The Issue of Equivalence

The Preamble

Long ago, the size of the film was large. So large that you did not need an enlarger - you just placed the negative film in contact with paper (hence the process called Contact Printing) and you produced a paper positive that was ready for display. Later on, Oscar Barnack invented the 35mm film - a dimension of 24mm x 36mm and it became the most commonly used and known film size by the general public.

When digital was born, the manufacture of sensors was difficult and expensive, so the common digital cameras for consumer used sensors smaller than this 35mm film (now given the name "Full Frame" - even though there are now digital sensors with sizes larger than this 24 x 36mm).  Any digital sensor size that is smaller than this Full Frame is called a Crop Sensor.

Usually, the minimum diameter / size of a lens is related to the Coverage Circle that the lens throws onto the sensor. That being the case, comparing two sensors, the bigger sensor requires a bigger lens diameter.

To bring some real cases into this discussion go look at a 70-200mm f/2.8 Canon L lens and a Full Frame Canon 5D DSLR body. And then go look at at 35-100mm f/2.8 Panasonic X lens and a Panasonic GH-3 MIrrorless body. There is quite a difference in size and weight isn't there? And if you chose carefully, there might be a big price difference in favour of the smaller camera as well.

Now, let's look at points and questions that plague the enthusiast community - whether they be serious nerds or casual shooters.

The Q & A

Q: Is the Canon 70-200mm lens the same as the Panasonic 35-100 lens?
A: No, of course not. They are not the same, one is so much larger and heavier.

Q: Is the Canon 70-200mm lens equivalent to the Panasonic 35-100 lens?

A: Yes and No. Used with their respective camera bodies, they cover the same angle of view. That is, you can stand at the same distance to your subject and get the same framing of your scene. They are also both f/2.8 lenses if you used an external light meter, setting the same f/no and the same ISO, you would get the same image brightness.  In other aspects, they are completely different. 

1. They don't have the same background blur. They don't have the same Depth of Field.
2. They don't deliver the same amount of light to the sensors. (Think of different size buckets collecting rain)
3. They don't cover the same sensor dimensions in their Circle of Coverage. 
4. Depending on the specific autofocus mechanisms in the lenses and the bodies, they don't focus the same way or with the same speed or track moving objects the same.

So, if these two lenses are not 100% the same, why would retailers market them as equivalent? 

Focal Length Equivalence

To give the consumer a reference point, a "handle" to understand the distance where they stand and how bright a lens was in regard setting f/no and shooting in dim light, manufacturers relied on focal length equivalence and f/no equivalence. It's a classic and traditional reference point when switching between different film size cameras.  In fact, the 2x Crop Factor yields understandable focal lengths - like 12mm (equivalent to 24mm). When we discuss the phone camera or the little pocketable digital cameras, the lenses are down to 3mm or 6mm - it just boggles the mind. So manufacturers refrain from even mentioning the real optical focal lengths.

One proposal is that instead of using focal length as a reference point (because it doesn't embody what some people want to infer), that manufacturers use the Angle of View. That's might seem to be worthwhile. But we have two handicaps with that idea:

1. Do you know the Angle of View of a Full Frame 50mm lens on a Full Frame Body? Some do, most people don't have a clue.
2. What happens when you take a Full Frame 50mm lens and fit it on a Crop Sensor camera like an APS-C body or a Micro Four Thirds body? The Angle of View changes

For these reasons, the manufacturers decline to use Angle of View. So they keep using the Equivalent Focal Length.

f/no Equivalence

The smaller lens, the 35-100mm Panasonic lens,  is f/2.8 - that's the correct, physical optical ratio. The 70-200mm Canon lens, is f/2.8 - that's the correct, physical optical ratio. 

The whole aim of designing both lenses as  f/2.8 is to create an image of the same tone on their relevant bodies, 

But the two lenses produce different visual images at f/2.8. Yes they do. But those are implications that we weave - the background blur amount, the impact on image noise and so on. We don't even have to think that hard to baffle brains. One lens is huge and heavy. The other is not. They can't be the same. But isn't that the reason why people want a smaller camera and body? And others want a bigger lens and body?