Jon Tarrant

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F-stop numbers explained

Lens diagram - light collection and projection

Almost two years ago I had a go at explaining F-stop numbers but on re-reading that article I’m not sure how useful it was (and its over-simplification was sometimes so crude as to be inaccurate) so I’m going to tackle F-stop numbers again now…

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Macro Sharpness

This picture of tiny thyme flowers (6mm across) was taken as a vertical composition and has been cropped top and bottom but is shown full width.

Trying to evaluate the sharpness of macro lenses at macro reproduction ratios is tricky. Here is a quick look at the nature of the problem – and the real-world performance of Pentax’s weather-resistant full-frame D-FA 100mm f/2.8 Macro WR lens.

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Pentax Lenses

Graphic showing Pentax's current KAF-mount lenses and future launches for 2012-13.

Pentax has been the name on some outstanding photographic products over the years and now the company has unveiled its intended launches for the next two years. Let’s take a look at what Pentax has done so far and what it has up its sleeve for the coming months...

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Chromatic aberration

MTF test target showing areas being examined.

Nothing ruins a good picture quite like chromatic aberration, which reveals itself as colour fringes. Let’s see what chromatic aberration looks like on technical targets and in real-world pictures.

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MTF graph of Sigma lenses on Nikon bodies, plotted in cycles/pixel

Having set the scene with various technical definitions and explanations we are now ready to consider how best to compare the performances of pro-spec and consumer lenses.

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Example image captured using the Samsung 85mm f/1.4 lens, exhibiting excellent sharpness.

Having looked at resolution figures quoted in line-pairs-per-millimetre and in cycles-per-pixel attention now turns to lines-per-picture, which is a measure that defines the maximum number of picture lines and therefore indicates the amount of detail that any picture can contain.

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