Pentax Optio 750Z Review
The 750Z proved to have good automatic white balance in natural and fluorescent light with a slight pinkish color cast in incandescent light. It's a step forward but not the full revolution yet.
|Outdoor - Auto WB
Red -0.1%, Blue -0.2%
|Fluorescent - Auto WB
Red -0.3%, Blue -1.7%
|Incandescent - Auto WB
Red 2.1%, Blue -2.2%
The 750Z built-in flash unit has a specified range of 3.8 m at wide angle and 2.2 m at telephoto (both at Auto ISO) which is about average for this kind of camera. The results from our flash test were good white balance with no obvious color cast although the flash exposure metering system did seem fairly easily fooled by a white background, our skin tone test shot was fairly under-exposed (although this could be manually compensated for).
|Color chart - slightly under-exposed,
no color cast
|Skin tone - under-exposed, no color cast, slightly pale color response|
The 750Z features automatic noise reduction for exposures of 1 second or longer. It uses a 'dark frame subtraction' method which takes a second equally long exposure after the main shot (thus an 8 second exposure will take 16 seconds). This dark frame contains a similar noise pattern to the original shot and can be used to 'subtract' hot pixels from the final image. The results were good with only slight pattern noise and no hot pixels or black pitting.
|ISO 80, 8 sec, F4.2 (manual exposure)|
Although the 750Z's best frame coverage was achieved at wide angle we found the best results were to be gained at full telephoto, frame coverage was slightly less but there is no frame distortion and corner softness is considerably better.
Barrel and Pincushion Distortion
At wide angle the 750Z's five times optical zoom produces approximately 0.9% barrel distortion which is about average for a compact digital camera lens. However at telephoto the 0.8% pincushion distortion is above average and could well be noticeable in everyday shots. We also noted that pincushion distortion takes effect from mid-zoom to telephoto.
|Wide angle - 0.9% Barrel distortion
Equiv. focal length: 37.5 mm
|Telephoto - 0.8% Pincushion Distortion
Equiv. focal length: 187.5 mm
Vignetting / Lens Shading
Our vignetting measurement is made by taking the average luminance value of the darkest corner of the image and comparing it to the average luminance at the center of the image, any difference greater than 15% may be visible in everyday shots. Vignetting was certainly an issue with the 750Z, especially at wide angle where it affects all corners of the image (although the top corners more so), the 19.1% maximum fall off value is one of the highest we have recorded and means there's a strong possibility of shading being visible in everyday shots. Stopping down (smaller apertures) does help a little and so does zooming in, things were much better at telephoto.
Purple Fringing (Chromatic Aberrations)
While the 750Z did produce some very slight purple fringing on our standard test shot we could not find a single example of any fringing in our everyday shots. Indeed the two samples provided below should really have produced quite strong fringing but there's absolutely none visible. We were so impressed that I can only think Pentax must be using some kind of fringing removal algorithm. If you look at the edge of the window frame in the last crop you can see that some of it appears to have been lost, I will guess this was affected by fringing and has in fact been 'cleaned'.
|37 mm equiv, F2.8 (max aperture)|
|37 mm equiv., F2.8 (max aperture)|
|37 mm equiv., F2.8 (max aperture)|
Overall Image Quality / Specific Issues
The overall look of the 750Z images was quite neutral and appealing, good tonal balance, nice color response. It was hard however to ignore the few issues which affected the five times zoom lens; pincushion distortion, vignetting and corner softness (see below).
Lens corner softness
One thing we noticed fairly early on was significant softness in the corners of the frame at from mid-zoom to telephoto, this seemed to affect the right hand corners more than others. We suspect that this could be an issue with our test camera, either a lens element out of alignment or the CCD.
|98 mm equiv., ISO 100, F3.8|
|62 mm equiv, ISO 80, F3.3|
|62 mm equiv., ISO 80, F4.0|