It's my pleasure. One day I learn something from you, the next it's vice-versa, sharing knowledge is fun.Originally Posted by Sean Setters
Agreed.Originally Posted by Sean Setters
Kind of. Yes, at f/2.8 you will be letting in twice the amount of light *per area*, but the total amount of light captured will be much smaller. For example, the lens on a very tiny 1/1.7" Digicam with a 7mm f/2.8 lens also has the same perspective/FOV as your 20mm, and it also captures the same amount of light per area, but the total amount of light falling on the sensor is much less. In fact, f/2.8 on a digicam is the same amount of light as f/14 on your 50D! (And f/19 on the 5D2.) It's no wonder digicams struggle in low light. The reason is sensor area.Originally Posted by Sean Setters
In the same way, a full-frame f/4 lens focuses more light than an f/2.8 APS-C lens: it's just spread out over a wider area. So the full-frame camera has about a 1/3 stop advantage in low light at f/4: you could upgrade to the 5D2 and 24-105 f/4 and still get all the same DOF, noise level, and shutter speed. In fact, it's more than a 1/3rd stop better at f/4.
Of course, if you take it even further, and go with f/2.8 on the 5D2, then you'll have a 1.4 stops thinner DOF, and that much more low light power.
Yes. To get the same depth of field on a full frame sensor, one must stop down approximately 1.4 stops. Since f/4 is only 1 stop, shooting the 24-105 wide open will give you less depth of field (and more total light).Originally Posted by Sean Setters
I shouldn't have mentioned it: the front element just confuses things. But since I did, I should explain. The front element is the entrance pupil, i.e. the big piece of glass that your brides see on the front of your lens. Generally, the aperture (not focal ratio) determines the size of the front element. A 400mm f/5.6 has an aperture of 71mm (400/5.6=71), just like the 80mm f/1.2 (80/1.2=70), so the front element of both lenses will be a certain size. Sometimes they are larger for higher quality (less vignetting) or other optical design reasons. If you compare the front elements of the 17-55 and 24-105, you'll see that the L has a bigger one. Generally, this means that it is longer, faster, or has a bigger image circle. In the case of the L compared to the 17-55, it is longer, slower, and a bigger image circle. Clear as mud?Originally Posted by Sean Setters
Yes! I'm glad you asked. There is one web page that explains all of this, and more, in excrutiating detail.Originally Posted by Sean Setters
http://www.josephjamesphotography.com/equivalence/
But I would be happy to answer any additional questions about this topic.
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Daniel