If you use a crop camera, then you change to full frame, you WON'T have to use slower shutter speed or higher ISO. That's the whole point of the discussion above. An f/2.8 lens is an f/2.8 lens irrespective of the sensor size. The exposure doesn't change. Only the field of view and depth of field that the lens provides vary when you change sensor size.Thanks for great info. DPReview is the source where I came across this equivalence thing. They mention it in every crop camera review. As soon as I realized this equivalence comes at the expense of either slower ss or higher ISO, It felt something wrong to me. I stopped thinking about upgrading to FF.This is how I understand it (and feel free to correct me if I'm wrong):The EF-S 17-55/2.8 IS lens is f2.8, and there's no getting away from that. And it also has a focal length range of 17-55. And when shot on a crop body at ISO 5000, it is shot at ISO 5000.
- For exposure, no difference between F2.8 on crop or on full frame.
- For depth of field, there is a difference, but only if you maintain the same field of view.
The reason for the difference in depth of field is that to maintain the same field of view between crop and full frame, you have to change your distance from the subject, and that is what changes depth of field.
When you see someone say "F2.8 on a crop sensor is really F4.5 equivalent on full frame," they're talking about the depth of field for equivalent framing only, not exposure.
Teleconverters are another story (and might contribute to the confusion for some), because they DO affect exposure. I believe the reason for this is that it changes focal length (one of the inputs for the aperture value), which changes the ratio, affecting the light that can hit the sensor, and thus, exposure.
Any experts want to chime in to set me straight?
While these cold hard facts are indisputable, it's only if you want to compare to different sensor formats that you have to start doing maths. If you don't, stop reading here.
While you can say it's the equivalent of a 27-88mm lens, due to the physical size of the entrance pupil (which can't alter at a given focal length and aperture setting), the equivalent aperture changes. (Take the lens set to 55mm, f2.8. The entrance pupil is 55mm/2.8, or 19.64mm. If you insist on calling 55mm 'equivalent' to 88mm, that fixed entrance pupil size means that at '88mm' the aperture ratio is 88/19.64, or 'f4.48'). Equivalent ISO can be calculated by multiplying by the sensor area ratio.
But, as I said, equivalence is only useful if you want to know how two different systems compare. If you only shoot one sensor size and/or have no interest in how your system is equivalent to anything else, it is irrelevant. As I said, f2.8 is f2.8. 55mm is 55mm. ISO 5000 is ISO 5000.
And image noise. So, DOF and noise change just like changing fstop and ISO on full frame. That's why the discussion above from rs is absolutely correct.