Depth of Field and Diffraction

 


Small Sensors - Depth of Field and Diffraction


Panasonic camera models in the FZ and ZS/TZ range have a sensor size of 1/2.3” (6.17 x 4.55mm) or for a few models, 1/2.33” (6.08 x 4.56mm).  The total number of pixels on the sensor varies between 12MP (such as for the FZ38) and 18MP (such as for the ZS30).  This type of sensor is classed as “small” in comparison with the sensors used in DSLRs which are much bigger, such as the APS-C (23.5 x 15.6mm) sensor in the Nikon D7100 DSLR.


There are inherent differences between small and large sensor cameras in relation to depth of field (DOF) and the effects of diffraction, as described in detail on the Cambridge in Colour website here:

http://www.cambridgeincolour.com/tutorials/digital-camera-sensor-size.htm.


Some specific implications for the ZS30 in relation to DOF and diffraction are discussed below.


ZS30 and Depth of Field


The shorter the focal length (FL) of a camera’s lens, the greater the DOF.  However, small sensor cameras have much shorter actual FLs than large sensor cameras.  This is not so apparent when “35mm equivalent” focal length is used to describe the focal length range of the ZS30.  While the ZS30 has an FL range of 24mm to 480mm in “35mm equivalent” terms, the actual FL range of the lens is 4.3mm to 86mm.  The DOF is also affected by the aperture setting.  At a given FL, a narrower aperture (higher f number) has a greater DOF than a wider aperture (lower f number).


To calculate the DOF it is necessary to use the actual FL, not the “35mm equivalent”.  The actual FL for any zoom setting on the ZS30 can be determined from the table which I constructed here in the section on the ZS20, since the ZS20 and the ZS30 have the same lens and zoom range.  The 35mm equivalent FL length as read from the table needs to be divided by 5.58 to give the actual FL in mm.  That table also shows the widest available aperture for a particular zoom setting/FL.


The DOF can easily be determined for a particular FL and aperture setting by using a DOF calculator, such as can be found here: http://www.dofmaster.com/doftable.html.


Using such a calculator it can be seen that at full WA (actual FL 4.3mm) and widest aperture (f/3.3) for a subject distances of 1 metre, the total DOF is about 8m (0.5m in front of the subject and 7m behind).  At subject distances beyond 1m, the total DOF becomes infinite. 


However, if the FL  is increased slightly to 2x zoom, giving an FL of 8.6mm (48mm in 35mm equivalent terms) then the DOF becomes much shallower.  At f/4.2 (the widest available aperture at that FL setting) and a subject distance of 1m, the total DOF is only 60cm (22cm in front and 38cm behind).


At a zoom setting of 5x (129mm equiv.) the actual FL is 23mm, and the widest available aperture is f/5.4.  At these settings, for a subject distance of 1m the total DOF is only 11cm (5cm in front and 6cm behind).  At the same settings and a subject distance of 2m, the total DOF is 43cm (19cm in front and 24cm behind).  At the same settings and a subject distance of 3m, the total DOF is 98cm (41cm in front and 57cm behind).


Shallow depth of field therefore needs to be taken into account at short subject distances, especially for longer FLs and for closeups.  However, shallow DOF can be beneficial for isolating the subject from a distracting background, and that is certainly possible with the ZS30 as these example calculations show.


ZS30 and Diffraction


A good explanation of diffraction and how it affects digital camera resolution is given on the Cambridge in Colour website here: http://www.cambridgeincolour.com/tutorials/diffraction-photography.htm.


Diffraction depends on the f number (aperture setting), and is independent of the FL.  The effect of diffraction increases as the aperture becomes narrower, that is, as the f number becomes larger.  Because the ZS30 has a very compact lens it has a restricted aperture range, so that as the zoom setting is increased to longer FLs, the widest available aperture at each zoom setting becomes narrower.  This can be seen more clearly by viewing the table here which shows the widest available aperture for every zoom setting (the ZS30 has the same lens as the ZS20).


At full WA on the ZS30 (1x zoom and 24mm equivalent FL) the widest available aperture is f/3.3.  Using the Diffraction Limit Calculator shown on the Cambridge in Colour website here, for the ZS30 (a digital compact with a 1/2.3” sensor) at aperture settings of f/3.3 and f/4, the ZS30 is shown to be not diffraction limited.  At f/4 the diameter of the Airy disk is 5.3 microns.  At f/5.6, the Airy disk is 7.5 microns in diameter and thus diffraction begins to have an effect, although in terms of image IQ this effect is only very slight.  At aperture settings above f/5.6 the effect of diffraction becomes slowly more pronounced.


The visual effect of diffraction as the aperture is set narrower can be seen by viewing my resolution chart crops here for the ZS20 at different aperture settings.  It can be seen that there is a gradual increase in “fuzziness” above f/5.6, and this becomes worst at f/8. However, it should be emphasised that the crops are shown magnified to 300%, and at normal viewing sizes the actual effect of diffraction is only slight, even at f/8.  Therefore, although the effect of diffraction should be borne in mind it does not mean that an aperture setting of f/8 must always be avoided.  There are situations where f/8 would be quite appropriate to use, such as in very bright light or when maximum DOF is required for macro shots.


The ZS20 has the same lens and aperture range as the ZS30 but the pixels are slightly different in size.  The pixels on the ZS20 are 1.40 microns wide while those on the ZS30 are 1.26 microns wide. That means the effect of diffraction will be greater for the ZS30 by the ratio 1.40/1.26 = 1.11.  However, the resolution of the ZS30 is greater than that of the ZS20 by the ratio 2448/2160 = 1.33, as shown in my resolution comparison results here.  Therefore, the increased resolution of the ZS30 compared with the ZS20 is offset by an increased effect of diffraction at narrower apertures.  Differences in apparent overall IQ between the models are however also affected by different JPG processing in the two models, and that effect may be more important.





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