fz200 vs dslr for low light zooming?
The F stop determines the shutter speed required for a full exposure at a specific ISO setting. The smaller the aperture (ie. the bigger the F stop number) the longer the exposure required in the same lighting conditions.
The collected amount of light for a full, correct exposure, is proportional to the area of the sensor. The area of the FZ200 sensor is 4.6x6.2mm=28.52 sq mm. The area of a Canon cropped-format DSLR such as the 60D is 14.9x22.3mm=325.58 sq mm. Therefore the Canon sensor collects 325.58 / 28.52 = 11x more light. It has more than a 3 stop advantage. 3 stops is equal to 8x more light; 4 stops, 16x more light.
The amount of light collected will determine the amount of noise in the image, all else being equal, which it rarely is. The tests at DXOMark suggest the FZ150 sensor at its base ISO of exactly 100 is roughly on a par with the Canon 60D at some point between ISO 400 and 800. To be more precise, the DR and Color Sensitivity of the FZ150 sensor at base ISO is equal to the 60D at ISO 500, and the SNR and Tonal Range is equal to the 60D at ISO 800. The FZ200 sensor might be slightly better.
At 300mm and F5.6, the Sigma 70-300 on a Canon 60D at ISO 400, used at the same shutter speed as the FZ200 at F2.8, will probably be slightly underexposed because the real ISO is only 320. You'd probably need to set the ISO at 500, 1/3rd stop higher. Over-all noise in the two sensors should then be very similar, with a slight edge to the Canon. However, a 300mm lens on the 60D is only 480mm in full-frame 35mm terms. The 600mm equivalent of the FZ200 is noticeably longer to a degree that makes a difference when shooting wildlife. A closer equivalent would be a 400mm lens in the form of the Canon 100-400 zoom.
On the other hand, the 60D (or T3) sensor has 50% more pixels than the FZ200. If one crops the 60D image to 12mp that will turn the 480mm equivalent into a 580mm equivalent, which is close enough to 600mm.
Hope that helps clarify the situation.