P Mode and iISO with the ZS20


P Mode and Widest Available Apertures on the ZS20

If the ZS20 is set to Aperture Priority (A Mode) and to the widest available aperture of f/3.3 at full WA (24mm equivalent), then as the zoom is changed, the aperture also changes but gives the widest available aperture at each zoom setting (focal length).  I utilized this to determine the 45 discrete steps in focal length (FL) as the cam is zoomed in, as described earlier here.  I have called this pre-setting in A mode, the "Widest A Mode" (WAM), and when using this mode for taking shots it provides the widest available aperture at any FL while adjusting the shutter speed auomatically.

I compared using WAM with using P Mode, where the camera sets the aperture and the shutter speed automatically.  I had used P Mode over several months with my ZS20, usually set to 100 ISO, taking shots with different of zoom settings, lighting conditions and subject distances, including closeup shots.  I analysed the EXIF settings of about 450 of these images to determine what aperture was used by P Mode compared with the widest available aperture at that  particular FL.  This taks was made easier by using the browser of PSE v8 (Mac) where as well as the usual EXIF details of the thumbnail image, the widest available aperture at that FL is also given.

My results showed that P mode preferred to set the widest available aperture at a given FL.  There were some instances of f numbers higher than the widest available aperture but in those cases the shutter speed had reached the maximum of 1/2000s.  Thus, in preference to narrowing the aperture, P mode tends to increase the shutter speed as required for the given light intensity.

A exception to this was at short FL settings, particularly at full WA (24mm equiv.) where there were often instances of slightly narrower aperture settings being used.  At 24mm (widest aperture f/3.3), an aperture of f/4 was often used, with various shutter speeds, not only 1/2000s.  However, as the table from my resolution tests shows (here), in the short FL range the resolution remains at or near its maximum value from the widest aperture up to about f/5.0.  Therefore in terms of resolution there is no disadvantage in using those aperture settings.

I carried out further tests on my ZS20 to determine how high an f number P mode would select under extremely bright lighting.  To test this in normal outdoor light I used a high ISO setting, thus forcing the camera to use a shutter speed of 1/2000s and to then reduce the aperture as required.  I found that in P mode the aperture would not set an f number higher than f/6.4 at maximum zoom (480mm equiv.) and to f/6.3 at all other zoom settings.

In P Mode in very bright light requiring an exposure of 1/2000s and an f number higher than f/6.3 (or

f/6.4 at 480mm), a warning in red text appears on the LCD indicating overexposure.  At 480mm the reading is shown in red as "f/6.4, 1/2000" and for all other FLs it reads "f/6.3, 1/2000". In other words, in P Mode it is not possible to use aperture settings of f/7.1 or f/8.0.  Presumably that is to avoid the loss of resolution caused by diffraction at those narrow aperture settings.

To sum up, P mode will preferentially use a wide aperture and provide the optimum resolution at all zoom settings. In very bright light it will increase the shutter speed and if necessary may narrow the aperture, but it will not narrow the aperture beyond f/6.3 (or f/6.4 at the maximum zoom setting of 480mm equiv.).

Comparing P mode with WAM, the difference is that no aperture setting other than the widest available is used in WAM mode and therefore higher shutter speeds are needed to avoid overexposure in bright light.  Under very bright light in WAM, using the widest aperture and 1/2000s, the situation can arise where overexposure occurs.  This is much more likley to occur in WAM than when using P mode, because P mode can close the aperture (within the limits described above) under such conditions.


My results indicate that using P Mode is generally preferable to using WAM since it is more flexible in bright light and at the same time it gives optimum resoluton at all zoom settings.

Examining iISO on the ZS20

I carried out some tests on the iISO setting of my ZS20 since, as with the other "i" features, Panasonic does not explain how iISO actually functions.  iiSO can be used in either P mode or A mode but not in S mode.  In P mode when using iISO, the "iISO" symbol appears on the LCD together with the aperture and shutter speed selected by P mode.  When the shutter is fully depressed the actual ISO value used for the shot is momentarily displayed on the LCD in place of the iISO symbol.  In other words, it is not until the shot is taken that the ISO setting is (very briefly) displayed.  I think this is not a particularly useful arrangement!

Using iISO it is possible to take shots at intermediate values such as 160 ISO and 640 ISO which cannot be set using manual ISO.  However, testing in both good light and low light indicated that it is usually preferable to set the ISO manually, as detailed below.  

In Good Lighting Conditions

In normal to bright light, 100 ISO can generally be used.  If iISO is used in good light, it can tend to set the ISO above 100 to provide a faster shutter speed, especially when zoomed in where the apertures are smaller.  Therefore unless subject motion requires a fast shutter speed it is preferable to set the ISO manually to 100 ISO and avoid using iISO.  If necessary, 200 ISO can be used to provide a faster shutter speed without causing objectionable noise for most situations.

In Low Light

In low light, shutter speeds using iISO tend to be faster than required if compared with the slowest shutter speeds that are possible to achieve hand-held.  Therefore the ISO setting with iISO tends to be correspondingly higher than necessary.  Thus iISO appears to "play safe" in terms of the user's ability to hold the cam steady. 

In low light the best strategy therefore seems to be to set the ISO manually, using burst mode (2 or 5 fps with AF) and to use the slowest shutter speed that can be hand-held without causing blur.  This may be 1/8s, 1/4s or 1/2s depending on the person and the zoom setting.   This will allow the lowest possible ISO setting and increase the likelihood of a sharp shot within the set of burst shots.

An additonal strategy in very low light would be to set EV to say -1, and to correct in  PP.  This can allow a one-step lower ISO value to be set, although depending on the scene it may increase the noise level.  The benefit of this strategy would need to be evaluated against simply using a higher ISO without any EV offset.  Possibly shots could be taken using both strategies and the better result determined by later evaluation.  With subject movement in low light conditions, a fast shutter speed and high ISO setting would be needed, or else flash could be used.

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