Dispelling the Myths Around Depth of Field and Crop Factor

There is a lot of misunderstanding and confusion on the subjects of depth of field, the difference between full-frame and crop sensors, and the ‘crop factor.’

Time and time again I see arguments surface over these topics—in YouTube comments and on camera forums.

This confusion is partly caused by the camera manufacturers themselves, due to their bad marketing habit of stating focal length equivalence in adverts, and sometimes even on lenses, or crop sensor cameras themselves. Then they fail to complete the full equivalence equation.

I wanted to create a clear and concise response to this issue, so I decided to make a video that breaks down all of the basic principles using some simple and (hopefully) clear illustrations, and then cover exactly what is needed to get a true equivalent image from completely different sized sensors.

If you follow these steps you will be able to match:

  • Depth of field
  • Field of view
  • and noise levels

between a full-frame and crop sensor cameras. Plus, you will have a much better understanding of what gear you will need, to produce the image that you want.

Also, I want to touch upon the topic of ’speed boosters,’ as this is another area that seems to have a slight fog of confusion hanging over it. I will get onto that shortly….

So, what do you need to do to get the look, the field of view, the noise levels, and the depth of field of a full-frame camera on a crop sensor?

Crop Factor Equivalence

The recipe is pretty simple; understanding why we need to go about it this way is a little harder to get across though. That’s why I felt the need to brake it all down into the fundamentals in the video, and I highly recommend you watch the full thing to make the basics of this stuff crystal clear.

Here is the equivalence recipe:

Apply the crop factor to the focal length. Example = 2x if you are going from full-frame to M43. So that means 50mm on FF is 25mm on M43.

Apply the crop factor to the aperture. Example = f/4 on FF to f/2 on M43.

Apply the square of the crop factor to the ISO. Example = crop factor of 2x² means 800 on FF goes to 200 on M43.

And that’s all there is to it.

This shouldn’t be new information—a fair few ‘YouTubers’ have covered this before—but hopefully my video also helps demonstrate the underlying principles at work. This is what happens to the light that travels through the lens to create the D.O.F., and covers the basics you need to know about focal length, near and far focusing, aperture, how crop sensors behave when compared to full-frame, and how a basic understanding of the circle of confusion will really put the cherry on top of the cake of knowledge.

Here are a few real-world examples. As you can see, everything is identical, you just have to do the right conversion:

What About Speed Boosters

Lastly, allow me to touch upon another much misunderstood topic: speed-boosters.

Firstly regarding DOF, many people seem to think that a speed-booster magically gives a narrower full-frame like depth of field. It doesn’t. Technically, a speed-booster will make the DOF WIDER than the same lens would have with a non ‘speed-boosted’ adaptor. Its main advantage is that using a booster will waste less of the light that would normally be spilled around the sensor, and, importantly, it widens the FOV.

The wider FOV allows similar framing to a camera with a larger sensor, which then in turn allows you to use the longer focal length, which WILL give you a similar DOF to a larger sensor. It’s the lens doing the work, not the speed-booster.

As to the extra stop of light a speed-boosters will give you, people claim you are getting more light out of the lens compared to a full-frame equivalent. This is of course completely wrong; you can’t get more light out of the lens than what happens to be going through it in the first place.

The booster is simply shrinking the circle of light to better fit the smaller sensor, so that less light is wasted by spilling around the sensor.

Here’s an example:

The reason the exposure is now different and about one stop brighter (compared to the same lens on a full-frame camera at the same settings) is because the ISO number is only telling you the light fall per square inch of the sensor, not the total amount of light hitting the sensor, and the total amount of light is most certainly what is important to our image.

Put simply: the M43 sensor is gathering one quarter of the light of the full-frame sensor. So in terms of total light hitting sensor, an ISO of 200 on M43 will equate to a ISO of 800 on FF.

This shows why we need to square the crop factor when doing our lens equivalence with no speed booster in order to match exposure and noise levels. AND it shows why the speed-booster appears to be brighter than the same lens on FF… because the ISO number doesn’t represent total light, only light per square inch.

Not to say speed-boosters are bad, they are great! I own a Metabones XL booster and think it is a fantastic bit of kit, but I’m just pointing out that the increase of one stop we see (compared to FF) is more about the ISO lying to us, than the booster making the lens magically gather more light.

Anyway, I hope you watch the video, and please do leave some feedback in the comments if I either got something wrong (we are human and we ALL make mistakes), or if I helped enlighten you. This is pretty basic stuff that we ALL should know before buying some new gear, or get into an online argument with someone on these topics.


About the author: Blake Evans is an animator and YouTuber. You can see more of his work by subscribing to his YouTube channel.

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