Originally posted by 24X36NOW I invite you all to:
[...]
Your photo will be underexposed.
[...]
Then, add a stop
[...]
Your photo will now be properly exposed.
[.../.]
It is the light that doesn't hit the sensor in an APS-C dSLR that causes the reduction in light gathering ability, because of the small-sensor-in-camera-designed-for-bigger-sensor design and the resulting much-bigger-than-the-sensor image circle.
@24X36NOW,
I get it now. Understand every jota. I see how this belongs to your set of core truths. Unfortunately, it is wrong. Just like earth turned out to be not flat. Not everybody accepted it.
As you are a thinker, please accept the challenge to accept that you may just be wrong.
1. The claim that APS-C underexposes where FF doesn't is probably rooted in experience. However, it must have been with diferent vendors rather than different sensor sizes. Yes, there can be up to 1 stop difference in ISO calibration in different cameras. But this doesn't relate to sensor size at all. The ISO standard is such that sensor size differences must cancel out.
And the mount has absolutely
nothing to do here (provided it isn't too narrow for wide extreme apertures or inhibiting ultra wide angles). Please, think it through. Pls.
And just to be on the safe side... the mount does
not create (or determine) the image circle. You may think so. Would be amazingly wrong.
2. DxO measures calibrated ISO sensitivities. You may want to have a look at the D300/D700 comparison yourself. at dxomark.com No difference. However, you'll see a difference between D3X and D700 which are both FF.
3. If I have to try to find the point where your line of reasoning breaks down, I believe it is here:
You believe that the FF image circle is cropped by the APS-C sensor which leads to less light being captured. This sounds correct. But isn't. You forget to take into account that the APS-C lens would also have a shorter focal length which bundles the available light onto a smaller area exactly cancelling out the loss due to cropping. By available light I mean the light rays (photons) hitting the front element from within the field of view.
At constant f-stop, this front element is larger for the FF lens hence the advantage in light collection. But not anymore for a lens of same physical size, like f/4 on APS-C and f/5.6 on FF. Then exactly the same amount of light is shed onto the overall sensor area.
Please, in replying, refrain from the reflex to shout "no". Try to understand every word I wrote here and further up. I did it with your posts. Listen, don't speak. I already heard you
Originally posted by 24X36NOW Marc, a teleconverter does not change the focal length of the lens; this is an inaccurate oversimplification. Rather, it magnifies the image circle of the lens
Don't disagree with your magnification statement. Except that it absolutely makes no difference by which optical means you implement a given focal length. Adding a magnifying rear element is just part of standard lens construction. Go measure your 300mm lens. Is it 300mm long? Not? How can it be? Magnifying rear element?
The fact that you call teleconverter changing focal length "oversimplification" tells me that you don't have an accurate understanding of optical physics.
I really invite you to read the thread from the beginning. It is good lecture.
Anyway, I cannot spend more time to discuss it with you. Seize your chance. Or not.