Crop Factor In Photography Explained - Sunday School

I have been been wanting to do a Photography lesson regarding Full Frame vs Crop Sensor and the crop factor equivalence for awhile now using a 3 panel window.

Well I have been going to my wife's side of the family for years and never took advantage of the fact they have several there....not to mention the outside view is of Toms River which serves as a good backdrop.

The reason I wanted to use a 3 panel window was because I always felt it was a good representation of how a full frame sensor captures a scene and that closing two of the 3 blinds can represent a crop sensor

The most important aspect of this lesson is that once you close the blinds, in order to see in the one blind what you originally were able to see in the 3 blinds, you would have to move CLOSER to the window. 

This is a perfect example as it then gives you a physical example of what needs to happen. Now just associate you moving closer to the window to see a larger field of view to the camera needing to move closer to the LENS (not the subject matter)

Many photographers mistake the crop factor equivalance to simply moving back but this changing ones relationship to the subject matter. The moving back needs to happen INSIDE THE LENS.

You see a crop sensor is 1.6x smaller than a full frame. (1.5 on Nikon). While there are lenses made specifically for crop sensor cameras, their measurements are still based on full frame sensors. A 50mm lens is a 50mm lens.

However when that 50mm lens is put on a crop sensor it's going to give you a 1.6 size smaller field of view. You may not want that smaller field of view so how do you compensate?

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This is where the crop factor equivalance comes in. To get the same field of view in the crop sensor that you had with the 50mm on the full frame, you have to lower your focal range by 1.6, which is around 32mm. (The two red lines in the above graphic should be shorter)

Of note -

1. Just as you adjust focal range by 1.6  ...so to should you adjust the rest of your exposure settings 

2. The 1.6 crop factor is multiplied as you go through the focal range (50mm behaves like 75mm on a crop sensor, 100mm behaves like a 150mm)

I hope this lesson helps and I look forward to future Sunday School posts.

Scott RothComment