Sensor size X Depth of field

More often than not you see the discussion on how the sensor size affects the depth of field and I really can’t see that, since the light doesn’t know where it’s gonna land (if a full frame, an APS-C, a micro four-thirds …). So I think there’s a big misconception on the topic.

Depth of Field, a nice tool (photo: Cris da Rocha)

Depth of Field, a nice tool (photo: Cris da Rocha)

Having said that, I tried to understand where does that come from. I packed that with the concept of “zoom” for smaller sensors and equivalent focal length (and now the recent argument of equivalent aperture).

Being a bit hard on that, but … the difference between full frame and APS-C (for example) is the sensor size. The image is not magnified (in the optical sense), your image is cropped!!!

What you see in your APS-C sensor (Nikon flavor) with a 50mm at f/4 is the same you’d see in a full frame sensor with a 75mm at f/6 (that’s the “equivalent” number you’re using, keep the reduction of aperture in mind also). OK! But that doesn’t mean you are using a 75mm at f/6 you’re using a 50mm at f/4.

Comparison between the field of view of a full frame sensor and an APS-C sensor. (Photo: Cris da Rocha)

Comparison between the field of view of a full frame sensor and an APS-C sensor. (photo: Cris da Rocha)

Most of the available comparisons I see around rely on recompose the shot, or zoom in, so the content in the image is the same, but the camera specs change (for example one is at 50mm, other is at 75mm, and so on). Once you move the camera, or zoom in, all the relative distances or internal specs are different and the comparison is no longer fair!

After some discussion with several friends I partnered with one of them, Vincenzo Forchi, that happens to have a Nikon FX (full frame) camera and a Nikon DX (APS-C) camera, so we could make the test using the very same lens.

The shots were taken at the SAME DISTANCE from the subject, so no recomposing.

We took the same shot with different configurations:

1- Full frame camera at 50mm f/4
2- DX mode (which reduces the used area to a APS-C size) with the full frame camera at 50mm f/4
3- Full frame camera at 75mm f/7.1 (to emulate the “equivalent” of a DX camera)
4- APS-C camera at 50mm f/4

For technical reasons we lost the shot for 75mm f/4.

Can’t see the difference between the shots.

They all use an aperture that has the physical size of about 12.5mm and that’s what light cares about.

FF sensor in FX mode @50mm f/4 (photo: Vincenzo Forchi)

FF sensor in FX mode @50mm f/4 (photo: Vincenzo Forchi)

FF sensor in DX mode (cuts down to APS-C size in camera) @50mm f/4 (photo: Vincenzo Forchi)

FF sensor in DX mode (cuts down to APS-C size in camera) @50mm f/4 (photo: Vincenzo Forchi)

FF sensor in FX mode @75mm f/7.1 (photo: Vincenzo Forchi)

FF sensor in FX mode @75mm f/7.1 (photo: Vincenzo Forchi)

APS-C sensor @50mm f/4 (photo: Vincenzo Forchi)

APS-C sensor @50mm f/4 (photo: Vincenzo Forchi)

Any thoughts about it?? Please share!

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About Cris Da Rocha

Astrophysicist, DB manager, cyclist, musician and, why not, "photographer to be". Back to enjoy photography after many years ... it's cool. Might share something nice and get something new.
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