Cellphone Image Size

[rgans]

<p>I feel a bit stupid but I'm just not clear why this is the case. The pictures my Android takes (at 8MP) are like 45" x 27" and are at 72DPI. Now, I do resize this in PhotoZoom but I'm curious: What does this size mean? And why isn't it, say, 360 and 7"x4" (as an example)?</p>

<p>RON</p>

[rgans]

<p>DUMB and dumber; I asked this in January. SORRY! (But I'm still confused about it).</p>

[mike dixon]

Here's a link to the previous thread with the same question (so people won't waste time providing the same answers): http://www.photo.net/digital-darkroom-forum/00ZoMs

[scott_ferris]

<p>A sensor has pixels, a printer need dots. When you take a picture in pixels most software (for some unfathomable reason) turns those pixels into a relative number of dots per inch, dpi. The dpi figure is only relevant for a printer or a few software programs that have "show as print" options in the zoom dialogue. DPI and PPI (pixels per inch) have become interchangeable/confused over the years, strictly dpi relates only to printing, it is dots of ink on paper.</p>

<p>The total number of dpi x the inches will equal your pixels. The dpi figure for anything but a printer is irrelevant, it is just a tag in the file giving instructions to a printer. The 72dpi figure is an archaic number derived from old CRT monitors and meant that at full size the image displayed on the screen at the same size it would in print, however nowadays most monitors are displaying at over 100 pixels per inch and few people print!</p>

<p>As a hands on example, if your sensor is 4000 x 3000, or 12mp. To display on screen the figure 55.5" x 41.6" @ 72dpi is exactly the same as the figure 16.6" x 12.5" @ 240dpi, the only time it makes a difference is if you send it to a printer and the "scale to fit" option is not selected. Both give you the 4000 x 3000 pixel number of your sensor, and both display exactly the same on a screen. If you printed them the 55.5" print will look horribly pixilated whereas the 16.6" print will look good. Common wisdom says any dpi below 150 when sent to a printer starts to visibly degrade the image.</p>

[rgans]

<p>Scott, I greatly appreciate your answer. I think my question is more: why have 72 pixels per inch and with my 8MP sensor I get 45x27 inches? Why not 360 pixels per inch and smaller dimensions? I understand (only dimly) that the 45x27inch numbers are purely a mathematical artifact (right?), but it still confuses me. Not sure why, but it does. I mean, why do the high capacity sensors on cameras like mine at 8MP, shoot at 72ppi? As opposed to 360ppi? Or does it not make a difference? If it shot at 360ppi and the image were like 7x9, you wouldn't have to worry about resizing. Or, am I missing something?</p>

[scott_ferris]

<p>72 dpi is just a legacy number relating to old printing and crt specs.</p>

<p>Your camera does not shoot at a dpi, its sensor dimensions and numbers are fixed. The relative camera figure is a ppi and is what the sensor is, so a FF camera with 4000 pixels on the long side is 4000/1.42, or 2,817 ppi, that is the native resolution of your sensor. In the example I gave the sensor collect 4000 bits of information along one edge and 3000 bits along the shorter edge, it is just the software after the fact that is suggesting laying those pixels of information at 72 dpi or 360 dpi, it is exactly the same pixels with exactly the same information.</p>

<p>An analogy might be laying out a chessboard with 64 tiles, it doesn't matter if the tile is 1" x 1" or 12" x 12", the chessboard still contains 64 tiles. The only difference is the size of the chessboard, there is not more information in the bigger one, you just used a bigger tile. To transfer that to a digital picture, the 1" tile would have a dpf (dots per foot) of 12, the 12" tile would have a dpf of 1, but they give you the same thing with the same resolution, the same detail etc etc.</p>

<p>The ONLY way the image is tagged with a dpi number is in processing, this can happen in the camera or externally, but it is entirely arbitrary, some software (Photoshop, Lightroom etc) allows you to change that arbitrary number on import to a more suitable arbitrary number. I have my software set up to change images to 240dpi and my preferences set to my actual screen resolution, 108 ppi. This means I when I clich "Print Size" the image on screen is an exact copy of a print.</p>

[richard__]

<p>Yes you are, and as most everyone in the previous thread said, that DPI is meaningless except to a output device, like a printer (and also you obviously) so I'll try to not to restate it here again.</p>

<blockquote>

<p>OK, I get that, sort of. BUT, in PS, my camera's image is 3264x1952 pixels. And it says the resolution is 72 pixels/inch. So, my question is: what is the best way to get the pixels/inch number up to, say 360? If I just change it to 360, the width and height pixel values go up accordingly.</p>

</blockquote>

<p>The problem you are having here is that you are rescaling the image when assigining a different DPI and thus changing the pixel data when all you should do is change the DPI without scaling the data. It's easy enough to do that correctly with Photoshop.</p>

<p>But since you don't, that procedure is leading you into the confusion that you have between the original photo data and your now altered data set of a pixelated image.</p>

<p>So stop doing that and you will then notice the rather meaningless disconnected relationship that DPI has to the actual original image size in pixels.</p>

<p>Changing the DPI should never change the number of pixels in your image, and is generally considered a poor practice when it's done. You always should act to preserve the original pixel data and not degrade it by resizing the number of pixels.</p>

<p>72 dpi was originally a monitor's output display resolution, and thus became a standard number for so long it has clearly outlived it's useful life for all practical purposes except conformity.</p>