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PostPosted: Thu Nov 19, 2009 6:56 am 
hi,
i need to know what does it signify when i say nikkor AF-S DX 18-70mm f/3.5-4.5G IF-ED , i know the term f/3.5-4.5G IF-ED in it states the focal some thing.... , but in what physical terms, like visual term do u differentiate , and explain the logic of lenses going from 18mm to 70mm , i know the basic terminology but i do not know the scientific explanation to this mechanism of coverage , like if i say 18mm how much in reality say for example physical distance i can cover in focus , and i need to know a detailed explanation to the terminologies given in a lens....

all of that will be very helpful as i need to know them , and wishing that u all will respond to this , iknow its a stupid question , but i need to know the basics .
thank you
sreejith.


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PostPosted: Thu Nov 19, 2009 7:48 am 
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The f actually stands for focal length and f/x notation is used to denote the maximum aperture size. On a lens the f/x number denotes the maximum aperture possible. e.g 50mm f/1.8. On a zoom lens where the focal length is variable ie 18-70mm then the maximum aperture usually changes as you zoom in and hence the max aperture range is also written as f/3.5 to f/4.5 (eg f/3.5-4.5). Zoom lenses with one f/x number have a constant max aperture throughout the zoom range and are usually expensive as it costs more to manufacture.

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PostPosted: Thu Nov 19, 2009 2:15 pm 
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If memory recalls the F ratio is lens diameter/sensor diameter. So the lower the F number, the larger the lens is compared to the sensor. This also explains why full frame lenses have to be bigger than cropped frame lenses to get the same aperture.

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PostPosted: Thu Nov 19, 2009 3:12 pm 
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Citruspers wrote:
If memory recalls the F ratio is lens diameter/sensor diameter...

Nearly right - it's the focal length divided by the effective lens diameter (taking account of the iris). But these sort of questions are easily answered by the OP doing his own research, an essential skill for a student and an engineer. :idea:

If Googling doesn't do it then, as long term members here know, a good reference site for the mechanics (as opposed to Gordon's DSLRTips which concentrates more on the practicalities) is CambridgeInColour.

Bob.

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PostPosted: Thu Nov 19, 2009 3:33 pm 
Bob Andersson wrote:
Citruspers wrote:
If memory recalls the F ratio is lens diameter/sensor diameter...

Nearly right - it's the focal length divided by the effective lens diameter (taking account of the iris). But these sort of questions are easily answered by the OP doing his own research, an essential skill for a student and an engineer. :idea:

If Googling doesn't do it then, as long term members here know, a good reference site for the mechanics (as opposed to Gordon's DSLRTips which concentrates more on the practicalities) is CambridgeInColour.

Bob.


hi,
thanks bob, and everyone who posted , well i always knew somthing about the topic i just need certain confirmation on those , as a fresh mechanical engineer , i just needed to study something like this it is an amazing thing to learn the mechanism , but thanks again i will ask only
those questions which are not available on sites after googling ,
and Mr.gordon sirs dslr tips also is very helpfull and also the site cambridgeincolour is a nice site to refer to
thank you
sreejith


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PostPosted: Thu Nov 19, 2009 8:44 pm 
same thing it means in just about every other context. :wink: :lol:


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PostPosted: Thu Nov 19, 2009 10:25 pm 
Here is what I don't understand:

Increasing f-numbers, in one-stop increments; each aperture has half the light gathering area of the previous one.

So we have:
F1.4 - F2 - F2.8 - F4 - F5.6 and then F8?

If this was a standard IQ test and you were to guess the next number in the series:
1.4 - 2 - 2.8 - 4 - 5.6 ___ Then you'd choose F7.6, would you not?

Cheers :-)


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PostPosted: Thu Nov 19, 2009 10:53 pm 
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It's logarithmic, not linear, Lahlah :)

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