this post was submitted on 26 Oct 2023
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what confuses me is aperture diameter in aps-c 23mm f/1.4 is 16 while 35 in f/1.4 is 25. we all know that the bigger the aperture diameter, the more light comes inside the sensor, the more light, means faster shutter speed.
aperture diameter is a part of mechanical (size) that affects the light.
Aperture scales at the same scale as the sensor size so it’s a wash.
Yes, a larger entrance pupil diameter lets through more light than a smaller entrance pupil diameter.
But don't forget that focal length also affects light transmission. A longer focal length gathers light from a smaller area of the scene and therefore less light than a shorter focal length.
That's the whole reason the f-number is set up as a ratio between focal length and entrance pupil diameter, to normalize the effect of focal length so you don't have to account for it separately when figuring exposure. So an f/1.4 aperture should be the same contribution to exposure no matter what the focal length. That's why nobody needs to specify the focal length when communicating exposure settings. A 16mm entrance pupil diameter lets in less light while a 23mm focal length lets in more light, and that equals out to the same as a 25mm entrance pupil diameter letting in more light and paired with a 35mm focal length letting in less light.
And none of the above has anything to do with different format sizes. At 23mm and f/1.4 on full frame format you have the same brightness at 35mm and 1/1.4 on the same full frame format.
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.