Drawbacks of flash  
  Low Intensity - The level of light produced, with the built in flash may often not suffice for good pictures at distances of over 3 meters (10 ft) or so. Dark, murky pictures with excessive noise or "grain" will result. In order to get good flash pictures with simple cameras, it is important not to exceed the recommended distance for flash pictures. You get this information in the camera booklet or calculate as per the flash guide number. To get better results, even with external dedicated flashs it is not a very good idea to use very long focal lengths. Most of the strobes work well upto about 100mm. If somehow the subject or the background, which may be further back from the subject is still underexposed, it is best to move closer to the subject while changing the focal length. this way both the subject and the background will be well within the range. keep in mind that you move closer only to the extent that you are still using a relatively longer focal length.i.e you may come down to maybe 70mm from 100mm but not all the way to 28mm!

solution-1.do not use very long focal length 2 be closer to the subject , some advanced flashes will expose well for both the subject and the background

The Red eye effect is another problem. Since the retina of the human eye reflects red light straight back in the direction it came from, pictures taken from straight in front of a face often exhibit this effect. It can be somewhat reduced by using the "red eye reduction" found on many cameras (a pre-flash that makes the subject'sirises contract). However, very good results can be obtained only with a flash unit that is separated from the camera, sufficiently far from the optical axis, or by using bounce flash, where the flash head is angled to bounce light off a wall, ceiling or reflector.
  Delay due to red eye reduction mode- when you shoot in this mode, the camera fires some preflashes to eliminate/reduce red eye leading in a delay in the actual flash. this delay is particularly noticeable in compact cameras.  
  Some camera's flash exposure measuring logic fires a pre-flash very quickly before the real flash. In some camera/people combinations this will lead to shut eyes in every picture taken. The blink response time seems to be around 1/10 of a second. If the exposure flash is fired at approximately this interval after the TTL measuring flash, people will be squinting or have their eyes shut. Many compact cameras  suffer from this problem. One solution may be the FEL (flash exposure lock) offered on some more expensive cameras, which allows the photographer to fire the measuring flash at some earlier time, long (many seconds) before taking the real picture. Unfortunately many camera manufacturers do not make the TTL pre-flash interval configurable.  
  Hard Shadows - if your subject is too close to the background or a wall you may get a hard shadow on the wall. You may bounce the flash from the ceiling or soften the flash. Otherwise  simply ask the subject to step ahead away from the wall. since the wall is now quite away the shadow is lost in midair. solution-
1.soften the flash using readymade softboxes for flash, 
2. diffusing sheet at a little distance from the flash, (if it is close, touching the flash, it may burn due to heat of the flash if fired continuously) 
3. Bounce the flash if using an external one,
4. create distance between the subject and the wall.
  Shutter Speed limitation - Electronic flash units and some bulbs have durations so short that slower shutter speeds must be used on focal plane shutter cameras. This is because the slit of light that traverses the film to expose it travels slower than the duration of the light provided from the flash. The resulting photograph will have anything from only a narrow slit exposed, to one side dark, or both sides dark, depending on the exact scenario. Some cameras and flash units solve this problem, but most do not.  
  Flat Lighting -You need to learn how to balance the flash lighting with the ambient lighting so that both kinds work together. On camera in built flash generally results in flat lighting as the hard light is coming from the camera axis. The external flash is relatively higher than the camera lens and falls from a higher angle in comparison to the inbuilt flash. further you may use the external flash off camera with the help of synchronisation cords or wireless remotes enabling you to set the flash at the desired angle. This will add modeling to the subject resulting in a pleasing photograph unlike the regular flash pictures.  
  Slow Recycling - In built flashes generally do not have a very fast recycling times in comparison the the external flashes. This happens when the the flash is firing at its maximum capacity and you are shooting a bit too fast. Move closer if shooting slower is not possible. The flash will need to work less hard and not use its full capacity to fire, leading to faster recycling as it need to recharge only the used charge and not what is left out from the previous flash.  
  Light Fall Off - The subject is too bright or in other words the background or the part of the subject further away from the camera is too dark. Although there is not much that can be done with a built in flash but with an external flash on the camera you may tilt the head towards the neutral ceiling which will act as a large light source redirecting the flash light. This will result in almost uniform light all across a large area resulting in a very uniform exposure.You should have a strong enough flsh for the purpose as you will loose a lot of light.  
  Lost Ambient Light mood - You don't have to use the flash as the main light source but learn to use it as a fill giving more importance to the ambient light. With dedicated external flashes and some experience you can learn to strike the right balance between the two lighting kinds so that your pictures look natural  
  Burnt out details- although this is less likely to happen with a dedicated flash as the output is thoroughly calculated and controlled through the lens  


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