My slr lens will not work with my pentax k-m dslr camera, it will not let me take photos, the lens is a manual lens Optomax 300mm. and a F keeps flashing on the screen

before buying ask the sigma salesman or dealer about it, i know only for nikon camera- it's lenses are compatible for all camera, digital or film base slr cameras.

If I am not mistaken all lens can pretty much work with other cameras provide that the attachment is compatible. Having said that with the new SLR cameras with auto focus of the lens the lens itself has a motor and it would be difficult to attach it to a manual SLR. Save yourself the headache and if you can purchase same manufacturer for camera body and lens life would be wonderful. In the past I thought that I could save money and do the swap thing and it just became too much. In addition the new cameras that are out has wonderful features like anti vibration, smooth focusing and the sensors are just dynamite. Good Luck

Safest to get it fixed by an expert.

It is unlikely that the manufacturer sells spares to the public - safest to get it fixed professionally.

Any Pentax K-mount lens will fit any Pentax DSLR. The DSLR will fire the shutter.  There is a menu setting (RTFM) which allows the use of older lenses, but you will lose some of the automation (again RTFM). Remember to allow that the effective focal length when using a "film" lens on a DSLR body goes up by 50%.
A standard 50mm lens becomes a 75mm equivalent, but stays a large aperture.

It could Canon, this is one of the edge Canon over other brands. Its versatility on lens mount is better.

If it doesn't work then sorry that is how it is. I heard is not really a good reason, I hear a lot of things most is not true.

Depending on how new your Minolta SLR film camera was, the lens may or may not work on a Minolta Digital SLR. In many cases, a lens with the correct mount (in this case, a Minolta mount) can be used on the same brand of camera in the digital format.

One thing you should know is that DSLR sensors are, generally speaking, smaller than the size of a 35mm film negative. Long story short, that means that your lens will have a magnification factor on the DSLR. Usually, it is in the range of 150%, so a 70-300 lens from a film camera would cover 105 to 450 on a Digital SLR.

To be sure about the mount, you'll need to seek advice specific to Minoltas -- probably best to take your lens to a local camera shop and see for sure if it fits and what features will work (aperature, auto-focus, etc) and which won't work on the DSLR.

http://shop.ebay.com/items/_W0QQ_nkwZminoltaQ20canonQ20
adapterQQ_armrsZ1QQ_fromZR40QQ_mdoZ

here is a link to an ebay search for minolta to canon eos adapter, they sell for just about every camera. i know it sucks, buying a new camera that wont work with your favorite lens, but i think you might be better served focusing (couldnt help myself) on camera features and pricing. i own a canon dslr and love it. sigma has a cool dslr. it has a three layer image sensor, with a removable on sensor ir filter. the ir filter thing is really cool.

Make sure the aperture ring is set to the "A" position. If the lens doesn't have an auto aperture setting then I don't think the lens will work. There is a setting called "Using Aperture Ring" that you can setup using the menu and configuration that can be set to "Permitted" that will allow you to take pictures even if the aperture ring is not set on "A", but not sure how well this work for you, but should allow the shutter to work.