Weird issue with Pentax DSLR and flash

[megagram]

Hi there. I hope someone here can figure out what I'm talking about and shed a little light on the issue for me.

Well, I have a Pentax *ISTD with a Sigma 18-50 3.5-4.6 lens. The weird thing that happens is when I use this lens with the flash in dark rooms. When I take a picture of the room as a scene, everything lights up wonderfully and the picture turns out quite well. Now, when I take a picture of that same room except with a person in the scene now, only the person is lit up with the rest of the room blacked out and the lighting is very very dim.

In that situation, with a subject in the scene, it sometimes works nicely, most of the time not. It seems I can't figure out what makes it do it and what doesn't but i have tried EVERYTHING and can't set on one individual aspect that would be causing this.

Other lenses that I have tried, a Sigma 55-200 and a full manual old 50mm lens do not have this effect.

I think it might be the lens then, but is this at all a possibility? Could the lens be screwing this up?

Here are some sample pics:

1. This one is the desired effect that only comes up once in awhile. I believe i got this one to work by using the timer on the camera (however, this doesn't always work). It's out of focus, and a terrible picture, but we're not focusing on that ;)

http://members.shaw.ca/gfleming/light.jpg

2. This is what happens with most of my flash-enabled shots, ugly ugly ugly!

http://members.shaw.ca/gfleming/dark.jpg

3. One more "dark" one but without a comparison "light" shot. In my mind, this shot should be far more brighter with more of the background lit up as well.

http://members.shaw.ca/gfleming/friends.jpg

PS, I have tried compensating for this problem using full manual mode, full auto mode, everything in between, with lens hood, without lens hood, the list goes on. I really do feel this is a lens problem but I just can't see how.

I really appreciate you all taking your time to help out a new addition to these forums. Let me know if you have ANY ideas. Thanks! :)

Graham

[grays]

It seems you're not the only one. Lots of reading and some suggestions here:

http://forums.dpreview.com/forums/read.asp?forum=1036&message=11744792
and
http://forums.dpreview.com/forums/read.asp?forum=1036&message=11554571

[graigsmith]

i think what the camera does. since it has ttl. it adjusts the shutter speed automatically, so that the subject is properly exposed. this eliminates the subject being overexposed. if you remove your hand, and take a picture of the background, the camera will automatically adjust (either the shutter speed or the length that the light remains on) and the subject of the photo will be properly exposed.

[graigsmith]

hmm. i have a pentax *ist d. and i tested my camera to see if it would make dark shots, with just my hand up close to the lens like that. and my camera takes correct exposures.

i kinda think it must be that lens too. but i have no idea why that lens would do that.

mabey the aperature on the lens is wearing out or something. have you tried setting the camera to aperature priority, and choosing 3.5aperature. does it ever happen with that aperature? i noticed the picture was dark, it was using aperature 4. higher aperatures use less light, mabey the lens diddn't pick the right aperature when the shutter button gets pressed

try this. if you look thru the viewfinder, does the brightness look consistent every time you move the depth of field preview button? if the brightness looks different every time, then its definately a lens problem.