P900 and Depth of Field

 


P900 at and near 2000mm: DOF Considerations


Depth of field (DOF) refers to the range of distance of a subject that appears acceptably sharp in the image.  DOF varies depending on the sensor size of the camera, aperture setting and subject distance, as well as print size and print viewing distance. A good tutorial about depth of field is given on the Cambridge in Colour website here: http://www.cambridgeincolour.com/tutorials/depth-of-field.htm.


In my earlier tests on the Panasonic FZ38 I used a nearby power pole at a distance of 30m as a test subject.  The pole has wires with fittings which are separated by some distance as viewed from the tripod position.  The FZ38 had a maximum zoom setting of 486mm EFL (86.4mm actual FL) and like the P900 it had a 1/2.3” sensor.  At its widest aperture and 486mm, the total DOF at 30m for the FZ38 was 5.5m.  The separation of the wires was far less than that total DOF.  Even with a 1.7x teleconverter attached to the FZ38, giving an effective 826mm EFL (147mm actual FL), the DOF was 1.9m and still exceeded the separation of the wires.


However, the P900 at 2000mm EFL has a much shallower DOF.  I calculated total DOF values at both 1800mm and 2000mm EFL and widest aperture for a range of subject distances as shown in the table below.  I used the standard circle of confusion diameter for a 1/2.3” sensor of 0.005mm.



Above: Table of Total DOF versus Subject Distance at Widest Aperture and f/8


As can be seen, the total DOF at close subject distances is very small, being just 16mm and 12mm, for 1800mmm and 2000mm respectively, at 5m and widest aperture.  I plotted the results of the table in a graph, as shown below (click on the image for the full sized view):



Plot of Total DOF versus Subject Distance at Widest Aperture


The total DOF increases rapidly with distance, and at 30m it is 55cm and 45cm for 1800mm and 2000mm EFL, respectively, at widest aperture.  It is also to be noted that at 30m distance there is a difference of 10cm in total DOF between 1800mm and 2000mm EFL which, depending on the subject, can be quite significant.



Conclusion

DOF is a major factor to consider in relation to images taken at or near max zoom with the P900.  At close subject distances the DOF is very shallow.  At greater distances, such as around 10-30m, which might be the case when taking shots of birds, DOF is still likely to be very important.



Other Considerations


Other factors which may impact on the IQ of P900 images of distant scenes taken at or near maximum zoom may include air turbulence, smog, haze and mist.  


An explanation of air turbulence in relation to limitations in the accuracy of laser surveying is given in online exerpts of this text.  Air turbulence is caused by time-dependent variations in the refractive index of the atmosphere, due to dynamic variations in the air density, and the direction and speed of propagation of light beams in the atmosphere.


While air turbulence generally is more of a problem at long distances, it has been noted that air turbulence can occur even at relatively short distances.  Air turbulence (called heat waves here) was observed while testing a lens outdoors at a distance of 30m (100ft) over thick green grass.



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