on the user, how large the image files are, and how much iCloud space is available. It will be interesting to see how many users will be forced to upgrade to paid iCloud storage in order to use the feature: Apple is selling 20GB for $12 per year, 200GB for $48 per year, 500GB for $120 per year, and 1TB for $240 per year, though it bills monthly and lists its prices by month rather than by year.

For now, Apple is allowing users to stick with My Photo Stream and continue using the new Photos app. If you don’t upgrade, you’ll still see the “My Photo Stream” album in the new Photos app, and won’t get to take advantage of Apple’s new system of storing photos with improved syncing, the iCloud.com Photos app, and more. Apple also notes that you can disable iCloud Photo Library in order to recover iCloud storage and (again, for now) just use My Photo Stream. After doing so, iCloud will continue storing the photos for 30 days to give you an opportunity to download them.