[richardstringer]

I've got just a quick question about raw files. As far as I know, the higher the dpi number the bigger the image or better the image quality right? I'm just wondering why my Pentax K30 raw files show only a 72dpi horizontal and 72dpi vertical resolution. Is that right? I mean surely because the image is from the brilliant K30 it should be about 300dpi right?

[Adam]

I've got just a quick question about raw files. As far as I know, the higher the dpi number the bigger the image or better the image quality right? I'm just wondering why my Pentax K30 raw files show only a 72dpi horizontal and 72dpi vertical resolution. Is that right? I mean surely because the image is from the brilliant K30 it should be about 300dpi right?

DPI is a print setting controlling the physical size of the printed image. It has nothing to do with the files coming out of the camera apart from the fact that low-res files will print tiny at high DPI settings.

[richardstringer]

Ah ok so it doesn't affect the end image quality then, ok. I was thinking maybe the dpi being so low might be affecting the image quality of photos taken with my K30.

[Kobayashi.K]

Dpi's are a common source of confusion. As Adam says, the dpi value is only important when printing, because a print is measured in inches. When the photo is shown on a screen only the number of pixels is important because a screen is measured in pixels.

The maximum size of a photo from the K-30 is 4928x3264 pixels. If you print it at 300 dpi it is 4928/300=16.4" wide and 3264/300=10.8" high. Printing it at 150 dpi makes it twice as big. So choosing the dpi value defines the print size at a given number of pixels.
As you say, the dpi value affects image quality. So, its up to you to select the dpi value for a compromise between image quality and size.

On the other hand, when you are displaying the photo on your screen (via an editor or in a web page) only the number of pixels matters because the size of your screen is in pixels. If your screen is 1920 pixels wide and the photo is 4928 pixels wide it cannot be shown fully and it must be zoomed out (about 30%) to show a complete picture.

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Last edited by Kobayashi.K; 08-30-2013 at 03:36 AM. Reason: Display in web page added.

[richardstringer]

If I was printing onto A4 photo paper, what would be the best image quality dpi to set it to print at?

[Kobayashi.K]

Usually 300 dpi gives a good quality print. Print shops sometimes use higher values like 360 dpi.
However, as said before, it depends on the size of the available crop (in pixels).

In the following calculation we want to know the maximum size of a crop for a photo on A4 with a white border of 5 mm and printed with 300 dpi.

1" = 2.54 cm
A4 = 29.7 x 21.1 cm

You need a border of about 0.5 cm, so printed area is:
Width is 29.7 - 1 cm = 28.7 cm or 28.7/2.54 = 11.3".
Height is 21.1 - 1 cm = 20.7 cm or 20.7/2.54 = 8.15".

If you select 300 dpi you will need a crop of:
Width = 11.3" x 300 = 3390 pixels.
Height = 8.15" x 300 = 2445 pixels.
This will fill the complete available area.

Of course, when you start with a given crop size you must select another dpi value to make the photo fit in the A4.
Also, when you have another width/height ratio of your crop you will end up with other sizes for the white borders.

[JimJohnson]

If I was printing onto A4 photo paper, what would be the best image quality dpi to set it to print at?

That depends entirely on the printer's capabilities, and on the device driver used to communicate with the printer. The setup panel for most printers usually has settings for the specific surface type of your photo paper and another one for print quality. Sometimes a specific dpi setting is available for one printer and not for another, but nearly all have settings like 'draft' (or quick), 'normal' and 'best'. Best means best for that printer, which may or may not be the highest dpi setting.

A number of studies show that when it comes to printing the best and longest lasting photos, you should use the manufacturer's supplies - both ink and paper. If all you are doing is creating proofs, not so much.

[richardstringer]

I used to print out onto A4 Ilford Glossy photo paper when I had my Nikon D5100, I always selected no border.

[Tako Kichi]

In addition to all the above people get confused about pixel count and dots per inch (DPI) and the two are totally separate entities. DPI is a rating for printers only and quantifies the number of dots the printer is able to print per inch. Pixel count is an on-screen value.

I used to have an Epson inkjet printer 10+ years ago that could print at 1600 DPI and the results were impressive but boy did it drink ink if you made a letter size print at that resolution. As others have said 300-360 DPI is pretty good for most people's needs but like everything else you get what you pay for. Increasing the DPI for printing increases the printed quality but the ink costs can rise exponentially too.

[JimJohnson]

Hey to all my Canadian friends, slightly off topic but here goes.... Are paper goods in Canada (specifically photo paper) generally sold to metric or inch sizing?

I manage to cross the border here and then and have never paid much attention. I'll have a chance to visit Windsor in a couple weeks. We have both relatives and close friends in Europe, some of which are elderly with no Internet access. Shipping almost anything is cost prohibitive, but I know they would love the occasional printed photo of the people they love on this side of the pond. Of course unless I pay an extravagant price, photo paper here is in inch sizing and won't fit metric sized frames very well. But since I will be in Windsor....

[richardstringer]

In addition to all the above people get confused about pixel count and dots per inch (DPI) and the two are totally separate entities. DPI is a rating for printers only and quantifies the number of dots the printer is able to print per inch. Pixel count is an on-screen value.

I used to have an Epson inkjet printer 10+ years ago that could print at 1600 DPI and the results were impressive but boy did it drink ink if you made a letter size print at that resolution. As others have said 300-360 DPI is pretty good for most people's needs but like everything else you get what you pay for. Increasing the DPI for printing increases the printed quality but the ink costs can rise exponentially too.

Yeah I hear that, I used to have a canon 5 ink carttidge photo printer and that thing consumed more ink thank an alcaholic consumes alcohol.

[Tako Kichi]

Hey to all my Canadian friends, slightly off topic but here goes.... Are paper goods in Canada (specifically photo paper) generally sold to metric or inch sizing?

Even though Canada is metric in many cases (fuel in litres, speed in KPH etc.) they adopted the North American standard for paper sizing so we use letter size, legal size and all the rest.

Unfortunately US paper size and ISO paper size are incompatible as you have found out and they are so different that you can't get one out of the other either. US Letter is wider than A4 and A4 is longer than US Letter. This page gives a good comparison between the two: Between Borders

You could, in theory, print an A4 image onto Legal size paper and then crop back to A4 size.

[JimJohnson]

Even though Canada is metric in many cases (fuel in litres, speed in KPH etc.) they adopted the North American standard for paper sizing so we use letter size, legal size and all the rest.

That is a darn shame.It is likely because Canada was a late comer to adopting the metric standard and America is its biggest trading partner and vice versa.

As a citizen of one of only three countries in the world not using the metric standard, I am frankly ashamed that my previous generation and many of my generation (I'm a baby-boomer) refused to make the change when we could better afford to do so. At one point our interstate highway system began replacing signs with dual KM/miles and that stopped, going back to strictly miles. Car speedometers show both, but only for the convenience of folks who drive across the border into Canada or Mexico. Many of our food products still show both weights/volumes IF the product is traded outside the U.S.

It is no surprise to most of the world that the metric system makes a lot more sense than the American system based on fractional amounts. Even folks here in the U.S. that rally against the metric system have trouble with some of the more specialized measurements like knots, fathoms, leagues, rods, and chains. Although for you folks in the U.K., the U.S. never officially adopted stones.

For what it is worth Tako Kichi - when we visited the Soo a couple years ago and took the Algoma railway up to Agawa Canyon (beautiful ride in the fall!), I was surprised to see the trackage is still maintained in miles.

Ah well, at least time, f/stops and light sensitivity is measured consistently worldwide for photography or we would all be in trouble!