PS9 and Picasa have two different sizes of photo

[--pv--]

I have a photo I would like to enter in a contest. The requirements are no larger than 16MB 3300 x 2550 pixels (11 in. X 8.5 in. at 300 dpi) I have satisfied all the requirements re: 300 dpi and 3300 x 2550. However, when I look at the photo in PS9, it states it is 24M. I assume that means MB. When I right click on it or look at it in Picasa, it says 4.5 MB. Which is it? Thank you.

[MichelBParis · Accepted solution]

24 M is 24 megapixels, the space used in memory to store and edit the 3300 x 2550 pixels in each of the 3 red, green, blue layers. which gives  25 245 000 bytes. Divide by 1024 to get roughly the MB count in memory.

When you save to your disk in various photo formats, you find that:

- in .PSD or .TIFF formats, the size in MB is about the same on disk

- with non lossy compression TIFF, you reach 9 MB

- with jpeg (so lossy compression) you get sizes from 0,5 to 4 MB depending on the compression level. I would suggest to use 10, not the max 12 for good compression and quality compromise.

[--pv--]

Thank you for that. It is a jpeg so I am "assuming" the 4.5 MB I see in Picasa is the correct number.

[MichelBParis]

Thank you for that. It is a jpeg so I am "assuming" the 4.5 MB I see in Picasa is the correct number.

By @--pv--

Yes.

[Greg_S.]

@MichelBParis said:  24 M is 24 megapixels.

I hate to correct Michel on anything to do with math but I think he intended to say that 24 M= 24Mb which is measuring Megabits.  As Michel has explained, this is different than 24MB which refers to Megabytes.

(Megapixels measures the resolution or pixel dimensions of the image.)

[--pv--]

Mega pixels much smaller entity in this language, correct? Megabytes much larger. Thank you.

[Greg_S.]

Megapixels (MP) refers to the dimensions of an image, e.g. 3000px X 4000px is a 12 MP image. 

Megabytes (MB) and Megabites (Mb) refers to the size of the file. There are 8 bits in 1 byte. 

There is no direct relationship between MP and MB.  A 12 MP image can be a file of 1MB or 10 MB, depending on the format and compression level of the file.  For example a non-compressed TIFF file will be many more Megabytes than a compressed JPEG file.

The "size" of a pixel is defined by the number that can fit within a unit of measurement e.g. 300 pixels per inch (ppi).  So, printing an image at 300 ppi will result in a much smaller printed photo than printing the same photo at 72 ppi. 

I hope I'm not confusing you too much, but it is important to understand these concepts.

[Test Screen Name]

If you want to know the size of a file use Windows Explorer or Mac Finder. Don't try and use any other editing app, because the size is actually not known until the file is saved. It will be different every time.