A guide to photo resolution explained

Forget your Camera 'pixels' for a moment. The biggest single deciding element in your camera is often the most overlooked. 'The Lens'. The better the lens the 'sharper' the image, and the ability to get the images 'in focus' becomes a lot easier.

A higher resolution does not fix 'focus'...

Fantastic (as it is) to display an image on your PC Monitor screen - it does disguise an ugly truth!. You may have a 'Mega pixel' camera but the Monitor has a relatively poor resolution of 72 dpi (for Macintosh) or 96 dpi (for all PC users). This is actually 'not ideal' viewing resolution and would make your pictures ugly indeed were it not for a small utility that 'improves' the display. This common computer utility is called 'Anti-Aliasing' and is applied to all 'on screen 'images..

, (which is present on all recent computers over the past ten years or so), is a little algorithm that 'dithers' the pixels expertly to create a smooth pattern around all the pixels on screen which removes the apparent 'jaggies'. The jagged pixels is how the computer sees the images itself, just as spots of colour (pixels) with no defining edge. So successful has this been, that it's widespread adoption has transformed Digital camera photography and images and Internet browsing possibilities enormously. It does not 'improve' the printed quality however... just the text and pictures when viewed 'on-screen'.

Anti-alias is not a 'fix' for poor 'resolution'!

However using another print technique called '
Stochastic dither' it is able to reproduce subtle mid tones better than 'patterned dither' which has been a common adoption for all but the earliest of Camera colour printers. The upshot of this technique is being able to 'enhance' poor resolution images for printing, particularly those web images from the 'Internet' you happen to find at 72dpi.

High resolution photo printers?

Low cost ink jet printers can have a very high nozzle 'resolution' i.e. 1440 dpi seems high! And you know it's high, (because it's also very slow). It's also using more than four colours, perhaps six! Pictures can be impressive, surely this is high resolution? This can be misleading. The actual resolution is the headline 'target' resolution divided by the number of print nozzles. i.e. 1440 divided by 6 = 240dpi

You may be surprised to learn that Photographic images can be a very low resolution and still perfectly visually acceptable thanks to the Photographic RGB process which performs just as well at 200 dpi as it does at 400 dpi. The advantage of higher resolution is 'cropping an image' and maintaining optimal resolution for print. The sharper
(better lens detail) the better the enlargement options of any cropped image. This is often overlooked by cheap hi-res digital cameras. A good lens can be more expensive than the camera itself!

Photographic imaging is not just about using maximum dpi.

In fact trying to print them at any higher resolution will not yield much 'improved' detail at all, just a larger digital file size. So you can see 'apparent' resolution has it's limits. Photography is not about maximum dpi. But it does help to have the correct resolution for the output device used.

And a high resolution Printer does not 'necessarily' make an image prints 'sharper', it just prints (the dots) better, and 'dithers' the colours to make dots 'sharper'. The print nozzles are much smaller so they can 'despatch' tiny droplets on the paper more accurately.

A simple calculation for image resolution is 1.3 (varies in practice from 1.3 to 1.5 and maximum 2) times the screen frequency, known as LPI (lines per inch) which is anything from 133 to 200 LPI. This is how a professional views image size. If for example you have a colour magazine printed (using the CMYK process) at 133 dot (LPI) screen frequency and times the size you will get ( LPI x2). This then becomes; 133 x 2=266 dpi. This is what we need to print your cmyk images at the best print resolution.

However we do not scan or print images at 266 dpi either, for example (Colour Litho printers have 2,400 dpi film resolution)! The point here is that 'sharp' texts require a far higher resolution than those of camera images which are ''down sampled' to just 266 (or 300 dpi to allow for re-sizing). If we were to 'image out' or increase 'resolution' of a digital camera to 2,400, this would be counter productive as your eye cannot distinguish the detail between the higher end and the lower! Plus it gives you a ridiculous file size without any obvious benefit.

Shown above: Photo prints need only the minimum detail for each output device

In the same way real Photo prints need only the detail applicable for the output device. If 200 dpi (minimum or equivalent) is the final result then you need at least (2.49 Mb) RGB file to print a minimum size (140 mm x 100 mm) as a digital photo print and you need at (7.46 Mb) to print the 'same size' on a Digital Colour laser printer at 300 dpi. CMYK).

Although a laser laser printer may have a resolution of 600 dpi, - so the file size is much larger than your camera, yet 600 dpi photo image files are almost indistinguishable from a 300 dpi image size. Technically speaking this could even be reduced to 266 dpi without noticeable loss! However you can certainly see a noticeable increase in TEXT quality at 600 dpi compared to 300 dpi. Professional Imagestting often is normally set at 2,400 for film print imagesting for example for text and as low as 300dpi for photographs.

Our eyes are actually quite good at rendering a 'good image',
even though the actual image may be less than perfect. There is no doubt a better 'sample' can be achieved with a MEGA pixel camera of 6Mb or more but do not expect a significant increase in visible image quality without a good lens, as the lens enlargement and detail focus are more important than camera resolution. A high resolution camera will enable 'enlargements' of any detail captured by the lens - other factors still apply exposure, light quality, focus etc..

The high cost of 'LOW COST' of ink-jet photo printers...

What is not obvious 'at first' is the high cost of both paper and ink, not with standing that almost all ink jet photo printers have a very short working life (and if a fault develops it is usually almost impossible to fix without a replacement part or an entire unit). On top of this is the 'card compatibility' and colour balance even if your camera is capable of the 'output resolution' which looks high but is very likely to be 'set' 'automatically at it's lowest 'default' setting. (The default being a lot lower than the big number on the box). The reason for this is, is that all ink jet printers are painfully SLOW, and the Inks very expensive at their highest stated resolution (which uses the most ink). They are not really photographic print devices in the accepted sense of photo film prints - but they are capable of a higher resolution to print at the higher setting. However it's not generally used by default as its so slow. Results may therefore vary widely, make-to-make, batch-to-batch, user to user unless professionally maintained and target calibrated.

Copyright 2007 Art Services

A guide to digital Photo resolution

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