priorities: more highlight detail from digital photos to get
Digital cameras have replaced in the consumer area Knips with film loaded miniature cameras almost completely, while in amateur and professional sector is still some resistance fighters swear by the chemical way of life. Even digital cameras have certain weaknesses that is against the shooting on film. Which will highlight the priority mode of new Canon SLRs meet, the more Dynamics without HDR multiple recordings and the work involved promises.
With ISO 100 normally exposed shot. The yellow of the tower is slightly faded in the sun, the clouds in the sky drawing weak. 
the exposure latitude and tonal range of negative film is still regarded as unmatched. In particular negative film responds to over-exposure (including a particularly light parts or "lights" in the subject) with a gradual "zadelka" or a flattening curve. Digital cameras tend to contrast more or less violent, "cutting off" the lights, or "clipping" called - or in photo lingo: erode. In less extreme cases, just only the "light drawing" such as a sky pale and inconspicuous. Other hand, can be done but a lot - with special recording techniques and, where appropriate, in a photo editing program.
cope better order the implementation of high subject contrast in a well-drawn picture today like HDRI (High Dynamic Range Imaging) is used - that's only case of stationary scenes and requires a great effort in the recording - usually several differently exposed images a tripod. If something moves in the picture, creates the problems.
With ISO 2oo (HPM) is exposed. The yellow of the bell tower is a bit rich, measurement technology can be found in the lights a little lower RGB values. The sky appears to be little changed.
But there are other ways of tracing more than usual, extract from a digital recording. For amateur photographers provide the latest cameras from Canon, the EOS 40D and the 450D called a so-called HP-mode (highlight priority mode or HPM), also called "Tonwertpriorität" in the German language version. The effect of this recording is, however, with a gain of an aperture stop in the highlights relatively low and not dose. For this work They finished with the camera coming from JPEG images without any further post-processing and without these lenses when shooting or something else would have to consider.
arise generally softer photos. Among the tested RAW converters dominated this was only Canon's in-house Digital Photo Professional (DPP). The same can be achieved with deliberately underexposed images in RAW format and subsequent image processing with appropriate Tonwertausgleich.
The ratio of the brightness in a scene - or, more photographs, the object contrast - can be very small - about 1:100 or even considerably less (it's weather, fog, paper documents), but also extremely great, eg in sunlit landscape, which is adjacent to a shady forest, or in backlit situations: Contrasts of 1:10,000 and 1:100,000 or even more are possible. The latter contrasts the human eye can not detect "simultaneously" (overall, his play between successful adaptation in the moonlight or the sun in the snow scene a million times). A landscape design with sun behind the other hand, can move at "normal" range 1:500 to 1:1000.
Perceived contrast
photo paper itself can only brightness range 5-6 f-stops (contrast 1:32 to 1:64) play between paper white and maximum darkness. The art of drawing photos or prints, then, lies mostly higher exposure to curtail the scope of the recording so or "bend" to that a high contrast image is produced and acting in forming important parts of both light-and shadow detail still present. Monitors can contrast some. 8-10 f-stops constitute (1:256 to about 1:1000).
taken with ISO 100 and processed in Photoshop with various functions. Sky lights and draw much better, sunny tower area was a bit too reddish.
Since dealing with these large and Exterm different speed ratios is impractical and not very clear, use a logarithmic measure, the exposure value (EV) or English "exposure value (EV), a difference equivalent to 1 between two light levels (eg between 10 and 11) by 2 and is also referred to as "one stop". Instead of the change in aperture of f eg / 8.0 to f/5.6 to double the exposure time, about 1 / 250 second to 1 / 125 s. So you could just as easily speak of time periods. A level "+" always corresponds to the doubling of the amount of light a stage "-" to halve.
changing the camera's combination of aperture of f / 8 and the speed of 1 / 250 sec "same direction" to f/5.6 and shutter speed 1 / 125 sec, then one has set a change of two "stops" or EV values and thus a quadrupling of the effective "amount of light." An "opposing" change from baseline to f/11 aperture and shutter speed 1 / 125 sec or f/5.6 and shutter speed 1 / 500 sec is, however, the same exposure. The aperture numbers themselves have to specify a ratio of 1 to (root 2) or 1:1,414 per f-stop, since it is the ratio of focal length to the effective light transmission area. doubling the diameter of the aperture, quadrupled the amount of light the aperture, the ratio of 1:2 (eg f / 4 or f / 8). Therefore, cover this area the "stops" 4, 5.6 and 8 from.
slide film has an exposure range of about 6 stops (Helligeitsverhältnis or subject contrast 1:64) - significantly less than that which provides a digital SLR, the current models in amateur and mid-range now at low or medium speed 90-10 f-stops create (1:512 to 1:1024), while negative films about 10 or 11 f-stops absorb (about 1:1000 to 1:2000) - so big the difference is not anymore, at least not for color films.
unpleasant, however, is a phenomenon out in the titillating play of shadows on the lower end of the exposure latitude - Banding called the formation stage, clearly separated islands of different sounds that can occur in strips or blocks-like bounds and therefore very much disturbing than coarse grain at high speed film. Even in the shadows like the existing "color noise" is quite unattractive. So today is an improvement in the tonal range herbeizuwünschen much more than more pixels.
light value levels, f-stops (1 / 3) and exposure times
| EV (EV) | f-stops (in 1/3-stop increments) Exposure time | |
| -4 | 22 | 1/2000 |
| | 20 | 1/1600 |
| | 18 | 1/1250 |
| -3 | 16 | 1/1000 |
| | 14 | 1/800 |
| | 13 | 1/640 |
| -2 | 11 | 1/500 |
| | 10 | 1/400 |
| | 9 | 1/320 |
| -1 | 8 | 1/250 |
| | 7,1 | 1/200 |
| | 6,3 | 1/160 |
| 0 | 5,6 | 1/125 |
| | 5 | 1/100 |
| | 4,5 | 1/80 |
| 1 | 4 | 1/60 |
| | 3,5 | 1/50 |
| | 3,2 | 1/40 |
| 2 | 2,8 | 1/30 |
| | 2,5 | 1/25 |
| | 2,2 | 1/20 |
| 3 | 2 | 1/15 |
| | 1,8 | 1/13 |
| | 1,6 | 1/10 |
| 4 | 1,4 | 1/8 |
the "traditional" values are f-stop increments), the latitude of the 40D is about 9.5 EV, the Fuji Finepix S5 pro is as high as 10 eV and more.
camera exposure curves
Modern digital cameras already show a "film-like" exposure characteristic, which is attributable to the processing of raw sensor data (RAW) by software in the camera or on the PC. If you look at the so-called "raw data" in a RAW image file, without interpreting visual tools, but for a "linear" conversion to a TIFF file, this is clearly (See box diagrams). Such a "linear" conversion is possible with the in-house program "Digital Photo Professional (DPP) from Canon, or the freeware converter dcraw [7] . Won, the data presented from all stage lighting series, which each about an "f-stop (+ / - 1 EV, see above) were compared with the marked 0 EV exposure obtained on a gray card, in the range from -6 eV to + 4 EV (under-or over-exposure, sometimes -7 EV to +5 EV). It turns out that the sensor in the camera reacts almost linearly on the exposure.
for later playback of images on a monitor or printer, the data in this form, but useless. There are two reasons: First, these devices can not express the exposure range is shown, on the other disappear in the reduction to 8-bit RGB data, especially in the shade almost all the nuances (intermediates). In fact, the raw data are from these two camera models in a 14-bit data format before.
These nuances can be in the conversion into a 16-bit TIFF preserved, which is also useful if you then in an image editing program still plans to make further edits. As a final product but typically requires 8 bits per channel (particularly in JPEG files, which are intended for the web). For this Fundamental data is subjected to the conversion into an image file of a logarithmic conversion, the more reserved values for shadows and less for the lights. This also happens when the camera stores have a ready JPEG. This can also be seen on the curves in the diagrams.
a contrast ratio in order to convert f-stops, such as 1:10,000, use the base 10 logarithm of * 3.32, the results for log (10,000) * 3.32 = 13.28 stops! Such a thing hardly creates a film and a digital camera, but the human eye already. Rather, we would have at 1:4000 = 12 f-stops and light levels (EV LW or English) a chance, even with the eye should be able to capture contrast ratios of 100,000:1, or 16.6 stops at the same time. The conversion of stops in a linear brightness ratio (with each calculator) simple: Calculate the corresponding power of 2, so 2 ^ f-stops, such as 2 ^ 9.5 = 724th
brutality
The RAW formats (raw data) contains the digitized brightness value directly from the sensor, ie that which "comes back", if pushed forward to the analog sensor voltages with a certain gain. A doubling of the brightness value in the motif here, a double value (at least in theory). The values shown were obtained by a bracketing obtained, in which a gray card was taken.
Based on the measured exposure (0 EV) were determined by shortening the exposure time by up to 7 f-stops and by extending by up to 5 f-stop values for the various stops. In the conventional transmission of this absolute brightness data in an 8-bit data format (RGB, 3 x 8 or 24 bits per pixel) non-linear transformations are applied that correspond to a first approximation of a logarhitmischen scale.
The mean gray value in a motif (defined as a gray surface that reflects 18% of the incident light) should thereby be mapped to a value of 118 (ranging from 0 to 255) when it is anmisst in the admission and the exposure to + / - 0 EV stops. This value is slightly below the "middle" between 0 and 255 By contrast, the linear value for middle gray located much further in the lower part of the value space (about 20 based on 8-bit representation).
why should you when the exposure - exposure latitude normally put as much as possible in the upper range, where it is not at the expense of the tracing in areas important for the picture lights - on the histogram-read. As this are much more per f-stop values to distinguish between shades of gray (and shades) available than in the case of a short exposure.
HPM for more highlight detail
This is unfortunately contrary to the desire to provide a highly nuanced detail in the light to achieve. The first chart shows the direct (linear) response of the camera sensors, EOS 40D and 450D. Since the absorbed light intensity is doubled by doubling the exposure time when recording a gray card for EV-EV-stage to stage as well, the curves rise exponentially.
compared linear RAW conversion EOS 40D/450D - normal and HPM
are seeing two bands of three curves: Left yellow (theoretical course at 0 EV), orange (450D) and dark green (40D) for an exposure at ISO 100, and further to the right blue (theoretical progress in under-exposure by -1 EV), red (450D) and green (40D) for the HP- mode (Tonwertpriorität) for alleged ISO 200 Shown are the respective "linear" from the RAW file values determined for different exposure levels.
The yellow curve shows the - more or less accurately applied - in theory shifted to the expected values for a linearly responsive sensor, so a doubling of the absolute values for each aperture setting, the blue curve the same values for one stop to the right (exposure).
The orange Curve of the EOS 450D is in the range of about +1 to +3 almost identical to the lights out it is slightly flatter, and in the shadow of something higher. The dark green line (EOS 40D) is in the shade with the 450D almost identical, resulting in the central area lights slightly higher values, but runs with the highlights a little flatter. At +5 EV clipping occurs here at a value read from 219th
curve sketching
The second curve band (blue: theoretical curve, red: 450D, green: 40D) for the HP mode, the curve of the 450D analogous to the exposure at ISO 100 and is the theoretical linear sensor response very similar to the curve 40D shows the a minimal increase at 4 EV then flattening curve to the exposure limit at about +5 EV is again something less steep than the 450D. rise later that the curves for the HP mode by about one stop general than at ISO 100 entlarft the "trick" the application by the Canon here - see text and the next chart.
The 450D exposed when measured on the gray card at 0 EV, according to the so-obtained sensor data on almost two thirds of apertures. The diagrams were recorded reduced their values to the exposure by 2 / 3 stop and appear in the central region, therefore, slightly smaller than the 40D.
Nevertheless, it provides in the lights return to higher values - their linear sensor curve is steeper and more the case as the theoretical curve, while the 40D a somewhat more pronounced flattening indicates that intensified in the HP mode significantly. The exposure values in the table to be at ISO 100 f/16 for 0 EV on the 40D 1 / 5 sec and the 450D 1 / 4 sec (or 1 / 10 versus 1 / 8 in the HP mode).
BACKGROUND
After release of the 40D guessed it soon became clear that the HPM or the recording mode "Tonwertpriorität" otherwise than under-exposure by one stop and "push" the sensor data with the lights gently running curve. This is achieved by that is set out amplifier before A / D conversion as ISO 100, the light meter of the camera but a sensitivity of ISO is pretended 200th Therefore, the linear values in the RAW file in the HPM 450D/40D consistently reduced by one f-stop and only reach the limit at about +5 EV, which occurs usually at 4 EV (clipping limit).
This explains also why it can be set at "Tonwertpriorität" no less sensitive than ISO 200 (default setting for the exposure), because the amp would be so set to ISO 50, ISO 100 HPM alleged that the hardware of the sensor, but not hergibt.
Comparison linear / logarithmic RAW conversion EOS 40D/450D - normal and HPM
The second chart "Comparison of linear / logarithmic RAW Conversion" shows the resulting curves at implementation in an image file (8-bit RGB). Right again, the linear curves - red and orange for the 450D, turquoise and blue for the 40D. They are clearly separated by an aperture stop (in HP mode a step underexposed). It calculates the software (in this case from Canon DPP) remains in the shadows and midtones almost exactly the same values, but the orange curve in the highlights (+3 to +4 EV) is well below the red and continues up to the clipping limit (255) continued to +5 EV.
Thus one gains about one stop over exposure latitude and a (slightly) softer tracing of lights. However, in the shadows is also about one stop momentum lost because the values go down more in the noise.
More bits for better shadow
The exposure latitude of digital cameras is limited by various factors. The values that can be useful to read from an electronic sensor to be bounded from above by the saturation of the sensor elements. More incoming photons can not increase the stored charge. go down towards the After deleting the stored charge before reading and after exposure in addition electrons stored in the noise gradually under that arises by including thermal effects.
Another confounding factors, the noise of the readout amplifier itself must finally be the downstream A / D converter (Analog to Digital) is also able to convert the voltage difference in clearly distinct digital values. Also, the A / D converter can add noise, especially at small values. In addition, the A / D converter only a limited value range (usually 12 bits, with newer cameras such as those mentioned here 14 bits), in which the scanning of the sensor elements be pushed by different gain (ISO) read voltages.
suppressed in the diagrams of the fact, that occur at very low exposure levels barely distinguishable figures and they remain simply 0, and high exposure levels do not exceed a certain limit will also increase if the exposure continues. Since the linear values of exposure level of exposure level in the shadows move together more closely, an extension from 12 to 14 bits with a gain of two exposure levels down with a similar tonality value (differentiation of shades) which - assuming the noise of the sensor or Amplifier makes it destroyed again. And you can also use these profits only if one uses a 16-bit destination format, or before conversion to JPEG or another 8-bit format, for example, making an increase of the shadow.
Critical motifs
test pattern to investigate the tone-Priority mode with critical "lights"
For the tests we have a special test pattern with very high contrast set up, among other things, made of black velvet (as background), a color chart with gray and color stairs and shiny objects (Two Christmas balls, a furniture knob and two with gold metallic paint garden sprayed bullets made of plastic) is. The range of contrast between the black velvet and the lighter highlight is slightly above 9.5 f-stops between velvet and the dim light shine just 8.5 stops. That should be a digital camera that is actually still can cope.
However, the exposure range is not used in any case until the very end - which makes sense, because in the shadows threatening stage tone reproduction and noise in the highlights or even color fading / damage (clipping) on reaching the limit. Thus, "normal" shots of this sample document without further run post-processing in the shade and / or erode in the highlights. To determine
the "normal" or different exposure and contrast range as a whole, we have used a gray card with the pictures. as here - - A problem of such motives, if the distance of the lights from the mean is very high gray (here +5.3 stops for the great highlights of the gold balls), because then you have to underexpose quite strong, a / damage of Lights to avoid. This is "normal" areas too dark and need to be raised in the subsequent image processing - basically, what also makes the drive mode "Tonwertpriorität.
detail of a montage of the second shooting at each test subject -0.6 EV, up in the "normal" mode at ISO 100, ISO 200 at the bottom HPM The purple line segments illustrate the boundary line between the two images. should
When the shots you look at the histogram in the camera and watch the flashing overexposure warning. This is especially important to visual lights, which should not erode or should have correct drawing and colors, such as sky with clouds.
exposure on the gray card with over exposure display in the highlights (Adobe Camera RAW)
In the adjacent diagram for overexposure indicator or area histograms (screendumps of Adobe Camera RAW from the Photo Suite CS3) are over-exposed areas marked by red warning color (screendumps of ACR). Very nice is also possible in ACR or in Photoshop histogram for arbitrary cropping. So you can check specific example highlights the Golkugeln.
underexposed 2 / 3 stop steps and HP mode. Only the two great highlights clipping on the gold balls.
When shooting in HP mode generates Adobe Camera RAW (green line), no gentler, but even a steeper curve than when shooting in normal mode - ie is almost "counterproductive". The curve of DPP (red), however much smoother curve in the lights on. Between +4 and +5 EV uses clipping.
The EOS 40D can use the "Tonwertpriorität" to create one or more of the custom shooting modes on the Mode Dial. With the 450D you have unfortunately always go to the menu of so-called custom functions to the on or off, which is quite cumbersome. Always on, one should not let this mode too, because it brings benefits only in high-contrast subjects with a high proportion of important picture lights, otherwise they will worsen the noise in the shadows.
comparison of the conversion of Digital Photo Professional (DPP) versus Adobe Camera RAW (ACR)
who prefers to take in RAW format and the images nachzubarbeiten itself (rather than up to the camera and use their ready JPEGs), only with Canon's DPP (Digital Photo Professional) to be happy. ACR from Adobe is indeed the Brightening the image by one stop from about right, but not the gradation of light to soften it - the shots in the HPM are on the contrary, even "harder" than without. Here you can indeed intervene manually, but it's more practical to dispense with this mode. Dear when shooting by hand if necessary by about one stop under exposed and then offset this by raising the brightness.
John Leckie
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