DO I need 300 DPI?

I like Guy Gowan but I think he has been a little too "rambling" in this video.

It's really quite simple to explain without too much info-overload.

Dots Per Inch (DPI) relates specifically to PRINT resolutions as digital images are measured in pixels (or megapixels). DPI is a useless reference when by itself. Why? Simply because there is no qualifier identifying how many INCHES are being referred to (or other appropriate measurement if Dots Per Centimetre is being used for metric people).

It is often used within editing programs like Photoshop as a method of resizing images but I avoid the use of those in favour of actually calculating the pixel size values myself (as you'll be able to do if you understand what DPI is).

I cringe whenever I get asked by a client to provide them with images at "300 DPI". What they need to say is one of a couple of things;

"I want the image to be XXXX pixels by XXXX pixels.",
"I want the image to be XXXX pixels on the long/short side.", or
"I want the image to be XXX DPI for a print size of XX inches by XX inches."

In the case of the DPI request you just have to multiply the DPI by the inches to get the pixel size values your image needs to be in order to print at the size requested.

The DPI value MUST be accompanied by the size the image is expected to be printed at otherwise it means nothing.

So 150dpi is enough? If so, I'm stopping using 300dpi *blush*

He did qualify what he was saying by mentioning he used to print in post script, which is like vector graphics. What he didn't mention is quality in raster graphics is directly related to DPI when you increase the size of your print. Vector graphics can be increased as large as you want with no detriment to quality, but raster graphics can't. There is never a situation when you have DPI without having a sized file, as you always have a dimensional file that can be measured in pixels that goes along with the DPI. The less DPI you have the less you can enlarge your print above its original size no matter what the pixel dimensions. With monitor viewing it makes no difference as he seems to imply, but with printing it is still how it is done, and without post script printing now, you need high DPI to print large quality above the original print size.

The higher DPI in your file the more options it has for use.

I do not know what kind of printing he used to do, but we used to scan at 600DPI - 1200DPI to get crisp graphic files for printing commercial work back in the early 80s, on a $12,000 scanner. Then those were 4 color separated, and printed on a Heidelberg press.

Double post.

I have changed to 150dpi but don't see a great difference in file sizes, so I am really not getting the video at all if he claims that there will be a significant size difference

Its all about the Inches. You can change a photos metadata "DPI" value all you want & it'll do nothing (except maybe a printer might pick that up). You have to also calculate the print size value. Doing so can drastically change your file size.

I save my photos as Quality 12 in photoshop (max quality), should I go lower than that?

That particular setting probably (although you don't say so) relates to saving your files as JPGs. That is a completely different line of enquiry. The quality setting for digital files (in this case JPG) relates specifically to how much of the image data your application (in this case Photoshop) is going to "throw away" (most especially important if you shoot raw as JPG should only be used as an export/final image option) in order achieve the desired digital image quality & digital file size (in digital units eg megabytes, kilobytes etc).

I suggest you read an analysis by Jeffrey Friedl related to Lightroom JPG export quality (Google = Jeffrey Friedl JPG quality, it's the 1st result). You'll get a good understanding of JPG quality & what setting to choose from him.

If there was only one medium that Demotix supplied to then it would be able to say, yes do it this way, and no you don't have to save at that high of quality, but that is not the case with Demotix.

Demotix sells to a wide range of mediums, so it is best to make your material as salable as possible, saving at the highest possible quality, and size you can without artificially increasing image size.

If your camera will take pictures at 16 megs then don't shoot at only 8 megs because it is easier to transfer, shoot at full resolution. This is the best you can shoot at, not a situation of I can shoot at a better quality, but I don't have to. Demotix does not have a consistent requirement with all buyers, other than shooting at the best your camera will do.