Aperture and the Nikon 18-55mmVR G Kit Lens: This may be a dumb question, but I learning Manual shooting on my D3100. I noticed at the 18mm focal lengh the camera allows me to "open...

Nikon_DonB Loc: Chicago

This may be a dumb question, but I learning Manual shooting on my D3100. I noticed at the 18mm focal lengh the camera allows me to "open up" the aperture to the widest f3.5 stop. As I zoom out to maximum 55mm focal length the aperture stops down to the smaller f5.6 automatically and I cannot open up the aperture unless I reduce focal length again.
Can anyone explain why it does this. I have a wild guess, lens flash????otherwise i'm at a loss......thanks my friends.

sinatraman Loc: Vero Beach Florida, Earth,alpha quaudrant

it is a variable apeture, and is designed that way so the lens is afforadable, smaller and lighter. check out the price of the same lens with a fixed apeture and have some sticker shock. also you will notice the lens will be larger.

J. R. WEEMS Loc: Winchester, Virginia

As the above, but-- this one place where exposure comp. can come into play. However, this will not change you DOF, at least I do not believe so. :)

Nikonian72 Loc: Chico CA

Nikon_DonB wrote:

This may be a dumb question, but I learning Manual shooting on my D3100. I noticed at the 18mm focal lengh the camera allows me to "open up" the aperture to the widest f3.5 stop. As I zoom out to maximum 55mm focal length the aperture stops down to the smaller f5.6 automatically and I cannot open up the aperture unless I reduce focal length again.
Can anyone explain why it does this. I have a wild guess, lens flash????otherwise i'm at a loss......thanks my friends.

Here is a similar question that is part of our (pending) Frequently Asked Questions:
Why does my aperture change from f/3.5 to f/5 when I zoom from wide-angle to telephoto?

When your lens is focused to far distance, it should read f/3.5. As you focus closer, the lens physically extends, which also changes your actual focal length. At the same time, the physical aperture does not change, but the ratio between aperture diameter and focal length changes, causing aperture ratio to increase from f/3.5 to f/5.

Nikon_DonB Loc: Chicago

Nikonian72 wrote:

quote=Nikon_DonB This may be a dumb question, but... (show quote)

Thanks Douglass and everyone else for your input. makes sense.

MtnMan Loc: ID

Take a look at the number on the lens and you will see it shows the range from 3.5-5.6. It is telling you that the min aperture (max effective opening) varies as you zoom. It is due to the physics of the light going down the tube but I haven't tried to understand it deeper than that.

It may relate to the change of length as you zoom; maybe something like the ratio of the diameter of the lens (which stays fixed) to the focal length (which changes as you zoom).

Nikon_DonB wrote:

This may be a dumb question, but I learning Manual shooting on my D3100. I noticed at the 18mm focal lengh the camera allows me to "open up" the aperture to the widest f3.5 stop. As I zoom out to maximum 55mm focal length the aperture stops down to the smaller f5.6 automatically and I cannot open up the aperture unless I reduce focal length again.
Can anyone explain why it does this. I have a wild guess, lens flash????otherwise i'm at a loss......thanks my friends.

MtnMan Loc: ID

Zooming changes the DOF because both aperture and focal length affect it. Someone just posted a great link in another thread that lets you check it out. Note that the numbers on the diagrams below the input change as you change the inputs.

http://www.dofmaster.com/dofjs.html

You might want to see what happens at the same distance for the two zoom extremes and apertures. It doesn't have my D5100 but most Nikons have the same crop factor (1.5) which is what the camera selection controls.

J. R. WEEMS wrote:

As the above, but-- this one place where exposure comp. can come into play. However, this will not change you DOF, at least I do not believe so. :)