it is said that full frame aperture equivalent of 2.8 to aps-c is 4.2. does it mean that shutter speed of aps-c is one stop slower that full frame on the same aperture? given the same focal length equivalent e.g. aps-c 23mm and ff 35mm.
it is said that full frame aperture equivalent of 2.8 to aps-c is 4.2. does it mean that shutter speed of aps-c is one stop slower that full frame on the same aperture? given the same focal length equivalent e.g. aps-c 23mm and ff 35mm.
This is not the case in this context as it was specifically about APS-C vs FF. 23mm APS-C lens and 35mm FF lens have the same field of view, thus the same light reflects from the scene to the lens.
A 16mm entrance pupil of 23mm f/1.4 on APS-C collects about 2.3 times less light than 35mm f/1.4 on FF with it’s 25mm entrance pupil - the same amount of light reflects from the scene to the lens, but the system with the larger entrance pupil collects more light.
For exposure purposes this is of course not relevant, but for image quality and DOF it is.
Focal length affects transmission in any context. The f-number is calculated the same regardless of format size, incorporating the focal length. Exposure settings values, including the aperture as expressed as an f-number, work the same for every format size.
This discussion is about exposure purposes.
It doesn’t, at least in principle.
Of course. Have I said anything else?
Really? You went way beyond that for example when you wrote:
This has nothing to do with exposure purposes at all, but about how much light is collected. And as the context was FF vs APS-C and the size of aperture, what you wrote was hardly helpful.
It’s built into the aperture f-number because it does, just like entrance pupil diameter does. Thus, it answers OP’s real question about why exposure isn’t changing even when the entrance pupil diameter changes: because focal length is also changing and ultimately you’re arriving at the same f/1.4 f-number.
https://en.wikipedia.org/wiki/F-number
Yes, you said: “This is not the case in this context”
I’m just talking about how focal length and entrance pupil diameter both go into the aperture f-number for the purposes of exposure. That’s literally how the f-number is mathematically defined.
How much light is collected is not the same as exposure, indeed. But it’s a component of exposure that affects the exposure.
What I wrote applies in the context of different format sizes. It also applies in the context of the same format size.
Well, I simply wanted to clarify what you presented in a way which seemed to me to be causing more confustion to OP in this context than necessary.
Basically this was the problem for me:
In the context of format comparison, it can get confusing to the OP as in this context the focal lengths collect light from the same area of the scene, not different.
We interpreted OP’s question differently. Not saying either interpretion is better or worse.