Originally posted by alvaro_garcia ...
Why the ...
ISO is the same in both cameras and you wonder why exposure is different
Nikon : f/5.6 , 1/160
Pentax : f/2.8 , 1/20
This is up to a 5 stop difference and it is weird
You mention that the Nikon lens is an f/2.8 and the Zeiss is f/2. This is a significant difference in maximum aperture but maximum aperture makes no difference when the lenses are stopped down (f is bigger). The f/2-ness of the Zeiss lens makes no difference when the lens is set at f/2.8. This paragraph does not help solve your mystery but I thought the infos might be interesting...
One lens might let in more light than another lens. The Nikon lens might transmit more light to the camera. Maybe the Zeiss lens transmits less light because it has more glass elements. When taking a picture with two different lenses that let in different amounts of light, the image brightness should be the same but the exposure settings will be a little different to achieve this.. usually this is not a great difference and the results might be Nikon = f/5.6 , 1/160 & Zeiss = f/5.6 , 1/90. The differences lens light transmission (called t-stop) do not explain the great difference in exposure you have told us about. [also, I am guessing the Zeiss lets in less light to illustrate the point]
I have started exploring a Canon point.shoot and in its AUTO mode it does stuff to lift shadows and whatever other special sauce even if exposure settings are the same as when I use Av with the camera.. Still, no amount of sauce will equal the 5 stops you describe...
As far as the mystery :
Do the Pentax & Nikon pictures look the same brightness on your computer
Do you have exposure compensation set on the Pentax
Do you have something like spot meter set on the Pentax
Try using M mode
Check that both cameras are set for matrix metering. Check that both cameras have exposure compensation set to 0. Use the same scene & light (a wall inside the house even if you need the tripod) and be sure the scene is framed the same. For each camera : set ISO as you have, 100, use M and pick an aperture, use f/4, and then adjust the shutter speed until the camera meter tells you the exposure is correct. Then compare shutter speeds...
Once your mystery is solved, you will find that the Pentax will need about one stop more f to have the same depth of field as the Nikon. The Pentax at f/5.6 should about equal depth of field of the Nikon at f/4 but this all leads to a different discussion... **
You described more depth of field as 'better'. For some people less depth of field is better. To avoid confusion, I might describe more depth of field as 'greater' or 'deeper' and less depth of field as either 'shorter' or 'less'.
** adding the paragraph above about matching DOF : the Pentax to f/5.6 to match the DOF of the Nikon at f/4 means that the Pentax will need a slower shutter speed or higher ISO to offset the bigger F number. But this is about DOF equivalence and has nothing to do with your mystery. Setting aside DOF, a picture of the same scene with the same light and same camera settings should give you a picture of roughly the same brightness and certainly not a picture 5 stops different.]