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Old Oct 4, 2006, 11:07 AM   #11
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The lens diameter affects thesmallest f-stop number or maximum light transmission achievable. The iris reduces the amount of light (increases the f-stop number) all other things being equal.

When the zoom is increased and the iris kept the same the f-stop number increases. This is because the pupil diameter also stays the same but the focal length has increased. (Remember f-stop equals pupil diameter approximately*divided by focal length)

Some digital cameras don't have irises, the maximum lens diameter is in used all the timefor light transmission and the shutter speed only is used to regulate the total amount of light or amount of exposure. Regardless of whether the camera has an iris, the two f-stop numberssuch as f/2.8-f/4 on a zoom camera's lensrepresent the maximumlight transmission or smallest f-stop numberfor the widest angle versus the maximum zoom settings respectively.

* when the iris is located near or between the lens elements.

Most zoom cameras don't have a dial or readout to tell you where along the zoom range they are set. Repeat: There are so many different sizes of sensors in digital cameras that the zoom setting or focal length in absolute millimeters is extremely difficult to work with. Theonly hope for partial easeof calculations isfor the camera to have and forthe photographer to use 35mm film camera equivalents on a readout.

Further confusion is hadbecause most cameras made nowadays have as their "standard angle" somewhere between 35 and 38mm focal length in 35mm film camera equivalents. Meanwhile professional photographers consider 50mm as "standard angle" and therefore would consider 38mm focal length to be "wide angle".

A 35mm film frame is about 36mm wide. If the focal length is 36mm the field of view is about 53 degrees. So any camerawith a "normal"a field of view at 53 degreeshas a 35mm camera equivalent of 36 mm focal length. A zoom of 2x means half the field of view, a zoom of 3x means a third of the field of view compared with the reference whcih for our example is 53 degrees.


50mm focal length for 35mm film cameras means a field of view of about 40 degrees. 70mm is for a field of view just under 30 degrees. 105mm is just under 20 degrees.









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Old Oct 4, 2006, 10:52 PM   #12
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Do you agree that for the sequence listed above, (and with the above parameters of my camera):
F 7.1 means focal length = 5.8 mm (as I see it totally zoom out)
F 12.4 means focal length = 17.4 mm (as I see it fully zoom in)
?

If you say 'yes' then move to the second question:

People say that the higher the f-number, the smaller the apparent diameter (not the actual diameter - which is fix), which means here D2<D1. Is this true ?

If the answer is 'yes' then my above question is not answered yet, as I caculate: D2>D1
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Old Oct 5, 2006, 6:32 AM   #13
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You are still confusing aperature with focal length. F stops have nothing to do with focal lenght/zoom. F stop refers to the size of the aperature opening, with smaller numbers being the widest openings and the largest numbers being the smallest openings. Focal length more accurately refers to field of view, that is more commonly known as zoom. Focal length is not technically related to aperature (fstop), however, with most digicams the maximum aperture becomes smaller as zoom (focal length) increases.
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Old Oct 5, 2006, 8:05 AM   #14
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nobtiba, when you change the aperture, you're not changing the focal length; you're changing the setting of the iris inside the lens assembly. When you set the lens to f/7.1, it means ONLY that at whatever focal length you're using, the iris in the lens will be open to a diameter 1/7.1 of that focal length.

The focal length does NOT change when you change the aperture; it ONLY changes when you use the zoom control.

Assuming N is the "aperture number" (5.6, 8, etc.), your equation N=f/D is correct. D in this equation is the diameter of the internal iris, though, not the maximum lens diameter. When you set N, that doesn't do anything to f; it tells the camera to set D (the internal iris diameter) so that wherever you set f (by means of the zoom control), f/D equals the N you've set.

In your equation, you set N using the aperture control. You set f using the zoom control. The camera calculates and sets D based on your settings for N and f. Changing N does not change f; it changes D. The only way to change f is by using the zoom control.
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Old Oct 5, 2006, 9:13 AM   #15
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Yeah, when I press zoom button (W or T), I see the changing in f stop (N).

The numbers in the sequence F 7.1 7.7 8.4 9.1 9.8 10.6 11.5 12.4 appear contiuosly as I zoom in (or out).

Example: When I totally zoomout I see F 7.1 on the LCD

when full zoom, I see F 12.4 on the LCD

(Camera sets in Manual mode).
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Old Oct 5, 2006, 10:02 AM   #16
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nobtiba wrote:
Quote:
Yeah, when I press zoom button (W or T), I see the changing in f stop (N).

The numbers in the sequence F 7.1 7.7 8.4 9.1 9.8 10.6 11.5 12.4 appear contiuosly as I zoom in (or out).

Example: When I totally zoomout I see F 7.1 on the LCD

when full zoom, I see F 12.4 on the LCD

(Camera sets in Manual mode).
Ok, here's the issue. The lens in question has a variable apeture. The possible aperture values when at it's widest zoom are 2.8 to 7.1.

As you zoom out, the possible aperture values change. When you are totally zoomed out, the possible aperture values are 5 to 12.4.

So, if you are zoomed out and set camera in manual mode and set aperture to 12.4. Now as you reduce the zoom, the camera can no longer maitain the aperture value of 12.4 so you see it start creeping down.

But, 7.1 should be maintainable throughout the whole range - so if you set it to 7.1 and zoom in or out it should remain at 7.1

But, I have to admit I'm not used to seeing the larger aperture number as variable. It's very common for a lens aperture to go fom say 2.8 to 5 as you zoom out. But there must be something in the design which forces both numbers to change.
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Old Oct 5, 2006, 3:56 PM   #17
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Another way you can think of f-stops for a zoom camera is to consider that their are four important numbers:

Maximum aperture at widest angle, say, f/2.8

Maximum aperture at full zoom, say, f/5.0

Minimum aperture at widest angle, say, f/7.1

Minimum aperture at full zoom, say, f/12.4

If the camera was set to f/12.4 and full zoom and then you started to widen the angle, the f-stopmust start moving towards f/7.1.

If the camera maintains constant f-stop as you zoom, the f-stop will not creep until it violates aparallogram shapedgraph based on the four numbers above.

If the camera maintains constant aperture diameter as you zoom (more likely), the f-stop will always creep.

Whendiscussing exposure or f-stops, ignore the number in millimeters (the focal length).




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Old Oct 5, 2006, 8:43 PM   #18
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"However, there are two numbers involved here: aperture (hole) diameter and focal length. It is not enough just to say that the higher the f-number, the smaller the apparent diameter. This can safely be concluded only when the focal length is held constant. If the two f/numbers are for different focal lengths - and if you are zooming the lens, you are changing its focal length - a higher number may also mean a longer focal length, even when the apparent aperture diameter stays the same or is even made slightly bigger." <Alan Ball>

May bethe aboveanswer of Alan Ballis theappropriate answer for my question. We have to admit that when the f-number gets higher value "the apparent aperture diameter stays the same or is even made slightly bigger" :?.
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Old Oct 6, 2006, 6:18 AM   #19
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Yes, that is true; as you zoom, the diameter of the opening of the iris will change--to maintain your set aperture number. f/7.1 while fully zoomed in does require a larger "hole" than f/7.1 when at full wide-angle, because that aperture setting sets a relationship between the size of the hole and the chosen focal length, and to keep that relationship (ratio) constant, as the focal length changes, the size of that hole must change.

Changing your aperture setting without touching the zoom control will not zoom the lens in or out. It only changes the size of the iris opening.

Changing your zoom setting (focal length) will not change your aperture setting until you reach the physical limits of the iris. It WILL change the diameter of the opening in the iris, but the ratio of that diameter to the focal length as you zoom remains constant until the iris can't open or close any further.

Now... if you have the camera in shutter-priority mode, when you zoom, the camera may need to choose a different aperture, depending on what part of a scene you zoom in on. If zooming in fills the viewfinder with more dark areas, the camera will have to open the aperture to keep the exposure constant. If zooming in gets rid of some of the dark parts of the scene, the camera will close the aperture down to compensate.

If you're really in manual mode, the only thing that should change if you change the aperture setting is the aperture setting. The camera might be recommending a different setting, though--maybe that's what you're seeing. The actual aperture setting should not change, since you set it in manual mode.
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