that not all brands and types of film reflect the light to the same amount although the actual difference between brands was less than half a stop. The one exception was Polaroid's instant slide film which had a black surface and could not be used in TTL flash mode. Nevertheless, for most applications analogue TTL flash exposure metering was more advanced and accurate than systems used previously and permitted far more flexibility - with bounced flash exposures in particular being more accurate than manually calculated equivalents. With digital, this way of direct reflection metering is not possible any more since a CMOS or CCD chip, used to collect the light, is not reflective enough. There are a few older digital cameras which still use the analog technique, but these are getting rare. The Fujifilm S1 and S3 are the most well known digital cameras to use this technique. Digital TTL works as follows: Before the actual exposure one or more small flashes, called "preflashes", are emitted. The light returning through the lens is measured and this value is used to calculate the amount of light necessary for the actual exposure. Multiple pre-flashes can be used to improve the flash output. Canon referred to this as Evaluative TTL (E-TTL) and later improved the system with E-TTL II. The first form of digital TTL by Nikon, called "D-TTL", was used in a few early models. Since then, the superior "i-TTL" system has been used. When using front-curtain flash (when the flash fires immediately after the shutter opens), the preflashes and main flash appear as one to the human eye, since there is very little time between them. When using rear-curtain flash (when the flash fires at the end of the exposure) and a slow shutter speed, the distinction between the main flash and the preflashes is more obvious. Some cameras and flash units take more information into account when calculating the necessary flash output, including the distance of the subject to the lens. This improves the lighting when a subject is placed in front of a background. If the lens is focused on the subject, the flash will be controlled to allow for proper exposure on the subject, thus leaving the background underexposed. Alternatively, if the lens is focused on the background, the background will be properly exposed, leaving the subject in the foreground typically overexposed. This technique requires both a camera