75-200mm Sigma Lens Darkens

This is an odd problem if you're using a standard lens. I've been told that it's called photo chromatic aberration and it's a fairly common problem with inexpensive big (600-1200mm) telephoto lenses.

If it's happening with a good quality standard lens this suggests that there may be a problem with the autofocus not being quite set when you take the first shot then being fully adjusted by the time you take the second shot.

I'd suggest contacting Nikon support at www.nikon.com to see what they suggest. Be sure to provide them with information about what kind of lens you are using and what the light conditions are.

Could you be a little more specific about "does not work"? Does the lens not fit? Can you not view through the lens when it's attached? Does it not autofocus? Does it not meter the exposure properly?

Remove the lens and fire a shot...look at the mirror....does it return? If not, the viewfinder will remain dark. If it's stuck in the up position, you'll need to find the cause but be careful...do not touch the surface of the mirror.

First thing to check is the shutter control or maybe what speed your shutter is on. Check your settings and you'll probably find something is switched to the wrong setting. Try switching everything to Auto, including anything menu operated your manual should explain how to get to your on-screen menu. Try that and if no good, post back and we can take it from there.
With cameras getting more compact and buttons getting closer together and smaller it's very easy to knock a switch the wrong way without realising it.

Silvanus is correct - I have a D200 and often by accident adjust the exposure bias dial when taking the camera out of my backpack resulting in dark shots. I bet this is your problem.

The Penta K110D has a 1.5X crop factor so a 17mm lense would be the equivalent to 25.5mm (in 35mm standard), therefore may not give you a 103 angle view. You would not really notice that much difference between 17 and 18mm.

You can adjust the number of focus areas so that it will not try to compensate for many objects in the pic at 1 time

This normally means that you have forgotten to set the lens aperture control to it's minimum aperture (the highest number). On the D100 the aperture control is via a wheel on the camera body instead of via the physical control on the lens aperture ring.

Wow, that's a long lens! It it a Nikon 500mm?

I use the full manual mode on my D70 sometimes, and I think the operation will be very similar. Spin the mode dial to M, first.

Now half-press the shutter and you should get metering at the bottom of the viewfinder. The meter will tell you if your current settings are over or underexposing. You can use the front control wheel to set the shutter time, and the back (thumb) control wheel to set the aperture.

If you've used S or A modes before, you'll be familiar with these controls. More aperture (smaller F numbers) lets in more light, as does (obviously) increasing the shutter exposure. If not, play around and watch what happens to your exposure meter in the viewfinder as you dial aperture and shutter duration up and down.

You're going to want a sturdy tripod or some very bright light with a lens that long!

You may have to use manual focus, too, if your lens does not have modern Nikon guts for the camera to talk to. I guess you'll cross that bridge when you come to it. Good luck!

Sounds as if your diaphragm may be sticking due to either the camera or the lens. Look at the back of the lens and work the small tab with the aperture ring set to f32 (or 22). See if the tab moves smoothly but with resistance from a spring, or if it sticks or moves roughly. If it does this with other lenses too, another possibility is the aperture lever on the camera. Mount the lens and set it for f22 or so and use your depth of field preview to see if the lens is stopping down smoothly. In either case it may need professional repair.