Tuesday, 27 February 2007

Vista metadata issues

Reports continue to appear about the problems with Micro$oft's new OS, Vista. Vista uses metadata as part of its search mechanism, but it is also possible to make changes to that same metadata. Many of the reports focus on the security implications, but maybe more worrying for digital camera users is that changes in the metadata can cause problems with files from digital cameras being recognised properly by other software. This could even render RAW files useless until a fix is provided.

The most recent mention is on Rob Galbraith's site. Further reading on the subject can be found on PhotographyBlog, and more general information on CNET.

Thursday, 22 February 2007

Going full circle (image circle and choosing a lens)

With PMA (the Photo Marketing Association exhibition in Las Vegas) just a couple of weeks away there are going to be quite a few new lenses announced (Canon and Pentax have already started the ball rolling), so it seems like a good time for a little piece here on the relevance of image circle to the format (sensor size) of the camera. Images produced by digital cameras are rectangular, yet the image formed by the lens is round. To get an image without any information missing at the corners you need a lens that produces an image circle at least as big as the diagonal or the rectangular image recorded by your camera. In fact since the image quality normally falls off towards the edges you need a lens to produce a circle that is a bit bigger than the format.

Now why should you care? You fit lenses to your camera and you get an image, right? That's fine until the time comes to buy a new lens. If you have a camera with a cropped frame sensor (currently all except a couple of Canon cameras) you have a choice of lenses that cover the full frame and those designed exclusively for the smaller format of most digital cameras. The lenses with the smaller image circle are no problem and are usually smaller and lighter (oh, and cheaper) than those designed to cover the 35mm format. It sounds like a good idea, and it may be, but you had better polish up your crystal ball, because you are making an investment in the future. What you need to decide is if you are likely to own a full frame camera, at least within the life of your lens, or be prepared to by a similar lens with a wider image circle when the time comes. A lens with a larger image circle will cover all formats up to it's maximum (the format for which it was designed). The smaller format lenses are restricted to that format or smaller.

Now you may wonder if this is such a big deal, as most cameras have cropped frame sensors. Go back to the crystal ball and find out if the maker of your camera system is going to introduce full frame cameras in the foreseeable future, and ask yourself if you are likely to be tempted by one. Larger sensors will get cheaper to make as the process of making them become still more reliable, and they may well find their way into cheaper cameras, and as we have seen, all cameras are tending to work their way down in price anyway. But cameras with cropped frame sensors have some very real advantages. Reduced size, weight and cost are all in their favour, and are likely always to be so, however far prices fall.

Photo: Lake district 2005

Monday, 12 February 2007

Out, damned spot! - sensor dust (part 1)

Most people using DSLRs will at some time have to deal with sensor dust. I say "most", because an increasing number of cameras employ some sort of anti-dust mechanism. In fact I think it is likely that within a very few years sensor dust will be a non-issue on new cameras. Of course the dust isn't on the sensor at all, but on a piece of glass in front of the sensor. But how does the dust get there? The particles that you see will have come either from dust in the atmosphere, or may have come from inside the camera itself. It is not unknown for new cameras to shed a few particles as they loosen up after manufacture. Sometimes these particles are even a bit oily from the shutter mechanism. After some use this tends to settle down and most dust found is from the atmosphere outside the camera.

You can do things to reduce the amount of dust entering the camera. Keep the lens throat covered as much of the time as possible, either with a lens or with the body cap. When changing lenses keep the camera pointing down. Try not to change lenses more than necessary particularly in dry dusty environments. Don't pull your big zoom lenses in and out more than you need, particularly if they are the "push-pull" type, as they act like a bellows, pulling air (and dust) into the lens and the inside of the camera. Finally, always turn the camera off before changing lenses. The sensor gets warm when the camera is on, and this increases static build-up, which in turn attracts dust.

After you have used the camera for some time it is almost inevitable that there will be some dust on the sensor and this will need to be removed (unless you like using the healing brush in Photoshop). But you are not always aware of sensor dust. Some circumstances make it more or less visible on your images. I had very few problems with dust on any of my (Canon) SLRs until I got a 5D. There could be a number of reasons for this. It may be that the glass filter in front of the sensor is nearer to the sensor on the 5D, so bringing any pieces of dust into sharper focus. It may be that the bigger sensor retains more heat. There is a larger mirror in it so it will probably stir up the air inside the camera with greater vigour each time a shot is taken. When the camera was new some of the dust was obviously greasy and my first attempts left smears on the sensor. This seems more common with the full frame cameras. They have shutters which are designed for a longer harder life and are greased better. Some of the particles inside the camera had probably come from around the shutter mechanism.

Another factor is that if you tend to shoot with wide apertures you are less likely to see dust as it will not be rendered as sharp. If you want to check if dust is on your sensor just set the lens to it's smallest aperture and take a shot of a plain white surface (out of focus) overexposing about a stop or two. View the image on the computer and you will see what is on the sensor (increasing contrast in levels will make it look even more obvious, if you can stand the shock). Even after cleaning don't ever expect to see it looking perfect. If there is noticeably less dust and it is not visible in normal use that is as much as you can hope for.

There are essentially four methods of removing dust (the camera maker may employ other ones in the design of the camera). The first is to use a blower to try to blast the dust off the sensor. The second is to use a statically charged dry brush which is specially made for the purpose. The third is to remove pieces of dust with either a vacuum device or a sticky probe (sounds worse than it is). The forth is to use some sort of swab (either dry or wetted with a cleaning agent similar to isopropanol).

In the followup to this piece I will go into more detail about the ways of removing dust and also link to some information from other sources.

Picture: Denham snow 2007