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```				   In a combination of two lenses(focal length f1 and f2) separated by a distance 'd', there seem to be two formulas to arrive at the effective focal length:
1. f = f1*f2/{(f1+f2)-d}
2. f = (d-f1)*f2/{d-(f1+f2)}

Can someone explain the difference?
Thanks!
```

6 years ago

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```										One of them is FFL and other is BFL.I have given you den=ifnition BUt i wnat you to derive it.If still doubt lingers tell me.Front focal length (FFL) or Front focal distance (FFD) is the distance from the front focal point of the system to the vertex of the first optical surface.    Back focal length (BFL) or Back focal distance (BFD) is the distance from the vertex of the last optical surface of the system to the rear focal point.Difficult to understand - post it here and we will get you the answer and detailed solution very quickly. We are all IITians and here to help you in your IIT JEE preparation.DO APPROVE IF THE ANSWER HELPS YOU.Now you can win exciting gifts by answering the questions on Discussion Forum. So help discuss any query on askiitians forum and become an Elite Expert League askiitian.Regards,Askiitians ExpertsRahul- IIT Roorkee
```
6 years ago

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