Thank you all.
Graystar wrote:
Just compare F-numbers and don't worry about the actual physical sizes of things.
Bad information.
Does it mean that with my sx100is which has f/2.8 on 6mm will let more light in then 18-55 f/3.5 on 18mm?
No, it does not. The reason, of course, is that the aperture diameter is much larger at 18mm f/3.5 (5.1mm) compared to 6mm f/2.8 (2.1mm).
A slight niggle is that 6mm on the SX100IS has the same AOV as 24mm on 1.5x. Thus, the difference in aperture diameters for the same AOV is 6.9mm vs 2.1mm, so 10 times as much light will fall on the 1.5x sensor than the SX100IS sensor for the same shutter speed.
Let's give a specific example. Assume you take a shot of a scene at 6mm f/2.8 1/100 ISO 80 with the SX100IS. If you took a pic of the same scene from the same position at 24mm f/11 1/100 ISO 1250 on a 1.5x camera, both systems would have the same AOV and DOF, and the same amount of light would fall on each of the sensors, and any differences in noise would be due to the differences in sensor efficiencies.
The reason that DSLRs have less noise is because they either use a larger aperture to get more light on the sensor (with a concomitantly more shallow DOF), and/or use a lower shutter speed since the larger sensor can absorb more light before oversaturating.
However, if you need the same DOF and shutter speed, then
all systems put the same amount of light on the sensor, and differences in noise are
entirely due to differences in sensor efficiencies.
Contrary to popular belief, the sensors in compacts are usually every bit as efficient as those in DSLRs, and often more efficient. Furthermore, the higher ISO used by the larger sensor system is at best a secondary effect, as a higher ISO, for some sensors, simply serves to make the sensor more efficient. More on that point here:
h ttp:
www.josephjamesphotography.com/equivalence/#iso
I'm actually an civil engineer, and i like to know technical stuff and mathematics on all things.
Read all about it here:
h ttp:
www.josephjamesphotography.com/equivalence/
DPR has recently banned the link due to all the controversy that this causes (as you can see by Graystar's posts). People think in terms of exposure (
density of light on the sensor) as opposed to the total amount of light that falls on the sensor, which leads to a poor understanding of what's actually going on.
for example:
AF Zoom-NIKKOR
80-200mm f/2.8D ED
so on 80mm aperature will be 28.5mm and on 200mm will be 71mm. does it mean that it lets more light in on full zoom and i need faster shutter speed or lower iso?
At 200mm, you collect light from less of the scene due to the more narrow AOV, but this is exactly cancelled out by the larger aperture. This point is discussed in detail here:
h ttp:
www.josephjamesphotography.com/equivalence/#fratio
. i'm buying a camera tomorrow, so, they are in the same price level, what would you recomend me: d3100 with a kit lens, or those prosumers. (i actualy dont need that much of a zoom, im no paparazzi, but its fun to have).
