Fstop Confusion
Hello all, I have been browsing these forums for a while now trying to get something straight in my head. I an relatively new to Dslr and I understand the concepts of iso, shutter speed and fstops(aperature). I just got a new Xti (400d) and a 17-80 IS lense for christmas and am having lots of fun with it..

But my questions is about fstops. I understand that this lense has a fstop range to it because it is a zoom lense. I dont remember the numbers off the top of my head but lets just say it's f4-f5.7. Ok, now when I am in AV mode on the camera it lets me adjust the f value alot higher and lower than that range. So what am I a missing here? or is that range on the lense the "ideal range" for use?.

Thanks in advance for any replys, I apologize if this is a stupid question or has been answered before...

Comments (6)

The f-stop numbers are the range of your minimum f-stop. You have a minimum f-stop of 4 at the wide range and of 5.6 at the telephoto end...

Comment #1

Drno wrote:.

When I am in AV mode on the camera it lets me adjust the f value alothigher and lower than that range..

It will never be numerically lower than the first number (f4)Regards,Hank..

Comment #2

Hmmm ok, I might have to check that again. I was positive I was going alot lower than the value on the lense..

Thanks for the responses...

Comment #3

Drno wrote:.

I understand that this lense has afstop range to it because it is a zoom lense. >.

Not quite true. Some zoom lenses have the same maximum aperture (smallest f-stop number) throughout their zoom range. I suspect that it has to do with the way the internal pieces of glass move with respect to each other. But it is a matter of the way the lens is designed, as to whether it maintains the same f-stop at it's wide end and at it's telephoto end..


One does not achieve success by being at the right place at the right time, but rather by being ready when the right place and time present themselves for your inspection...

Comment #4

The f-stop range is the range of MINIMUM f-stop numbers (actually, maximum aperture) available to you at different focal lengths (zoom). The maximum aperture is at it's greatest (the smallest number) at the shortest focal length (17 mm) and at it's smallest (the larger of the two numbers) at the longest focal lenfth (85 mm). The camera should not let you set the aperture lower than that limit, depending on what focal length you have the lens set at..

However, you CAN set the aperture smaller than the maximum (that is, larger f-stop number), up to the minimum aperture of the lens (probably f22). That should always be available to you, regardeles of the focal length..


Comment #5

First and foremost, all lenses have f stops whether telephoto, Zooms or primes.its just that in this day & age of electronics we let the Camera change them instead of a manual selection like the old lenses..

F stop numbers relate to the AREA of the hole that lets the light onto the sensor. f2.8 large hole f22 small hole..

The bigger the hole the more light the smaller the hole the less light.Sounds pretty basic doesnt it, well now lets complicate it..

F2.8 (a nice wide aperture lets in a certain amount of light) and the Camera's exposure meter says for that amount of light you need the aperture open(shutter speed) for 1/180 th of a second. Great, now if you lengthened the time it would be over exposed shorten the time and it will be underexposed..

So it's all about the total amount of light that strikes the sensor..

Now if I halved the AREA of the hole, to f4 then I MUST increase the time it's open so now f4 at 1/90th second gives me an identical exposure..

Same total amount of light. I/2 the hole area double the time, it's easy..

Now each f stop halves the amount of light of the previous stop so for each "stopping down" decreasing the area of the hole we must hold the shutter open for longer by the same ratio..

To prove my point I just went outside, stood the camera on a stand set it to "P" program mode, half pressed the shutter release and got 1/125th at f5.6 moved the adjustment wheel and it said 1/60th at f6.7 next move 1/45th at f8 next move 1/30th at 9.5 next move 1/20th at f11.

For the purpose of this explanation each one of these settings would have produced identical results..

Now you say if they all produce identical results why have the range, well now we have a lesson on DOF (Depth of Feild).

Without overcomplicating it large aperture f2.8 = shallow depth of feild that is to say only a very small portion of your shot will be in sharp focus..

Small aperture say f22 produces a deeper depth of feild, ie more of your shot will be in sharp focus..

After you have digested all of this then post a question on understanding ISO settings which introduces a third variable into EV (Exposure Value).

I have deliberatly over simplyfied the explanation and dont want to get into complicated discussions on 1/2 stops, or variations caused using long or short focal length lenses...

Comment #6

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This question was taken from a support group/message board and re-posted here so others can learn from it.


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