New Lens Enigma

[BLS]

I got a new lens a few weeks ago, and I like it a lot. But it tends to result in underexposured shots, and I can't understand why.

My "old" lens is a Tamron 28-300mm 3.5/6.3. The new one is a Canon 24-70mm 2.8 L. With the Tamron I seldom got an underexposure. But with the Canon I often get underexposures. I use both lenses on the same camera, a 300D Canon. I've learned to compensate by bracketing with +1/3 to +1 exposures on the Canon.

Another difference I noticed is with the LCD display on the camera. Using the Tamron gives an LCD image that is close to the brightness displayed by the same image on my laptop. But the Canon lens gives an LCD image that is much brighter on the LCD than on the laptop. I don't think I'm imagining this, but who knows.

Can anyone explain why this is happening?

 

[ferny]

The only difference (other than the lens being covered in cack) that the lens will make to the exposure is the aperture. Is it all correct? I guess there is a chance that it isn't calibrated properly and it's really setting itself to 5.6 when it says 2.8. Something silly like that. Take a series of shots with the same shutter speed and see if the exposure is different for each one.

Short of taking it back to the shop and asking them to test it I can't think of anything else. Someone else may though.

 

[BLS]

Thanks, Ferny. I'll try that.

Anyone else have any ideas?