Carl Skepper Posted February 17, 2015 Share Posted February 17, 2015 This has been brought up several times so I thought I'd try and show the maths behind it and how it can be changed to recalculate new sizes, should Alamy change the minimum size requirement. I'll start with the basic formula for full size images then move on to show how to work out for cropped images. Basic Formula (File width (pixels) x File height (pixels) x 3 (for the three channels, red, green and blue) ) ________________________________________________________________________> = 17MB uncompressed 1024 x 1024 The 1024 x 1024 comes from the fact that each pixel of information is stored in a byte of data and there are 1024 bytes in a kilobyte (KB) and 1024 KB in a MB So for my Nikon D200 an uncropped image saved as an 8bit TIFF would be: 3872 x 2592 x 3 ____________ = 28.71MB in size, well over the current minimum of 17MB 1024 x 1024 The slightly trickier part comes in if you need to crop the image. For images with the same aspect ratio we use the properties of similar triangles: Ax / AD = Ay / CD where Ax and Ay are the cropped width and cropped height sizes, respectively. Substituting what we know for my D200: Ax / 3872 = Ay / 2592 and transposing: Ax = (3872/2592)Ay (equation 1) Where does this get us? If we substitute this for the width in the basic formula at the start of the post we can find the minimum height for the cropped image: 3872 x Ay x Ay x 3 ______________ = 17 which becomes: 2592 x 1024 x 1024 11616 Ay2 __________ = 17, then cross multiplying this becomes: 2717908992 Ay2 = 3977656 so the minimum height for a cropped image from my D200 would be: Ay = 1994.4 or 1995 pixels Substitute this back into equation 1 above to get the minimum width: Ax = (3872/2592)x1995 = 2980 pixels If you keep the same aspect ratio for your crop then you only need one of the dimensions if you crop is Photoshop as there is a preset for this. Should Alamy change the 17MB minimum file size just substitute the new size in the basic formula at the start of this post. If you do not keep the same aspect ratio then you will need to put what data you do have into the basic formula and transpose for the unknown dimension. Please remember that when you convert your images to JPEG the file sizes will be much smaller than this. I hope this helps those struggling to work out file sizes. Link to comment Share on other sites More sharing options...
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