Post Processing Raw - Developed Pixels per CM/Inch

[wwalford]

Hi All

I am just wondering if someone can tell me what is the best pixels per cm to develop a picture . If you look at the pictures below they are the same size (800pixels X 533pixels) but they are developed with different pixels per inch out of light room.

150 pixels per Inch (328kb)	1500 pixels per Inch (328kb)

What I am trying to work out is when is too many and what is the advantage of developing it with more pixels or less pixels. The above pictures to me look the same and they the same size? Any info on this setting will be much appreciated. Thanks

Wesley
wwalfordphotography.weebly.com

 

[HughGuessWho]

There is no pat answer to your question. It depends on what you plan to do with the picture.
Generally, 72 is used for pictures that are going to be displayed online or on a computer. If they are going to be printed, again it depends on several factors, including how large the print is going to be. It is best to consult your printer manual or the company that is doing your printing for you.

 

[wwalford]

Thanks for the response. But what is it actually changing then. Because in my mine the more pixels per square CM/inch the better the quality and therefore the bigger the picture. Am I wrong in assuming this? Because pixel wise the photos in the forum post are totally different 150 vs 1500 yet size and quality to me doesnt differ ? So what is the pixel resolution really changing then ?
Thanks

 

[HughGuessWho]

Most monitors can only display around 72 PPI. You can view a picture with more than that, but you arent "seeing" the additional pixels. More pixels = larger file size. So, if you want to display pictures on your website for example, there would be no point in using huge files when the viewer would not receive any benefit from viewing the larger file, because their monitor will only display 72 PPI. If you posted pictures that were saved at 320 PPI, the file would be MUCH larger, take up much more space on your web server (some hosting companies have a limit for storage), yet you viewer would still only be viewing 72 PPI.
Regarding printing pictures, generally, the higher the number, the larger picture you can print and still be sharp... HOWEVER, all printers have a native print resolution. Therefore, if your printer prints at 240 (many do) and thing over that is wasted. As in my earlier response, if you are sending prints off to a printing company, I would suggest reading the documention on their website or asking them what their requirements are.

Lastly, it is really DPI (Dots per Inch) but most everyone, including manufactures get the two confused.

I hope this helps.

Some reading for you that might better explain;

Understanding Printer Resolution

Dots per inch

 

[Judobreaker]

Umm... PPI does not make a difference in filesize at all. It only makes a difference in image dimensions when printing.

 

[wwalford]

Thanks a lot all . I will check with my printing company what they require and what kind of PPI I should be sending them. Much appreciated.

 

[KmH]

Pixels per inch (PPI) is meaningless for electronic display, as evidenced by the 2 photos you posted having very different PPI values assigned to them but displaying at the same size .
For electronic display, size wise, only the pixel dimensions have meaning.
File size is about image quality, not image size.

Along with pixel dimensions PPI determines the size of a print.
The photo on the left will print at 5.33" x 3.55". 800 px/150 ppi = 5.33" and 533 px/150 ppi = 3.55" (the pixel units cancel)
The photo on the right will print at 0.533" x 0.355". The math is the same except the ppi value used in the calculations.

Pixels divided by PPI = Inches
Inches times PPI = Pixels
Pixels divided by inches = PPI

The resolution of displays can be changed. Most displays today have recommended resolution settings that approach 100 PPI.

My primary display is set to 1600 px wide and is 18 3/4 inches wide which is about 85 PPI. However, note that not all display or image pixels are the same size.