Suggest lithiums as best batteries to use when trying to eliminate such issues - they are the recommended disposable for Pentax cameras, and are much more stable.

Giovanni, it's not exactly clear what you mean by "set the aperture continuously". You mention that it's noisy, which sounds like you mean that the aperture ring is actually being stepped down constantly? When looking through the viewfinder, do you see the image getting darker when it happens?

If so, it sounds like you may have a problem with a stuck button or microswitch. In particular, the on/off lever has a third position, marked with an aperture symbol, that does step down the aperture - I wonder if the switch that this triggers has been stuck down, meaning that all the while your camera is on it is doing this?

One answer might be to try and un-stick the switch, if this really is the problem. I have heard that people have solved similar problems with the control dial by using isopropyl alcohol or the like to clean the contacts - however, please be very careful with this as it would be quite easy to damage the camera! I don't know if it might be possible to take the camera apart and mechanically clean the troublesome component?

What John says is right. In principle, 18mm at f5.6 is 18mm at f5.6 (if it really is set to 18mm). However, here are a few reasons why things will be different:

-Any distortion the two lenses have is likely to be different - in particular, for the 18-55, 18 is at its widest angle, whereas a 12-24 is bang in the middle of its range. This means that you are likely to have a less distorted image on the 12-24.

-Different lenses are likely to have different sharpness at the same settings - so one image may be flat-out sharper than the other, and additionally one may have better edge-to-edge sharpness. On top of this, differences in field curvatures may contribute here. Similar factors apply to contrast, and any abberations.

-Exposure is likely to be different as even if you set the shutter speed manually (you don't mention it in the question), different lenses transmit/absorb different amounts of light. They may also give a different colour tint to the image.

-Light conditions may change between the photos (e.g. clouds moving over) giving a different "look" - and auto WB might choose different settings for the different lenses/conditions.

-You'll probably knock the tripod/get dust on the sensor when changing lenses.

Lithium disposables are fantastic for Pentax cameras in general. In my K200D with review turned off, I got over 1000 shots on one set.

Pentaxophile, this is preciesely why I use SV mode. The K200 does not have TaV mode, and Sv mode makes it easy for me to make sure that I am not using a needlessly high ISO - the camera normally will prioritise 1/focal length shutter speed first, then start closing the aperture, so if my aperture is the wrong size I adjust the ISO.

I use SV mode in rapidly changing light conditions, such as sunrise/set - but this is shooting with a K200, so I only have one control wheel.

Chris,

You put in 2x the focal length that you are using - so if you have the lens at 200mm, you put in 400mm. If zooming, then you could choose an "intermediate" value such as 250mm, but I would recommend sticking with the long end - I assume you are trying to increase the effective length of your lens, after all!

Remember that the teleconverter is also going to affect your exposure - a 2x teleconverter essentially makes your f5.6 lens an f11 lens, so you will need bright light or high ISO to get a reasonable shutter speed. The viefinder will be correspondingly dimmer, and if the converter supports autofocus your camera may struggle to lock focus. Of course, your effective focal length is now 400mm as well, so you will likely need a shutter speed of at least 1/500s if hand holding.

I wish you good luck - but suspect you will be disappointed. A teleconverter "degrades" image quality - especially a 2x converter (c.f. a 1.4x) - and the 50-200mm is not fantastic at the long end to start with. Teleconverters are normally only recommended for top quality prime lenses that have sufficient quality to handle the degredation. You might have more success trading in the samsung 50-200 and converter for the pentax 55-300, which is generally better regarded, and gets you 50% longer - or holding out for the (rumoured) WR version.

The 35 is tiny, light and has a plastic mount - the 30mm is reasonably compact, but not small. Of course, the 30mm has a lot more glass - it's a 1.4 compared to a 2.4. The diagonal of an APS-C sensor is 28mm, so 30mm is closer to a "true natural", but the 35mm is slightly closer to the classic 50mm equivalent.

Steve,

Thanks for mentioning this - this was another point I wanted to touch on. Depending on where you go and who you go with, you may find that you can drive off the roads or only on the roads - I made my statements based on having gone off-road quite a lot. If you are limited to the roads, then longer lenses are probably important.

Furthermore, when I went on safari, it was just four to a land rover, so there was lots of space to store things and move about, as well as meaning that I had more control in general. If there are going to be more of you in the land rover, then I think that changing lenses would be more of a hassle.

I took my K200D on Safari two years ago. A few thoughts:

Everyone warned me against changing lenses - but I had no problems, and changed lenses several times a day, in the land rover. The trick I found is simply to wait until you are stationary for a while - the dust dies down. Also, keep the body facing down and change quickly! A rocket blower is essential for the front element, and if I did it again I would probably put filters on my lenses.

The DA 55-300 is a fantastic lens for safaris. I sometimes found 55 too long, but normally it was just right. Of course, 300 is a joke for birding, and the focus was painfully slow on many occasions. However, on my budget, I was very pleased. I would definately consider swapping that out for a DA* 60-250 (if I could afford it) for the speed, focus speed and sealing - using a separate solution for birds. I have heard good things about the 50-135 on safari, but suspect that 135 is not long enough and would probably leave it at home.

I bought the Samsung 12-24 at heathrow on the flight out, so played with it alot as it was a new lens - but I could easily have done without it. However, you have a gap in the short end that it fills quite nicely, and it would be nice to have for the coast etc. If weight isn't an issue...

It will get very dark back at base - consider a fast prime (e.g. 50mm), even if manual focus.

Give some thought to how you will carry the stuff around, and take the minimum - you won't want to leave lenses in your tent, after all! I ended up with back ache as I took a single strap sling bag. Don't forget spare batteries. You might find a battery grip useful - I did.

You should definately take a bean bag for stability, and consider a monopod - you can use them in the land rover when stationary. Also, don't forget to take a good pair of binoculars! Useful both for safari and stargazing at night, and SWMBO can use them whilst you're shooting? I took a great pair of 10*50s, which were just right - don't bother with a zoom pair. I'd have liked some 20*60s at times, but they are a bit too heavy to use comfortably.

Most importantly - have fun, and don't spend the whole time looking through a viewfinder!