Using a full frame lens on a cropped sensor body

A Quick FAQ for one of the most sensitive and hottest topics in forums. It stirs controversy not only because people don't fully understand, it causes anguish because people end up supplying answers to different questions.

Q1. Do I dial in a different ISO/shutter speed / f/no with the same lens on bodies with different sized sensors?
Ans: No, You use the same parameters

Q2. Do I get a different image "quality" if I use the same lens on different sensors?
Ans:

• It varies depending on some factors but overall, yes, the final image when enlarged from the sensor size to the final size of screen or print will show that the full frame image with less image noise, all things being equal. 
• It may not be exactly proportional to the sensor size ratios because all things are not always equal. 
• The increase in noise is due to the fact that the smaller sensor only sees a small part of the full frame image and you have to magnify the image more to fit the final size screen or print. 
• Some people want to explain it a different way and say that the smaller sensor has "lost" light - in one way, it has, because the smaller sensor cannot see part of the image. In another way, it has not - whatever light that does fall onto the sensor, is the same brightness per area.
• Also an issue is that people often want X megapixels (let us say 20 Mp) whether they use a cropped sensor camera or a full frame camera. That means that cropped sensor makers have to fit a higher pixel density (more pixels per area) onto a smaller sensor. This makes the pixel smaller on a cropped sensor camera. These are all theoretical design issues. At the end of the day, people compare real, practical cameras with real sensors - due to different technological edge, the superiority may not be proportional as prescribed by theory.

Q3. Will the amount of blur background be different when you use the same lens on different sized sensors for the same subject size in the frame.
Ans.

• Yes, there will be different background blur, all things being equal. This is where the notorious phrase - "f/2 on a MFT sensor is equivalent to f/... on a full frame sensor"
• A simplified visual simulator that you can interact with on the web is here:
  https://dofsimulator.net/en/
• An Android App that allows you to understand subject dimensions in the parameters of depth of field is the DOF and Hyperfocal Calculator by Cunning Dog.

Q4: If you fit a 50mm full frame lens on a cropped sensor body, what happens to the f/no?

Ans:

• The f/no stays the same - it is a property of the lens, not the camera body.

Q5: Isn't background blur the same as depth of field?

Ans: No, they are not the same. 

• Depth of field depends on camera to subject distance
• Background blur depends on camera to background distance

Q6. Will the inherent creaminess of a bokeh ball in the centre of the frame be different between the two bodies?
Ans. Likely the bokeh ball will be the same character of wiryness, onion skin, or bokeh ball shape.

Q7: Isn't bokeh the same as background blur?
Ans: Not, they are not the same. The original definition for bokeh is about the creaminess of the blur for the same amount of blur, not how blurred the background is.

Q8. Will the whole frame blur and bokeh effect be different between the two bodies with different sensor size with the same lens?
Ans. Yes, the full frame style of picture will be different because the smaller sensor does not show you the blurry bits and vignetting of the lens that is around the edge of the frame

Q9: If you fit a 50mm full frame lens onto say an MFT sensor body, will it become 100mm?
Ans: No.

• 50mm focal length is a property of the lens. 
• When you fit this lens on a cropped sensor body (whether it be MFT or APS-C), part of the image will not be seen by the sensor because the sensor is smaller (hence the name Cropped) than the Circle of Coverage of the full frame lens. 
• To ensure that you see the full height of the subject using a cropped sensor body, you will have to walk backwards - i.e. increase your camera to subject distance.
• People will then say that if you stand at the same spot but do not move back, you are using the equivalent of a 100mm lens on a full frame body, when you use a 50mm lens on a MFT body.

Having said all the above, let's look at an entertaining and illustrative video that combines some of these points together and..... potentially (if you didn't read above) fills your head with conflicting information (unless you sit down and calmly deconstruct the impact James is saying point by point)

Oh, Ok, so it wasn't that quick. Did you learn something?

For ease of access, here is a DOF calculator by PhotoPills

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?