Making fine prints in your digital darkroom
Digital cameras
by Norman Koren

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Making fine prints in your digital darkroom
Understanding image sharpness and MTF
Image galleries / How to purchase prints
Photographic technique
Image editing with Picture Window Pro

A simplified zone system
Canon FS4000US 4000 dpi scanner
Epson 2450 flatbed scanner
Digital vs. film | Canon EOS-10D
Building simple web pages
updated February 16, 2007
Color Management by X-Rite – Canon – Epson – HP – i1 – Eye-One - Densitometers - Spectrophotometers - Eizo - Canon Large Format

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Table of contents

for the Making Fine
Prints series

Getting started | Light & color
Pixels, images, & files | Scanners
Digital cameras
Specifications | ISO Speed | Other considerations
Depth of field | Table | Links
Printers | Papers and inks
Monitor calibration and gamma
Printer calibration | Scanning | Basic image editing
Black & White | Matting and framing
Tonal quality and dynamic range in digital cameras

Color Management: Introduction | Implementation
Profiles with MonacoEZcolor | Evaluating profiles




for Image editing with
Picture Window Pro
Introduction | Making masks
Contrast masking
Tinting and hand coloring B&W images
Example: Sunset, Providence, Rhode Island
 
Related pages:  The Canon EOS-10D Digital SLR | Digital vs. film | Tonal quality and dynamic range in digital cameras
Imatest: affordable software for measuring sharpness and image quality

Digital cameras are capable of excellent image quality; most digital SLRs have overtaken 35mm film cameras. Digital cameras vary enormously in size, shape, features, and operation. This page focuses on image quality; I can't keep up with all the latest models— there are simply too many. For detailed product information, news, and reviews, check the Links.

With digital cameras you don't have to buy film; you don't have to make one trip to drop it off for development and another to pick it up (or mail it out and wait). Of course you have to buy enough storage (cheap) and make sure to charge the batteries. I particularly like the ability to view images immediately after you've capture them. You can see if your composition is OK and you can check exposure with the help of histograms. For some reason, still a mystery, I find that I can make more consistently sharp images with digital than I could with 35mm film.

Technical terms are explained in other pages on this site, particularly the Understanding image sharpness series. I compare digital cameras with film in depth in Understanding image sharpness Part 7: Digital cameras vs. film. Their detailed performance can now be measured with the Imatest program.

I resisted buying a digital camera until March 2003, when I bought the Canon EOS-10D. In November 2004 I replaced it with the Canon EOS-20D.

Digital cameras fall into two broad categories.
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How to purchase prints
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An excellent opportunity to collect high quality photographic prints and support this website
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The image on the right was taken with the Canon EOS-10D.
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The key specifications that affect digital camera image quality

Sensor designations and sizes
(for small sensors with 4:3 aspect ratios)
Designation
(Type)
Diagonal
mm.
Width
mm.
Height
mm.
1/4"4.5  
1/3.6" 5.0 4.0 3.0
1/3.2" 5.68 4.54 3.42
1/3" 6.0 4.8 3.6
1/2.7" 6.59 5.27 3.96
1/2.5"6.9 - 7.2  
1/2" 8.0 6.4 4.8
1/1.8" 8.93 - 9.1 7.18 5.32
2/3" 11.0 8.8 6.6
1" 16.0 12.8 9.6
4/3" 22.5 18.0 13.5
35mm 44.3 24.0 36.0

You can find individual sensor sizes on manufacturer's websites, though it sometimes takes a little math. Sony's spec sheets for CCD sensors lists diagonal in mm and the type (i.e., 2/3). To get height and width you have to open the individual data sheets and  multiply the pixel size by the number of vertical and horizontal pixels. Panasonic publishes similar data. Two of their 1/2.5 inch sensors have different diagonals: 6.9 and 7.2 mm. Kodak also publishes sensor data.

Imatest: affordable software for measuring sharpness and image quality
Exposure (dynamic) range is related to both sensor bit depth and sensor noise. Sensor noise, as we've mentioned, is closely related to sensor size— the larger the sensor, the lower the noise. Increasing the bit depth increases exposure range only if sensor noise is low enough. In the case of the aforementioned Sony DSC-F818, its 2/3" (8.8x6.6 mm = 11 mm diagonal) sensor has a wimpy 2.7 m pixel spacing— small enough to make me skeptical of the advantage of the 14 bit A-to-D converter. For comparison, digital SLRs have pixel spacings between 6.8 and 9 m; a 14-bit converter would be more advantageous in a digital SLR.

To take full advantage of the potential exposure range of a digital camera, you should capture images in RAW format. for further explanation, see the (still unfinished) page, Tonal quality and dynamic range in digital cameras. Sensor noise and dynamic range can be measured by Imatest's Q-13 Stepchart module. 

How is ISO speed determined in digital cameras?
The answer is found in Kodak Image Sensors - ISO Measurement (App note MTD/PS-0234), extracted from ISO standard 12232:1998. (Only 73 CHF— about $59 US for 131 kB; such a deal!)

There are two basic types of ISO measurement: saturation-based (also called "base") and noise-based. The saturation-based ISO is

ISO = (15.4*f#2) / (L*t)    [Exposed so an 18% gray card has a level 18/106 of full scale. See below.]
where  f# is the effective f-number of the camera lens, L is the luminance in cd/m2 of an 18% reflector (the familiar 18% gray card), and t is the length of the exposure in seconds. Saturation-based ISO corresponds to the camera ISO setting, which is controlled by the electronic gain of the system. You can use this equation to find the approximate luminance of an object, L =  (15.4*f#2) / (ISO*t), where f# and t are read from an 18% gray card. For a typical white surface (90% reflectance), L =  (3.08*f#2) / (ISO*t).

Digital image sensors are linear devices: their pixel voltage (and the pixel level in a RAW image file) is proportional to the light energy reaching the pixel up to the point where it abruptly saturates. Now here's the heart of the matter.

When an exposure is made according to the above setting, an 18% gray card has a voltage or pixel level 18/106 of full scale (e.g., a pixel level of 696 at the output of a 12-bit A-to-D converter, which can represent 4096 levels). In an 8-bit color space encoded for display at gamma = 2.2 (sRGB, Adobe RGB, etc.), the corresponding pixel level is 114 (from the formula, 255*(18/106)(1/2.2) ).

Any portion of the image with luminance equivalent to more than 106% reflectiance (5.9x brighter than the gray card) would be saturated— pure white; burnt out... gonzo. Since a white card is about 90% reflectance, that doesn't leave much margin. The Kodak document says, "Very demanding applications might need more headroom for highlights, and so might use a higher number when calculating base ISO." That's why exposure compensation— equivalent to increased ISO speed— is often required to avoid burnt out highlights for "very demanding applications" such as ordinary sunlit outdoor scenes.

The Kodak document has little to say about noise-based ISO measurement, which is related more to image quality than to camera ISO settings.

Other considerations

The performance of a number of High quality digital cameras is summarized the Sharpness comparisons page of the Imatest website.
Depth of field  is the range of distance where an image appears sharp. It is discussed in great detail in my Depth of field page. DOF is approximately proportional to the f-stop, and inversely proportional to image sensor size, i.e, the smaller the sensor, the larger the DOF at a given f-stop.

Probably the easiest way of thinking of DOF is to relate it to 35mm cameras. Let N be the f-stop (aperture) on your digital camera, let N35 be the f-stop on a 35mm camera that gives the same DOF, let d be the diagonal of your digital camera sensor (see the Sensor designation table, above), and let d35 = 43.3mm be the diagonal of a 35mm image. Then,

N35 = 43.3 N/d
For example, suppose you have a digital camera with an 11mm diagonal sensor, typical of the compact 5 megapixel cameras with 2/3 inch sensors: the Minolta Dimage 7i, Nikon Coolpix 5700, Sony DSC-F717, Olympus E-20, etc. At f/8, the DOF is the same as a 35mm camera at f/32. That is a huge depth of field: nice if you want everything in focus, but problematic if you need to take advantage of a narrow DOF to isolate a subject from a distracting background (this is typically done with telephoto lenses). At f/8, image sharpness is diffraction-limited. In digital cameras with small sensors (<= 11 mm), diffraction would severly degrade image sharpness at apertures above f/8. That's why f/8 is the smallest aperture on most compact digital cameras.

On digital SLRs with a "multiplication factor" M, typically around 1.6x (Canon EOS D60, Nikon D100), DOF is larger than a 35mm image with the same field of view by a factor M.

Expanding exposure range  According to Secrets of digital photography, Nikon's digital cameras at ISO 100 (the low end of their speed range) have a dynamic range of about 446:1, equivalent to 8.8-stops of brightness, better than slide film and approaching negative film. (I used to be skeptical of this number, but I've confirmed it using Imatest Q-13 Stepchart). They give a nifty technique that enables you to expand the effective dynamic range of images with stationary subjects by combining two or more exposures. My son figured this out for himself and used it to soften the contrasty backlit image of Rome and Saint Peters below (click here for enlarged image), taken with the Kodak DC4800. Combining images to expand exposure range can be easily accomplished with the Stack transformation in Picture Window Pro 3.5.
Rome, St. Peters: composite of 3 exposures. Click for enlarged image.
In mid-June 2001 I purchased a Kodak DC4800 for my son to take to Europe and India. It's a nifty little camera, capable of amazingly sharp 8x11 inch prints and respectable 13x19 inch prints. Colors are lovely, but it can't capture the tonal range of negative film with the default settings (3.1 Megapixel JPEG; 900k file size) unless you combine two exposures (see example above). The main thing that bothered me was the time delay of nearly a second between pressing the button and the exposure. I didn't have time to explore its many options; I hated to give it up. Nathan used it well: click here for examples.

Summary

No digital camera is ideal for everyone— your choice depends on a number of factors.

Links

The subject of digital cameras is too vast for me to keep up with. Here are the essential sources of news and reviews.

Imaging-Resource.com and Digitalcamerainfo.com both use Imatest (developed by yours truly) for measuring image quality in their reviews.
Steve's Digicams and Digital Photography Review and  also have extremely comprehensive reviews.
Luminous-landscape.com
  Michael Reichmann only reviews a few cameras— he focuses on the cream of the crop. His reports are always stimulating.
Photo.net digital camera reviews
Digital Outback Photo
RobGalbraith.com  Excellent source of detailed technical news in areas such as support software and accessories. His CompactFash Performance Database is particularly interesting.
Ecoustics.com has a comprehensive list of reviews on other sites. Useful for researching.

Additional links for digging deeper:

DCReviews.com | megapixel.net (excellent French/English monthly) | Digital Camera Resource
John Tinsley (high-end professional digital cameras) | fredmiranda.com | Digital PhotoCorner
Nikondigital.org. Information and reviews, including Canon SLRs. From Moose Peterson and David Cardinal.
DigitalCamera-HQ.com  A nice site for searching and comparing models, with links to reviews and price comparisons.
Short courses in Digital Photography 
LetsGoDigital News in English, Dutch, and Spanish.
Digital Photography For What It's Worth (excellent articles on selecting and using digital cameras, geared towards beginners)


Related pages: The Canon EOS-10D Digital SLR | Digital vs. film | Tonal quality and dynamic range in digital cameras
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Images and text copyright 2000-2013 by Norman Koren. Norman Koren lives in Boulder, Colorado, where he worked in developing magnetic recording technology for high capacity data storage systems until 2001. He has been working on Imatest since 2003. He has been involved with photography since 1964.