What makes a good print?

The histogram for the above image reveals blown-out highlights as a peak hard up against the right hand side of the graph. Shadows fall well within the tonal gamut of the image sensor. The blown-out highlights in this image will not be printable.

While many digital images are only viewed on computer or TV screens or portable devices, none of these display options has the lasting impact of a high-quality A3+ print. Fortunately, making high-quality prints is relatively straightforward once you have learned to balance colours and tones.

You must also understand the differences between how an image appears on a screen and when it is output to paper. Put simply, screens support a dynamic range at least 10x greater than paper. A typical contrast ratio for a monitor or TV screen is between 1000:1 and 3000:1, whereas printing papers average about 100:1

Interestingly, human eyes have a static contrast ratio of around 100:1 when vision is fixed on a single subject. Consequently, a print provides a natural balance of bright and dark tones that requires no special effort to view. Our eyes can actually encompass a dynamic contrast ratio of about 1,000,000:1 (equivalent to the difference between starlight and brilliant sunlight). But it takes approximately 30 minutes to adjust for the change in brightness levels, as the iris has to contract with increasing brightness and chemical changes occur in the retina, where the light is collected.

Taking these factors into account, in this feature, we’ll explain how to get the best possible output from digital photographs.

Image Parameters

It should go without saying that images for printing should contain the full gamut of colours and tones you want to reproduce. That means capturing all the image data from highlights through to shadows. Blown-out highlights will print out as stark white, while blocked-up shadows will print black. Both are to be avoided.

Use the histogram, either in your camera when you’ve taken the shot or on your computer when you are sorting out images for printing to see where the tonal range in the image is positioned. If the histogram has large empty areas at the left end of the graph, blown-out highlights are likely. Blocked-up shadows occur when the graph is pushed up hard against the left end of the graph.

 

The histogram for this image shows a relatively even distribution of tones across the baseline with few pixels at the black or white end of the graph. It’s easy to make a good print of this image as a result.

If the histogram is low in the centre with peaks jammed up against each end of the graph, the contrast is too high and shadow and highlight details will be unprintable. The histogram for a printable image will be fairly evenly spread, with a moderate peak around the centre of the graph and few or no pixels at the extreme limits.

It’s easy enough to adjust contrast and colour saturation while editing the image for printing. But the amount of adjustment is predicated by the amount of data recorded in the original image file. And that brings us to the first tip:

Shoot Raw Files. All digital cameras record images as raw (unprocessed) data but most of them discard roughly a third of this data when they convert the files into JPEGs. JPEGs are ideal for instant viewing and sharing and they can also make good prints when little or no editing is required (and viewers aren’t too demanding).

But each pixel in a JPEG consists of only eight bits per colour and the data is further compressed to eliminate information whose absence won’t be noticed. This often compresses the dynamic range in the image as well. In-camera processing also sets the white balance and may tweak contrast and sharpness to produce a picture suitable for spontaneous printing, sharing or posting on the Web.

In contrast, raw files contain at least 12 bits ““ and sometimes 14 bits ““ of colour information per pixel. And, where they are compressed, lossless compression is used. Effectively, you can access all of the information the sensor recorded in a raw file. But the camera doesn’t process them; that task is left to you and your expertise will bring out the image you imagined when you took the shot. Because they contain the full dynamic range the camera can record, as well as full colour information, raw files are the best starting point for perfect prints.

Equipment Parameters

Printers: It should go without saying that you need a decent printer to make good prints. However, you also need a good monitor. Over years of reviewing printers we’ve discovered it’s possible to make acceptable prints with a four-colour printer. However, they don’t quite match the output of a printer that uses eight or more ink cartridges.

This is particularly true for monochrome images. You need at least three black inks to obtain B&W prints that contain a full tonal gamut without unwanted colour casts. But it also applies to coloured inks, where the addition of light magenta and cyan inks allow tonal subtleties to be reproduced, particularly in portraits.

 

The ink set for Canon’s PIXMAPro1 printer includes three grey inks plus red ink and a Chroma Optimiser that applies a layer of resin to improve the appearance of prints made on glossy papers. (Source: Canon.)

Some ink sets add red, green or blue inks to fine-tune colour rendition or emphasise certain hues. Red may be added to impart a touch of warmth to skin hues, while blue and/or green can add vibrancy to these hues in prints of landscapes. Pigment printers may include resin coatings to fill gaps between the ink droplets and produce a smooth, more robust surface. In the long term, a capable printer with an extensive ink set will probably be one of your best investments.

Monitors: There’s not much point in spending money on a printer when you’ve skimped on the monitor. A capable monitor should display the full sRGB colour gamut and have adjustments for brightness, contrast and each colour channel. Without these adjustments, it can’t be calibrated.

Calibration: Calibration keeps track of how the monitor displays a wide range of colours and saves this information in your computer as a profile. The profile can be used by the printer to reproduce colours and tones as closely as possible to how they appear on your computer monitor.

Monitor colours can drift slowly over time, so monitors used for printing should be re-calibrated at least once per month (more frequently if you’re making a lot of prints). Calibration devices like DataColor’s Spyder series or the X-Rite ColorMunki are affordable and easy to use. (The ColorMunki also lets you profile printing papers.)

Editing Parameters

The following tips apply to the editing process and allow you to fine-tune images to ensure they will print well.

1. When adjusting exposure levels, try to obtain details in both the brightest highlights and deepest shadows. Use the histogram in the Levels adjustment to make sure the graph is contained within the 0-255 level baseline.

2. Once the exposure looks about right, move on to adjusting the contrast. This determines how quickly intermediate tones change from dark to light. Prints with insufficient contrast look greyish and lifeless but where contrast is too high they appear stark and lacking in intermediate tones. The Curves tool can help you to manipulate contrast across the tonal range in the image if the Levels adjustment proves a little too stark.

The illustration above shows the difference between JPEG and raw images. The JPEG image on the left has a large area of blown-out highlights in the upper left hand corner. The raw image captured at the same time can be tweaked to restore most (although not quite all) of the detail that was not recorded in the JPEG image.

3. As well as good overall contrast, a wellmade print will have good ‘micro-contrast’, which is indicated by how well adjacent tones are separated. In Adobe Lightroom and Adobe Camera Raw, the Clarity adjustment allows you to change the tonal separation among adjacent image areas. This control can also be used to make different intermediate tonal areas stand out against the background. Negative adjustments in Lightroom’s clarity control can also be used to decrease apparent sharpness, if desired.

4. Once you’ve made the initial corrections to exposure and contrast, go back and finetune colour balance to eliminate any overall colour casts. At this point you can also adjust the intensity of hues with the Saturation and Vibrance tools.

Saturation adjustments boost the intensity of colours (hues) linearly between –100 (monochrome) and +100 (double the saturation). This adjustment affects all hues equally, which can lead to clipping, where excessive saturation can cause tones to be lost (clipped) in strong colours.

The Vibrance control only saturates colours that need it. Only the least saturated colours in the image are boosted, while pixels that are already relatively saturated receive less adjustment. Over-saturation (and the associated tone drop-outs) is impossible when the Vibrance tool is used.

For landscapes, Vibrance adjustment can give a nice boost to cyans, greens and yellows, without touching oranges and reds much. Most skies become slightly darker as they saturate when the Vibrance control is used, but skin tones retain their subtlety.

Both Clarity and Vibrance are highly imagedependent. You need to try each one to know what they will do for any particular image. You obtain more control over the intensity of colour with Vibrance than with the Saturation tool. A small increase in Vibrance will usually improve prints from any image, especially landscapes.

5. The final step should be unsharp masking, which increases the contrast differences along edges in the subject, creating an impression of greater sharpness. Images from digital cameras need some unsharp masking to overcome the slight blurring that results from colour interpolation. So do most images scanned in colour.

Be careful not to overdo unsharp masking as it can produce sharpening artefacts that appear as conspicuous edge effects. Excessive sharpening can also increase the visibility of image noise.

Printing

Make sure you select the correct paper type in the printer driver and use the Photo quality setting. Uncheck the High-speed box if the driver includes one.

Then correct any problems with the overall brightness and colour balance of the image by printing a test strip or making a print on a small sheet of the paper you have selected. Some printer drivers will print out a colour ring-around to show subtle variations of hue based on the same exposure.

A few ring-arounds show both hue and brightness variations for printing that particular image on the paper you are using. While these tools use up a whole sheet of paper, they make it easier for novices to home in on the best hue and brightness balance.

A good monitor should be able to display the full sRGB colour gamut, as shown in this graph.

Some printer drivers can produce a colour ring-around to show subtle variations of hue based on the same exposure.

 

This article is an excerpt from  Photo Review magazine Issue 60  

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