Select your language
  • Deutsch

    Sämtliche Inhalte auf der CPN-Website sind auf Englisch verfügbar. Einige Inhalte, wie z. B. Produktbeschreibungen, aktuelle Produkteinführungen und einige technische Artikel, sind ebenfalls auf Deutsch, Spanisch, Französisch, Italienisch und Niederländisch erhältlich. Wählen Sie in der Liste oben Ihre Sprache aus, damit sämtliche darin verfügbaren Inhalte automatisch entsprechend Ihrer Wahl dargestellt werden. Ansonsten wird als Standardsprache Englisch verwendet.

  • English

    All content published on the CPN website is available in English. Some content – such as product descriptions, recent product launches and some technical articles – is also available in German, Spanish, French, Italian and Dutch. Choose your language from the list above and all content that is available in your language will automatically be displayed in your language, otherwise the default language will be English.

  • Español

    Todo el contenido publicado en la página web de CPN está disponible en inglés. Parte del contenido –como descripciones de producto, lanzamientos recientes de productos y algunos artículos técnicos– también están disponibles en alemán, español, francés, italiano e holandés. Elija su idioma en la lista anterior y todo el contenido que esté disponible en su idioma aparecerá automáticamente en ese idioma, o , si no, en el idioma predeterminado que es el inglés.

  • Français

    Tout le contenu publié sur le site Web de CPN existe en anglais. Une partie du contenu (comme les descriptions de produit, les lancements récents de produit et certains articles techniques) est également publié en allemand, en espagnol, en français, en italien et en néerlandais. Choisissez la langue dans la liste ci-dessus, et tout le contenu offert dans votre langue s’affiche automatiquement ; par défaut, le reste s’affiche en anglais.

  • Italiano

    Tutti i contenuti pubblicati sul sito CPN sono disponibili in inglese. Alcuni contenuti come descrizioni di prodotto, lanci di prodotti recenti e alcuni articoli tecnici sono disponibili anche in tedesco, spagnolo, francese, italiano e olandese. Seleziona la lingua dall'elenco in alto e automaticamente si visualizzeranno tutti i contenuti disponibili in quella lingua; diversamente la lingua di default sarà l’inglese.

  • Nederlands

    Alle inhoud die op de CPN-website wordt gepubliceerd, is beschikbaar in het Engels. Bepaalde inhoud, zoals productbeschrijvingen, onlangs gelanceerde producten en sommige technische artikelen, zijn ook beschikbaar in het Duits, Spaans, Frans, Italiaans en Nederlands. Kies de taal uit bovenstaande lijst, waarna alle inhoud die beschikbaar is in de gewenste taal, automatisch in die taal wordt weergegeven. Anders is Engels de standaardtaal.

Infobank

Flash: Flash exposure lock and compensation

Exposure for flash photography used to involve time consuming calculations based on guide numbers, subject distance and apertures. Evaluative through-the-lens (E-TTL) flash metering has changed all this. Flash photography, is now about as easy as a two times tables.

Evaluation

The evaluative metering system, used to take daylight readings, is shared by the E-TTL flash system. A low power pre-flash fires in advance of the exposure. Light from this flash is metered. The duration of the main flash is then controlled to give correct exposure with the selected aperture.

When you are shooting with daylight, exposure adjustment is sometimes needed with very light or very dark subjects. This is because the camera is calibrated to suit an ‘average’ scene. You can either take a reading from an ‘average’ area of the scene and use the exposure lock function to hold this for the main scene, or use the exposure compensation function of the camera.

Since E-TTL flash uses the same metering system, the flash meter readings can also be thrown by light or dark subjects. Here, adjustment is made using the flash exposure lock (FEL) or flash exposure compensation (FEC) functions.

Flash exposure lock

Flash exposure lock (FEL) enables the camera to remember the exposure for any selected area of the subject while you recompose the image in the viewfinder. To make an FEL reading you bring a mid-tone subject area to the centre of the viewfinder and press either the exposure lock or flash exposure lock button (varies with camera). The flash fires and a reading is taken without an exposure being made. This reading is held for 16 seconds while you recompose the image and take a picture.

FEL gives you total control over the flash exposure. You choose the area from which the reading is taken. If there is no average tone in the subject, you can take an FEL reading from a lighter or darker area and then apply flash exposure compensation to adjust the reading.

FEL is especially useful when there are highly reflective surfaces, such as mirrors, within the subject area. These can create bright ‘hotspots’ by reflecting the flash illumination directly back to the camera. A general flash reading will see this bright light and reduce the flash output to compensate. The result will be underexposure by the flash. This problem is avoided if you take an FEL reading from an average area of the scene which does not include reflective surfaces.

FEL is not limited to a Speedlite attached to the camera. It can also be used with the Speedlite on an Off Camera Shoe Cord or with the Canon wireless flash system.

Flash exposure compensation can be set on some Speedlites. If this is done, it will override any flash compensation settings on the camera.

The exposure for this image (1/250 second at f8) was set manually on the camera, based on a partial meter reading from the window. This has given detail in the view through the window. The first picture (Without FEL) was taken at these manual settings using normal E-TTL flash, linked to the centre focusing point. The background is well exposed, but the main subject is a little overexposed - the result looks like a flash picture. For the second picture (With FEL) an FEL reading was taken from the face of the subject, giving a well-balanced result.

Warning signals

Flash exposure lock provides a useful ‘flash in range’ check. When you press the exposure lock or FEL button, a green star will appear to the right of the flash bolt icon in the viewfinder. This reminds you that you have locked the flash exposure. However, if the flash bolt icon starts to blink on and off, it means that there is not enough flash power for adequate exposure at the current settings. You should either set a wider aperture or move closer to the subject and take a new reading.

One signal to treat with caution is the green flash confirmation lamp on the back of the Speedlite. This normally lights up for about three seconds after a flash exposure if correct flash exposure has been obtained. However, if FEL is used and there is no out-of-range warning in the viewfinder, the confirmation lamp will always show green, even if you are out-of-range with the recomposed image. The confirmation lamp is picking up its information from the FEL pre-flash, rather than from the main flash exposure.

Second curtain − the problem

E-TTL flash exposure is calculated before the exposure starts. This can be a problem if you use second curtain flash synchronization and the subject distance changes during the exposure. The exposure will be based on the initial subject distance rather than the subject distance at the end of the exposure, when the flash actually fires. The result will be an over- or underexposed image, depending on the direction the subject moved after the exposure was underway. If you have control of the situation, the solution is to take the FEL reading with the subject at their final position, then get them to move to their initial position before starting the exposure.

With a moving subject and second curtain flash synchronization, take the FEL reading with the subject at what will be their final position (End). If you take a reading from the subject’s start position (Start), exposure will be incorrect.

Flash exposure compensation

The metering sensors inside an EOS camera are calibrated for mid-tone subjects (often referred to as 18% grey). When the Speedlite pre-flash fires, it is reflected from the subject back to the camera. If the main area of the subject does not have an average tone, the flash exposure will not be correct.

Although flash exposure lock is a very effective method of overcoming the problem, you have to be aware of certain issues. First, the FEL reading is only held in the camera for 16 seconds. You can extend this by keeping partial pressure on the shutter button, but this is not always convenient. Second, you need to take a new FEL reading for each exposure, as the flash data is not stored by the camera after the shutter release button is pressed. Third, there may not be an average area of the subject from which to take the FEL reading.

An alternative technique is flash exposure compensation (FEC). Here, you simply enter the amount of compensation you want on the camera (or on some Speedlites) and it will be applied to every flash exposure until you reset to zero. Of course, this assumes that you know the amount of compensation required for different subjects. As with many aspects of photography, this only comes with experience. However, very light-toned subjects will need an increase in exposure of around +0.5 to +1.5 stops; dark-toned subjects will require a reduction in the flash output of around -1 to -2 stops.

If you are shooting in daylight and using flash to add detail to shadow areas or create catchlights in the eyes of a person or animal, the result can look a little artificial if you leave the camera to calculate the aperture and flash output. If you want a more subtle effect, flash compensation of between -1 and -2.5 stops can be effective.

Without flash exposure compensation, this portrait looks like a flash shot. Setting -1.0 stop FEC gives a more natural result. Even with the reduced flash exposure, the catchlights in the eyes provide an attractive sparkle to the portrait.

Auto flash reduction

You need to be careful when applying flash exposure compensation to cameras that use E-TTL metering. When you shoot in bright light, they assume that you are using flash for fill-in, and automatically provide flash reduction. If you apply further reduction, the fill-in effect may be too weak. You need to get to know your camera so that experience will tell you when to apply compensation and when to leave well alone.

Some EOS models offer a custom function that disables automatic flash reduction. This has two uses. First, it allows you to take control of flash reduction in all situations − you don’t have to guess whether or not flash reduction has been applied − you know it hasn’t. Secondly, you can turn off the auto reduction when photographing backlit subjects. Here, you need a good burst of light from the Speedlite to add detail to the shadow areas of the subject.

Flash exposure bracketing

Flash exposure bracketing (FEB) is a convenient feature offered by the Speedlite 550EX, 580EX, 580EX II, Macro Ring Lite MR-14EX and Macro Twin Lite MT-24EX. It makes it easy for you to shoot a sequence of three pictures, each with a different amount of flash exposure compensation. The variation can be between 0.5 and 3 stops. You must use single frame advance and wait for the flash to recycle after each exposure.

The bracketing sequence is normally standard, under and over, but this can be changed by a custom function on the Speedlite to under, standard and over.

Bracketing is useful when you are not sure just how much flash exposure compensation is needed. However, you often know in which direction the compensation should be. By setting exposure compensation on the camera as well as FEC, you can force the flash exposure bracketing to shift the standard position. For example, a sequence of -0.5 stop, 0.0 stop and +0.5 stop becomes 0.0 stop, +0.5 stop and +1 stop if +0.5 stop exposure compensation is set.

There is no need to be concerned if your Speedlite does not offer flash exposure bracketing, or if the calculations have left you confused. You can bracket manually simply by altering the flash exposure compensation setting between each shot.

Of course, with digital cameras, you also have the opportunity to examine each image immediately after exposure and base compensation level on these results.

When you are not sure how much flash exposure compensation is needed, the simple solution is to take several shots at different FEC settings. Here, three pictures were taken with FEC of 0.0 stop, -1.0 stop and -1.5 stops . Speedlites 550EX, 580EX and 580EX II, Macro Ring Lite MR-14EX and Macro Twin Lite MT-24EX offer automatic flash exposure bracketing (FEB), but similar results are possible with any FEC camera or Speedlite by manual adjustment.

Here is an example of flash exposure lock combined with flash exposure compensation. An FEL reading was taken from the white feathers around the eye of the duck. However, this gave an exposure which would have underexposed the feathers and lost their detail. FEC of -0.5 stop was applied to overcome this. An alternative would have been to take an FEL reading from the dark feathers and set +1.0 stop FEC.