Crop Sensor vs Full Frame – Focal Length?

Crop Sensor vs Full Frame: Do Crop-Sensors Increase Focal Length?

I see this question come up all the time on various groups and forums, whereby the answer is often given to people that crop-sensor cameras give the advantage of an increased focal length when using the same lens on a full-frame sensor equivalent. I have therefore written this short article to try and dispel some of the myths around this recurring question.

Crop Sensor vs Full Frame – Focal Length

When asked the question myself, the short answer I give is that a crop sensor absolutely doesn’t increase lens focal length. In Figure 1, I have illustrated what Focal Length is, whereby it is the distance from the point where light enters the camera through the camera lens, to the camera’s sensor. So straight away, you can see that Focal Length is a physical attribute of the camera and its lens, and completely independent of the camera sensor size.

Focal Length
Figure1 : Focal Length

So to really hit this message home, when you put for example a 400mm lens on a full-frame camera, the focal length is 400mm. When you put that same lens on a crop sensor camera, your focal length is not somehow magically 500mm or 600mm, it is still 400mm, because the distance from where light enters the lens, to when it hits the sensor, is exactly the same on both cameras.

Crop Sensor vs Full Frame – Field of View

So what does a crop sensor do, and why do so many people mistakenly think that it increases Focal Length? Quite often, you will also hear photographers describing that the reach of a crop sensor camera is better than for a full-frame sensor. Once again, this is misleading, What they are actually talking about here, is a difference in the Field of View between the two sensor types. Take a look at Figure 2.

Field of View
Figure 2: Field of View

What I have tried to illustrate above, is that a crop sensor has a different field of view than a full-frame sensor when looking through the same lens. In essence, the crop sensor is only looking through a smaller segment of the available lens than a full-frame sensor. The result is that it appears that the focal length is increased, or that there is an increased reach, but of course, it hasn’t, you are only looking at a smaller portion of the actual image that the lens is capable of.

Crop Sensor vs Full Frame – Pros and Cons

Now there are advantages and disadvantages to this let’s call it an apparent increase in focal length when using a crop sensor versus a full-frame sensor.

You can achieve the exact same image on a full-frame sensor camera as on a crop-sensor camera by cropping the image post-processing. The disadvantage of this is that the cropped image then drops in mega-pixels. You then get into a debate (and some of it tends to get heated on the web) as to whether the final image is still of higher quality, as full-frame sensor cameras tend to have higher quality sensors, resulting in better light sensitivity & higher dynamic range. Of course with that comes a higher cost, and in my view, it is the cost that is the main driver and the depth of the photographer’s pockets. In summary though, given comparable cameras and a telephoto lens, you can pretty much capture the same image on either a crop sensor or full-frame sensor camera.

The main disadvantage of the crop effect is when it comes to wide angle landscape shots, and its down to that reduction in field of view. You can get some wide lenses for crop sensor cameras, but there tend to be fewer options than on full-frame sensor cameras, especially if you are looking for that expansive super wide feel. You tend only to get this with full-frame sensors.

Crop Sensor vs Full Frame – Conclusion

In conclusion – Crop Sensor vs Full Frame:

  • Crop Sensor Cameras do not increase a lenses focal length.
  • Crop Sensor Cameras do have a smaller field of view than full-frame sensors equivalent, which subsequently give an apparent increase in focal length.

Ok, I could have gone into a lot more detail on this subject, but I purposely kept this article concise and to the point. I could at this juncture also go into the advantages and disadvantages of owning a crop or full-frame sensor camera, but I will leave this for another article.

I hope you found this useful, if so, please check out other articles in our photography life series

Reference material: Wikipedia


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