Really dumb question

[nwyman]

I have tried researching this on other forums, but apparently I'm too stupid to even know what I am looking for.

On front of the S2 lens, there is an engraving that says:
6.0-72.0mm - I know what that means.
And then it says:
1:2.7 - 3.5 USM

and there my confusion begins.
I think these are f/stops.
I know they describe the lens

I don't understand the "1" and why these numbers are mentioned when the lens goes from 2.7 to 8.0.

I gather this has something to do with "maximum aperture", but I just don't get what this has to do with anything.

Nancy (praying for an answer that makes sense to a math-befuddled mind)

[Ikinaa]

f/2.7 is the maximum aperture at 6 mm
f/3.5 is the maximum aperture at 72 mm

if the aperture is f/2.7 or 1:2.7, that means that the size of the hole (the aperture) is the focal length divided by 2.7
that's why the hole is smaller when the number is larger.

the minumum aperture for all focal lengths on you cam is f/8

[nwyman]

2.7 is the maximum aperture at 6 mm
3.5 is the maximum aperture at 72 mm


I think a light bulb just went off. Is that why I can't get the camera to read a 2.7 aperature when I'm zoomed out?

Nancy (feeling even dumber than before)

[Ikinaa]

I think a light bulb just went off. Is that why I can't get the camera to read a 2.7 aperature when I'm zoomed out?

Nancy (feeling even dumber than before)

when zoomed out, the focal length is 72, an maximum aperture is f/3.5.
an aperture of f/2.7 is larger than f/3.5, so it's impossible to have a f/2.7 at 72 mm, because it can't be larger than the maximum.

hope my explanation is somewhat clear...

[nwyman]

very clear - thanks. I have this mental block when numbers are involved, and fractions tend to make it worse. <g> But I think this is one mystery that you've solved for me.

Nancy

[adas]

To make it even clearer as to why your lens has to become darker when zoomed out, imagine a dark room where you holding a light bulb close to the wall at 6 inches, the wall will be well lit, but as soon as you move the bulb away to 70in, the wall gets darker and darker.
In this case the light bulb is the incoming light through the lens opening and the wall is your sensor.

So the aperture is the lens brightness. 1/2.7 = 0.37
That is, an object will be 37% bright on the sensor than in real life.

(Otherwise, for practical use, the numbers such as 2.8;3.5;5.6; are pretty meaningless)

[nwyman]

Fire hot - tree pretty.

Nancy ( wondering how she stumbled into this hobby anyway.)

[Jethro790]

USM= Ultra-Sonic Motor regarding the auto-focus mechanisim.

[RossW]

I think a light bulb just went off. Is that why I can't get the camera to read a 2.7 aperature when I'm zoomed out?

Small nit to pick: zooming in gives a close-up; zooming out gives a wide shot. So the S2 can yield an f/2.7 aperture only when zoomed out. Maximum zoom in = 72mm focal length; maximum zoom out = 6mm. The use of "zoom in" and "zoom out" in this manner goes back to the earliest days of the use of zoom lenses in movies and TV. (My other pet peeve in this regard is saying "pan up" or "pan down" instead of "tilt." Pannning is left/right movement.)

[nwyman]

I had NOT realized that! Gosh, there are a lot of pitfalls here.

Nancy

[RossW]

I had NOT realized that! Gosh, there are a lot of pitfalls here.

Nancy

Only if you're running a video camera on a crew when I'm director! :D At one time I worked in a studio that used a mirror to give an overhead shot -- not unlike for a cooking program to show the stove from above. The mirror reverses the image, so the camera had to be electronically reversed back... making all of the pan right/pan left directions "wrong" according to the viewfinder. Since the crew was trained to immediately respond correctly to the pan left/right command, we decided to use the term "image right" and "image left" for the mirror camera, and let the camera operator figure out which direction to pan. Not everything was better in the "good old days!" :confused: