Digital Photo Printing Guide

by W.A. Steer PhD
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Following the rapid take-up of digital cameras and digital photography, consumer in-store and online digital photo printing services have become widely available. This page will explain how to get the best out of these services, and how to avoid some common problems.

Introduction

High-street photographic shops, and on-line retailers now offer digital photo-printing services. They take your digital images in any common format (JPG, BMP, TIF etc) on CD or various memory-cards, or web-uploads, then print them onto traditional photographic paper in the familiar 6×4", 7×5" etc. sizes. Compared to home ink-jet prints, prints made on true photographic paper tend to look much 'richer', and are vastly more durable with regard to handling by sweaty fingers, humid environments, and fading in daylight. Although argueably more convenient, once you take into account the cost of speciality papers and inks, home inkjet printing usually works out much more expensive.

Photographic prints from digital files

Industrial digital photographic printing equipment, which you will find in specialist photographic labs and behind the counter in high-street stores, are designed to produce thousands of high quality prints per hour at relatively low cost. The machines use conventional colour photographic printing paper, but instead of exposing it through a negative, they use digital exposure systems. The Fuji Frontier uses a scanning laser system, as does the Noritsu, while the Kodak/Kis machines use an LCD-based exposure system somewhat akin to those used in digital office projectors. Either way, after exposure the paper is processed in the traditional way using wet chemicals to develop and fix the image, then washed and dried. Incidentally, prints can also be made from old-style film-negatives (and even slides) using an integrated high-speed negative scanner.

Fuji Frontier 355 Kodak/Kis DKS 1550 Noritsu QSS3302

Examples of industrial digital photographic printing equipment: digital exposure techniques are used to produce "traditional" prints on photographic paper (wet chemistry)

Be aware that as well as true photographic prints, many shops and malls feature stand-alone kiosks offering while-you-wait prints which you collect from a slot in the front of the kiosk. There is often no minimum-order, but these machines use a thermal printer technology which is inferior to proper photographic prints, and usually works out more expensive.

Stand-alone kiosks with integrated thermal printer

Print size and cropping

It may seem trivial, but people often have problems with heads or feet being cut off on the print, even though they are present in the digital image provided. This is frequently caused by the aspect ratio of the print being different from that of the image file. While 35mm film has a 3:2 aspect ratio which perfectly matches a 6×4" print, compact digital cameras commonly take 4:3 ratio pictures which are much squarer.

4:3 ratio digital image 6×4" print crop 7×5" print crop

Cropping caused by print aspect ratio (shape) mis-match to original image

By default most consumer printing services enlarge the image so that the paper is filled. If cropping is a regular problem then the easiest solution is to choose a print-size which better matches the shape of the pictures your camera takes (for 4:3 ratio images, 7×5" prints are a reasonable match). Otherwise you can usually ask for pictures to be printed with borders (choose black or white bars at top/bottom or left/right) so you see the whole picture. Finally of course you can crop the image yourself, either in the image file itself before you take it to the shop or upload, or using the shop/website do-it-yourself console.

Finally, even if you do supply an image in exactly the correct ratio (eg 3000×2000 pixels for a 6×4" print), the resulting print typically still does not show the extreme edges of your digital file. This is because the consumer has been conditioned to expect prints which "bleed" off the very edge of the paper. To allow some mechanical alignment tolerance and avoid thin black or white bands at the edge of the print, the image is actually scaled to between 1% and 5% larger than the physical paper for printing (meaning there's typically around 1/8th inch or 3mm of image missing off the edges of the print).

Image resolution - how many pixels do I need?

How many pixels do I need? How many is optimum?

Simple answer

Unless you have a good reason not to, I strongly recommend simply taking your original digital images to the printers at their original resolution. Crop them perhaps, but do not waste your time rescaling them. You've nothing to gain and lots to lose.

More comprehensive answer

Fuji Frontier (and Noritsu) printers print at 300dpi resolution for all paper sizes. To get the number of printer-pixels horizontally and vertically you multiply the print size, in inches, by 300. So a 6×4" print consists of 1800×1200 pixels.

Other manufacturers' printers may use a different resolution, or (as in the Kodak/PhotoMe DKS series) use different resolutions depending on the print size.

If you're critical, for the sharpest possible prints, realistically* you should aim for source images with at least 50% more pixels horizontally and vertically than the calculated number of print-pixels. However, unless you have a very, very good reason, do not 'resize' images before taking them for printing. Every resizing of a pixel-based image degrades its quality (some software is worse than others) and you're best off allowing the printer to do the one-and-only scaling between your original and the printed paper, rather than adding additional scaling steps.

The only exception to my no-resizing rule would be if your original pictures are so huge they're really stretching your storage space, and have several times more pixel-count than you need. In that case, downsize them to about 50% bigger than the printer resolution (i.e. number of pixels in each dimension about 450× to 500× the print-size in inches - the exact amount is not critical). It is rarely worth upscaling an image for making a big print - let the lab do it.

* In consumer/high-street printing services, because of the automatic scaling to fit the digital image to the paper format, and the 1%-5% enlargement to bleed off the edge, you will not (in practice) be able to match your image perfectly to the printer resolution. Believe me, I've tried! If you try, your image will invariably get scaled by just a few pixels, with a patchy loss of sharpness equivalent to halving the image-resolution. It's not worth the trouble!

The effective resolution of the human eye, for a person with normal ('20/20') vision, is equivalent to approximately 300dpi at a "reading" distance of about 14 inches (35cm). Since the visual acuity (resolving power) of the eye is defined by the anglular size of detail projected on the retina, the "equivalent dpi" resolution decreases in inverse proportion to the distance of the object. Double the viewing distance and only half the dpi print-resolution is required. Ink-jet printers usually boast thousands of dpi resolution (3200dpi / 4800dpi etc), but need to use many pixels which are individually either 'on' or 'off' to reproduce colours of varying density by a process of half-toning. Consequently their effective-resolution for full-colour images is only 300 to 600dpi. Photographic printing processes reproduce continuous-tone images natively, so a single '300dpi' pixel by itself can reproduce any desired colour or density.

Colour/tonal balance

A print will never be an exact match to a display screen because the print is reflective and the screen is emissive. There exist colours which can be displayed but not printed, and colours which can be printed but not displayed. Furthermore, the actual apprearance of the print and the subjective appearance of both can change subtley according to the type and brightness of ambient light, and the colours and brightness of the surrounding room or environment. Even so, with some care and a consistent printing service, your monitor can be a very good guide to how the prints will turn out.

Setting up your monitor

Just a quick adjustment of your monitor's brightness and contrast controls will reveal how considerably they can change the appearance of a photographic image. It should become apparent therefore that what you see on your monitor is not automatically a definitive representation of your image.

At the very minimum, you should set your monitor to "sRGB" and/or adjust the Colour Temperature (also known as the white point) to "6500K". Using my greyscale test (see link) you should adjust your monitor brightness and contrast until you can see all 17 greylevels distinctly, with black as a good black, "10" being just barely visible above black, and the "f0" just noticeably less bright than the white. You should do this under the lighting conditions in which you normally work - not in direct sunlight! Be aware that the tones and colours on an LCD screen may change as you adjust your viewing-angle, particularly as you move up and down - be sure to sit in your usual position while making adjustments.

Monitor-alignment greyscale test - from my LCD monitor section

Ideally the greyscale will remain colour-neutral from black through to white, although in practice very slight greeny or steely-blue tints are not uncommon. It is possible, but unlikely, that your monitor has adjustment for this.

Mobile/laptop screens: to achieve longer battery life for a given display brightness most mobile/laptop display screens have less saturated colours than sRGB desktop monitors and consequently will be less suitable for colour-critical work.

Getting some test prints

It is very helpful to get some "known good" images printed, along with some more technical tests. An extremely useful guide is to get a greyscale printed (similar to the one used to set up the monitor). If this doesn't print well then you've got problems which are outside your control!

[Insert link to a greyscale test image to download]

Get a print of the greyscale and see how it comes out.

Are the blacks a really dense black?
Is the white paper-white?
Is the first greystep just detectably brighter than black? The second greystep should be easily noticeably brighter than black.
Is the second-brightest step noticeably less bright than white?
How neutral is the colour of the greyscale?

To be honest, printing neutral greys is quite challenging for any colour printing process. You may find that some of the greysteps take on a slight tint of green, brown, blue or magenta. A well-maintained Fuji Frontier can produce remarkably good greyscales which you'd be hard-pressed to fault.

If there is an objectionable overall colour-cast or the whites saturate early (i.e. several of the light greys all come out the same white) then give up and find another retailer!

If the greyscale is good, start comparing the known-good images to your screen. They should be quite close. If there's a systematic difference, you may be able to slightly adjust the brightness /contrast settings on your monitor to improve the match.

[Add another test image - Quick colour triangle? Fine lines/text???]

Many photo-processing labs will apply either manual or automatic adjustment (or 'optimisation') to your images before printing, while others will simply print all images 'as is'. Better labs give you the choice. Properly exposed or prepared images should need very little optimisation to make good prints, but be aware that if adjustments are being made you won't be able to predict exactly how an image turns out.

Effect of ambient lighting

With exactly the same monitor settings, the image on your screen will tend to appear brighter, particularly the medium-dark colours will seem lighter (we say the image appears to have lower contrast), in a dark room compared to a light room.

A print will look darker if put in a light frame surround, and lighter if put in a dark frame. If your print is hung on a wall and illuminated with a spotlight for display, it will appear lighter (less contrasty) than if just illuminated by the ambient light. This is particularly noticeable for high-contrast black-and-white prints. You may even find that a particular black-and-white image, when reproduced at a large size needs to be reproduced at a higher contrast than a 6×4" print to look "right"!

It can be useful to be aware of these effects, though you normally only need to worry about them as your photography becomes more advanced...

Proceed to Digital Photo Printing - a technical assessment...

Background reading

Fuji Frontier 355 brouchure

Created: January 2007
Last modified: 24 February 2007 (a work in progress - more illustrations to be added)
Source: http://www.techmind.org/photoprinting/intro.html


Fuji Frontier 355	Kodak/Kis DKS 1550	Noritsu QSS3302

		Be aware that as well as true photographic prints, many shops and malls feature stand-alone kiosks offering while-you-wait prints which you collect from a slot in the front of the kiosk. There is often no minimum-order, but these machines use a thermal printer technology which is inferior to proper photographic prints, and usually works out more expensive.
Stand-alone kiosks with integrated thermal printer


4:3 ratio digital image	6×4" print crop	7×5" print crop