Tuesday, July 03, 2007

A brief histogram-based qualitative comparison between the Panasonic FZ-30 and entry-level DSLRs

The objective of this investigation was to compare the dynamic range of the Panasonic FZ-30 with entry level DSLRs. Lens flare is also briefly discussed.

Introduction of concepts
In general, cameras are devices that record light brightness values in the form of a continuous bounded spectrum of dark (black) to light (white). On typical (film or digital) cameras, dark areas are rendered as black while light areas are rendered as white.

The black and white areas represent the limits of the sensor- the sensor cannot detect light levels less than the black areas, and light levels more than the white areas. The ratio between light levels required to produce black and white areas on the sensor is known as the dynamic range of the sensor.
Dynamic range explained

Dynamic range is typically quoted in units ‘stops’. One stop represent 2 times of difference in light intensity; two stops represent 4 times; three stops, 8 times; 4 stops 16 times; 5 stops 32 times…

At first glance, it may appear odd that the dynamic range is quoted in multiplicative form (8 times, or 3 stops) instead of additive (3 lux, 4 lux, 5 lux…). The logic is demonstrated in the following example:

A hypothetical sensor would be black if 5 photons or less hit it, and would be white if 160 or more photons hit it. Once can see that the dynamic range is 160/5 = 32 times, or in a more conventional notation, 5 stops.

Consider two light sources giving out 5 photons per second and 3200 photos per second. Exposing a sensor to each of these sources for a second would expose them to give black and white, as the sensors would receive a total of 5 and 3200 photons respectively in that one second of exposure.

However, let the light sources be brightened significantly, so that the dim light now throws out 5000 photons per second, and the bright light is worth 3,200,000 photons per second. Exposing the same sensors to these light sources for a short period of time, 1/1000 second, would still result in 5 and 3200 photons on each sensor, again giving us black and white.

In both cases, the ratio of light intensities between the bright and dark sources is the same:
3200 photons per second / 5 photon per second = 32
3.2 million photons per second / 5000 photon per second = 32

However, the difference in intensity is not identical:
3200 – 5 = 3195 photons per second
3195000 photons per second

Thus the multiplicative manner of describing dynamic range makes more sense than additive.

Description of Cameras

The Panasonic FZ-30 is a regarded as a prosumer point-and-shoot superzoom camera with a fixed lens (7.4-88.8 mm f/2.8-3.7) of Leica design.

Entry level DSLRs used were the Nikon D40 and Canon 300D, both using their respective kit lenses (both 18-55 mm f/3.5-5.6).

Methodlogy and results

A scene with excessive highlights and shadows was captured at various exposure settings using all three cameras. The exposures were calibrated to be exactly the same: ISO 100, f/5.6 and exposure times of 1/200, 1/400, 1/800, 1/1600. In the case of the Nikon D40, it does not have the ISO 100 setting, so exposure times were halved to compensate for doubling the sensor’s sentivity.

The exposure corresponding to ISO 100, 1/800s and f/5.6 was arbitrarily chosen as the baseline exposure (0).

The captured images were cropped to contain approximately identical portions of the scene. The cropped scenes are presented in the figure below.

Click here for large size image

The histogram for each scene was observed in Photoshop CS2, and captured by using the Print Screen function. Distinct peaks (corresponding to large areas of similar darkness) were marked.
Histograms explained

Histograms show the frequency of various darkness levels within the image. The horizontal axis shows the darkness of pixels (dark on the left, bright on the right) and the vertical axis shows the quantity of pixels corresponding to each darkness level.

Click here for large size image

As the marked histograms show, the Panasonic FZ-30 has a remarkably narrow dynamic range. The main indicator is that where the DSLRs can accommodate 3 distinct peaks in the first two exposures, the FZ-30 is unable to do so even in the final one where the green peak is pushed closest to the left.

Curiously, the histograms from both DSLRs show that certain brighness levels are not represented (the histogram having zero height between the green and blue peaks) while the Panasonic one does not fall to zero.

Further investigation shows that this is due to lens flare. Internal reflections from the intense sunlight were not well controlled, resulting in light hitting the sensor where it should not.

A region of the scene was further cropped so that it contained a portion of the dark foreground and a bit of the bright sky. Ideally, the horizon should be a sharp division between the dark foreground and bright clouds. A histogram of this cropped region should have two sets of peaks- a dark one corresponding to the dark foreground, and a light one corresponding to the clouds.

Click here for large size image

The histogram from the Panasonic FZ-30 shows that some of the light from the sun has spread into the dark foreground, resulting in a gradual drop off in brightness instead of a sharp transition.

Summary of results
Compared to entry level DSLRs, The Panasonic FZ-30 has a sensor with comparatively low dynamic range and lens which suffers from noticeable flaring.

Concluding remarks
The results are not surprising.
The Panasonic FZ-30 has a very small sensor (approximately 7.4 mm across) while the DSLRs have much larger sensors (approximately 24 mm across). The resolution is of these sensors not remarkably different. The DSLRs’ larger sensors allows each pixel to have a larger area and collect more photons. As a result, large signal amplifications is not required, and noise is well controlled thus allowing usable data to be extracted from regions with low light.

The FZ-30’s 12x larege aperture (f/2.8 - 3.7) zoom lens requires more compromises in terms of various design objectives compared to the 3x zoom (f/3.5 - 5.6) on the DSLR kit lenses.

Overall, these are the inherent drawbacks of having a high resolution sensor and large zoom range on a relatively small camera body. The Panasonic FZ-30 serves as a general purpose camera which does many things (macro, wide, long telephoto) with a small, lightweight package. It lies in a different category and has a different target market from DSLRs.

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