Panasonic Lumix DMC FZ50 Digital Camera An assessment of the Extra Optical Zoom (EZ) and Digital Zoom (DZ) options Dr James C Brown CEng FIMechE 1. Introduction...2 Extra Optical Zoom (EZ)...2 Digital Zoom...3 FZ50 LCD Display of Focus and Metering options...3 2. The Effect of EZ and DZ on Focus...4 Practical confirmation...5 3. The Effect of EZ and DZ on Metering...5 4. The Effect of EZ and DZ on Resolution...6 The Effect of EZ...7 The Effect of DZ...8 5. Discussion...12 Extra Optical Zoom EZ...12 Downsized Digital Zoom...12 6. Conclusions...13 7. Appendix Optical Calculations...14 Copyright 2012 James C Brown -1-
1. Introduction The Panasonic DMC FZ50 Digital Camera has a 12x optical zoom lens with a focal length range of 7.4 mm to 88.8 mm and maximum apertures of F/2.8 and F/3.7. In conjunction with its 1/1.8 10MP CCD sensor that corresponds to a 35 mm equivalent focal length range of 35 mm to 420 mm. In the Operating Instructions for DMC FZ50, the section on Taking Pictures with the Zoom, in addition to describing the use of the zoom ring to adjust the magnification provided by the 12x optical zoom, describes two additional forms of zoom, Extra Optical Zoom (EZ) and Digital Zoom (DZ) Extra Optical Zoom (EZ) The FZ50 Operating Instructions include the following statements about Extra Optical Zoom This camera has a 12x optical zoom. However, if the picture size is not set to the highest setting for each aspect ratio [4:3/3:2/16:9], a maximum of 21.4x optical zoom can be achieved with no picture quality deterioration. Extra optical zoom mechanism When you set the picture size to (3M EZ) (3 million pixels) the 10M (10 million pixels) CCD area is cropped to the centre 3M (3 million pixels) area, allowing a picture with a higher zoom effect. In the table of Picture size and maximum zoom magnification the maximum zoom values for a 4:3 Aspect ratio are shown as 10M 12x, 8M 13.4x, 5M 17.1x and for both 3M and 2M 21.4x. That is because no EZ cropping takes place below 3MP and the image file for a 2MP image is downsized from 3MP to 2 MP by the software in the camera. In addition if a reduced image size is selected with the FZ50 set to minimum zoom the normal 10 MP image will be downsized to the reduced value selected before being written to the SD card. As this results in reduced resolution it seems to be of little or no value considering the high capacity and relatively low cost of SD cards. A similar effect may occur with various other combinations of focal length and reduced image size. Due in part to the somewhat misleading nature of the description in the Operating Instructions the Extra Optical Zoom feature of Panasonic digital cameras has been the topic of many discussions in the DP Review Panasonic Talk Forum. Together with the use of the EZ crop size instead of the sensor size in calculating the 35 mm equivalent focal length in the EXIF data, that has led some PT Forum members to make erroneous claims about their ability to take hand held shots at 748 mm focal length using the 21.4x zoom 3MP EZ setting. As the size in pixels of the subject of a 3MP EZ image is identical to that of the same subject in a 10MP image it is obvious that for a 3MP EZ image the 35 mm equivalent focal length of the FZ50 lens remains unchanged at 420 mm (88.8 mm actual). From these PT Forum discussions it is clear that although it is recognised by many that EZ images are simply in camera crops, the fact that the use of EZ Zoom provides a magnified view of the subject by enlarging the selected EZ crop so that it fills the entire frame of the EVF/LCD display has made it very popular. It is also claimed by some that by using only the reduced area of the sensor the use of EZ Zoom improves focussing and metering accuracy resulting in higher quality images. See for example this post by L Bradford, http://forums.dpreview.com/forums/read.asp? forum=1033&message=38598509, in which he wrote: "EZ Zoom is an in camera crop, but the real advantage is that the light metering and focus are on the cropped area only. Therefore the possibility of a better, sharper image." As a consequence of that statement and the contradictory experience described by Gary R in this post, http://forums.dpreview.com/forums/read.asp?forum=1033&message=38648910, I decided to investigate the effect of EZ Zoom on focus, metering and image quality. -2-
Digital Zoom For a full frame 10MP image selecting Digital Zoom crops the image by the chosen DZ zoom factor then up-sizes the crop to 10MP by interpolation, i.e. by creating additional pixels at intermediate positions with data values derived from those of the adjacent original pixels. It has been claimed by some that downsizing the resulting DZ image by the same zoom factor in post processing can improve the image quality by smoothing the noise and providing good IQ which can be useful for small distant objects such as the moon. In 2007 JJDiniz referred to the use of digital zoom followed by downsizing while describing his attempts to obtain high quality shots of the moon with his FZ50 and TCON17. See: http://forums.dpreview.com/forums/read.asp?forum=1033&message=25821527 In 2009, tymevest discussed the use of DZ followed by downsizing in relation to his shots of the moon and distant aircraft stating, I have noticed that when the camera is in digital zoom, it processes the image after it is increased in size. If you take the same image and then reduce it, the picture seems a bit more clear with less noise. See: http://forums.dpreview.com/forums/read.asp?forum=1033&message=33014822 In view of these claims I decided that for completeness I should also investigate the Digital Zoom (DZ) feature by taking 2X and 4X DZ shots of suitable test subjects and comparing these in both normal and downsized form with the equivalent crops from matching areas of 10MP full frame and EZ images. FZ50 LCD Display of Focus and Metering options As illustrated in the following images the LCD display of the FZ50 is used to provide information about the various focussing and metering modes. For normal light levels the size and position of the areas used for the various focussing modes are shown in green. For low light levels they are shown in red. The very large difference between these red and green areas may be very important for some subjects such as small birds in dark surroundings. For the Spot metering mode only the central + mark is shown on the display. As the Spot metering area does not appear in the display the dimensions indicated in the above image were derived from the drawing on page 90 of the FZ50 Operating Instructions. -3-
2. The Effect of EZ and DZ on Focus Suspecting that all nine AF areas might lie within the EZ Zoom areas of the sensor, I measured the size and position of the rectangles indicated on the LCD, calculated the dimensions of the corresponding EZ Zoom areas and marked them on a drawing of the LCD. As will be seen from the following image all nine AF areas are inside the 8MP and 5MP EZ images and only a very small part of the outer AF areas is outside the 3MP EZ image. That suggested that the same AF areas might be used for the 10MP and EZ settings and that as a result selecting an EZ setting would make no difference to the focussing accuracy. For both the normal and high speed single area focus options, pressing the FOCUS button on the side of the FZ50 lens barrel causes the nine AF areas in the above image to be displayed and allows any of the nine areas to be selected as the desired focus area. Careful measurement of the dimensions and positions of these AF areas on the LCD showed them to be identical for the 10, 8, 5 and 3MP images. That indicates that all nine AF focus areas are inside the area of the corresponding EZ crop which is magnified to completely fill the display. It was also found that for each of the EZ image sizes any of the nine AF areas could be selected as the desired focus area by using the cursor buttons. The table below shows for each image size the dimensions of the corresponding focus areas on the LCD and their equivalents in pixels. -4-
As with the EZ settings, for both the 2x and 4x DZ settings all nine focus areas were displayed and any one of these could be selected using the cursor buttons. However unlike EZ mode, when an attempt was made to focus in DZ mode, in both high and low light levels, focus was confirmed by the appearance of a 23.5 x 17.5 mm green rectangle instead of the AF area which had been selected. Practical confirmation Using a test chart of 160 mm x 120 mm and 90.0 mm x 67.5 mm rectangles to represent the dimensions of the 10MP and 3MP EZ images I adjusted the position of my FZ50 so that the outer rectangle just filled the frame. By placing a piece of dark thread at various positions across the width and height of the outer and inner rectangles I was able to check for the 10MP and for the 8MP, 5MP and 3MP EZ settings the position of the AF areas at which the camera would focus in each AF mode. As illustrated in the following images of the FZ50 LCD, these tests confirm that for the 8, 5 and 3MP EZ Zoom settings the relationship between the dimensions and positions of the nine AF areas and those of the corresponding EZ image area is identical. The TZ7 shots of the FZ50 LCD in the following section were taken while I was using a print of a bird in a tree to assess the effect of EZ Zoom on metering. These images provide further confirmation, for a more typical subject, that the use of EZ Zoom results in the size and position of the nine focussing areas being adjusted to match the size of the selected EZ crop. 3. The Effect of EZ and DZ on Metering If the use of EZ Zoom limits the area of the sensor used for metering exposure to that of a selected EZ image size the level of illumination on the pixels surrounding that area should have little or no effect on the exposure required. There should therefore be no significant difference between the exposures settings for a 3MP EZ image surrounded by either a light or a dark background. -5-
To assess the effect on metering of using EZ Zoom, I took 10MP and 3MP EZ shots of a test image consisting of a 112 mm x 84 mm picture of a bird in a tree placed against a light and a dark background. To provide documentary evidence of the results obtained I used my TZ7 to take the shots of the FZ50 LCD which are shown in the following images. For these shots the FZ50 was set to the 3MP EZ setting and its position adjusted until the 112 mm x 84 mm test subject just filled the frame. Using Multiple, Centre weighted and Spot metering modes, 10MP and 3MP EZ shots of the test image were taken against both the light and dark backgrounds. The exposure test results presented in the above table confirm that for EZ zoom images only data derived from pixels that lie within the EZ image is used for exposure metering. As will be seen from the table, where there are differences in the exposure readings these appear to be consistent with the differences in background colour and metering method and do not exceed a factor of 2, i.e. one stop. While I did not attempt to assess the effect of DZ on the Pattern and CWA modes a simple manual check confirmed that the Spot mode was still effective. 4. The Effect of EZ and DZ on Resolution To assess the effect of EZ and DZ on resolution I used the coloured Es test chart described in my report on the resolution of my FZ50. See:http://ianperegian.com/My_FZ35_38_Webpage/Home_files/FZ50%20Doc%202new.pdf To make a similar assessment with a more typical subject I used a picture of a bird made from feathers on a background painted in water colour. With the FZ50 mounted on a tripod I took full frame, EZ and DZ shots of the Es test chart and the picture of the bird. As the following images have been copied from unprocessed originals they can be examined in much more detail by choosing a high magnification. For example at 3200% the individual pixels can be clearly seen. It is also possible to compare selected images by locating them in the navigation pane on the left and clicking on any chosen point within the images of interest. -6-
The Effect of EZ As will be seen from the following images although the use of EZ Zoom provides additional magnification in the EVF and LCD it does not provide any additional magnification of the image which is recorded to an SD card. In addition as detailed examination will show, if there is any difference in resolution or image quality between the following full size and EZ images it is insignificant. -7-
The Effect of DZ The following 100% crops from the original images are intended to allow comparison at high magnification of the resolution of 10MP, 3MP EZ, 2x DZ and 4x DZ images. -8-
To allow detailed comparison of the resolution of downsized 2X and 4X DZ images with that of a full frame 10MP image I used the Lanczos3 option of the FastStone Image Viewer to downsize the DZ images by factors of 2 and 4 respectively before saving them as the lossless JPEG files which were used to form the following composite images. -9-
The composite image below consists of 684 x 912 pixel 100% crops taken from the preceding images. These allow direct comparison of the quality of a 10MP full frame image with that of 8MP, 5 MP and 3MP EZ images and of the appropriately downsized 2X and 4X DZ images. - 10 -
The 400 x 300 pixel 100% crops in the composite image below allow direct comparison of the image quality of a 3MP EZ image, a 10MP image and downsized 2x and 4x DZ images. Close examination of the above images, especially at high magnification, indicates that while the smoothing effect which results from downsizing the digital images appears to give a slight reduction in noise, the resolution of fine detail in the downsized DZ images is no better than that of an EZ image or a normal 10MP image. The small differences in the appearance of the brown feathers in these crops suggest that in addition to reducing the noise, the downsizing of DZ images may also affect their appearance in terms of sharpness, contrast and colour. - 11 -
5. Discussion Extra Optical Zoom EZ Magnification When EZ Zoom is selected by choosing a reduced image size it causes a central crop of the chosen image size to be magnified so that it fills the entire area of the EVF/LCD display. However as confirmed by the FZ50 images obtained in the test results presented in this report the selection of an EZ image size does not provide any additional magnification of the image recorded to the SD card. In that respect Extra Optical Zoom is a very misleading term which has led to significant numbers of Panasonic camera owners being confused by it. The Effect on Focussing Careful measurement of the size and positions of the focussing areas in relation to the EVF/LCD display showed these to be identical for each image size. It was also found that for each of the EZ image sizes any of the nine AF areas could be selected as the desired focus area by using the cursor buttons. As these findings were confirmed by tests in which a piece of dark thread was used to investigate the location of the focussing areas it is clear that for small remote subjects, such as a bird amongst the branches of a tree, the use of Extra Optical Zoom (EZ) is likely to result in more accurate focussing. It should however be noted that in poor lighting conditions much of that benefit may be lost due to the greatly increased size of the focussing area which is designated in red. The Effect on Metering From the tests conducted with the picture of a bird placed against light and dark backgrounds it is clear that when EZ zoom is selected only the area of the sensor specified by the EZ image size is used for metering the exposure. The differences in the recorded exposures which do not exceed a factor of 2, i.e. one stop, appear to be consistent with the difference in background colour and metering method. Downsized Digital Zoom The red, green and blue values for each individual pixel in a normal or EZ image are obtained from the value of that pixel and those of several adjacent pixels. In the case of 2x and 4x DZ images additional pixels are created at intermediate positions by using interpolation to derive their red, green and blue components from those of the pixels of a normal image. Taking account of these facts it is hardly surprising that, as close examination of the resulting images suggests, the averaging process involved in downsizing of a 2x or 4x DZ image may result in some minor alteration in the quality of the image in terms of noise, sharpness, contrast and colour. However it does not result in any increase in the resolution. Resolution and image quality As will be seen from a detailed comparison at high magnification of the 8, 5 and 3MP EZ images with the 10MP images presented in this report, the use of EZ zoom does not provide any increase the size or resolution of the image which is written to the SD card or result in any significant difference in quality between the EZ and original images. A similar comparison of the full 10MP image with the original and downsized DZ images shows that while as a result of downsizing there may be some reduction in noise in the image the use of DZ and subsequent downsizing does not provide any increase in the resolution or any significant difference in image quality. - 12 -
6. Conclusions From the findings discussed above it may be concluded that while the use of EZ and DZ may allow more accurate focussing and metering they do not provide any benefit in terms of increasing the resolution or the level of detail in an image. Neither, in the opinion of the author, do the results of the tests discussed in this report provide any evidence that the use of EZ or downsized DZ images results in any significant increase in image quality. The issue of image quality is however entirely a matter of personal opinion. For that reason I leave it to the individual to examine the evidence and use their own judgement to decide whether or not the use of EZ and or downsized DZ images may be beneficial for their own particular purposes. The use of EZ Zoom does however have a significant advantage of providing a magnified image of the subject in the EVF/LCD. That magnification can be extremely useful when composing the image and in deciding exactly when to release the shutter. It is also possible that the magnified image provided by the use of EZ may assist the user in keeping the line of sight more stable and so assist the OIS system by reducing the amount of correction required to eliminate or reduce blurring due to camera shake. A disadvantage of EZ and to a greater extent DZ is that with the reduced field of view and the resulting increase in the movement of the image in the display, it is more difficult to keep the line of sight sufficiently stable to avoid accidentally omitting part of a desired image such as part of the head or tail of a bird. Giving careful consideration to the various benefits and limitations described above I believe that Panasonic could have made the EZ Zoom setting of the FZ50 much more useful if they had provided an option to use an enlarged EZ crop for viewing, focussing and metering while allowing the information from the entire sensor to be recorded to the SD card as a 10MP JPEG or RAW image. - 13 -
7. Appendix Optical Calculations Calculation of the test subject dimensions and distance used to assess the focus and metering accuracy at 12X zoom for 10 MP full frame and 3 MP EZ Zoom images. Projected size of 10 MP sensor at 12X zoom and focussed at 2. 0 metres 1/u + 1/v = 1/f 1/ 2000 +1/v = 1/ 88.8 1/v = 1/88 1/2000 = (2000 88)/2000 x 88 v = 88 x 2000/1912 = 92.05 Scale = 2000/92.05 = 21.727 Sensor Size 7.144 mm x 5.358 mm Projected size of sensor = 155.22 mm x 116.41mm Projected size of central 3 MP of sensor at 12X zoom and focussed at 2. 0 metres 155.2x2048/3648 mm x 116.41x1536/2736 mm = 87.13 mm x 65.35 mm To allow for the inaccuracy of thin lens theory, I decided to use a 160 mm x 120 mm test area to represent the 10 MP area of the full sensor. The corresponding area for a 3MP EZ image should therefore be: 160x2048/3648 mm x 120x1536/2736 mm = 89.82 mm x 67.37 mm. For practical purposes these dimensions were rounded to 90 mm x 67.5 mm. - 14 -