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In "photos" on a macbook, there is an option to zoom in a photo up to 400 percent. Is it normal to see some pixelation when fully zoomed in?

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400% is not just fully zoomed in - it is much more than fully zoomed in. In some cases it may be beneficial to view photos at 100% but I can't think of a use-case for viewing at more than that. To answer your question, it would be very strange (or completely incorrect and misleading even) if you didn't see pixelation at 400% magnification.

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  • "I can't think of a use-case for viewing at more than that." – I often zoom in to 150-200% when working in areas where slight misalignments would be very noticeable, like around people's eyes. When using a high-DPI display, I'd have to zoom in even more to make sure pixels are aligned. – xiota Jul 29 '19 at 15:31
  • Also, in group photos, someone might want to zoom in on particular people. The amount of zoom required depends on the display characteristics and size of people in the photo. – xiota Jul 29 '19 at 16:04
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The iPhone 6's rear camera shoots 3264 x 2448 pixel photos.

If you zoom in 4 times, you're looking at a quarter of those pixels, which is 816 x 612 pixels.

Your monitor likely displays 1920 x 1080 pixels or larger. If you want to display a 816 x 612 pixel image on the entirety of such a monitor, it has to 'stretch out' these pixels so it fills up the monitor. If you assume 100% to fit the monitor exactly, 400% means you're showing 1 pixel of the photo on 4 pixels on the monitor- which would show as 'pixelated'

In short: yes, it's entirely normal

EDIT
In regard to John's comment

Some clarification about the difference between viewing at 400% and zooming in 4 times:

Seeing as how your iPhone's camera shoots photos with more pixels than the monitor you're using, viewing the photos at 100% would mean they're 'zoomed in' in your monitor.
Zooming in 4 times would not necessarily create the same image as viewing at 400%. This depends on the monitor size and the starting point before zooming in.

This aside, zooming in and viewing at a certain scale will always mean a certain amount of pixelation if the number of pixels to be displayed is lower than the pixels the monitor consists of

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  • But what happens if I start with an image of size 9000x6000? By your reasoning there's no stretching involved and I should see more detail. Zooming in 4 times is completely different to zooming in to 400%. Your short answer is right but your reasoning is wrong. – John Hawthorne Jul 29 '19 at 14:09
  • @JohnHawthorne Well, one would have to take into account that I assume 100% to fill up the full screen (which isn't quite accurate but it works to get my point across) – timvrhn Jul 29 '19 at 14:12
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    But assuming 100% means fitting the image to the monitor exactly is completely different definition to 400% meaning showing 1 photo pixel on 4 monitor pixels - two different definitions in one sentence - which is it? – John Hawthorne Jul 29 '19 at 14:20
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    You still don't understand I'm afraid. 100% magnification means the image size on the screen is 1920 x 1080, not 3264 x 2448. At 100% the image does not fit on the screen unless it is smaller than the screen size. – John Hawthorne Jul 29 '19 at 14:40
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    @timvrhn To zoom in n times isn't necessarily the same as an n-x zoom. For instance, if I hit the zoom key four times in this browser window, the magnification will zoom to 150%. But a 4x zoom would be 400%. Also, there's an issue with whether zoom references a single dimension, such as the width, or the area of the image. Your answer appears to assume it's based on width (or height) of the image, which seems to be what image editors and viewers report. But note that a 4x change in width is a 16x change in area. – xiota Jul 29 '19 at 15:54
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It is normal to see pixelation when zooming in past 100%. It's also normal to not see pixelation.

To zoom in to 100% means there is a 1-1 correspondence between the image pixels and display pixels. When zooming in past 100%, there are "gaps" between pixels that need to be filled in for display. (May be thought of as "stretching out" the pixels to fill the screen.)

Filling in the "gaps" between pixels is known as interpolation. Some methods result in a pixelated appearance (nearest neighbor). Others won't (bilinear, cubic, sinc/lanczos). The interpolation method can be selected in most image viewers. Some programs may refer to them by complexity instead of name (fastest, fast, good, better, best, etc).

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