Received my Tamron 17-70mm today...

jhorse wrote:

BeatX wrote:

Yes, I know that f/2.8 lens is still an f/2.8 lens, but my point is that comparing two lenses like: Tamron 17-70/2.8 vs Canon 24-105/4 IS L (closest competition) will give exact same results when they are used on APSC body vs FF body.

Is this correct? I do not think so. There are two aperture comparisons: depth of field and light gathering ability. In some circumstances a f2.8 can gather more light when set to f2.8 and thus for a give ISO can provide different, aka, better results.

This is my understanding if equivalence. Why do you think they give the 'exact same results', implying under all circumstances?

The important parameter for image quality is the TOTAL light gathered: i.e. the light intensity on the sensor times the size of the sensor.

The f/2.8 produces twice the light intensity on the APS-C sensor, but the sensor has half the area compared to the full frame camera. So the total light (light intensity times the sensor area) is the same as on the full frame sensor which has half the light intensity but twice the area. Hence the image quality is also the same.

Because the light intensity on the FF sensor is 1/2 the intensity on the APS-C sensor, the ISO must be increased by 1 stop to have the same brightness level (apparent exposure). So the potential one-stop advantage of the larger sensor is negated by having to boost the ISO one stop.

This equivalency breaks down when both sensors can be used at base ISO, such as very bright light, or when slow shutter speeds can be used. Then the FF sensor has the same light intensity as the APS-C sensor but has twice the area, so it has 1 stop more total light and 1 stop higher IQ. This is why the bigger the sensor, the better the camera for such uses as landscape, where slow shutter speeds can be used.