Photo Printing

One of the more frustrating and sometimes disappointing tasks that the digital photographer undertakes is that of printing images that they have taken. And it usually occurs when the user wants a result they just can't seem to get.

They can't get the right size or the right quality, and the colours look different to those on screen. Things just don't turn out quite as they expected or as they want. And each time they try, and fail, they know it's using ink and paper that is quite expensive, which gets annoying. 

So we hope the information here will help all of you that have found yourselves in this situation. And perhaps the knowledge that you are not alone in this, and that many have been down this route before you, will be of some comfort.

It might also be of some benefit if you also read this page on  Ink-jet Printing  if you have not done so already.

Printing Images  - Resolution

Deciding at what resolution and size to print an image is often many digital camera users biggest problem. An understanding of what resolution is, and its relationship with image size is needed in order to be able to fully appreciate what the effects are of printing at differing resolutions. There are in fact two types of resolution involved. The resolution of the image, and that of the printer used, and they are not one and the same thing. Which is where confusion often arises because no one explains properly or clearly to the user that there is a difference between them. Because the resolution of an image is allied to, but not tied to, the printers resolution.  Printer resolution is a means of expressing how coarse or fine the image detail reproduction will be. Image resolution indicates how much detail a print may contain. 

Printer resolution

Ink-jet makers have tried to help ease the printing process by writing most printer driver software so that all the user has to do is input the type of paper being used and the correct resolution will be set for the printer to use. However this is to ensure that the right amount of ink is laid on the paper, for the type being used. Different paper surfaces need varying amounts of ink for best results. It's no good printing at 720dpi on ink-jet paper for example. The paper just gets over-saturated with ink. Nor is it worthwhile printing at 360dpi on photo paper. Not enough ink is laid down and the image looks bland and colourless and lacking detail. These settings are called paper profiles. However the resolution used is not the only way that the amount of ink laid down is controlled. It's a combination of the resolution and the paper type. You have probably seen of late, newer ink-jets that have ever higher top resolutions, 1440dpi or 2880dpi and greater. This does not mean that with each increase twice as much ink is being put on the paper. Rather its the case that finer and finer dots of ink are being used, and being laid in greater numbers. To give a further example we often print on glossy and semi-gloss paper using the glossy film setting. The resolution is the same, the number of dots, but the printers paper profile has been set to only lay down roughly half the volume of ink as glossy film is less absorbent. This results in a print on gloss or semi-gloss paper that is less vibrant and with less contrast, but which is often more accurate with images from digital cameras. And for images that may only be used for reference, and spend most of their lives stored away in folders, this is a useful way of saving ink. Although the practice of working from screen images is widely used these days, it is often handy to be able to show the actual size that an image can or will be printed at, to give those viewing them a better understanding of the various sizes involved, and the choices available.

An important point to note here is that these printer resolution figures do not remain constant as they increase. And so are a further cause of confusion. As with scanners the top resolutions only apply to one direction, across the page, not down it. Whatever type size or make of printer you have it always prints sideways across the shortest edge. Take the model we use. It has settings of 360dpi, 720dpi, and 1440dpi. At 360dpi, 360 dots per inch are laid across the page at a rate of 360 lines per inch down . At 720dpi the same occurs, 720 dots per inch are laid across the page at a rate of 720 lines per inch down. But at 1440, whilst 1440 dots per inch are laid across the page, this is still at the rate of 720 lines per inch down. There are some printers that now lay down as many as 5760 dpi, but you will see that this is at a line resolution of around 1200 -1400. The figures are usually quoted as 5700 x 1200dpi or something similar. Because there are different figures involved across the page and down, both are given.

Here is a table giving some common printer resolutions.

When a printer is set to use a certain resolution this is fixed whatever the size of the paper used, it doesn't vary. Because the setting is in dpi - dots per inch, it doesn't matter if the paper is 6"x4", 8"x10", or any size you care to think of. We will return to these figures. As a general rule the first three steps do result in more ink being laid down, increasing volumes, depending on the paper type used, but as there is a limit to how much ink can be laid down on any particular type of paper, in most respects the very high figures do not result in more ink being laid down, but as we have said previously, the same volume in finer dots.

The paper profiles that the printer makers provide set the maximum ink levels for each type of paper. They can be overridden in many cases in the printer's driver options, some printers having many options, but this usually only ends up in grief if you aren't aware of what you are doing. And is usually only needed when you use a type or make of paper for which their is no suitable profile. There is of course a difference between the paper profile, which is concerned with the amount of ink put down, the printer resolution, which is concerned with the number of ink dots used, and the actual colours laid down, the tones that result. This is something different altogether, and we will deal with this last part individually further on.

Image resolution

As with printer resolution, image resolution is not a fixed amount, but varies depending on the size of the image file, the number of pixels in it, and the size it is printed at. So basically the larger you print an image file, the lower the image resolution becomes. Although image files are normally considered in terms of dpi, dots per inch, in this case we really should be talking in ppi, pixels per inch, because that is what image files are composed of indirectly, the information that each pixel on a sensor provides.

Here is the table from our  Sensor image resolutions  page which shows the ppi for different print sizes for all the major sensor pixel counts. As you can see, taking 300ppi as the benchmark any sensor past 2mp can produce images that printed at 6"x4" could be classed as high resolution, whilst at A4 anything below 11mp is low.  However, as we have stated previously, most ink-jets can produce really good quality output with image resolutions from 180ppi upwards. This suggests 3mp images and above are capable of A4 reproduction. In practice this has generally proved to be the case, although there are exceptions. A poor quality image remains so, whatever size it's reproduced at.

Those of you with a keen eye might spot that the 10mp sensor size produces just over 300ppi at the A4 size. Many recently introduced digital cameras are now at this resolution. This is what most camera makers have been aiming for, a resolution that is acceptable to magazines and print houses. They mostly use a resolution of 300dpi, which is fixed, they can't work with any other amount, because of the way commercial printing presses work, which is different to that of ink-jets, and they need a ratio of one pixel per ink drop for decent reproduction of images. It is in many respects considerably more than most digital camera users either need or want, but scant attention has been paid to this. Please see  Ink-jet Printing  for more details.

Mostly, the size an image is reproduced at, and the quality that results, depends to a large extent on the distance at which it is viewed. 3mp images printed at A4 size at 720dpi are quite acceptable when viewed at distances of 18" to 24" and greater. Examined any closer and yes they do lack detail sometimes, but this is often as much to do with the printer resolution as the image file size. We have used this as an example. Any print viewed at too close a distance will exhibit faults of one kind or another. 

Resolving the image resolution v printer resolution conundrum

As we hope you can now understand although printer resolution is separate from image resolution both can affect the print quality in their own way. So you can have a high resolution image printed at a small size with a low printer resolution at the one extreme, and a low resolution image at a large size printed at a high printer resolution at the other. Neither will produce satisfactory results as neither image will look good, but for different reasons. The high resolution image will look low resolution, because too few drops of ink have been used to represent each pixel, so there is a lack of detail, and the low resolution will look exactly the same for the opposite reason. Too many drops have been used to represent each pixel and again there is a lack of detail. So it's a relationship between paper type, image file size, reproduction size, and printer resolution.

Here's another table by way of illustration. This uses sensor counts and print size, and different printer resolution settings, to show either pixels per ink drop, or ink drops per pixel. We've chosen to use three sensor pixel counts. 3mp, 6mp and 10mp, and three printer resolutions, 360dpi, 720dpi,and 1440dpi, and two print sizes, 6"x4" and A4.

When you look at this table take particular note of the figures in the last two boxes, because that is what we are mainly concerned with. Any figures that appear in the first box are a particular worry, as are low or very high figures in the second. Both are indicative of a loss of image quality in the print produced for one reason or another which we discuss after the table.

The first point to note is that you don't want a situation where one ink dot is used to represent more than one pixel if you can avoid it. This just leads to a big loss in image quality and detail. Only a part of the information in the file is being depicted. If you look at the first set of figures in each division, for the 6"x4" print at 360dpi on ink-jet paper, the output from a 3mp sensor is just about at this level at 1 dpp. But the 6mp and 10mp sensors output is way above this. In the case of the 10mp one dot covers three pixels of information, 3 ppd. So the final output, the image quality, is no better than that from a much smaller file size, because at least two thirds of the files information is being lost even at this rate. The 6mp size is not quite so bad, only half it's information is lost at 2ppd. At the A4/360dpi size the 3mp has four dpp, an average amount, the 6mp, 2dpp, and the 10mp just over 1dpp. So it is only at this size that all the 10mp sensors pixel information is used. But even 1dpp is a poor representation of an image file.

Now lets look at the figures for the next print resolution, 720dpi. At 6"x4" the 3mp has 4dpp, the 6mp 2dpp, and the 10mp still has just 1dpp. At A4 the 3mp has 16dpp, the 6mp 8dpp, and the 10mp 5dpp, just.

And for the last resolution, 1440dpi. At 6"x4" the 3mp has 8dpp, the 6mp 4dpp, and the 10mp 2.5dpp. At A4 the 3mp has 32dpp, the 6mp 16dpp, and the 10mp reaches 10dpp, again just.

Conclusions

So what are the conclusions that can be drawn from this table? Well there are several. The first thing to note is that the best dpp ratio is around the 7-8dpp mark. At this ratio there is enough dots per pixel to provide good detail yet the area covered is not so big that large areas containing a single tone exist and the image appears to lack detail. As the figure reduces below this detail is lost, there aren't enough dots to represent all the fine detail that might exist, and as it rises the reverse occurs, there are too many dots covering each pixel, the image becomes over stretched, with too little information eventually resulting in the extreme case, of pixelization of the image. So a figure of 1dpp is the absolute minimum below which image detail is lost by the bucket load. And dpp's much over 16 just waste ink since they add no extra detail. This ratio is approaching the upper limit of usefulness.

As a result we can say that small files sizes are all that is needed with 360dpi ink-jet paper settings. Few will probably want to print their images at this setting on photo paper, which you can do, but some types of special ink-jet paper and ink-jet quality card besides ordinary ink-jet paper use this setting. So if your producing a calendar this information is of use. We can also say that 3mp and 6mp images printed on photo papers are best produced at a printer resolution of 720dpi. Going any higher doesn't really bring any noticeable benefit. No more detail is produced and the only increase is in the amount of ink used and a very slight increase in colour saturation. But the reverse is true with the 10mp image. The highest setting, 1440dpi, needs to be used to produce an image if all the detail that exists in the image file is to be depicted and produce a print that reveals more detail than a print from a lower pixel count can provide. Print at 720 and the dpp is not very high, even at the A4 size. Indeed at the 6"x4" print size the output from a 10mp sensor is totally and utterly wasted, whatever the printer resolution used.

And the bottom line.  The bigger a file size is, the more information it contains, the larger the printed size needs to be, and the greater the printer resolution, to benefit from the information contained in it. Print a 10mp image at 6"x4" and the detail present will be no greater than that from a 3mp image. 

Paper Sizes

Perhaps one of the most frustrating aspect of printing a photo is finding the right sized paper to print it on.  Whenever you go to print something, you usually find the image won't fit on the paper. It's either too long or too wide, leaving either uneven sized borders, or a border on only the sides or top and bottom. It's not something that's peculiar to digital photography. Anyone who has ever used paper will understand. There's Imperial sizes, Metric sizes, Photo sizes etc. What seems even worse though, is that even the photo sizes aren't standard. 

The reason, although not a solution, is because there is no standard camera sensor/film format ratio, and matching paper standard. If you've looked at our  Sensors  pages you'll know that there are two different digital sensor ratio's, 4x3 and 3x2. Film cameras add even more. Most photo paper sizes to date have been imperial. The most common being  6"x 4", 7"x 5", 8"x 6", 10"x 8", 12"x10" and 16"x12". Very few match the ratio's of the sensor/film ratios and of course there's no square format paper at all - used by one of the most iconic cameras of all time, the medium format Hasselblad - which produces 6cm x 6cm images on 120 film. To produce papers in all the sizes that the different formats use would just not be economic. The answer, in one form or another, is cropping.

Cropping

Cropping an image to fit a certain size, chopping a bit off, is an everyday occurrence that photo finishers and others use to produce printed images. Even if you take film or digital images to a high street lab for the common 6 x 4's it's almost certain that some amount of cropping will occur to ensure edge to edge prints result. The proverbial 10"x 8" requires rather more, whatever format used, something most camera users remain blissfully unaware of until they discover someone or something at the edge of a shot is missing in an enlargement, having been chopped/cropped so the image fits the paper.

Although ink-jet printers are made to use standard metric paper sizes - A4, A3, A2 etc - and most ink-jet paper is based on these sizes, this is of little help to digital camera users. These paper sizes suit no sensor/film format, so whatever type of camera you have, some form of cropping is usually required when you print full sized images on the papers. You either crop the image to suit the paper, if the paper size cannot be altered. Or you crop the paper to suit the image format after printing.

Interpolation and Resampling

Most software applications have a default dpi that they work to. The default for word processing and most others is 72dpi. Some use 96dpi and this is also often the monitor display setting size. When you open an image file on your computer it may be displayed at one of a number of settings. Photoshop now uses 300dpi as the default. We prefer 72dpi and change it. The size an image or document is displayed at on screen however is not the size it can or will be printed at. The user must set that themselves. 

With word documents the overall size, the resolution it will be printed at, is fixed in the application. As we have said this is 72dpi. And a document is a whole sheet, even if it only has one letter or word on it. You also have to choose the size it is, A4/5/6 etc, which way around it is, landscape or portrait, the font size, the font type, margins etc before you start.

With digital images it's different. Almost nothing is fixed. It can all be changed, even to the size of the file itself. Of course this is one of the cornerstones of digital imaging. That almost endless alterations can be made. But there is a downside. The sheer number of options can bewilder and confuse the less experienced. And this is particularly true when it comes to printing images.

There are some imaging applications that seem a very good option in this situation in that you can open an image, set the finished size required, and the application will automatically arrange it to fit on a certain sized sheet of paper, and set the resolution. You should be wary of these. They often use re-sampling, which can destroy your image file if you are not careful, besides not giving you the quality of image reproduction you may have been expecting.

There are two ways of re-sizing an image. One way is to alter the size, larger or smaller, whilst keeping the file size constant. Using this method results in the resolution altering, depending on the size the image is reproduced at. In other words if you have an image that is 6"x 4" at 600ppi and you alter the print size to 9" x 6", the resolution will then be 200ppi. Conversely, if you have an image at a print size of  9"x 6" with a resolution of 300ppi ,and you re-size it to 6"x 4", the resolution will become 450ppi. 

The other way is to alter the size whilst keeping the resolution constant. This may seem at first a better way of doing things since you can set a minimum resolution that's in keeping with the quality you wish. But it's not all it seems. Because this involves re-sampling - adding or removing pixels - which reduces image quality. And tonal range and image sharpness suffer as a consequence.

Take the previous examples. The 6"x 4" print at 600ppi increased to 9"x 6" by re-sampling would have it's image interpolated upwards - adding extra pixels to make up the numbers. So then it would be 9"x6" at 600ppi. This doesn't add any extra detail, the pixels created being made up by sampling the adjacent pixels and placing the extra ones in-between. There are two basic methods used. Bilinear and Bicubic. Bilinear is quicker because it samples line by line. Bicubic samples area. Of the two Bicubic is the slower, but produces smoother tonal graduations - less jagged edges. If interpolation of an image is used, applying un-sharp mask is required as it makes the image look rather fuzzy. Down-sampling is the opposite, throwing away pixels - image detail - as the image size is reduced. The 9"x 6" at 300dpi, remains 300dpi at 6"x 4", but again image quality suffers.

Perhaps the worst aspect however is that some software applications, like the type mentioned above that do things automatically, re-save the original image files as they re-sample, meaning a high resolution image can be lost, ending up low resolution, if you are not careful. 

There are some situations where re-sampling is needed. Producing a sheet of thumbnail images, where image quality is not paramount, would not be practical using multiple files at full resolution. The final file size could well be many hundred of Mb. In cases such as this, using duplicate image files created using drag and drop or copying ensures the original file is left untouched. Be sure to delete the duplicate files after use to avoid mistakes.

Another use of re-sampling is when image ratio's are altered, or perspective distortions are applied or removed. In these cases it is un-avoidable, but you should be aware that any re-sampling requires the application of un-sharp mask to restore a level of sharpness, which is generally lost.

Multiple Image Printing

There are many occasions, such as making thumbnail sheets, where multiple images are used. If you have many images to print it is often more economic to print them in batches on A4 paper than individually. For example, if you wish to print some images to put into a photo album, say the type that has sleeves to take the standard 6x4, then four 4x3 format images at 5.3"x 4" can be fitted on A4, and three 3x2 format 6"x 4". They can be arranged on the paper in Photoshop using layers before flattening as illustrated below. It is usually as cheap to buy a pack of 20 sheets of A4 photo paper as it is 20 sheets of 6"x 4" photo paper. 

After printing allow to dry fully, and then cut to the individual sizes. We find using a ruler and craft knife on a cutting mat the quickest and easiest way to do it.

If the images are only for reference or will spend most of their lives tucked away unseen, try printing on photo paper using the photo film setting if your printer has one, rather than the photo paper setting. Only half the amount of ink is laid down, making it very economic for large numbers of prints. The colours are not as deep of course, but we find it very useful. Some cameras and printers produce over saturated images anyway so it's a good counter in this case. 

Please read further on for details on how to set up to undertake multiple image printing on a single sheet.

Sizing, Re-sizing, and setting Image Resolution

Okay, now we have gone through all the main points we come to the crux of the matter. Opening an image file and printing it at the size and resolution you want. It might seem as if we have taken a long  while to get here but unless you understand the basic points to be aware of then you won't understand what the implications of the options you have are, and what ones to use to get what you want.

As normal we will use the commands from Photoshop Elements for the examples. Most image editors work in broadly the same way. If the one you currently use doesn't, and doesn't give you a range of options as to size, resolution etc, then it may be the type we mentioned earlier that you must be wary of. That make changes to original files without your knowledge. We cannot recommend such programs at all.

So you open an image file. To be able to print it you have to set the size you want it printed at, and the resolution. If you simply go straight to, file >print then all that will happen is that either a warning box will pop up telling you that your image won't fit the paper size, or when you print the image it will be at a small size in the middle of the sheet. It might even be that the image is the wrong way round on the sheet as well. All this can happen if the default page resolution size of either 72dpi or Photoshop's 300dpi has been set, and if the page orientation has not been matched with the orientation of the image, landscape or portrait.

If you haven't made alterations before in the manner we will describe, or at any time your not sure what you are doing, your trying something new or different perhaps, then a good practice to follow is to make a duplicate image and work with that. It is all too easy when trying to follow descriptions as to how to undertake a task, to get confused, decide to close the image and start again, and just click yes when asked if any changes made should be saved in the confusion. It is an easy way to destroy original image files in terms of size and quality without even trying. And once this happens there is no going back. Software applications can't undo saved file commands once a file has been saved and closed. Many experienced photographers tend to work this way, from duplicate files, when doing anything but very basic tasks. It is a lesson many have learnt through hard experience.

Once you have made a duplicate of the image you want to print, image>duplicate, then close the original so it's out of the way, don't just minimize it. You can end up restoring it and working on it without realizing otherwise. Now go to, image>resize>image size. This dialog box will appear.

In this example all the figures in the boxes refer to a 6mp DSLR image file. This is at a format ratio of 3x2 as you can see from the topmost boxes. The dialog is basically self explanatory, or so it seems on the surface, but it does contain quite a few options that are not at first immediately apparent, or the effects that changing them will bring. Anyway lets just look at this first as it stands. Then we'll make some changes to the options.

You have the actual file size, the pixel dimensions, indicating that this file is 17.3mb when open - it's a Jpeg. Then there is the size in pixels in two boxes. The width and the height. And then two drop down boxes. The choice here is just between pixels and percent. At the end it is indicated that these figures are locked together in ratio. Changing one will automatically change the other. This has been set by ticking the option box at the bottom - Constrain Proportions. The fact that they can be changed has been enabled by ticking the other option box at the bottom - Resample Image - which allows this to occur.

Below the Pixel dimensions are those that refer to the document size. What is meant by this is the size the image will be displayed and printed at, and the resolution used. Again these figures can be changed as both the option boxes are ticked, but only in ratio for the dimensions. The drop down boxes allow for a choice as to the indices used. These figures can be in percent/inches/cm/mm/points/picas/columns via the drop down menu. The resolution figure can be in either pixels per inch, or pixels per cm, again via the drop down menu, and because the Resample Image box is ticked the resolution can be changed to whatever you wish. Alongside this box is another drop down menu that allows resampling to be carried out using, nearest neighbour, bilinear, or bicubic. 

You should only make changes to an image whilst the options boxes are set like this if you are sure you know what you are doing. Any changes you make will result in the image being interpolated in some way, up or down, and the file size changed. For example as it stands this image will be printed at a size of 106.12cm x 70.84cm, (41.8" x 27.8") at a resolution of 72dpi. Change the document size to 12" x 8", and the image is re-sampled down to just 1.43mp from 17.3mp - it states this at the top, with a pixel size of just 864 x 577. And the only way to reverse this destruction of the image file is undo/CTrl+Z, or closing the image file without saving any changes. Re-entering the previous figures in the boxes is no good. It won't undo/reverse the last action. It will restore the image file size, but not the image file quality. Why? Because the image, which is now 864 x 577, will be re-sampled upwards. 

So please beware.

The same situation applies if the Constrain Proportions box is un-ticked as in the sample below. In this state not only can the image file be changed in size but the dimension ratios can also be modified. And changing one, either the width or height in pixels, will automatically change the corresponding values in the Document Size boxes, and visa versa. Changing the resolution will also correspond with a change in the pixel dimensions.

If you must change the file with the options like these, then please work with a copy image and not the original. Then at least no serious long lasting damage will occur.

In this third example the resample box has been un-ticked. As a result many of the options are now greyed out and unavailable. You cannot alter the file size or the ratio dimensions. And if you alter the width, the height, or resolution, all the figures keep in step with each other, they are locked together, as no resampling is undertaken. The file stays untouched. All that is being changed is that the image editor is being told how to interpret the file at the pixel size it is, and display and print it in the resolution that is chosen. The settings that it should use for this particular image file. This is the default state that you should use the image size option in. If you leave it like this Photoshop will open it like this. As is the case with many options/tools, Photoshop remembers the last settings used and will open an option/tool at those. When you use the image size tool remember to always check what the box option settings are before making changes.

Right, lets say you want to print this image out at 6"x4", or 15cmx10cm, it's virtually the same size, but not quite. Both are in a format ratio of 3x2 so they will match that of the image file size. Enter 15.0 into the top box gives the result that appears below.

Printing a 6mp image at this size gives a resolution of 509 ppi as you can see. Now using this technique you can find out what the print resolution would be for any image you wish to print, at any size and resolution. All you have to do is keep altering the figures in the boxes. It's a bit like a ready reckoner. And using this dialog you can set the image to be printed at either a certain resolution or a certain size.

All you have to do now is go to, file> page setup to make sure the page format orientation matches that of the image, portrait or landscape, portrait is the default, and then file>print to bring up your printers driver options dialog. If you have chosen to print the image at or near the maximum size on the paper you are using make sure that the printer has been set to print it in the middle of the paper otherwise one edge may be cut off. Printers are set by default to print from the top edge of a sheet of paper unless instructed otherwise in the print driver settings. This is to do with document printing. A short letter that only takes up a small proportion of a page would look rather strange if it didn't start until half way down.

When you go to print it, file>print, the box - print clipping warning - will probably appear. Ignore it. Do not on any account click, scale media to fit. If you do the print you have scaled will turn out the wrong size. We don't generally use the print preview dialog in Photoshop, except when we want to add a border around an image, or add crop lines, but the one our Epson printer produces. We only click to print when we have checked in this, that no clipping of the actual image files will occur. It indicates the limit of the area the printer will cover.

As a failsafe if you do use the Photoshop print pre-view dialog, set your printer driver to also give a pre-view if it is capable. Then if you find that some setting in the Photoshop settings is wrong, and won't work as you thought, then you can cancel it before printing and start again with different settings. Anything you can do to save wasting ink and paper is an advantage.

Multiple Image printing on a sheet

As we indicated earlier it is handy to print small image sizes in multiple on large sheets and trim them to size afterwards. This is considerably cheaper than buying small sheets of paper. To do this you use layers. We will give two examples covering the two format ratio's. The image file size is not an issue, as you will discover, because you set that to suit the images you are printing. The only proviso is that the images must all be of the same size, the same pixel dimensions, although it doesn't matter whether they are landscape or portrait as you can alter that for the purpose of printing. So you cannot mix say 3mp and 6mp images. Or 6mp and 8mp. Or 4x3 format and 3x2 format. But it is still better than the options you get even in Photoshop, where you can print multiple copies of the same image at different sizes on a sheet, but not different images at the same size on the same sheet.