Technically*, if you focus on aspect ratio, you only need two of each image:
These models are all 9:16:
- iPhone SE, 5, 5C, 5S, iPod Touch 5g
- iPhone 6, iPhone 6s, iPhone 7
- iPhone 6 Plus, iPhone 6s Plus, iPhone 7 Plus
- iPhone 8, iPhone 8 Plus
These models are all 3:4:
- iPad Mini 2, iPad Mini 3, iPad Mini 4
- iPad 3, iPad 4, iPad Air, iPad Air 2, 9.7-inch iPad Pro
- 12.9-inch iPad Pro
If you focus on diagonal size, you need 6.
If you focus on ppi, you only need 3.
If you focus on resolution you need 5.
As a side note, 3 class sizes (pairs) would cover those devices, landscape only.
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The question then is what is the lowest required number that satisfies your focus. 3 in one category may not be the same 3 in another, so if overlap occurs, the minimum could be higher than 3. Scaling, as an example, can be a good scheme, but it might not cover all your needs.
Example is applied aspect ratios vs. ignored diagonal size...when displayed on your target devices, 2 out of 6 will display correctly; 4 will be stretched. How those 4 look to the user depends on your image...a simple fill might be hard to detect if stretched, whereas a shiny or deeply contrasted gradient could display artifacts, warping, flashing etc. A distorted gradient might be acceptable in one case but visually disturbing if it sits behind one font vs. another. And remember, not all pixels for your target devices are square...some are rectangular and don't play well w/certain graphics when ignored.
It's the overlap that prohibits rule of thumb. Overlap when involved, can be entirely different based on which of the above are represented by your app and your images. Applicable results that matter to the dev, for a given use case, can only be found by testing.
>I'm going off of what I have learned from books, online sources.
Be careful - books often rely on beta code/specs and are typically outdated by the time they reach the shelf...keep the receipt, better yet avoid them - online references can suffer the same fate - specs change too frequently and the newcomer can waste time chasing beta and/or outdated material, unaware they've been misled.
>I'm curious on your thoughts on my overall process.
You're getting there...
Striving to simplify any task is a good thing, but risky as a primary driver when new to that task. I like to learn and work with a topic as much as I can first, and only when I know it inside/out will I see if it can be distilled and if it can, I'm all over it....it's the systems analyst in me, I guess.
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* Note 11.7.17:
- Aspect ratio for the iPhone 8/Plus is 9:16; diagonal size is 4.7"/5.5"; ppi is 326/401; resolution is 750x1334/1242x2208
- Aspect ratio for the iPhone X's 5.8-inch OLED display is 9:19.5; diagonal size is 5.8"; ppi is 458; resolution is 1125 x 2436
Updated for iPhone 8/Plus and iPhone X:
- If you focus on aspect ratio, you need 3
- If you focus on diagonal size, you need 8
- If you focus on ppi, you only need 4
- If you focus on resolution you need 7