I use a Canon 30D for wildlife. Sometimes when I have great light and think I am getting a steller image, it looks good in RAW. After processing, when I go to resize for NPN, I notice the resolution is 72 pixels per inch before changing the resolution. Many photos however, (usually in poorer light) originally show 180 pixels per inch before resizing. Does a smaller aperature in good light reduce the pixel resolution? I haven't figured out what makes the difference. Any help with an explanation is appreciated.
Ralph Kerr
Posted by Richard Ditch on July 17, 2009 at 12:05 PM
ppi is something you set somewhere in your work flow: raw conversion, photoshop, etc.
It has zero impact on image quality (as long as it isn't tied to resampling of the image) - it controls how the total linear pixels in an image (say 1200 pixels wide) are mapped to the printer.
For web posting (at NPN and anywhere else) PPI is completely meaningless and completely independent of the total pixels in your jpeg file.
Actually, PPI/DPI is a factor on your file size. If, through your work flow, you reduce the pixels per inch (PPI/DPI) from 300 (typically used for print resolution) down to 72 DPI, the file will be smaller so you can get under the limits NPN sets on your upload size. The vast majority of monitors around the world can only render a resolution of 72 dpi, so anything higher than that for web work is useless & just increases the file size, not the quality. If you were going to print any of the final images, it would be much better at a higher dpi.
I had a problem getting this too when I switched to digital. A couple NPNer's explained it to me, and I apologize if this is something you already know. Ppi only matters when you are sizing for a print...such as 300dpi x (10x8) equals 3000x2400 resized for a print or an 8x10...on the web ppi it doesn't matter. I have never noticed that the file is smaller when resizing for the web by changing it to 72, but maybe I need to test it again. What does matter, which has greatly improved my posts is the quality scale when you resize...mine was set at 7 and it greatly improved when I made my image size smaller than 720 (to fit the 200KB limit) and upped the quality to 9 or 10. I've noticed that some folks have these beautiful large images posted here that have great quality, but I've been told that may have something to do with using Photoshop tools and better compression to jpg than what I'm using (Canon software)...they also may take out lots of imbedded info which makes the file smaller than 200KB for posting. Someone feel free to explain why someone can post 720 photo using a 21MP camera on this site...and it's not fuzzy...maybe it's all about image quality with the latest camera technology. Also, I learned that sharpening after resizing greatly improves the look on the web. Regarding the 72-180ppi, my screen has a option to set a standard dpi such as 400 and it always defaults to that unless I change it for a specific image. I've never seen my software actually change the dpi on me based on image quality (I use a Canon 40D). Hope this helps, my knowledge is limited, so if I've said something incorrect I hope someone will correct it.
Regarding your small aperture, there is huge difference in image quality (that I've noticed) between using f22 and f16...I rarely use f22 anymore...you may get depth of field but it isn't as sharp. f16 works just fine, especially if you have a general idea of the hyperfocal distance...such as a lens set at 20mm should be approximately focused at about 5 feet for foreground and background depth of field...I'm sure other photographers are much more specific, but this really helped me get higher quality photos without relying on f22. I use f8 and f11 quite a bit, especially if my subject is focused at infinity...these f stops are usually the sharpest on most lenses.
Does a smaller aperature in good light reduce the pixel resolution?
Not at all.
Richard's explanation was correct.
The ppi figure you see in the Image size dialogue is an arbitrary and meaningless figure.
Just lower the resolution to 720 pixels on the long edge of the image you're going to post and ignore the ppi number.
Hi, Ralph--
Getting a stellar image to show up well on the web has more to do with resizing and sharpening (IMO) than anything else. Here's a good guide (right here on NPN); its PSE-specifics are easily recognizable in the interfaces of other image editors. As everyone has said, ppi depends entirely on your monitor and dpi is meaningless unless you're actually printing.
Posted by Richard Ditch on July 20, 2009 at 10:01 PM
Actually, PPI/DPI is a factor on your file size. If, through your work flow, you reduce the pixels per inch (PPI/DPI) from 300 (typically used for print resolution) down to 72 DPI, the file will be smaller so you can get under the limits NPN sets on your upload size. The vast majority of monitors around the world can only render a resolution of 72 dpi, so anything higher than that for web work is useless & just increases the file size, not the quality. If you were going to print any of the final images, it would be much better at a higher dpi.
This is simply incorrect.
1. reducing the ppi will have no effect on the file size or image quality provided you do not resample the image at the same time. Resampling the image to fewer pixels total will reduce the file size; simply changing the ppi setting will not.
2. to get your file size under the NPN limit (200KB last time I looked) you ned to compress your jpeg image more until it gets under the limit. ppi has nothing t do with jpeg compression settings.
3. monitors have a native ppi, and if your monitor is 72 ppi then a 720 pixel wide image will be 10 inches wide on the monitor irrespective of the ppi setting in the jpeg file. If your monitor is 96 ppi then that same 720 pixel wide image will measure 7.5 inches wide on screen.
4. for the web ppi is completely meaningless and has no effect on file size or on quality. Use whatever ppi setting you want in a jpeg for web posting.
Posted by Richard Ditch on July 20, 2009 at 10:04 PM
Image at 100 ppi.
720 pixels wide x 546 pixels high
jpeg compression: 1:11
File size 134.3KB
Posted by Richard Ditch on July 20, 2009 at 10:07 PM
Same image, this time at 400 ppi.
720 pixels wide x 546 pixels high
jpeg compression: 1:11
They look the same on screen irrespective of the ppi of 100 or 400 because tehey both contain the same number of pixels (720x546) and have been compressed the same amount (1:11).
Discussion
8 repliesRalph Kerr
It has zero impact on image quality (as long as it isn't tied to resampling of the image) - it controls how the total linear pixels in an image (say 1200 pixels wide) are mapped to the printer.
For web posting (at NPN and anywhere else) PPI is completely meaningless and completely independent of the total pixels in your jpeg file.
I had a problem getting this too when I switched to digital. A couple NPNer's explained it to me, and I apologize if this is something you already know. Ppi only matters when you are sizing for a print...such as 300dpi x (10x8) equals 3000x2400 resized for a print or an 8x10...on the web ppi it doesn't matter. I have never noticed that the file is smaller when resizing for the web by changing it to 72, but maybe I need to test it again. What does matter, which has greatly improved my posts is the quality scale when you resize...mine was set at 7 and it greatly improved when I made my image size smaller than 720 (to fit the 200KB limit) and upped the quality to 9 or 10. I've noticed that some folks have these beautiful large images posted here that have great quality, but I've been told that may have something to do with using Photoshop tools and better compression to jpg than what I'm using (Canon software)...they also may take out lots of imbedded info which makes the file smaller than 200KB for posting. Someone feel free to explain why someone can post 720 photo using a 21MP camera on this site...and it's not fuzzy...maybe it's all about image quality with the latest camera technology. Also, I learned that sharpening after resizing greatly improves the look on the web. Regarding the 72-180ppi, my screen has a option to set a standard dpi such as 400 and it always defaults to that unless I change it for a specific image. I've never seen my software actually change the dpi on me based on image quality (I use a Canon 40D). Hope this helps, my knowledge is limited, so if I've said something incorrect I hope someone will correct it.
Regarding your small aperture, there is huge difference in image quality (that I've noticed) between using f22 and f16...I rarely use f22 anymore...you may get depth of field but it isn't as sharp. f16 works just fine, especially if you have a general idea of the hyperfocal distance...such as a lens set at 20mm should be approximately focused at about 5 feet for foreground and background depth of field...I'm sure other photographers are much more specific, but this really helped me get higher quality photos without relying on f22. I use f8 and f11 quite a bit, especially if my subject is focused at infinity...these f stops are usually the sharpest on most lenses.
Not at all.
Richard's explanation was correct.
The ppi figure you see in the Image size dialogue is an arbitrary and meaningless figure.
Just lower the resolution to 720 pixels on the long edge of the image you're going to post and ignore the ppi number.
Getting a stellar image to show up well on the web has more to do with resizing and sharpening (IMO) than anything else. Here's a good guide (right here on NPN); its PSE-specifics are easily recognizable in the interfaces of other image editors. As everyone has said, ppi depends entirely on your monitor and dpi is meaningless unless you're actually printing.
This is simply incorrect.
1. reducing the ppi will have no effect on the file size or image quality provided you do not resample the image at the same time. Resampling the image to fewer pixels total will reduce the file size; simply changing the ppi setting will not.
2. to get your file size under the NPN limit (200KB last time I looked) you ned to compress your jpeg image more until it gets under the limit. ppi has nothing t do with jpeg compression settings.
3. monitors have a native ppi, and if your monitor is 72 ppi then a 720 pixel wide image will be 10 inches wide on the monitor irrespective of the ppi setting in the jpeg file. If your monitor is 96 ppi then that same 720 pixel wide image will measure 7.5 inches wide on screen.
4. for the web ppi is completely meaningless and has no effect on file size or on quality. Use whatever ppi setting you want in a jpeg for web posting.
720 pixels wide x 546 pixels high
jpeg compression: 1:11
File size 134.3KB
720 pixels wide x 546 pixels high
jpeg compression: 1:11
They look the same on screen irrespective of the ppi of 100 or 400 because tehey both contain the same number of pixels (720x546) and have been compressed the same amount (1:11).
File size 134.5KB