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