Brilliant. Glad to know I'm not alone in having to sit through dreadful TV programmes in this way!

Your sums piqued my curiousity too; I had a play and tried solving your equations assuming a focal length of 55mm and made u=216mm. I'm not entirely sure what this means either, other than 216mm would be somewhere inside the lens/camera in your set up and so, unfortunately, it's probably not as simple as the book says!

I did a bit of optics during my degree, but as it was a while ago and concentrated on optical fibres not camera lenses, I can't claim any sort of authority on the subject.

However, If my memory serves me correctly, it was possible to model an optical fibre with a set of comparatively simple propagation equations but not very accurately. The more accurate you wish to make the model and the more physical factors you take in to account, the more complex the equations become.

For this reason, I would assume (without having read it) that the equations in the book may use a simplified model of the camera lens, making a set of assumptions about it (i.e. a simple, fixed focal length optical scheme) that mean it will never be perfectly accurate.

Considering that the kit lens is a zoom and contains a complex optical design with an aspherical element, I think that getting a result of 47mm is actually pretty good!

Do you know what the speck on the rear element is? is it a chip on the outside surface or something internal?

Lens arrived today, in excellent condition as described and well packaged, thanks - very pleased!

Payment sent

You have a PM re the 105/2.8

I'm not that well up on PayPal fees, but as I understand it, gift transactions attract a lower/no charge than transactions linked to a purchase of goods and therefore aren't as profitable for PayPal.

With my cynical at on, I think that they have removed the sender's address details from gift transactions to discourage people from purchasing goods with a 'gift' transaction to avoid the fees. It might put a few people off using gift payments and is probably worth it from their point of view.



+1, especially if this is the only way there is to find their address now!

I bought one from RobE17 as well and am very pleased with it - a smooth and pleasant transaction thanks.

(now I have 4 K-5 batteries and am also sorely tempted by a project involving lithium polymer cells and a cheap grip... Thanks to everyone for their help and advice.)

+1

The 18-55 kit lens can be bought quite cheaply (compared to a K-x, anyway) and is a surprisingly good lens. It will complement your Sigma 28-70 nicely, with the Sigma's wider aperture being useful for low light or portrait shots.

I have one, it's not technically as good as the Pentax 12-24. I prefer the colours from the Pentax and the Tamron is not as sharp at the edges, but there is a significant difference in the field of view between 10mm and 12mm at the wide end.

I have recently got a copy of the Pentax 10-17 and can't really justify keeping both, so I'm planning to see how images from the 10-17 de-fished compare to those from the 10-24. I think that I can get a slightly wider FOV from the 10-17, but my early attempts at de-fishing with hugin give quite a lot of distortion (but this is probably down to lack of experience on my part).

True and, indeed, how well you can tell between left and right! (which never was my strong point - different dominant eye and hand)

I had a look at my M28/3.5 yesterday evening and the hyperfocal marking is on the right hand side of the infinity symbol, as expected (not the left, as per my previous post).

However, it does sound as if the lens is not focussing to infinity.

I think that checking the minimum focus distance is a good start, although due to the non-linear focus scale, a small error in location of the ring at minimum focus distance would translate into a massive difference at infinity.

Definitely sounds like lens problem, then, which is good news.

Another easy check would be to mount it on your K-5, on a tripod, aim at an object a long way away (I use a telegraph pole about 300m away from my window), use live view zoomed in to its fullest extent on a portion of the image where there is some detail. Move the focus throughout the range, starting from near, moving towards infinity.

If the image gets progressively sharper until you hit infinty, then the focus ring hits its stop before you reach infinity.

If the image gets sharper, sharpenss peaks and then falls off again as you move towards infinity then the lens is focussing past infinity.