Why are same-resolution photos different file sizes?
Asked 1/20/2016
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I took two photos on my smartphone at the same resolution: one of a plain table surface and one of a cluttered tabletop with lots of detail. The plain photo is about 3.5 MB, while the cluttered one is about 5 MB. If both images have the same number of pixels, why does the more detailed photo create a larger file?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
10y ago
2 Answers
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This is not only possible, but extremely likely, when you're using a compressed image format such as JPEG. Data compression methods in general become more efficient as the data to be compressed decreases in entropy (try creating zip files of a large page of actual text vs. the same sized page of a single repeated character).
The more features or fine textured detail that an image contains, the less the compression can "cheat" by simplifying the data that it stores to represent the image when it's uncompressed. On the other hand, cameras that store images as uncompressed RAW files tend to produce similar-sized files per image, regardless of image content.
A lossy format like JPEG, because it actually discards some inessential data in a very clever way while compressing, can often achieve ratios up to 10:1 while still giving a decent representation of a typical photograph, while a lossless compression format that retains all data might only achieve 2:1 compression, or less.
Originally by user37649. Source · Licensed CC BY-SA 4.0
user37649
10y ago
0
Generated from our catalog & community — verify before relying on it.
Yes—this is normal, especially with compressed formats like JPEG. Resolution only tells you how many pixels the image has, not how efficiently those pixels can be stored.
Compression works better on simple, repetitive image data. A plain surface has large areas of similar color and tone, so the file can be compressed more efficiently. A cluttered scene has more edges, texture, and fine detail, which is harder to compress, so the file ends up larger.
This happens with both lossy and lossless compression, though in different ways. JPEG is lossy, so it throws away some image information while trying to preserve the overall look. Even so, detailed scenes usually still need more data than simple ones. Lossless formats and many RAW files also vary in size based on image content, though often less dramatically. Truly uncompressed formats would stay about the same size at a given resolution and bit depth.
So: same pixel count, but different image complexity = different file size.
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