Printing in relation to DPI & PPI The 300dpi conundrum.
The amount of times I have received support calls or requests for information on the ins and outs of printing resolution. I have read many an article and to be frank none of them put it very simply. Here is my version.
You own a printer and you own a digital camera or have some digital images you wish to print, just how big can you print and still have it look good? Well it amounts to several factors but the best one is whatever looks good to you. This sounds very amateurish and I am sure some of you might wince at my short answer, but really if you are happy with the result then that's an important factor. But if you are not happy then maybe you are viewing it too close, viewing distance is also another factor. Seriously do you look at an A3 picture from 10 inches away or do you stand back and admire the picture for what it is? Well you should view it from a distance that is right for the size of image. If you're too close then you may see the digital artefacts or pixels that the image is made up from, this is most likely to happen on larger photos than 10x8 inches or on posters.
Printers often have an enormous printing resolution of for example 4800x 2400 dpi, dots per inch. You can immediately get confused if you try and equate this with the resolution of you image, for example your cameras resolution is 2816 x 2120 or 6 million pixels, so if you print the image using the printers maximum resolution then surely your image will appear less than an inch big, well yes but that is not what should happen. You cannot literally equate the two devices in this way, I'll explain...
Commercial printing presses used to publish books etc always request images at 300dpi as that is what works for them, actually this is a good optimum printing resolution for us too and this can be used as a base for our printing equation. Simply what ever your image is in size it can be printed at 300 dpi. Well actually its ppi or pixels per inch, as your camera or image is digital and in pixels, not dots like a printer. For example even a 640 x 480 resolution image can be 300 ppi but will only measure just over 2 x 1 inches achieved by simply dividing the dots per inch into the pixel size of the image. When we get larger images such as 2816 x 2120 we can see that at the optimum printing resolution of 300 ppi we spread those pixels over 300 pixels per inch and get a image size on paper of 9.4 x 7 inches.
The simple printing rule.
Scale your image across the page at somewhere between 250 to 300 pixels per inch to give you an optimum print. If you need to go bigger then just scale to 200 pixels per inch or even less. Do a test and see how it looks. If it looks good go for it. When you set your printer going, set to maximum resolution and use the best paper you can afford.
NOTE: Don’t get confused again with the maximum resolution of your printer being 4800 x 2400 dpi as this is just how much ink going down on the page used to print those 300 pixels per inch you scaled across the page earlier.
The continuing conundrum
I often speak to printers who tell me they need a large file size in order to print to a predetermined size. It is cast into the conversation "it must be at least 18mb". When asked why, the response is more often than not because I need it big to work with I know what I can do with a large file. My point I try to make is that I can make you a large file if you wish but do you actually need it. A typical TIFF of a 6 mega pixel image is 17MB. Anyway the print houses want large files but we can make them larger but the dimensions will remain the same so how will they benefit by having a larger file. If we convert the file to 16bit per channel in Photoshop the file size will swell enormously as the colour information contained in the image is now so much greater. I am certain that this will not benefit the printers in anyway as they still have the same 6 million pixel image we started with but a file size up in the 34mb region, then if we convert to CMYK it reaches a massive 46mb!
Here is a break down of how this works:
Photoshop was used to convert to the 16bits per channel. This was completely unnecessary but done to illustrate a point that a large file can be obtained for 1.7mb JPG. Most software’s (in fact I don’t know of any) cant edit a 16 bit per channel image, as there is just too much information to process, it is normally converted to an 8 bit per channel image 8 RED + 8 GREEN + 8 BLUE = 24 bit colour for RGB or 32 bit colour for CMYK which can then be processed by most editing software’s. For an explanation of bit depth see below.
| 6 | MPixel | 2816 | x | 2120 | 1.7 | Mb | JPG | 8 | Bit/channel | RGB |
| 6 | MPixel | 2816 | x | 2120 | 17 | Mb | TIFF | 8 | Bit/channel | RGB |
| 6 | MPixel | 2816 | x | 2120 | 35 | Mb | TIFF | 16 | Bit/channel | RGB |
| 6 | MPixel | 2816 | x | 2120 | 46 | Mb | TIFF | 16 | Bit/channel | CMYK |
Bit-Depth
The signal produced by each pixel on the camera’s sensor is a voltage (analogue),and must be given a discrete value by the Analogue to Digital (AD) converter. How closely the resultant values for each pixel represent the original brightness of each pixel depends on the accuracy of the AD conversion, and the number of steps it has to choose from. In the simplest example, the AD converter would use only two steps, representing black and white. Any pixel with an output of 50% or under would be white, anything higher would be black. Using such a system would produce a recognisable image, though it would have no colour and no grey tones (true ‘black and white’). As each pixel is either one state (black) or the other (white) only 1 bit of data is needed to record it.
A bit is the smallest possible unit of digital data, essentially ‘on’ or ‘off’ (1 or 0). Of course, more than two steps are needed to represent the full range of possible brightness between black and white. Most cameras use 256 levels for each pixel, meaning the final full colour image can contain a maximum of 16.7 million different colours (256 steps each for red, green and blue). This is more than adequate for most normal users, but can result in a lack of subtle tonal detail in unusual scenes (such as those dominated by shadows or highlights). Incidentally, it requires 8 bits of data to record 256 levels, which is why the output from most digital cameras is known as 24-bit (8 bits each for the red, green and blue components).
However, some professional digital cameras can use higher bit depths, with 10 (total bit depth – 30) or 12 bits (total bit depth 36) per colour at their disposal, giving the possibility for up to 68 billion colours per pixel. Using a high bit depth is important because the more colour information you have, the finer the steps between colours in an image. This really comes into its own when working with raw files and with unusual images (such as those mentioned previously with excessive highlight or shadow areas). Here, the finer gradation of tones can be used to tease details out of areas that would – with a lower bit depth – appear as a single, solid colour.
