Some Digital Printing Basics

This page is a very simple introduction to some digital printing basics, designed to give "newbies" some starter information.

New owners of digital cameras may be confused about the possibilities when it comes to getting prints of their images.

Nearly all photo labs provide printing of digital files as a normal service. You can hand them your camera or just the memory card and ask them to print the lot after you have previewed the images in the camera and deleted the duds. Another way is to use one of the many photo booths appearing at photo print service providers and preview and crop and improve your images using the software in their booth which is usually a computer attached to their lab printing machine. Or you can upload all the images to your home computer and use photo edit software (usually provided with the camera) to crop and improve images and then re-write those images to a CD to hand to the photo lab to print. Another possibility is emailing image files to labs and then having prints mailed back to you, or collected at the shop.

Of course with lab printing you are at the mercy of the machine and the operator and the results may not turn out the way you like them. It may take some time and a few re-tries to get things the way you want them and to establish a working relationship with the lab operator. The advantage of lab prints is that they are usually printed onto normal photo paper so will have the same feel and durability as traditional film prints.

More and more photo booths are appearing at shops, so you do have some more control as to how your print turns out, crop it the way you want, enlarge a bit to get rid of junky edges etc, it's your choice. Often the standalone machines print out a dye-sub 4"x6" print, but many booths are tied to a photo lab machine of the ilk of Fuji Frontier, check carefully what you want and how long you have to wait for the results.

To take lab printing to the highest level of quality involves "profiling" their printing machine and then only provide files to print that effectively incorporate that profile. More on profiling on a future page or meanwhile go to www.drycreekphoto.com for more information on this process..

This option now works so well that the results can usually be made better than a photo lab can provide. Good results can be obtained from newly released 3 or 4 colour ink-jet printers but the prints are not as good as the best 6 colour (or more) printers. Small dye-sublimation printers can also provide excellent results. In the best circumstances, these home printers will provide a wider colour gamut than the lab machines using traditional photo paper.

Ink-jet 3 colour printers (CMY) use Cyan, Magenta and Yellow inks mixed by spraying a fine pattern of very small dots close together to obtain all colours. Better density of blacks and shadows are obtained by using a 4 colour printer (CMYK) where Black is added. Better still are the 6 colour printers that add Light Cyan and Light Magenta to the mix to get a more subtle variation in tones. Lately more elaborate printers have added extra colours like Red and Blue to the mix to achieve a wider colour gamut (colour range) that will better match the wide colour gamut that digital cameras (and scanned film) can provide.

Most ink-jet printers use dye inks that have the widest colour gamut possible but have the disadvantage of shorter print life under some conditions. Some use pigment inks to achieve longer life prints, that can last well past the expected life of conventional wet lab colour prints. Pigment printers do have a bit less colour gamut than dye inks so can't produce the really snappy colours that dye inks can, but the difference is very close now and will probably practically disappear over the next few years.

Many printers are now designed to work directly from a digital camera, either by direct USB connection from camera to printer (PictBridge) or by direct connection from same brand camera to same brand printer via company specific means. Some printers also have memory card readers so memory cards can be plugged in and prints made directly from them.

The best performance comes with using a printer connected to a computer, that way a photo edit program can be used to improve the digital files and to get the print looking "perfect". There's many photo edit programs out there and all will do a good job, it's just that some are more convenient to use than others. Photoshop CS2 (version 9) is the heavy duty industry standard, but it is way too expensive and top heavy with features for the average home user. Photoshop Elements 4 is an excellent low priced alternative that provides all that a home user requires. Paint Shop Pro X has many features and is very easy to use and is also a reasonable price. For a different approach the very capable Picture Window Pro was designed as a photographic digital image file manipulating program from the beginning whereas most of the normally used photo edit programs came from a background of graphics design and not so much from digital camera file manipulation.

These photo edit programs all can print the file, but sometimes a specialist printing program manages to do a better job of printing. You review and enhance the digital image file with the photo edit program and then use the separate printing program to do the final output onto paper, or use the special printing program to produce a new file to be sent to an external photo lab. Qimage is an excellent example of a stand-alone printing program.

One normal problem is that a very small digital file is sometimes required to be used to make a big print. If nothing is done to the file, the output print gets the "jaggies" due to the individual pixels being "enlarged" too much directly onto the print. To avoid this problem interpolation can be performed on the file to increase the number of pixels by inventing new pixels and inserting them between the original ones. Various mathematical treatments can be used to make the best new file possible, according to the type of image and the extra size required. No program can put real detail into the image that wasn't there in the first place, so an over-enlarged image always looks lacking in detail and soft when examined closely.

The best way to guarantee good sharp prints is to try for a certain number of original camera pixels per inch of the final print. Too low a count per inch and the print always looks soft despite careful interpolation. Above a certain number of pixels per inch and there will normally be no visible improvement. The standards appear to be - around 300 original camera pixels per inch the print will look very sharp, above about 400 pixels per inch there will be no visible improvement, below about 200 pixels per inch the print will look soft. Most agree that probably about 240 pixels per inch is the reliable low limit to suit all occasions. Based on that we can make a small table showing how many camera megapixels are required to achieve various print sizes.

Pixels per Inch	4"x 5.3"	5"x 6.7"	6"x 8"	8"x 10.7"
200 least acceptable	0.85 megapixels	1.3 megapixels	1.9 megapixels	3.4 megapixels
240 OK safe minimum	1.2 megapixels	1.9 megapixels	2.8 megapixels	4.9 megapixels
300 good sharp	1.9 megapixels	3 megapixels	4.3 megapixels	7.7 megapixels

The peculiar print sizes are the exact sizes yielded by the 4:3 image aspect ratio of most digital cameras. Be aware that most photo labs will, for example, print the full 4"x6" size and to do so will enlarge and crop the image, thereby losing some pixels and upsetting the above table. There's not a lot of interest in expanding the table past the 8"x10.7" column as that size is the normal limit required. Larger prints should be viewed from larger distances so the resolution required for larger prints actually falls. If the file is sharp at 8"x10.7" size then it's OK at any larger size as long as it is viewed at the correct distance for the size.

If wanting larger prints, the rule is..... if you can print a sharp print at 8"x10" then that print can be made to any size as long as it is viewed at the normal distance of the diagonal of the print or 10", whichever is greater. To me that means 8 megapixels is the minimum camera to own if you regularly want larger prints.

Please realise that only 35mm film and APS film and many digital SLRs have the 'normal' aspect ratio of 3:2, all the rest of digital cameras have the aspect ratio of 4:3 just as in TV sets. Added to that aspect ratio confusion is the fact that some very common print sizes all have different aspect ratios. Examples below:

Print Size	Aspect Ratio	Remarks for Landscape Orientation Prints from Digital 4:3 Ratio Images.
6"x4"	1.5:1	Digital 4:3 ratio images often are evenly cropped off top and bottom to fit the paper. (Suits 3:2 ratio better as in 35mm film or some digital SLRs, but labs usually still crop a little).
7"x5"	1.4:1	Again some cropping top and bottom for digital 4:3 images.
8"x6"	1.33:1	Exact same ratio as 4:3 found in most digital cameras.
10"x8"	1.25:1	Cropping off ends from a digital 4:3 image.
A4 full bleed	1.4:1	Crop top and bottom from 4:3 ratio.

So what happens? If you have a Nikon or Canon 6 megapixel single lens reflex camera (SLR) with the same aspect ratio as 35mm film, then printing to 10"x8" size means that some cropping needs to occur. If you could print the full image then it would be 12"x8" to achieve the full image with 35mm film or with the Nikon or Canon digital SLRs. To fit to the very common 10"x8" size then 2" gets lost. In other words the 6 megapixel camera has become 5 megapixels, losing 16.7%. If using the common digital camera with the 4:3 ratio sensor then the "natural" print size would be 10.67"x8", so only 0.67" gets lost to get the 10"x8" size, that's a loss of 6.3% - that would make a 6 megapixel 3:2 ratio camera into 5.6 megapixels.

Some of the better compact digital cameras do allow a shooting choice via in-camera menus of 4:3 ratio or 3:2 ratio images. By choosing 3:2 ratio you will be shooting with a few less pixels but those pixels would have been cropped off anyway in the printing process. Some cameras now also provide a 16:9 ratio image, so as to match the 16:9 ratio display on large LCD and plasma home TVs.

Even though Australia may be officially a metric measurement nation, we are swamped by the US market and its choice to stick with inch measurements, so 4" print paper is 4" wide print paper, measuring exactly 101.6mm. The print sizes may be quoted as 100x150mm in Australia but believe me, the actual size will be 101.6mm wide and the length can vary from a true 6" (152.4mm) to something less. The lowest I've found is 148mm (about 5.83"). That way the sneaky lab can save a few dollars and get more prints off a roll of paper. One of my own little printers is a Canon CP-200 dye-sub and it produces (after snap-off ends are removed) an exact 100mm x 148mm print (about 3.94" x 5.83"). So be prepared for random cropping of your image if sending off to a lab to be printed. To control things better you have to do your own cropping to suit the required print size better. Remember that at all times (unless borders are requested) the labs will print full bleed (overshoot a little to get no white borders) and that will always lose a thin slice off an image anyway. If you want borders on the print then you will generally get the whole image if you sent the image at the correct aspect ratio. Do a few tests and try to establish a good working relationship with a decent lab, that way you will get more predictable results. If printing at home then you only have yourself to blame for errors.

Prognathous wrote:
> Guy, do you happen to know the recommended DPI values for typical
> viewing distances (per print size)? Is there a formula to calculate
> this?

From my own and others' experiments the "formula" really boils down to....
a). Consider any DPI figure to mean the original camera pixels per inch of print, interpolation does not add anything new, it only makes the result smoother. [In my text it is always DPI in capitals for this exact meaning].

b) The "normal" close viewing distance of a print is 250mm (10") or the diagonal measurement of the print, whichever is greater. That means the 150mm x 200mm print (6"x8") has indeed a 250mm (10") diagonal so could be considered as the largest print that could be viewed at that 250mm (10") distance. But we all know that make an A4 or 200mm x 250mm (8"x10") print and people will still stick their nose against it, so I choose to use the 8"x10" size, or more exactly 8"x10.67" (4:3 aspect ratio) size viewed at 250mm (10") as the base for printing standards, even though that print size should be viewed at 339mm (13.3").

c) At that 250mm (10") viewing distance we need the following DPI.....
300 DPI or above is a very sharp print. Fuji Frontier and some other photo lab machines lay down the data at 300 DPI so it's a valid way to think.
240 to 300 DPI is very good, and is quite acceptable 99% of the time.
200 to 240 DPI is good and will work OK if careful with sharpening.
180 to 200 DPI is borderline quality it will be acceptable if desperate for a result, but will look soft if viewed a bit too closely.
Around 150 DPI and below is definitely a soft looking print, despite any careful interpolation via Qimage and other programs.

d) In the real world with the Ricoh R4 that has 2816 x 2112 pixels the net result for printing various sizes would be (common USA sizes as also seen in Australia) and guessing at the way they would be cropped to fit the format for a borderless print (using Picasa for calculations).....
10 x 15 cm (4"x6") 476 DPI
13 x 18 cm (5"x7") 397 DPI
20 x 25 cm (8"x10") 268 DPI
Full A4 page with borderless 240 DPI
The Canon printer I have can't be set for 6"x8" borderless size so I cannot give a result for that.

e) So going back to the fact that we can print a sharp 8"x10" (at 268 DPI) then that file can be printed to any size as long as it is viewed properly.

f) If you want large prints that you can examine closely for detail, then always think in terms of aiming for 300 DPI. Of course that makes a 406mm x 508mm (16"x20") print come in at 4800 x 6000 camera pixels which equals 28.8 megapixels. Of course you can now buy over 30 megapixel backs for medium format cameras, or you can stitch a dozen R4 shots together to get there.

g) Following the above reasoning the ideal camera is 8 megapixels to get a 300 DPI 8"x10" print and that's why I waited for the 8 megapixel Olympus E-300 as my "serious" camera, but the R3 (and R4) give results that are so good (and convenient) that the Olympus is not worth carrying sometimes.

Regards.......... Guy

Later... point d) mentions the result DPI seen for selected print conditions. I used the freebie Picasa from www.picasa.com to see that. In Picasa, select an image by double click on its thumbnail, do any edits required, then click the Print button at the bottom of the window. Select the printer if you have more than one connected and click Printer Setup to select paper type, paper size, borderless or not and any other usual printer property settings you wish to use. Then select the print size you want, the print size can match the paper size in the printer or you could print a small print on part of a larger piece of paper and cut it up later. After the print size is selected you then can click the Review button to see the DPI of the print, and a comment as to likely quality. Picasa does automatically internally interpolate any file to suit the printer being used. Canon uses 600 dpi internally, and Epson uses 720 dpi internally for photo quality. The only other program that I am aware of that does automatic interpolation is Qimage from www.ddisoftware.com which can also provide the resulting DPI for the selected print size.