Many users are puzzled by the numbers found on lenses. They go something like 1:3.5-5.6. What do these numbers mean? In short, they refer to the minimum aperture values that your lenses can go. The values “3.5-5.6″ can normally be found on a zoom lens. For entry-level dSLRs, the standard kit lens that comes together should have these values too.
3.5 refers to the lowest aperture value that you can set for this lens when it is zoomed out and 5.6 is the minimum value you can obtain when the lens is zoomed in to its maximum. As a general rule of thumb, the lower the aperture value, the more expensive it is. And the lower the aperture value, the more “blurred” effect (bokeh) you can achieve for your background, which means better isolation of the subject.
On the other hand, the higher the value, the larger your Depth of Field will become and the better your lenses will perform (up to a certain value). When controlling the Depth of Field is of priority, switch to the Aperture mode on your camera. Find out more about Aperture mode.
However, as a general guide, it is recommended to use aperture values between 8-11 for landscapes. The reason is that most lenses perform at their best and are sharpest at these values. Sure, you are free to use any value you like, and we tend to think that if we want the largest Depth of Field, it makes sense to set the values at 22 or 36. That is theoretically true, but note that the higher the value (above 11 or 16), the quality of the images may gradually degrade due to diffraction, which means your image may not be as sharp as at 8 or 11, although you will obtain a large Depth of Field view. For more information, read http://www.truphotos.com/2010/01/04/photography-basics-mode-dial-aperture-priority.
In some instances, photographers still use values above 11 or 16. This may be because their lens still performs excellently at those values, or it could be because they deemed the larger Depth of Field view to have more priority over the slight degrade in image quality. Which you choose boils down to your priorities.
Lenses that have low aperture values are used to achieve the “bokeh” effect in order to isolate a subject or to increase the shutter speed in low-light photography where flashes are not preferred, such as in wildlife photography.
Aperture values for such lenses can go as low as 1.2 and many of them can be found to be around 1.4 to 1.8. Again, note that lenses normally do not perform their best at the extreme values. Those that do cost a bomb. For the majority, it is better to “stop down” a little, meaning to increase the aperture value, to ensure better image quality (Note that increasing the “aperture value” is the same as narrowing down the “aperture size”.
For example, for a lens with a minimum aperture value of 1.8, around 2.5 should be a good place to start.
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