Casio Exilim EX-Z75

Color (7.47)
All cameras reproduce colors differently, which is why we test color accuracy. We tested the color accuracy of the Casio Z75 the same way we test all our cameras, by photographing an industry standard GretagMacbeth ColorChecker test chart and comparing the colors the camera reproduced with the known colors of the test chart. The ColorChecker chart consists of 24 color tiles that represent a range of colors from around the color spectrum, including some sky blues, foliage greens, and skin tones. The image below shows how the Z75’s colors compare to the actual colors of the ColorChecker. The outside squares are the colors the camera reproduced, the inside squares are the actual colors of the ColorChecker corrected for the exposure, and the small rectangles are the ideal colors of the ColorChecker.

The Z75 had most accurate colors when it was slightly underexposed, which is why the little rectangles in the images are brighter than the other squares. When comparing the inner squares with the outer squares, several of the colors look very similar, while several others look quite different. This color error is represented graphically in the chart below. The chart shows the whole color spectrum with the real colors of the test chart specified as squares. The corresponding colors the camera reproduced are shown as circles, and the line connecting squares and circles shows the color error for each color tile on the chart.

The chart shows significant color error in blues and yellows, as well as a general undersaturation of all colors. The mean color error of 8.03 is respectable, but watch out for yellows and skin tones drifting a little green in your photos.

Still Life Sequences
Click to view the high-resolution images.



Resolution (2.76)
We tested the resolution of the 7.2-megapixel Z75 by setting it to best quality and full resolution and photographing an industry standard resolution test chart. We varied the focal length and exposure settings and ran the photos through Imatest to determine the settings that produced the sharpest image. The chart is shown below.

Click to view the high-resolution image

The sharpest image was recorded at ISO 50, f/5.9, and a focal length of 19mm. Imatest measures resolution in terms of line widths per picture height (lw/ph), which refers to the number of equally sized alternating black and white lines that could be fit across the frame before becoming blurred. The Z75 produced 1674 lw/ph horizontally with 17.7 percent oversharpening, and 1703 lw/ph vertically with 18.4 percent oversharpening.

In this resolution test, the Z75 fell flat on its face. The sharpness values are not very good to begin with, but more importantly, there is way too much sharpening being applied inside the camera. This visual effect of this oversharpening is image artifacting, clearly visible on the Z75 resolution chart above (click to see the full resolution version) by the white "ghost" lines that surround edges of high contrast. You can see these ghosting artifacts next to every black edge on the resolution chart. This will look very poor on large prints, so don’t buy this camera expecting to print big. The Z75 tallied the worst resolution score of any camera we have tested yet this year.

In addition to the artifacting problem, note the considerable barrel distortion apparent around the edges of the resolution chart. The bottom black edge of the chart should be a straight line, but is instead bowed significantly inward. This is evidence of problems with the camera’s optical system, rather than its processing.

Noise – Auto ISO (2.81)
In addition to our color accuracy test, we use the ColorChecker test chart to measure image noise. Noise refers to the amount of random signal in a picture that doesn’t come from the scene, but rather from the camera sensor itself. It usually takes the visible form of unwanted snowy or sandy splotchiness that uniformly covers an entire image. Due to the physics of camera sensors, the higher the ISO sensitivity, the more noise is created. Imatest measures noise by the percent of the image it completely drowns out.

We set the Z75 to auto ISO and shot the ColorChecker under bright studio lights to see how much noise it produced. The camera shot at ISO 200, with 1.16 percent of the image lost to noise. This is a decent score, and should keep noise from showing up too visibly in brightly lit scenes. It might be a good idea to adjust ISO manually in order to keep noise as low as possible.

Noise – Manual ISO (4.88)
We also shot the test chart at all ISO settings to determine the noise levels throughout the ISO range. Noise stayed nice and low from ISO 50-200, and then jumped significantly at ISO 400. Try to keep the ISO under 200 when possible, though this will not help you in low light conditions. Overall, the noise performance of the Z75 was mediocre.

White Balance (8.61)
Poor white balance accuracy can ruin an otherwise perfectly nice photo. Think of a group photo of your best friends at the beach, but with a blue cast over it that makes everybody look like a cadaver. We tested the white balance accuracy on the Z75 by photographing our ColorChecker under four different types of light: flash, fluorescent, outdoor shade, and tungsten. We took shots using auto white balance, as well as the white balance presets located in the white balance menu.

Auto (8.88)
The auto white balance did quite well on the Z75, so there’s no need to worry about any beach cadavers with this camera (and if there still is, you need to find a different beach). Despite lacking a flash white balance preset, the auto setting did very well using flash. Also, under white fluorescent light, the auto setting was more accurate than the white fluorescent "N" preset.

Preset (8.32)
In outdoor shade, the preset was much more accurate than the auto setting, so make sure to use the presets when you’re outside. Under tungsten light, both the auto setting and the tungsten preset were very inaccurate, with auto causing a yellow cast, and the preset turning things very blue.

Low Light (4.30)
We dimmed the studio lights to see how the Z75 performed in less-than-ideal lighting. We photographed the ColorChecker at light levels of 60, 30, 15, and 5 lux at the highest ISO sensitivity possible, which in the case of the Z75, was ISO 400. 60 lux corresponds approximately to a room lit softly by two lamps, while 30 lux is roughly the light from a single 40 watt bulb. 15 and 5 lux are very low light, such as dark night scenes.

Low Light Tests
60 Lux	30 Lux
15 Lux	5 Lux

The Z75 could not properly expose at 15 or 5 lux, as you can see in the images above. If you can avoid using this camera in very low light, you will be happy with the color accuracy and noise levels at 30 and 60 lux, which only degrade slightly from their values in bright light.

We also test camera performance in long exposures under low light, but the Z75 would not comply. Using some of the Best Shot modes, the camera can take exposures as long as 4 seconds, but only with major problems autofocusing. We couldn’t get reasonable exposures, or even proper focus, when the camera decided to use shutter speeds longer than 1 second in duration. For half second exposures in auto mode, the picture was still underexposed, and suffered from strong undersaturation, color error, and high noise levels. Overall, low light performance was poor.

Dynamic Range (5.00)
An important aspect of image quality is how many shades of gray the camera can distinguish before blowing out or losing all information completely. This measure is called dynamic range, and we test it by photographing a backlit Stouffer step chart which consists of a row of rectangles all slightly different shades of gray. The more rectangles the camera can distinguish, the better its dynamic range. We shot the Z75 at all ISO sensitivities to see how the camera performed over the entire range.

Dynamic range in the Casio Z75 decreased slowly and steadily over its short ISO range. Measured dynamic range was almost 7 Exposure Values (EV) at ISO 50, which is quite respectable, but then fell off from there. Overall, the dynamic range of the Z75 was mediocre. It scored worse than the Casio Z1050 and the GE G1, and may have some trouble rendering detail in very bright or very dark parts of an image.

Speed/Timing – All speed tests were conducted using a Kingston Ultimate 2GB 120X SD Card.
Startup to First Shot (8.5)
The Z75 took approximately 1.5 seconds to take a shot after turning on. However, this speed was obtained by continuously pressing the shutter button, because the camera will not shoot a photo if the shutter is just held down right after it is turned on.

Shot-to-Shot (8.7)
With the Continuous option set to "On" in the Record menu, the camera took shots every 1.3 seconds until the memory card filled up. This is a fairly slow burst speed, and won’t give you a lot of options if you’re trying to capture a great action shot of your kid taking a swing at a baseball, for example.

Shutter-to-Shot (9.0)
There was no measurable lag between pressing the shutter and recording a shot, either with the camera prefocused or not.

Processing (7.8)
The Z75 took 1.1 seconds to process one 4.2MB full resolution image shot at ISO 200.


Video Performance (4.56)
Bright Indoor Light – 3000 lux
We shot video of our test charts under bright studio lights to determine the Z75’s video color accuracy. We shoot video color with auto white balance, and, as you can see in the images below, the colors were far from accurate. Mean color error was 16 and saturation was 122.8 percent. Even though these are frightening numbers, they’re actually just about average for camera video in bright light with auto white balance.



Low Light – 30 lux
We dimmed the lights to test the color accuracy and noise in low light. The Z75 shot with a mean color error of 18.1 with saturation of 62.22 percent, which describes how underexposed the video is. See the images below for the color accuracy and a frame taken from the video of the video color test chart.



Resolution
We also recorded video of our resolution chart. The Z75 recorded 258 lw/ph horizontally with -15.5 percent undersharpening, and 406 lw/ph vertically with 23.4 percent oversharpening. This extreme oversharpening is apparent when looking at the 100 percent crops, where you can see white "ghost" lines above and below hard edges on the resolution chart similar to what you can see in the still images described above. The main reason the video resolution numbers are so much smaller than the still resolution numbers is that video is recorded at a much smaller resolution than the full resolution still images.



Motion
We took the Z75 outside to see how it recorded the motion of cars and moving people. The video had very good exposure and contrast, and the motion was smoother than most point-and-shoot camera video. However, there was some motion moiré, as well as a lack of detail in more distant objects. Overall, the video looked quite good for a camera.