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Barrel and Pincushion Distortion

One of the compromises we must accept when choosing a compact camera with a large zoom range is the likelyhood of lens distortion. The HP 850's 8x zoom lens exhibitted a larger than normal 1.4% barrel distortion at wide angle and approximately what we would expect to see at 8x telephoto zoom, 0.8% telephoto distortion.

Barrel Distortion, 1.4% @ wide angle Pincushion Distortion, 0.8% @ telephoto

Vignetting / Light fall off

Our vignetting / light fall off test is very simple, a shot of a blank wall from two meters away, vignetting will always be most visible at wide angle and maximum aperture and will start to disappear at smaller apertures and/or further zoom. The HP 850 exhibitted some vignetting, more so at the bottom right corner of the frame. This went away with smaller apertures 'lens stopped down' but was still visible at telephoto focal lengths which was surprising.

Slight corner vignetting visible at wide angle and maximum aperture (F2.8), more at bottom right Vignetting better at wide angle with a smaller aperture (F5.7)
Slight corner vignetting visible at wide angle and maximum aperture (F2.8), more at bottom right Some vignetting in the bottom right corner of the frame still visible even at full telephoto

Purple Fringing (Chromatic Aberrations)

Just as with distortion we would expect to see some chromatic aberrations from a long zoom lens. Indeed, we managed to reproduce fairly strong blue chromatic aberrations on our test shot and our 'foil torture test' (below), however browsing through the few hundred real life test shots taken for this review it was difficult to identify any single image which exhibitted particularly bad CA.

Some fringing visible along the edges of this high contrast detail, although nothing distracting Our standard chromatic aberration test shot, strong and consistent CA

Foil "torture test" comparison

We first used this test to compare the CA visible on the Canon G3 with its predecessor the G2. However it's a useful test because it's easy to carry out at different apertures. It's simply regular aluminium foil 'scrunched' into a ball and shot in direct sunlight at a range of apertures.

As you can see from the 100% image crops below the exhibits visiblem blue CA on the edges of highlights at wider apertures but that these are reduced at smaller apertures, although still not totally eliminated.

F2.8 F4.1
F5.7 F8

Overall Image Quality / Specific Issues

The HP 850 is clearly better than the 812, it's fairly obvious that HP's engineers have been working hard to deliver better image quality and produce incrementally better digital cameras each time. Resolution seems to be good, color balance is accurate if sometimes a little saturated, best of all is exposure control and white balance. The 850's automatic white balance is considerably better than that found on many other digital cameras, and that can count for a lot when we talk about good image quality.

Complements aside it's also clear that HP still have some way to go to be matching the better established names for image quality, it appears that a little development on their demosaic algorithm would improve matters greatly.

Jagged diagonals / Demosaic artifacts

One thing I noticed fairly quickly were jagged diagonals against single color backgrounds, such as branches and wires against a blue sky. This is clearly the fault of the demosaic algorithm which isn't smoothing the transition between each pixel and instead sees them as individual black pixels. This can leave certain parts of an image with a very digital appearance.

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