Hard to say without specific information and sample images, but it very well could be normal.
The problem with wide angle lenses arises because generally we use them for landscape shots at a distance. In that circumstance the subject matter is relatively small in the frame, and occupies a small segment of the AF sensor. That may make it difficult for the system to lock focus. There can also be multiple low contrast targets at varying distances on the same AF sensor which could also cause problems and a miss-focus.
Also with shorter focal lengths, the harder it is to correct aberrations which could affect AF performance, especially in zoom lenses which are often optimized for a specific focal range at the expense of the others.
Then again, it could be a malfunction in the lens that occurs only at the wide end.
So for me at least, you need to provide a bit more information.
Pete, Your explanation gels with me, I was musing on this topic while you were writing. Unless the subject is something very large and dominating with no visual depth it is going to be very hard to decide how to achieve automatic focus. Perhaps a large well lit factory wall would be about the only practical object to fill the sensor? I am sure that subjects that fooled automatic sensors used to result in what was once known as subject failure, e.g. snow that appeared too white and slewed the 18% grey issue, black cats in coal mines, etc. Does anyone else remember that term?
A sample shot or two would help. If you know someone with another compatible Nikon, or your local camera store, try your lens on another body and another fairly wide lens on your body, and see if the same issue occurs under similar conditions. Lack of contrast is a major culprit in autofocus issues and not confined to wide by any means. Pick a good contrasty target, a dark building against a bright sky for example, aim so half your focus point is in the light and half in the dark areas and test the focus ability on a variety of subjects in this manner. The 18-70 being an F3.5/4.5 lens looses about 2/3 of a stop of light over an f2.8 lens so light transition through the lens as a focus aid is a little more limited. In low light/poor contrast situations all Nikon models can have AF issues. Think driving down a road on a foggy day. The details you need for safe driving are harder to acquire. Under less than ideal conditions AF struggles more to acquire a good focus lock.
____________________________________________________________________ When no one is looking, Pigs can walk on they're hind legs
Say I'm sitting in my easy chair, fiddling with the camera. I have a fireplace/bookcase thing on the wall on the other side of the room. Its filled with lots of good contrasty lines and shapes and stuff...the sort of stuff that AF can grab instantly.
I set the lens to 18mm...point and click...and "whirrr....whirrr...." camera hunts for focus from end to end and gives up. Won't fire.
I set the lens to 25mm...point and click...and "POP!" Instant focus lock and exposure.
This is with my AF point set to the center, and with the little center block on the viewfinder positioned right over some good high-contrast (easy to AF) stuff.
>Here's an example... > >Say I'm sitting in my easy chair,
Its nice having a specific scenario, but sample images would be better.
In your scenario, what appears bright and contrasty to you might not be so bright and contrasty to the AF sensor. How far is the bookcase, and how magnified are the subjects?
There is just too many variables involved, and without specific sample images to say OK this shot the AF failed, and this shot it could AF were just going to go round and round in an academic exercise, not a determination and solution.
Perhaps use a wide angle setting that AF fails on, use manual focus to take the shot then provide us with that sample. We could then determine if there may be a lens malfunction when set to the wide end. That will at least help eliminate one of the variables.
If it is a bookcase across the room then I am going to guess it is illuminated only with normal internal light and is likely to be a challenging condition. I think Pete is right that an example shot of the scene would be 'worth a thousand words'.
Your lens or your camera is faulty. My D70 and 18-70 combo works perfectly at any focal length unless there is some egregious issue with the subject such as others have already described. Since that lens is variable-aperture, it ought to focus most reliably at 18mm because that focal length gives the AF sensor the most light to work with.
My first reaction to this post is to ask exactly what your Autofocus settings are. You're dealing with a lot of variables, so let's eliminate the ones that can be easily and clearly quantified. Specifically - AF-Area Mode, Servo Mode, frame advance and which sensor in the array are you relying on? All the other factors would rely on the situational environment (lighting, subject, etc)
Well I am glad that you have been able to put the issue to bed. My earlier suggestions were all aimed at making sure that the old lens was properly tested. Will you explore repair options on the old lens or just bin it as not worth further action?
The old one is going in the drawer. Sort of hate to throw it away, but since the new one covers 100% of the focal range of the old (and then some), and seems to be doing it better, I see no use for the old one.
And its not worth even a diagnostic bill from a repair shop, so in the drawer it sits.