Before you buy either the sigma 120-400 or 150-500, you might like to consider the failure rates experienced by lensrentals.com:

http://www.lensrentals.com/news/2009.11.01/lens-repair-data-35

Of course, these figures aren't for Pentax mount, but I don't know how much of a difference that makes. The 50-500 is not that much more expensive, and is meant to offer as good if not better sharpness, whilst being five times more reliable!

I may be mistaken, but isn't the 60-250 an Internal Focus lens? If so, does that not mean that it's more like a 60-210 when focussing on objects closer than five metres... something to consider, perhaps.

I see that you are comparing the A 1.7 with the FA 1.4. You might like to check out this link:

http://www.pentaxforums.com/forums/pentax-slr-lens-discussion/81528-fa50mm-f1-4-...

Where a FA 1.7 is compared to the FA 1.4. I had thought that the A and FA lenses were optically identical? Indeed, Dimitrov's website seems to indicate that this is the case.

You should know that the image may still be recoverable, even if you have used the card since. There's software out there that can pull a lot of data off even a heavily corrupted card!

Steve, what lenses do you already have?

Regarding the 100 to 300, as axl mentons, you will get Auto Focus and exposure, but most of the power zoom functions will not work. It'll be a fine lens if you can get if for cheap. If you can stretch to ~£250, you could get a DA55-300 new, which is very good, or as axl also mentions the "L" version is also very good, but might be available ~£150 second hand.

You say that you want to shoot birds at a distance. What kind of distance? I should point out that even 300mm on a crop body is not very good for serious birding - you would typically be looking at around 500mm or more for looking at birds at any kind of distance. Of course, 300mm is better than nothing!

You could consider picking up a mirror lens - the Sigma 600mm mirror lens typically runs at around £90. However, mirror lenses have lots of problems; their main advantage is that they can give you extreme focal lengths for a fraction of the price!

Stefan,

Unfortunately, by sharing your idea with the world, you will no longer be able to patent it in most countries outside the US!

I'll be more accurate in my explanation to show what is going on.

F-number is determined by the size of the "Entrance Pupil" - that is, the apparent size of the aperture as seen from the front of the lens. For telephoto lenses, this generally works out as approximately the size of the front element - which is why 300mm f/2.8ers get narrow towards the camera, and aren't over 100mm wide the whole length of the lens.

If you think about it, this is why wide angle lenses have surprisingly high F-numbers despite the ruddy great size of the front element - the extreme curvature of the glass makes the aperture look very small!

Steve,

You might find you get a much better response if you give us at least a rough idea of how much you are looking for.

The equation for the f number is focal length over diameter. 135/58 comes out to 2.4, so it is plausible that the aperture is correct.

There is a setting in the camera (at least, there is on my K200d) that changes the pop up to "controller only", so it does not affect the exposure and just triggers the flashes. This only works if you use the remotes in P-TTL mode.