I don’t think any of these answers are correct, because I don’t think we’re answering the Beginner’s question of Equivalence.
What the Beginner is really asking: “Do I need to buy a full-frame camera, or will an APS-C (or a 4/3) be just as good?” Because the term “equivalence” means “the same as” or “just as good.”
And the correct answer explains when the smaller sensor will be equally good, and when the full frame will be better, and even when the smaller sensor will be superior.
No, it doesn't, if properly qualified, and James and others qualify it well.
Equivalence does not provide the basis for answering that question, nor does it pretend to.
Then how does it benefit the Beginner at all?
It benefits the beginner who believes the claim that to produce a similar looking picture using different formats focal length should be adjusted but f-number should not be. For example, the claim that a 12mm f/2.8 lens on a mFT camera will produce a picture looking like that produced by a 24mm f/2.8 lens on a FF camera (all else being equal).
Helps understanding what is possible with different formats, helps to understand how to spend the money.
Well, if we were dealing with complete beginners and we had photon-counting cameras with only two exposure/brightness-related controls, exposure time and aperture size, we might not even need to mention "equivalence". Equivalence as a concept is a way of correcting myths that thrive in a world where people conflate noise with ISO and/or sensor size, angle of view with focal length, and f-number with pupil size.
One answer would be that the FF camera has more potential to go into a range of visible photographic qualities that come from having a larger pupil - less diffraction, shallower DOF, and less photon noise. If you don't go into that range, which requires lenses that are generally more expensive, heavier and larger, then the size of the FF sensor is no longer an extra value. If you have a high-MP FF, though, you may have more total pixels than is available with smaller sensors, and your base-ISO shooting when light is abundant will capture more light (regardless of pixel count). Of course, the lower diffraction potential of FF is often moot because the fastest lenses for FF are generally aberration-heavy wide open, making them soft when you go for that shallower DOF and extra light.
Most of the situations in which a smaller sensor is superior final-IQ-wise are ones where you aren't going to utilize the entire FF sensor area (through higher pixel density). You simply need to extend the idea of equivalence to "sensor area actually used for the final composition" rather than "sensor size".
So it helps the Beginner decide whether to buy an APS-C or a Full Frame camera.
I liked Richard Butler’s explanation: That Equivalence is an alternative to the standard exposure model. In the standard exposure model, we know that a given scene can be exposed consistently by using the same aperture and shutter speed, regardless of sensor size. The Equivalence model shows us that, since the smaller sensor camera uses a shorter lens to achieve the same FoV, the aperture can be opened accordingly to achieve an equivalent photograph, with the same FoV and depth of field.
I just find that the Equivalence Model isn’t especially helpful for a beginner, because they are rarely trying to recreate a photo shot with a full frame camera.
or m4/3, or 1", ...
also may help with lenses, flashes, tripods, ...
Folks trying to sell things and trying to recreate something they perceive as a professional look, using cameras that have smallish sensors, their question is: how to achieve that clean look? I met such people. I met people trying to shoot landscapes, portraits, trying to get shallow DoF, and without explaining equivalence it's hard to answer their question in a way that puts them onto the right track (like averaging shots when needed).
Best answer and nothing has changed, saves all the waffle that goes on around it.
Simple really unless you want to branch out like it seems in here.
Danny.
I believe you mean f-number rather than aperture. For example, mFT 12/2.8 is equivalent to a FF 24/5.6.
You have already answered your own question. 16 days and 23 posts ,someone should post that dead horse image here as well.
No, I mean aperture. Because Beginners measure aperture as an f-number, not by measuring the size of the aperture opening in their lenses.
In the Standard Exposure Model, an f-stop is equal to an f-stop, no matter the format of the sensor. In the Equivalence model, it is not. But I remain doubtful as to how this discussion benefits Beginner photographers, beyond answering their basic questions about the advantages of various sensor sizes.
In the “Equivalence Model” a 12mm aperture diameter is always a 12mm aperture diameter, no matter what the physical focal length of the lens.
Thus a 25mm lens at f/2 and a 50mm lens at f/4 both have the same 12.5mm aperture diameter.
I'm pretty sure a beginner without prompting will not measure aperture at all and without a prompt do not know what an f-number is, what the '/' stands for. So do we try to give them a correct or an incorrect prompt?
As frequently as equivalence comes up in any discussion of digital photography - equivalence is relevant to any discussion of crop factor - it's beneficial to photographers of all experience levels to have a basic understanding of what equivalence is. One of the benefits of that basic understanding is being disabused of the notion that different format cameras can't be used to produce the same photo. Equivalence is a tool for understanding how that is possible. It's information any photographer can use in their decision-making process for choosing a camera that best meets their needs.
If one asks, "Why are they different?" that's the first step in a journey to a destination of understanding that exposure is not the same as the total light used to make a photo.
Different format camera can work with the same f-stop and same exposure (the same scene brightness per unit area of the sensor) but that does not guarantee those cameras will make the same photo. In fact, it pretty much guarantees the opposite. The photos will be different...potentially significantly so to the eye. At the same exposure, a smaller format camera captures less total light and makes a photo having more prominent shot noise. At the same f-stop but with different focal lengths, a smaller format camera at a shorter focal length will be working with a smaller diameter lens aperture. Depth of field in the resulting photo will probably be greater.
Equivalence helps a photographer understand those differences and how to use a camera of any format to make the photo they're envisioning.