Originally posted by normhead This is only an issue if noise at the ISO you need to shoot is an issue for you.
What the numbers mean is, if you're not happy with the noise in your images, you may be better off with a larger format. Rent something try it out.
It's quite possible that you just learn to live with what you have. If you're not happy with the noise of a 300mm on a K-3, it doesn't mean you're going to be happy with the weight of 600mm lens on your 645z. The problem with these kinds of questions , is, you can't get everything you want. You have to decide what's most important to you. For absolute IQ you want the 600mm on the 645z,,,, but honestly, most of use are not interested in that. To much weight, too much cost, all the sudden a 400mm lens on a FF looks better. Less weight less cost, or maybe you decide given the alternatives a 300mm lens on a K-3 isn't so bad.
You just have to find what you can live with. To me, part of that is what your customers want. I'll buy what they'll pay for.
With the 645z, D3s and A7s, you certainly have more choices now than ever before.
Until there's a K-3s APS-c is not giving you a workable APS-c option if you're having to bump up your ISO to get the images you want, and that's causing too much noise. But keep it practical. Base it on your images, not on a bunch of numbers.
Too practical, Norm
Still, you must admit that there is at least the occasional situation where one wishes one could shoot at an ISO step lower, no?
Aside from that, it could be said that this is simply academic - understanding for its own sake.
---------- Post added 16-10-14 at 00:42 ----------
Here is my simplified take on this entire matter. If what I’m saying is nothing that hasn’t been said before, then all I can say is that at least I’ve clarified things for myself
Also, these are my thoughts, and are being presented as such. I don’t claim to have verified the validity of everything that follows.
Imagine a box, one meter on each side, with a window of 25cm square in the centre of one of the sides. Outside the box, directly opposite this window is a uniform light source that is larger than one meter. i.e. larger than the side of the box. The window will permit a certain amount of light to enter the box, and the centre of the side opposite the window will be hit by light of a certain intensity.
If we now increase the size of the window to 50 cm square, there will obviously be more light inside the box. However, the intensity of the light at the centre of the opposite side will be the same as it was with the smaller window (this is an assumption on my part).
I believe the above illustrates the difference between “light intensity” and “total amount of light”. What this means is that one can have more light, but equal exposure.
Now, a 150mm/f4 lens has an aperture of 37.5mm. This size aperture will permit a certain amount of light to enter the camera. On a FF body, this 150mm lens will have an angle of view (AOV) of 15 degrees. To get the same AOV using a cropped sensor body, you need to use a 100mm lens. In order for this 100mm lens (or any other lens, for that matter) to permit the same quantity of light to enter the camera as would the 150mm f4 lens, the aperture has to be the same size i.e. 37.5mm. This means that the lens would be 100mm f2.67 lens (100/37.5). Hence the argument for applying the crop factor to the f stop as well as the focal length. We can say that the 100mm f4 lens is equivalent to 150mm f4 on APS-C for the purposes of exposure (light intensity), but not in terms of total amount of light. As stated above, a 100mm lens would have to be f2.67 in order to permit the same amount of light to enter the camera as a 150mm f4 lens.
Now I ask myself the following: what if don’t care about perspective or DOF in some situations, so I simply move back from the subject until I get the same AOV with my APS-C 100mm f4 as the FF would in the original position. Would this not effectively retain my 100m f4 as a 100mm f4? My guess (and that’s all it is) is that in this example, increasing the subject-to-camera distance reduces the light that enters the camera, and you are no better off.
Sometimes I find that I can be satisfied with a logical explanation, even if i's wrong
I don't know if this is the case here.