INTRODUCTION, TERMINOLOGY EXPLANATIONS AND
PIXEL DENSITY WITH TELEPHOTO LENSES AT THE SAME FOCUSED DISTANCE

Not too many years ago photographers had quite a variety of film equipment to choose from. 35mm film had become a very popular format since the late 1950's. There were also four popular medium format film sizes and three large format film sizes. Each of these film size formats had a fairly well accepted photographic niche. All of these camera formats had lenses that were optimized for the dimensions of the film used.

Although there is some debate and discussion concerning digital camera sensor height to width ratio, the major discussion topic seems to be about the size of the sensor relative to the original format that the lenses were designed for. For most of the DSLR's currently on the market the sensor size comparison is relative to 35mm format (24X36mm film area). I am not going to work with or discuss digital backs for medium and large format although the same image sensor size and lens design issues apply to some of these systems too.

Some photographers argue that there are advantages to smaller sensors for some types of photography. All of my photographic experience has leaned toward the theory that bigger is better when it comes to image size. However, in the digital realm there may be other factors involved in the equation besides overall image size. As an example, I was absolutely astounded three years ago to find that a 6mp DSLR image could be upsized to equal a 4000 dpi 35mm film scan (19mp) and the DSLR image looked as good or better than the native 4000 dpi 35mm film scan.

What are the advantages and or disadvantages, if any, to smaller sensors? Is this multi-sensor size camera lineup just a manufacturing expense vs marketing thing?

I'm going to explain some of the terminology and then I'm going to use comparison images, a small amount of math and some dialog to demonstrate how different sensor sizes can affect image quality and the practical aspects of photography. We will supply a generous number of comparative images. This is not a scientific test. While I have used some math to explain sensor resolution and some concepts related to sensor resolution, I'm not going to use optical formulas or software generated charts and graphs. This article is just a simple comparative pictorial essay. I will offer comments and draw some conclusions. You may see things differently...:^) If you find verifiable technical inaccuracies, please bring them to my attention via Email. - Thanks

Canon currently makes three DSLR sensor sizes and utilizes them in many different DSLR camera models. As stated in the title of this article, we are going to be working with the 1Ds Mark II, 1D Mark II and 20D.

The 1Ds Mark II's 16.7 megapixel sensor is 'full frame' (same size as 35mm film) at 24X36mm. The sensor is about 1.41 inches wide with resolution of 4992 x 3328 pixels. Dividing the width resolution by the width of the sensor in inches shows us that we have about 3540 ppi (Pixels Per linear Inch) on this camera's sensor. Canon's DSLR cameras produce pixels with an equal height and width, square pixels.

The Canon 1D Mark II's 8 megapixel sensor has a 1.3 crop factor and a sensor size of 19.1X28.7mm. The sensor is about 1.13 inches wide with a pixel resolution of 3504 x 2336. Dividing the width resolution by the width of the sensor in inches shows us that we have about 3100 ppi on this camera's sensor.

The Canon 20D's 8 megapixel sensor has a 1.6 crop factor and a sensor size of 15X22.5. The sensor is about .89 inches wide with resolution of 3504 x 2336 pixels. Dividing the width resolution by the width of the sensor in inches shows us that we have about 3937 ppi on this camera's sensor.

FOV
Every lens focal length has a predetermined 'angle of view'. Telephoto lenses have a narrower angle of view than wide angle lenses, thus the term wide angle. The angle of view determines the Field Of View (FOV) or just how much of a particular scene is 'in view' in your camera's viewfinder at any given focused distance. Cameras with sensors smaller than 24X36mm (a crop factored sensor) will show less of a particular scene with any given focal length lens at the same focused distance.

CROP FACTOR
This commonly used term denotes the size ratio between a smaller sensor and a full frame 24X36mm sensor. As an example a 20D with its 1.6 crop factor. If you take a full frame camera's sensor width of 36mm and divide it by 1.6 you get 22.5mm which is the sensor width of the 20D. Again, since the smaller sensors have less area they have a narrower FOV for any given focal length lens and focused distance.

The crop factor is most often used to determine the "35mm equivalent" field of view for any given lens when used on a camera with a sensor smaller than 24X36mm. As an example a 24mm lens on a 1.6 crop factor camera will produce the same field of view as a 38mm lens on a full frame 24X36mm camera. The math is simple, just multiply the crop factor by the existing lens focal length (1.6 X 24 = 38). If you need to know what lens you'd need for an  equivalent field of view on your sub 35mm size sensor camera just divide the focal 35mm equivalent focal length necessary for that field of view by your camera's crop factor. As an example you need the field of view and perspective of a 50mm lens on your 20D. Divide 50 by 1.6 and you find that you'll need a 31mm lens.

See examples of the 2 different crop factors we will be dealing with below. In this case a 180mm lens was used for this photo. As a practical example the 20D + 180mm lens produced the same field of view as a 288mm lens at the same focused distance on a full frame camera. It is important to note that this is not an increase in the lens's actual focal length or magnification power but just a smaller field of view due to the 20D's smaller sensor.

PIXEL DENSITY
We are working with one 16.7mp camera and two 8mp cameras and each one has different length and width dimensions for its imaging sensor. Pixel density is nothing more than the dpi (Dots Per Inch) rating of the sensor where each dot is an individual photo receptor on the sensor that will produce one pixel. Higher sensor dpi numbers = higher pixel density.

Using two of these cameras as an example, the 8mp 1D Mark II has 3504 pixels across its 1.13 inch sensor while the 8mp 20D has the same number of pixels across its smaller .89 inch width sensor. To fit the same number of pixels across the smaller 20D sensor the pixels (photosites) must be smaller and therefore they are more 'dense' on the 20D sensor than the pixels on the 1D Mark II sensor. As an imaging device, the 20D sensor has higher input resolution than either of the other two sensors in this article. However, it's output resolution is only equal to the 1D Mark II (both at 3504 x 2336 pixels)  and less than the 1Ds Mark II (4992 x 3328). Putting this information into more traditional imaging language you can think of the pixel density as input resolution and the final image pixel dimensions as output resolution. This input/output resolution and sensor size variable make for some interesting cross camera image scenario comparisons.

From a theoretical point of view pixel density should affect the amount of random digital noise present in an image and the dynamic range in the image the camera produces. Dynamic range is how much detail can be held in shadow areas before highlights begin to loose texture or, how many F stops of light range the camera sensor can capture. Again, in theory, larger photosites (less pixel density) on the sensor should produce less random noise and more dynamic range for any given sensor size.

I chose to use Canon cameras for this article because all three of these models use the same image processing system, Canon's DIGIC II. All three cameras having the same image processing system levels the playing field to the basic issues of optics, sensor size and pixel density. The 1Ds Mark II used in this test is my own camera. The 1D Mark II and 20D were loaned to me from the very well stocked rental department at Calumet Photographic in Escondido, CA.

All of the photographs presented in this article were taken using my 17 pound studio tripod. I used manual focus to the exact same spot in each image. As an extra focus control measure I took three shots of each subject and re-focused in-between each shot. I examined the images and looked for the best focused shot. In truth, there was almost no difference in focus in the series of images. I have been successfully using manual focus on a variety of camera types for over 25 years.

I used manual exposure at 1/320 sec @ ISO 100 and F8 with the mirror locked up for all of the outdoor comparison images. All of the photos were shot in RAW. I used Capture One Pro  to convert  and save to 16 bit TIFF. I used Capture One's default settings except for using 'film extra shadow' and 'standard look' sharpening at 20. I used ETC's low sat profile for the 1Ds Mark II images. All of the full frame photos and thumbnails were saved as level 9 JPEG in Photoshop with the EXIF info intact. The thumbnail full resolution examples were cropped out of the original TIFF images.

PIXEL DENSITY AND TELEPHOTO LENSES AT SAME FOCUSED DISTANCE
Some nature photographers like the Canon 20D's 1.6 crop factor. Because of the 1.6 crop factor they can fill the frame with their subject with any focal length telephoto lens at a longer camera to subject distance than with a full frame DSLR. In the first set of image comparisons below I used a Canon 180 macro lens and photographed the bear from the same distance with each camera. The point of focus was the bear's nose. This first image set demonstrates how the crop factor of the three cameras affects the field of view if you couldn't get any closer to your subject and you only had your trusty prime telephoto lens along. Sorry, couldn't muster up a grizzly bear or bald eagle so you'll have to live with my wife's Victorian Valentine bear.

On the next page we'll have a look at how the three cameras do when you are able to move in closer or zoom in and fill the frame equally with your subject.

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ARTICLE LINKS:
PAGE 1 - INTRODUCTION, TERMINOLOGY EXPLANATIONS AND PIXEL DENSITY WITH TELEPHOTO LENSES
PAGE 2 - DEPTH OF FIELD, PERSPECTIVE AND SAME FOV WITH TELEPHOTO LENSES
PAGE 3 - FOV AND LENS RESOLVING POWER WITH NORMAL TO WIDE ANGLE LENSES
PAGE 4 - DIGITAL NOISE AND DYNAMIC RANGE