What DPI actually is.

**Leo** · 08-03-2006, 11:26 AM

The problems in the latest photo contest underlined this subject isn't exactly understood by quite a few people. I count myself among those folks. So I did a bunch of research and actually got a program myself that would edit DPI. I downloaded infraview from the link posted on the "problem with contest" thread. Thank you ruttinbuc

What DPI is:

1. DPI stands for Dots Per Inch. DPI times the inch size of the photo yields the number of pixels. So a photo that is sized to 7"x7" at 72x72 DPI will be 504x504 pixels.

2. DPI is an attribute tag. Like the date of modification.

What DPI is NOT

1. DPI does not affect file size. The exact same 504x504 pixel photo can be set to 96DPI. As long as you use the same parent photo (see Note) to make the 72 and 96 DPI versions, the file size will NOT change. But the inch size of the photo will change from 7"x7" to 5.25"x5.25"

2. DPI has nothing to do with screen resolution. Whatever your desktop size is set to the photo will show up on the screen in full pixel by pixel size. Unless the pixel x pixel size of the photo is LARGER than your desktop size. Then whatever viewing software or browser you are using will rescale the photo appropriately.

3. DPI also doesn't affect print size of the photo. Printing software programs will fit the pixels into whatever size paper you select. The more pixels the better the printed points per inch and higher the quality of the photo.

NOTE: JPG format is a compression format that gets applied to the PHOTO when you save it. The amount of compression is dependent on the Quality settings of your photo software. Most of the time your DPI changes to whatever the default is for that software. But it is the JPG quality settings that change the file size NOT the DPI value. This is why you should use the same Parent Photo to make two different DPI photos. Otherwise it is possible to recompress a already compressed photo.

I hope this has been helpful and not too repetitive for some. I just felt pretty confused about this before and thought I'd share with those who may have been in the same boat.

**VermontHunter** · 08-03-2006, 04:01 PM

Makes absolute sense to me....

[ QUOTE ]
2. DPI has nothing to do with screen resolution. Whatever your desktop size is set to the photo will show up on the screen in full pixel by pixel size. Unless the pixel x pixel size of the photo is LARGER than your desktop size. Then whatever viewing software or browser you are using will rescale the photo appropriately.

[/ QUOTE ]

Correct.. but DPI will affect picture resolution...

,, if a picture is 96 DPI which is standard screen resolution,,, it will be a much sharper image than the same size image set at 72 DPI....

The MORE DOTS per INCH the sharpewr the image....

**wtnhunt** · 08-03-2006, 04:19 PM

Yep, Luke is right. The higher the dpi the sharper your image will be. Knew what dpi was when Bill posted the thread about the problems, but still not sure how to use my software to manipulate the dpi in my photos. Other than the contest, not sure of any reason I would want to really. Can resize with no problems.

**Leo** · 08-03-2006, 04:41 PM

Depends on what you consider "SIZE" to mean. A 7x7 inch photo at 96DPI will be sharper than a 7x7 photo at 72DPI. This is certainly true. But the 7x7 photo at 96DPI will need to have a pixel size of 672x672 versus the 7x7 photo at 72DPI having only 504x504 pixels. Your computer cannot add the detail from 504 to 672 pixels. Increasing a 72 DPI 504x504 photo to 96DPI simply shrinks it's inch size to 5.25 x 5.25. The photo is still 504x504 pixels. Your file size will remain exactly the same. DPI is only valuable if you are resizing photos on an inch x inch basis. DPI and inchxinch sizes are merely attributes of a digital file and have no bearing whatsoever on the file size.

Contrary to what quite a few believe, your DPI value won't make a difference in how your photos print out. That's controlled by the PPI setting of your print and the actual pixel x pixel size of your photo. PPI and DPI are different and that's another source of confusion.

Bottomline, a 7x7 photo at 72x72dpi is 504x504pixels.

**VermontHunter** · 08-03-2006, 05:01 PM

Leo,,, I think we could really take this to another level and really mess some heads up...

But you have explained it about as clear as it can get in laymens terms so we'll leave it at this level for abit....

But, you are right a computer can't add pixels, only remove them....

,, this is where eye deception comes into play, and confusion mounts...

**OJR** · 08-03-2006, 05:06 PM

VH, you are confused enough, so just leave it alone!! LOL!!

**billkay** · 08-03-2006, 07:01 PM

OK, here's what I think, for what it's worth. Pixels per inch will be determined by the size of the CCD in your camera. Larger chip, more pixels. However when you have 2 limiting factors, in this case, the longest side, (7 inches), and 2, 72 DPI, you have a specific image size. If you try to crop a 96 dpi image from an original 72 dpi, you will lose resolution. However, you really need to be cropping from a saved version of the original which will be much higher in resoltion. The reason behind the 72 DPI in this contest is that is by far the most common computer screen resolution in use. Any larger is usually just a waste. It was, and still is, in my opinion the best way to allow all images in this contest to be viewed on an equal basis. Sorry if that is a problem but I felt it needed to be done.

**VermontHunter** · 08-03-2006, 07:59 PM

Bill, your analysis and conclusion of DPI and size cropping is totally correct...

I think it's just that so many are so use to seeing HIGH Resolution Pictures, that anything less than 96 DPI looks grainy to them...

I thought everyones pictures looked absolutely GREAT...

and enjoyed viewing them all,,, THANK YOU for yeat another great RT contest...

**Leo** · 08-03-2006, 08:21 PM

Bill,

I'm sorry if you took this post as a personal attack. I had a lot of confusion about this issue my self. I thought I clued into the fact that I was not alone. So I'm hoping to clear it up for everyone.

That being stated, here's a real example of what I'm hoping will finally clear it all up.

This picture is 72DPI 504x469 pixels

This one is 180DPI 504x469 pixels

Both look the same on everyones screen regardless of their computer screen resolution. Both files are also exactly the same size. If you decide to print them both the same physical size they will look exactly the same. There will be no difference in print resolution. This is because both pictures are the same pixel x pixel size and have undergone exactly the same level of JPG compression.

Please feel free to save copies of these photos to your hard drive and examine their properties.

This is no computer geek trick, this just illustrates what DPI really is.

What DPI is not is something that affects how something looks on a computer screen, how it prints or how big a file the photo is.

With two real examples I hope this makes sense to everyone.