[woodyneale]

Hi,

I'm not sure where this question ought to go but hope it's acceptable here.

Can anyone on the forum clarify the variation in horizontal/vertical image resolution between cameras? Having looked around I’m unable to find anything that makes sense to my simple brain.

I've just acquired a long awaited K-5 (what a camera!) and am beginning to get to grips with it. Uploading some K-5 images into Photoshop Elements I discovered that it is working with a horizontal/vertical image resolution of 300dpi. This threw me slightly as my Pentax K-m and Panasonic Lumix DMC-LX1 work with 72 dpi. My son has a Canon EOS 7D and that also works at 72 dpi.

Is there a reason for this difference as I was expecting similar file data between the DSLR’s.

[Adam]

DPI has nothing to do with files that come out of the camera- it's a metric used to determine the resolution of a printed image.

[chalion]

woodyneale, if you were to change the DPI to 72 in the Image properties (Photo Shop), the overall size displayed will multiply by 3 times.

The images from a K5 would let you crop then resize the image and not lose too much of the image quality, compared to images set at 72DPI.

[Wheatfield]

If you are importing via camera raw, there is a dialogue at the bottom of the window that tells you what the import criteria is. If you double click on that, you can change the DPI, among other things.
As Adam said though, this is not changing the actual image size, merely the DPI tag, which is actually an output criteria.

[Marc Sabatella]

I'm always trying to find good analogies to help clarify this topic, since it is such an endless source of confusion for a lot of people. Let's try this:

The number recorded in the Exif as the resolution in dpi is like taking an empty cup and writing on the side of it, "this cup is capable of being filled 50% full". Well, yes, it is, but it is also capable of being filled 10% full, 80% full, 100% full, or being left completely empty. If you want to know how full it actually is at any given moment, this is not determined by what someone wrote on the side of the cup, but by simply measuring the contents and dividing by the total capacity. The constant here is the total capacity of the cup, the amount of liquid you put into it then determines what percent full it is, using simple division. The number written on the outside of the cup does *not* come into play. It makes no sense to call it a "50% cup" because depening on how much liquid you pour in, it might at any moment be 50% full, 10% full, 83% full, or whatever. But it *does* make sense to label it a 16 ounce cup (total capacity), because that at least is constant.

Similarly, the number 72 in the Exif is saying, "this image is capable of being printed at 72dpi". OK, but it's also capable of being printed at 300dpi, or 180dpi, or 100dpi, or 1200dpi, or 1dpi. If you want to know the actual resolution of any given print (or screen display), this is not determined by the number that was put in the Exif, but by simply dividing the number of pixels in the images by the print size. The constant here is the number of pixels in the image; the size at which you print it then determines the actual resolution, using simple division. The number in the Exif does *not* come into play. It makes no sense to call it it a 72dpi image because depending on what size you print, it might be 72dpi, 300dpi, 517dpi, or whatever. But it does make sense to call it a 12MP image, because that at least is constant.

[Schraubstock]

I'm always trying to find good analogies to help clarify this topic, since it is such an endless source of confusion for a lot of people. Let's try this:

The number recorded in the Exif as the resolution in dpi is like taking an empty cup and writing on the side of it, "this cup is capable of being filled 50% full". Well, yes, it is, but it is also capable of being filled 10% full, 80% full, 100% full, or being left completely empty. If you want to know how full it actually is at any given moment, this is not determined by what someone wrote on the side of the cup, but by simply measuring the contents and dividing by the total capacity. The constant here is the total capacity of the cup, the amount of liquid you put into it then determines what percent full it is, using simple division. The number written on the outside of the cup does *not* come into play. It makes no sense to call it a "50% cup" because depening on how much liquid you pour in, it might at any moment be 50% full, 10% full, 83% full, or whatever. But it *does* make sense to label it a 16 ounce cup (total capacity), because that at least is constant.

Similarly, the number 72 in the Exif is saying, "this image is capable of being printed at 72dpi". OK, but it's also capable of being printed at 300dpi, or 180dpi, or 100dpi, or 1200dpi, or 1dpi. If you want to know the actual resolution of any given print (or screen display), this is not determined by the number that was put in the Exif, but by simply dividing the number of pixels in the images by the print size. The constant here is the number of pixels in the image; the size at which you print it then determines the actual resolution, using simple division. The number in the Exif does *not* come into play. It makes no sense to call it it a 72dpi image because depending on what size you print, it might be 72dpi, 300dpi, 517dpi, or whatever. But it does make sense to call it a 12MP image, because that at least is constant.

Hi
Well explained. A 12Mp image is still a 12MP image at any DPI or more correctly PPI.
DPI (Dots Per Inch) is usually applicable to printing output meaning that the given number is the max resolution the printer is capable of. (See printer specs)
PPI (Pixels Per Inch) is usually applicable to the image itself.

And this is what you should always remember: The PPI/DPI count is of meaning only if you print the image, or to say it the other way around until you print an image the PPI/DPI number is of no consequence.

Therefore what your cam records 72 or 300 is, as they say in the classics, "6 of one or half a dozen of the other".

Greetings

[snostorm]

Hi woodyneale,

If you want a historical explanation for the 72 DPI convention, then read this article, especially the section @ 1/2 way down on the page under "Computer monitor DPI standards".

Dots per inch - Wikipedia, the free encyclopedia

Suffice it to say that the DPI standard established in these early computers is both obsolete and irrelevant with the resolutions that are common in the most basic displays and printers now, so it's really only a matter of historical interest at this point, and can (and should) be disregarded.

Scott

[noblepa]

I find it best to simply ignore the PPI value from my camera completely. IMHO, it is an utterly meaningless number. The only time it even approaches having a meaning is during printing. Even then, I have never, ever found a reason to concern myself with the ppi number for a particular image, no matter what size print I am making.

... but then, maybe I'm missing something...

[RioRico]

There are just two instances when DPI matters: scanning, and printing. The EXIF metadata has a DPI field that's used for printing and scanning. Any numbers plugged in there by cameras are just filling up the space. One design team may use 72dpi since it's the Web standard; another may use 300dpi because it's *a* printing standard; another may use 120dpi because it's a lucky number; whatever. Try 666dpi and see if The Beast appears, eh? But I digress.

When editing a picture, I worry about total pixels, not DPI. When printing a picture, I'll think about DPI depending on what distance I expect I expect the image to be viewed, but that's all. Almost anything printed|shown small enough, or from far enough away, will look good.

In other words: Don't sweat it.

[geoffpem]

i think dpi subject is ie:- 72 dpi per image is default on some cameras and is used univesaly for viewing and 200, 300, whatever dpi converted image size is used for printing depending on size of print and quality of print

[cats_five]

i think dpi subject is ie:- 72 dpi per image is default on some cameras and is used univesaly for viewing and 200, 300, whatever dpi converted image size is used for printing depending on size of print and quality of print

No, when you view an image in a web browser 1 pixel in the image gets displayed in 1 pixel in your browser, assuming you haven't zoomed the browser view in or out and the web designer has either put the correct pixel dimensions in or not put them in at all.

The 'universally for viewing' myth comes about as it used to be believed that all monitors had 72 pixels (dots) per inch. Also the belief that DPI matter for web site - had a big argument with a chap at the camera club about that, withdrew as he looked about to have a heart attack.

When you view an image in an editor the relationship between pixels in the image and on the screen depends on how you choose to view the image.

The DPI doesn't necessarily get used for printing either - you can tell many print drivers to stretch or shrink an image to fit the paper size.

I have a feeling Word does use it to initially size an image in a document, but it's easy to resize by dragging the corners.

If you Google 'DPI Myth' you will find plenty of articles about this that explain (in slightly different ways) why DPI is meaningless except (maybe) when you print. The only place I know of where it really, really matters is scanning when it is vital to use the right figure. What that is depends on the scanner, what you are scanning and what the scan is going to be used for. I use 4,000 dpi when scanning 35mm negatives or slides with my Nikon scanner.

[Marc Sabatella]

DPI is meaningful when you print, but generally, only the *real* DPI (number of actaul dots/pixels divided by number of actual inches, just like DPI literally means) matters, not the mostly meaningless "thisimage is capable of being printed at" DPI figure stored in the Exif. That is, when printi g, you ideall want 300 or more *actual* dots per *actual* inch. The number in the Exif. Ight be 300, 72, 667, 42, or 3, and it won't matter a lick. Just the actualnumber of dots divided by the actual number of images.

[RioRico]

Easy overview: Everything You Want to Know About Megapixels, Megabytes, and DPI

DPI Measures the Density of the Pixels

Finally, there's the thing that Sue actually asked about: dots per inch, or dpi. Dpi has no inherent value of its own when describing the size of a photo. The only thing that dpi does is help you to understand how large a photo can be printed or displayed, and--here's the key thing--it refers to the display medium, not to the photo itself.

What am I talking about? Suppose you take the 10-megapixel photo I mentioned at the beginning of this article and display it on a computer screen. Computer screens tend to have a resolution of around 72 dpi, which means the screen has about 72 pixels per linear inch. If you show the photo at its "full size" (so every pixel in the photo uses a pixel on the screen) then you'd divide 3872 by 72 and find that the photo would be about 53 inches across. But send that same photo to a 300-dpi inkjet printer, and you would expect that you could make a high-quality print that's about 12 inches across (3872/300).

... Dots per inch (dpi) is just plain meaningless most of the time. You can use this number, along with the photo's resolution, to find how large it can be printed or displayed on a particular device. But to be useful, you need to know the dpi of the device in question--for example, most inkjet printers give good results at no more than about 300 dpi.

Here's the annoying thing, and what is no doubt tripping up Sue: A dpi value is usually stored as metadata with your digital photo. That's really misleading, especially when a program resets the dpi value for some mysterious reason. As a general rule, you should ignore the dpi value and pay attention to the photo's resolution in pixels. That's the real indication of a photo's size.

[woodyneale]

Thanks for all your comments and links guys, it’s much appreciated. I’ve had a far broader response than expected to aid my understanding.

For me this begs one question “why does a dpi factor even get included in exif data?” Dpi is, as others have said, an output measurement for printing devices and we know from the exif data how many pixels of width and height an image comprises. I gather the use 72 dpi was a Macintosh convention, connected to early computer screens when only text and numbers were displayed. The value 72 originates from typography where picas and points measured font size and line spacing, i.e. 12 points equalling a pica and 6 picas one inch: as stated “all history now”.

One thing that I think would more useful is knowing how many linear pixels a camera sensor has per inch or mm along its x/y axis’s as it could give a more realistic comparison of the capturing image resolution between camera models. After all, what is the actual size of a pixel on any given sensor?

[RioRico]

For me this begs one question “why does a dpi factor even get included in exif data?”

Tradition.

One thing that I think would more useful is knowing how many linear pixels a camera sensor has per inch or mm along its x/y axis’s as it could give a more realistic comparison of the capturing image resolution between camera models. After all, what is the actual size of a pixel on any given sensor?

Bingo! I memorize the (approximate) pixel dimensions of images from my most-used digicams, and their pixel density in megapickles per square centimeters (mp/cm2).

Compared to my K20D (about 4 mp/cm2), an ancient 1.1mpx (1216x912px) cam is only slightly denser at about 6; newer 5mpx cams are rather denser at about 14; 7.1mpx cams are in the 20-25 range; and modern P&S's (which I don't have) are around 40-50 mp/cm2.

Lower density within any generation of sensors means less noise. Noise certainly limits effective resolution and enlargement. Big fat juicy pixels on my K20D (4672x3104px, 4.13 mp/cm2) and the ancient 1.1mpx Sony DSC-P20 (1216x912px, 6.23 mp/cm2) both allow more 'photographic' enlargement than do the more squashed & constipated pixels from 5-10-15mpx P&S's.

But a basic truth remains: Almost anything looks good if shown small enough or distant enough, or if it's moving. I blow-up those 1216x912px images to poster size, and seen from poster distances, they're fine. Just don't peer too closely.