Understanding Full-Frame vs Crop-Sensor Impacts on Depth-Of-Field And Perspective

There has been a lot of discussion going on about what are the impacts of using a crop sensor vs full frame when using a particular lens. How are crop factor sensors impacting depth of field and what are they doing to composition. In fact if you went to any photography forum on the web, you are likely to get as many answers as forum members.

Of course, the answer to that question really depend what you are comparing and how you are doing your tests. Photographer Neil van Niekerk did a thorough test accompanied with clear explanations on what actually makes a difference when using a crop sensor vs a full frame and the answer is not that simple.

When the differences between full-frame and crop-sensor cameras are discussed, there is an inevitable question about whether the crop sensor multiplies the focal length. Whether a 50mm lens on a crop-sensor acts like a 75mm lens (on a 1.5x crop sensor) or 80mm lens (on a 1.6x crop sensor)…

… And we’ll analyze whether there is actually an equivalency between certain focal lengths, when using a crop-sensor camera. In other words, whether your 50mm lens becomes “equivalent to” a 75mm or 80mm lens when used on a crop-sensor camera.

If you are at the edge of your sit, here is the final conclusion, it has to do with actually moving your legs as well as the sensor used:

Yes, a 50mm lens does indeed behave like an equivalent focal length of a 75mm lens (on a 1.5x crop sensor), or an 80mm lens (on a 1.6x crop sensor) … however, the depth-of-field increases by about a stop.

Yes, a 100mm lens on a crop-sensor camera will give you the same perspective as a 150mm / 160mm lens (on a full-frame camera), if you don’t change position … however, the DoF increases. (i.e., less shallow DoF)

Neil took this comparison to an interesting place.

Once comparing a shot on the D810 done with a Nikon 85mm f/1.4 lens and with a Nikon 58mm f/1.4 lens cropping the image to achieve the same composition
And once using the Nikon 85mm f/1.4 lens on the Nikon D4. The D4 has 16,2 megapixels vs the Nikon 58mm f/1.4 lens which gives an equivalent of 87mm on the Nikon D810 when it is set to 1.5x crop in the camera. The D810 then has 15,4 megapixels

Here are the results from comparison #1 (FF on left, crop on right):