How many pixels are in 10MB for photos?

+6 votes
13.2k views
I am having problems uploading photographs.  They are in .jpg format but I don't show my pictures in MB, rather in pixels.  Can someone please tell me where to go to learn acceptable sizes in "pixels?"  Thank you!

 

Margaret Cline Harmon
in WikiTree Tech by Margaret Cline G2G1 (1.1k points)
retagged by Keith Hathaway
Hi Margaret :)

Pixels would be a measurement such as across your computer screen... 400 pix by 400 pix would be a square.

MB (Megabytes) is a measurement of how much memory or disk space it is comprised of... how big the file is on your computer's drive.

As you adjust the size up and down in pixels using your imaging software, you can check to see how much disk space the file uses in MBs.  Often its shown right there in the same program if you look around the screen or click a little.

I hope this makes sense and is useful :)
There should also be an option to adjust how large a pixel is; how many pixels per square inch is common.  Having control over this allows you to choose whatever size you want the image to normally be displayed, and to separately choose the quality of the image including visible detail and range of colors.

I personally use YouTube often when I wish to learn anything... It's how I found and learned to use Paint.net which is now my favorite free image editing software.  Perhaps you can find a video tutorial on the program that you use and learn all kinds of cool tips and tricks in minutes.

Let me know if I can help in any way.

Cheers!
Also, my Internet speed isn't overly fast, if I tried to upload a 10MB photo to Wikitree, it would take a long time, and then later if I tried to look at it on wikitree it would also take a long time to "come up."

As an example http://www.wikitree.com/photo.php/9/98/ABGE-58.jpg is

10x8 inches or
1800 x 1382 pixels
180 resolution (this number is important. the lower the resolution, the less space it takes
This photo takes 1.2 MB of space.
Thank you, Keith.  I never thought about using YouTube to this question but a point well taken.  I appreciate you time and good answer.

2 Answers

+4 votes
 
Best answer
by Peter Roberts G2G6 Pilot (491k points)
selected by Margaret Cline
Thank you, Peter, the link was excellent.

Margaret
+4 votes

Hi Margaret,

The question you have posed is somewhat impossible to answer. It would be like saying you have a box that is 1 foot in length, width, and height and asking how much it weighs. It depends what's in it.

If you really want to get into the weeds:

  • s = file size in bytes
  • h = image height in pixels
  • w = image width in pixels
  • b = bit depth (24 bit is typical for a color jpg) 
  • z = compression ratio (varies by 'quality' setting and image)

the value b (which is in bits) should be divided by 8 to convert it to bytes.

s = h*w*b/8/z

The resolution does not per se change the file size - a 300x300 image is the same file size regardless if those 300 pixels are displayed/printed in 1 inch (300 ppi) or 100 inches (3 ppi).

*corrected formula to read divided by compression ratio

by Rob Ton G2G6 Pilot (273k points)
edited by Rob Ton
Just to expand on compression ratios (pun intended):

Lossless compression (such as used in png files) has lower compression ratios, but also does not degrade image quality. File sizes might be 10% to 50% less (typical compression ratios between 1 and 2.) Conversely, lossy compression - typical of jpg files - sacrifices quality (a problem if the file is re-edited and saved) to achieve much smaller files sizes (compression ratios between 2.5 and 25). Confusingly, photo editing software usually lets you choose a 'quality setting' for lossy files, but "100%" quality does not produce the same results in different photo editing programs - and generally has no direct correlation to a specific compression ratio.

Also, the content of the image itself can factor into compression - if large areas of adjacent pixels have very similar colour, like the sky, it can usually be compressed more effectively than when adjacent pixels are very different, like leaves in a forest in autumn.

 

One other factor in image file size that I forgot to mention is metadata, all the information in the file that is not the actual image  - for example many/most digital cameras today will include as part of the metadata the camera model, lens and settings, date and time, and sometimes even the coordinates of where a photo was taken.
Thank you for a very comprehensive answer.  It does shed more light on the subject.  I appreciate your taking the time to explain it.

Margaret
Thank you for explaining all that Rob

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