Flash Exposure Modes

There have been four basically different modes of flash exposure control commonly used.

1) Manual

In manual mode, the photographer pre-sets the flash energy level, which is related to a Guide Number, before taking the picture. Some flashguns have only one manual level (which will correspond to the unit's maximum Guide Number) even if they also have other more sophisticated modes. This is a time honoured method of course, dating back to flash bulb days when the light output of the bulb was fixed. Good modern flash units will allow several pre-set manually controlled energy levels. The photographer probably wants a free choice of distance from the subject, and given this the other variables that can be controlled are the lens aperture and the sensitivity of a digital camera. The Flash Formula will advise acceptable combinations of these.

The relationship between the Guide Number, film or sensor ISO, lens aperture and subject distance is defined by the Flash Formula, which is discussed in more detail on another page and which arises from the laws of physics. Practically all electronic flash units will have an aid to evaluating the Flash Formula: older ones often had a look-up table engraved on the side, others a linear or circular slide rule, and from about the year 2000 many have it in software with a digital display.

There are many pitfalls to evaluating flash exposure manually, one being that the environment has an influence. Flash units spill a lot of light outside the intended beam angle and this will reflect off walls and ceilings, increasing exposure. The flash unit maker may allow something for this, but in a very large room or out of doors this returning stray light will be less. The use of indirect (or bounce) flash is far more difficult to pre-evaluate.

Although this article is not intended to cover studio lighting, it is worth mentioning that studio flash is often manually controlled. Partly this is because multiple automatically controlled lights can interfere with each other, and there also the fact that the installation may be more-or less fixed such as in a commercial portrait business. In film days, studios often used Polaroid test shots to adjust lighting before the main shoot.

Manual control has made a perhaps surprising come-back in the digital era. One reason is that it is now easy to review a shot using the rear display. Another is that flash units from independent makers are far less expensive than those from camera makers themselves, and methods of automation requiring signal exchange with the camera are patented.

2) Auto

"Auto" can refer to many things in the wider context, but when photosensors were first added to flash units to control exposure in the 1970s the technology was called Auto, so I shall keep to that meaning. Beware that others may refer to TTL control as "Auto", especially if they have not come across this older method (and TTL is also automatic of course); but I shall refer to TTL automation as just "TTL". The Auto mode was an option on all good flash units made up until around the year 2000 but has since fallen out of favour. In Auto mode, once the flash unit and camera are set up the user can take photos, such as of an event, without regard to subject distance (within reason) or taking any further notice of the settings.

The Auto method works by the unit's own photosensor shutting off the flash when it reckons the camera film or sensor has received enough light. For this it needs to "know" the camera lens aperture and the film or sensor ISO, and in practice this is usually by the user first telling the unit the ISO being used, and the unit then telling the user the value to set the lens aperture. This is usually done by some additional or alternative markings on the same table or slide rule as is used for manual mode calculations.

Note that the flash tube always emits light at the same brightness; the variation in exposure is achieved by variation in the length of time it emits for. The duration of the flash is typically less than 1/1000th of a second and can be as little as 1/10,000th of a second.

The user is usually offered a small choice of Auto levels, perhaps only two or as many as six. For a given film/sensor ISO value this allows the user some choice in lens aperture and maximum distance.

It is common for the different Auto levels to be given different colour codes. Perhaps surprisingly, they all have the same maximum energy level (or colloquially, "power"), and hence Guide Number, as each other, so at their maximum ranges they are all using the unit's entire light energy. The design may obtain the different Auto levels by means of a slider placing different sized apertures in front of the unit's sensor, or they are implemented electronically.

Changing the film or sensor ISO value in a flashgun body-mounted slide rule has no effect on the unit's internals, any more than consulting an engraved table does, it only changes the displayed recommendation of lens aperture and does not change the maximum range of the flash. In fact the maximum ranges of the flash for the different Auto levels are often painted in the appropriate colour at fixed positions on the distance scale. Each Auto mode also has a minimum distance, because there is a limit to how quickly the flash can be shut off. This is about a sixth of the maximum distance and is usually also marked on the scale. In Fig 3 for example, in the yellow Auto mode the useful subject distance is from 1.5 to 5.5 metres (shown in the second numerical column from the left) irrespective of film speed, and in the green mode it is from 2 to 11 metres likewise. Notice that the yellow and green Auto modes have the same Guide Numbers as each other at their maximum range - 22 metres at ISO 100 - the same for every other (manual) entry in the table when adjusted for film speed. At the Auto modes' minimum distances, indicated by tops of the yellow and green tails on the left of the table, their Guide Numbers are both around 4 metres at ISO 100.

Auto mode does not need any electrical connection to the camera other than the trigger to set the flash off. Generally, the user is expected to make the appropriate settings on the unit and camera, although some later proprietory camera-flash unit combinations could do this automatically via auxilary contacts in the hot shoe.

A weakness of the Auto mode is that the choice of levels and hence of lens aperture is restrictive; for example it will probably not allow you to use the maximum aperture of the lens unless you are using a very low ISO value film or sensor setting. The fact that very shallow depths of field (requiring large lens apertures) has become fashionable is one reason for the demise of the Auto mode.

Auto control of flash can be fooled by non-typical subjects, just as automatic control of exposure in natural light can be. Wedding photographers will be aware of the problem when taking pictures of the men in dark suits and then of the ladies in white and pastel dresses. Unless compensation is dialed in, Auto exposure tries to expose scenes to an average of 18% grey. The Auto sensor can also be wrongly influenced by items just outside the frame because its field of view is likely to be wider than the camera's lens and the flashgun itself spreads light wider, so a bright object outside the picture may cause underexposure. Manual control, once it is set correctly, will give better consistency.

Another weakness is in close-up or macro work. Because the Auto sensor is out of line with the lens to subject axis it can give erroneous readings. In addition to that, the normal exposure calculations need to be modified when the lens is extended out significantly beyond its position for infinity focus - whether as a macro lens or by extension tubes. There are lengthy tables of such modification factors, which of course the Auto sensor circuit knows nothing about and I am not going into them here.

3) TTL (Analog)

With Through The Lens (TTL) flash metering the flash exposure is controlled by the camera. This method was introduced from around 1980, during film days. In most film camera implementations the flash is monitored by a photo sensor set into the bottom of the camera's mirror box and aimed at the film surface. When the photo sensor circuit has measured that enough exposure has been reached, it sends a signal to the flash unit to stop emitting light. This film version of TTL is sometimes calle A-TTL, which either means "Analog TTL" or is a reference to the Canon A-Series of cameras.

Obviously both the camera and the flash unit must be designed to incorporate TTL control, and at first only the more expensive cameras and their own-brand flash units featured it. It required an additional contact in the hot shoe and a protocol for the signal, and again there was no standardisation of these between the camera makers. Consequently a flash unit with TTL had to be designed for a particular camera brand or even model.

The advantages of TTL flash control are that any lens aperture can be used, and the exposure is measured exactly where it matters - at the film. A less mentioned advantage, for flash unit makers, is that it makes the design of the flash unit itself very simple, because exposure control is done by the camera. TTL does not need a photo sensor or its circuit to be in the flash unit, nor any provision to switch between different Auto levels. This cost saving for the manufacturer is another reason why Auto control was dropped from flash units after a transition period.

There are disadvantages to TTL. One is that it relies on the reflectivity of the film being consistent, which between different brands it is not. This can be corrected by exposure compensation in the camera if you are familiar with the behaviour of your regular type of film. TTL also needs the film surface to be matt, which it is, but the sensors of modern digital cameras are not matt; therefore the TTL systems of the film era cannot be used with them. Makers of digital cameras therefore had to adopt the following fourth basic mode of flash control.

4) TTL (Digital)

While this article is mainly centred on older flash units, digital TTL is mentioned here for completeness.

The reflection of light from CMOS digital sensors, ie the picture sensor, cannot be relied upon as an indication of how much light has fallen on them because they are somewhat mirror-like, so a flash control sensor aimed at the picture sensor would be unduly influenced by light in particular areas of the subject. The makers' solution to this is to send out a low power pre-flash, the result of which is measured by a photosensor within the camera behind the lens without involving the picture sensor. This photosensor may be by the pentaprism in a DSLR, or aimed at the first shutter curtain in a mirrorless camera. The result of this measurement enables the camera to calculate the required flash duration of the main flash and to control the flash unit accordingly. All this happens in a very brief space of time.

The algorithm used to calculate the exposure should also take account of the ambient light measured separately before the pre-flash (as opposed to simply measuring them together)and decide on an appropriate lens aperture and flash duration.

Again there is no standardisation of digital TTL between camera makers. Nikon call their system i-TTL, Canon call it E-TTL, Pentax has P-TTL and so on.

An ideal digital system would have the main picture sensor itself telling the flash to stop when enough exposure has been reached, but that would be a future development and would be the fifth basic mode in this series.