My 55-200 automatic lens will not auto focus on the lower focal lengths, but will on the longer ones. It hasn't been dropped or mishandled and the standard lens works fine. Nikon D40

You may be trying to hand-hold the camera in situations that call for a long shutter opening. These are usually indoor situations with with limited light, dark walls, etc or night time shots. To get an idea about the longest time you can hold a camera by hand with creating blurred pictures (without a tripod or other stable mount), look that the focal length of the lens. If it is a a zoom lens, this value changes depending on how far zoomed in or out you are. A 70mm-300mm zoom would have a focal length of 70 if zoomed out or widest, 300 if zoom in or narrowest or something between 70 and 300 if your not fully zoomed in or out. The lends will have indications as about the focal length you're current at. If it is not a zoom lens, it is a fixed or prime lens. You can only shoot at the focal length of the lens. Typical primes are 35, 50, 60, 85, 100, 105 etc. (but there are many more). Once you've determined what the focal length is, multiply by 1.5 (for DX sized sensors like the D3000 and others; FX bodies and 35mm film cameras skip this step of multiplying by a factor of 1.5). Let's assume you've got either a 50mm prime or an 18mm - 70mm zoom lens currently zoomed to 50mm. With the info above, 50 x 1.5 = 75. Next, find the reciprocal which is simple to do - just make it a fraction with a 1 on top, like this: 1/75. This fraction is the longest length of time in seconds that most people can hold a camera before shake appears in the captured image. The shorter the focal length or wider the shot, the longer it takes for the shake to be noticeable in the captured image. A few more examples follow: When the 18mm - 70mm lens is zoomed in to 20mm x 1.5 = 30; or 1/30 sec, when a 80mm-300mm lens is zoomed in at 100, 200 or more, that time plummets quickly: 200mm x 1.5 = 300; or 1/300 sec. 300mm x 1.5 = 450; or 1/450 sec. Use of a tripod, monopod, or other bracing is highly recommended. Additionally, you can try opening the aperture wider (a lower f stop number) and / or increasing the ISO value to 200, 400, 800 or more. The drawback to increasing the ISO is the introduction of digital "noise" or graininess. How much graininess that is acceptable is something only you can decide. Experiment by taking a number of pictures of the same subject (preferably with the camera on a tripod or table top) with a range of different ISO settings. Look at the results on a large screen - like your computer monitor - to get an idea about how the graininess or noise increases with each bump up of ISO. You will probably find that once you get to a certain value, it's not worth taking pictures. This will be your no go value - and you'll want to shoot at a lower ISO than this. it is not uncommon for this number to be as low as 200 or 400 with P&S (point a shoot) cameras and 800 on some entry and mid range dSLRs like your D3000 to as high as 3200 (or even more) on some higher end prosumer / professional bodies.

You may also be having an issue with focus. If you have turned AF (auto focus) off, you'll have to focus manually. If you have AF on, but do not hear the focus motor in the lens - there could be a problem with the lens. Try removing and reaseting the lens on the camera body again. Try other lenses to determine if it is lens specific or camera body specific. Also, if there isn't enough contrast in the subject, the lens will not be able to find focus. Check again taking pictures of well lit subjects. You may find that your lens will not open wide enough to reduce exposure time. This is where the expensive f1.2, f1.8 and upwards to f2.8 shine. They gather 2, 3 or 4 times as much light in the same time as a f5.6 lens can. The drawback to these lenses is their cost.

I hope this helps and good luck! Please rate my reply, thanks.

The answer to this is Yes and No. Yes there is an adapter to take the FD mount lens which is Canon's Manual lens to an EOS mount which is auto focus. As much as these third party manufacturers claim comparability there is really no way you are going to have "good" results. Here is the No part. To give you infinity focus there must be a lens mounted in the adapter this lens no matter how good the third party says they are you loose light, focal length and sharp focus. Your F3.5 lens will loose at least a stop and a half because the light is traveling through another piece of glass, so you are looking at a lens that is now about F5 at the short end to F6 at the long end. About the focus because now this lens has become your rear element is is more critical to flaws then the front element, any imperfections in that piece of glass is now magnified it still works but the results aren't as good. Also the focal length of the lens has now changed. If you are installing this on an EOS l7D the focal factor would be 1.6 is some cases this would be beneficial but in most others you loose again, it will take that 35 to 70mm lens and give you the focal length equivalent 56mm to a 112mm lens. Yes Canon made these adapters for a short period of time to help the pro shooters and their high priced lenses get through the transition BUT those adapters were only made for the professional lenses that was F2.8 or faster.
Yes it will work to some degree but No it's not worth it specially if you are installing it on a New Canon 7D.

At longer focal lengths, when the aperture is smaller, and in low light, many A/F zoom lenses struggle. Is it OK outdoors in daylight ? If so, lens is OK.

Some of the Rokinon (and other ) zoom lenses could only go into MACRO at certain focal lengths- the lens you have "zooms" from 80 to 200 focal length,but is that done by "sliding" a ring forward and back on the lens, or by twisting a different ring that the one that focuses?

It will default to manual through the viewing screen. It will only work on automatic, using the viewfinder. Make sure you also set the focal length (lens to shutter distance). You might also have a mf / af switch on the lens. This also only works through the viewfinder.

The camera is unable to focus properly in low light at long focal lengths. You might have to throw some light on the subject with a flashlight until the camera locks in the focus or resort to focussing manually.

All cameras have a minimum focal length. That is the minimum distance they have to be from an object in order to focus clearly. If you have a digital camera with changeable lenses, that minimum focal length is determined by the lens and how it is constructed. If you have a fixed lens digital camera then the manual should tell you what the minimum focal length is.

One other thing to keep in mind is that on most fixed lens digital cameras, there is what is called a macro setting. If you switch to macro mode, it will allow you to get closer to the object and still get a good focus. Your manual should tell you how to turn on that mode.

It will not focus on anything closer than 5 feet.

(From Sigma lens literature) Capable of macro photography, this lens has a 1:2 maximum close-up magnification at the 300 mm focal length. It's the ideal high performance lens for portraits, sports photography, nature photography, and other types of photography that frequently use the telephoto range. It also has a switch for changeover to macro photography at focal lengths between 200mm and 300mm with a maximum close-up magnification from 1:2.9 to 1:2. The minimum focusing distance is 1.5m / 59 in. at all zoom settings.

How does it stop working? Do you mean it doesn't auto focus anymore?
Many longer focal length lenses wont auto focus with 2x teleconverters on them.

The focus information is available at: http://www.ssdonline.com/... ...m?Productid=B111021F&affid=A01&urlid=B111021F