Lens Specification Question

camerai

Greetings -- I have a Nikon kit lens which has a focal length going from 18mm to 55mm. The f/stop rating on the lens is 3.5 to 5.6. However, when I look at the specs of the pictures I take before and after I shoot, the best I ever see is 4.5 to 5.6. Shouldn't I be getting more light? ISO is 200 the lowest I can get with the camera. Changing the shutter speed makes no difference. I am sure there is a ratuioinal explanation for this, but I am a neophyte.

camerai

craig_d

If your pictures are not using the wide end of the lens (18-20mm or so) then you won't see f/3.5. The maximum aperture varies across the lens' zoom range, from f/3.5 at 18mm to f/5.6 at 55mm.

Also, depending on what mode your camera is in, it will make different choices to get what it considers an optimal exposure. Some modes, such as portrait and sports modes, favor a wide aperture; landscape mode, on the other hand, favors narrower apertures.

The easiest way to see that your lens can do f/3.5 is to put it in aperture-priority mode (A), zoom out to 18mm, and dial in f/3.5. If you then zoom in slowly, you should be able to see about where f/4 or f/4.5 becomes the maximum aperture.

Qarik

the 3.5 is at 18mm the 5.6 is at 55mm. Try zooming to 18mm and you will be able to get to 3.5. What that means for you is that you typically don't shoot at the wide end.

chuckinsocal

I'm far from an expert, but basically speaking on zoom lenses the aperture is tied to the focal length of the lens. The longer the focal length the smaller the aperture (larger fstop number).

Try setting the camera to Aperture Priority and select the largest aperture, 3.5 in your case. Then zoom your lens in and out. You should see the aperture changing as the focal length changes.

A pretty basic explanation and I'm sure others will provide far more detail. I hope this helps in the mean time.

Edit: Ooops ... It looks like three of us were typing at the same time.

camerai

chuckinsocal wrote:

I'm far from an expert, but basically speaking on zoom lenses the aperture is tied to the focal length of the lens. The longer the focal length the smaller the aperture (larger fstop number).

Try setting the camera to Aperture Priority and select the largest aperture, 3.5 in your case. Then zoom your lens in and out. You should see the aperture changing as the focal length changes.

A pretty basic explanation and I'm sure others will provide far more detail. I hope this helps in the mean time.

Edit: Ooops ... It looks like three of us were typing at the same time.

Thanks for everybody's help. I got it at 3.5 now. I guess as you say, I typically don't use the wide end of my lens. I only use manual on this D40 -- a carryover from my old Nikon F days. I like to make all the settings myself although I usually allow autofocus.

camerai

ziggy53

You got three great and similar responses and they are all correct. The lens you are using is called a "variable aperture" zoom lens.

A lens is normally rated by the largest aperture, but on a variable aperture zoom the lens efficiency changes with the focal length, and the lens is least efficient at the longest end of the zoom range. This results in a dual aperture rating for the lens; one rating for the shortest focal length and another rating for the longest focal length.

Another type of zoom designs is a "constant aperture" zoom and they attempt to maintain the same "effective aperture" regardless of focal length.

A Nikkor 17-55mm, f2.8G ED-IF AF-S DX, a very expensive professional zoom lens, is a constant aperture design and it's actually "less" efficient at the wide end on purpose. The "relative" aperture size varies as a function of the zoom range in order to keep the "effective" aperture constant through the zoom range. The optical quality of the lens is excellent, another function of the particular zoom design, and another reason for the lens' cost.

Prime lenses, lenses of a single focal length, only have one aperture rating of course, except for some specialty lenses like some macro lenses which extend so far that they lose efficiency due to image magnification.