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ISO (Sensitivity) Adjustment

ISO equivalence on a digital camera is the ability to increase the sensitivity of the CCD to allow for faster shutter speeds and/or better performance in low light. The way this works in a digital camera is by "turning up the volume" on the CCD's signal amplifiers, nothing is without its price however and doing so also amplifies any noise that may be present and often affects colour saturation.

The Optio 330 has two ISO sensitivity choices of ISO 100 and ISO 200. There is also Auto ISO which will vary the sensitivity automatically depending on available light. It's a shame Pentax didn't include an ISO 400 option.

ISO 100, 1/50 sec, F2.6 ISO 200, 1/125 sec, F2.6

It's interesting to note that the exposure difference between the two shots was actually 1.3 EV, rather than the 1.0 EV you'd expect from a doubling in ISO sensitivity. This tends to hint that either Optio 330's ISO 100 is closer to ISO 80 or that its ISO 200 is higher than that. Overall the camera did a good job of keeping shadow noise to a minimum, even at ISO 200 the image is nice and clean. Kudos.


White Balance

The Optio 330 has a good set of white balance choices including manual preset white balance which we really wouldn't expect to find on an ultra-compact digital camera. As you can see from the results below the Auto white balance worked best in natural light. The pre-programmed white balance settings were excellent, indeed the incandescent and fluorescent settings are probably the best I've seen on a consumer digital camera. Manual preset white balance also worked very well.

Outdoors, Auto Outdoors, Daylight Outdoors, Manual
Incandescent, Auto Incandescent, Incan. Incandescent, Manual
Fluorescent, Auto Fluorescent, Fluorescent Fluorescent, Manual


Flash Performance

The Optio 330's internal flash has a specified range of 0.14 - 3.7 m (0.5 - 12 ft) at wide angle and 0.4 - 2.0 m (1.3 - 6.6 ft) at telephoto. In our tests it repeatedly produced underexposed or colour-cast images. This is a sign of either an inaccurate white balance (which should really be locked for flash photographs in low light) and / or poor flash exposure metering. Note that underexposure wasn't just a flash problem, I've written more about it below.

Skin tone test: Underexposed, good colour balance no visible colour cast on this image. Wide angle 2 m wall test: Looses out in the corners and also throws a noticeable cyan (light blue) cast over the whole image.


Barrel and Pincushion Distortion

The Optio 330 lens barrel distortion at full wide angle was measured at 1.2%, this is towards the high end of what we'd expect. The news is better at full telephoto, no measurable pincushion distortion.

Barrel Distortion, 1.2% @ wide angle Pincushion Distortion, 0% @ telephoto


Purple Fringing (Chromatic Aberrations)

The Optio 330's lens appears to be fairly good, it wasn't hard to detect some blooming (a white halo which runs into dark areas against overexposed areas) but that's fairly normal for this level of digital camera. The good news was that chromatic aberrations were extremely difficult to see / locate. I had to scan through quite a few images to find the example below, and that is very, very mild in comparison to some that we've seen.

Hard pressed to find evidence of chromatic aberrations in "every day" shots
Our now standard chromatic aberration test shot - mostly blooming


Overall Image Quality / Specific Issues

The Optio 330's image quality is relatively good, especially for its compact side. The camera clearly employs some kind of flat area noise reduction system (which you'll be able to see more clearly on the resolution charts on the next page). Resolution isn't the best in the world (see next page). Looking at an Optio 330 image at 100% you get a feeling of 'video image' rather than 'film like', despite the good colour the images suffer from unsophisticated image processing algorithms (Bayer interpolation, sharpening etc.).

Underexposure / Contrast

One thing that struck me when I first started to shoot our test charts was how underexposed they were. When shooting the colour patch test chart I normally use a +0.3 EV compensation (to adjust for the white background), on the Optio 330 this wasn't enough. It appears as though the 330's metering system takes a very cautious approach when faced with a 'flat surface' image such as the one below. This is true of both Multi Segment and Center-Weighted Average metering modes.

0.0 EV compensation +1.0 EV compensation

Even with a whole stop of compensation (+1.0 EV) the gray scale patches are only just starting to show any detail at the black end, and that's A LOT of compensation. This points to a secondary problem, that is that even when you have a good exposure the Optio 330's gamma curve is pushing dark coloured grays too far towards black:

+1.0 EV compensation

Every day shots were less affected, metering for well lit outdoor shots appeared to be fine, however it was still clear that the 'black end' of the grayscale was being pushed too far towards black which causes a loss of shadow detail. To counter this I've developed a simple Photoshop (6.0) curve which 'pulls' some of the detail out of the black end of the grayscale.

Before applying curve
(camera original)
After applying curve
(improved shadow detail)
Click here for a combined side-by-side image (800 x 600)

The difference is subtle but significant, the image on the right has better colour in the hull of the larger ship, better detail in the bottom of the smaller boat and detail in the shadows. Pentax Optio 330 owners are welcome to download this curve (it's still a bit primitive, you'll probably do well to test / change it yourselves):

Click here to download Photoshop
curve file (70 bytes)
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