The generally accepted figure in the publishing and photographic industry is 300 PPI. However, this tells very little of the real story of resolution and image sharpness/quality.

The key issue here is resolution - that is, how much fine detail is recorded by the capture system, and can be seen in a print. It's also important to separate detail itself from apparent sharpness (i.e. edge definition) - as poor use of the unsharp mask so often demonstrates, fine detail and sharpness are two very different things.

The first thing I would say is that very often pin sharp results just aren't required. A fair percentage of images simply do not rely on fine detail in any critical way to achieve their expressive goals. That said, most images do benefit from fine detail, especially when viewed for extended periods - fine detail rewards the eye as it moves through the image, and allows your image to work more successfully when viewed from closer distances. Fine details gives your image greater longevity and interest.

The first and most important thing that will affect your image's sharpness (and thus its quality when enlarged) is your photographic technique. No amount of spending on fancy lenses will save you if your basic technique is poor.

The old maxim of being able to hand hold at (1/focal length) is fine for adequate results with small prints. The reality is, however, that even when using a 35mm camera with a 50mm lens, there will typically be an obvious visible difference in sharpness between shots taken (hand held) at 1/60th, 1/125th and 1/250th. Fundamentally, the faster shutter speed will result in a sharper image, and most people grossly over-estimate their ability to hand hold their cameras if shooting for anything but the smallest prints. There is no substitute for steady support, so the first step to take is to always shoot from a stable platform (e.g. tripod) if at all possible. If that is not an option, aim for a faster shutter speed if possible.

The second most important thing that will affect the final resolution of your image is focus and depth of field. Most people just aren't that good at focusing really well, and with small camera systems, it's very hard to focus properly because fundamentally it is very hard to see your subject through a small hole!

In any picture, there is only one plane of absolute focus (parallel to the film plane in all cameras without movements). Areas is front and behind this plane of absolute focus may appear acceptably sharp through depth of field, but are not truly in focus. When calculating what is 'acceptably' sharp, many people use systems like hyper-focal distance, circles of confusion etc. While these are useful aids, be aware that most of these systems/tables were created long ago when imaging systems and lenses were of lower quality - film and digital have come a long way, as has lens design (with higher quality lenses, anyway), and so modern systems are capable of capturing and printing far higher detail than ever before. With this in mind, the goal should be to take advantage of this higher recording ability, and thus to use a greater depth of field in most situations. Blur is easy to add later, detail lost however is lost forever.

The third determining factor is the quality of your recording system - that is, the quality (and size) of the system you are using to capture your image. Fundamentally, bigger pieces of film (or bigger sensors) record more fine detail. This is why two cameras both having 8 mega-pixels may be far apart in quality terms - the digi-cam with its small sensor has far less resolving power than the SLR with a large, high quality sensor. The ISO of your system is also important - with film, the smaller grain allows for recording of finer detail, and with digital, less noise is created at lower ISOs. Finally, the quality of your lenses is of course very important - but less important than the elements mentioned so far. Buying a better lens will not help you take sharper, more richly detailed images unless you address all of the above points first.

The second to last important factor after technique, focus, and capture system resolution, is how carefully you deal with your image in Photoshop. Poor Photoshop technique can very quickly ruin fine detail in an image - poor retouching, not understanding the image size dialog box, and bad sharpening can all be catastrophic to an image's fine detail.

The final factor, of least but still significant importance, is the printing system's ability to resolve detail and it's native resolution (PPI). This is where 300 PPI comes into play. Of course, 300 PPI of unsharp, out of focus data will result in an unsharp, out of focus print with no detail. But if you're managed the process properly up until the point of printing, then the ideal situation is to send the data to the printer with no further scaling by the printer's driver or RIP system. Some popular printers and their native resolutions:

• Epson Inkjet Printers - 360 PPI
• Lamda - usually run at 200 PPI, you should ask for your prints to be run at 400 PPI prints for the finest results
• Pegasus - 250 PPI

Beyond these resolutions there is very little increase in resolved detail. N.B not none, just very little!

As a good default, 300PPI is a sensible figure to use if you don't know the resolution of your printing system, and in general will result in very high quality results. In many ways, it is best to flip this discussion on its head and ask - how large can I print my files? Assuming you have shot a truly sharp image using good technique and high quality systems, then depending on your capture system, you will get a truly sharp print using the following formulas:

• Digital Camera at low ISO - divide the number of pixels your camera captures by 240
• Digital Camera at high ISO (> 400) - divide the number of pixels your camera captures by 260
• High Quality Film Scan of 35mm - divide the number of pixels you have from your scan by 300
• High Quality Film Scan of Medium Format - divide the number of pixels you have from your scan by 280
• High Quality Film Scan of Large Format - divide the number of pixels you have from your scan by 240

I.e. the lower number you divide by, the less pixels you need to make a sharp print. This is because the quality of those pixels is higher in those systems - more of those pixels are being used to actually record detail, and less to record noise/grain etc.

There is in fact much more to the story of resolution, detail and sharpness, but these are the basics. Master these are you will be well on your way to sharp, richly detailed prints.