ZS20/TZ30: looking into P Mode

Started Jan 6, 2013 | Discussions thread
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ZS20/TZ30: looking into P Mode
Jan 6, 2013

Several months ago I determined the widest available apertures of the ZS20 at its 45 discrete steps in FL from 24mm to 480mm (equiv.) as posted here: http://forums.dpreview.com/forums/post/41241136. This was obtained by using A mode with the aperture set to its widest value (f3.3) at 24mm. Zooming the lens then provides the widest available aperture at each zoom setting. At that time I had not considered using that technique for taking shots, but Erik Ohlson has reccomended that recently, as discussed in this thread here: http://forums.dpreview.com/forums/post/50566809.

I've called Erik's recommendation the "Widest A Mode", and I've attempted to compare it with P mode. I noticed on reviewing my images taken with my ZS20 in P mode in bright lighting that the aperture setting never exceeded f/6.4. To investigate that further I set the cam to a high ISO in bright light, and I found that P mode limits the aperture to f/6.4 at 480mm (equiv.) and to f/6.3 at all other FL settings. That is shown by the red overexposure warning text which appears on the LCD. At 480mm FL it reads "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. Presumably that is to avoid the loss of resolution caused by diffraction at those narrow aperture settings.

To further investigate the way P mode sets the aperture I looked at the EXIF of a large number of my images. Since last April I've taken several hundred images, mostly using P mode with ISO set to 100, at a range of distances, FLs and lighting conditions. I live in a subtropical region where the light level is usually very high, and yet I found that in the vast majority of cases the aperture set by P mode was usually the widest available at that particular 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.

An exception to this was at short FL settings, particularly at full WA (24mm FL), where there were often instances of slightly narrower aperture settings being used. At 24mm (widest aperture f/3.3), an aperture setting of f/4 was often used, with various shutter speeds, not only 1/2000s. At 45mm FL there was an instance of f/4.5 and a shutter speed of 1/1600s, where the widest available aperture was f/3.9. However, as the table from my resolution tests on the ZS20 (details at: http://www.ianperegian.com/My_FZ35_38_Webpage/ZS20_Resolution_testing_2.html) showed, in the short FL range the resolution remains high from the widest aperture up to about f/5.0, and therefore there is no disadvantage in usiing those settings.

To sum up, P mode will generally provide the optimum resolution at all zoom settings, but occasionally in very bright light it will increase the shutter speed and may narrow the aperture, but it will never narrow the aperture beyond f/6.3 (or f/6.4 at max zoom). On the other hand, using the "Widest A mode", the aperture will always remain at its widest available setting. This widest setting will however be f/6.0-f/6.4 in the zoom range 409-480mm FL. This mode therefore always gives the highest resolution at a given FL but at the cost of overexposure in very bright light once the shutter speed has reached 1/2000s. This is likely to occur more often in the low FL range where the available apertures are wider.


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