FULL FRAME VS APSC DEPTH OF FIELD (THE TRUH)

[noctilux888]

In this video I want to compare the difference in depth of field that exists between a Full Frame sensor and an APSC sensor.

For this comparison I will use two cameras, a Sony Full frame and a Fuji APSC, both cameras have a Normal lens attached.

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[Bill Ferris]

At 53mm, the Fuji would have captured a field of view (FOV) equivalent to that captured by an 80mm lens on the A7III. That's a 60% longer focal length than the 50mm you used with the Sony.

Here are two screen captures from your video:


Photo made with Sony A7III and 50mm lens




Photo made with Fuji X-Pro2 and 53mm lens (equivalent to 80mm FOV on a full-frame body)

The similarity in the FOVs of the two photos is not what I would have expected. Based on the opening statement in your video and a basic understanding of the relationship between focal length, sensor size, and the resulting FOV, one would expect the Fuji images to show a significantly narrower FOV.

The fact that the Fuji image so closely matches the FOV of the photo made with the Sony suggests you may have been farther from the subject (giving the resulting image greater DOF) with the Fuji to match the FOV of the A7III image or cropped the full-frame image (making its DOF more shallow) to roughly match the FOV of the X-Pro2 photo. There are other possible explanations.

Would you care to expand on the details of the test to explain the similarity in FOV between the above photos despite one being made with a 50mm focal length and the other made with effectively an 80mm focal length?

Thanks.

--
Bill Ferris Photography
Flagstaff, AZ

http://www.billferris.photoshelter.com

 

[noctilux888]

I checked the video ... I forgot to specify that the 53mm focal length of the Fujifilm is EQUIVALENT but not its REAL focal length which is actually 35mm, on the Fujifilm I kept its 35mm 1.4. I'll try to add something on the video if I can ...

Thanks for your comment!

 

[Bill Ferris]

I checked the video ... I forgot to specify that the 53mm focal length of the Fujifilm is EQUIVALENT but not its REAL focal length which is actually 35mm, on the Fujifilm I kept its 35mm 1.4. I'll try to add something on the video if I can ...

Thanks for your comment!

Thanks for the clarification.

A 35mm f/1.4 lens has an entrance pupil (aperture) of 25mm. A 50mm f/1.4 lens has about a 36mm aperture. The larger aperture at the same distance from the subject delivers an image having a shallower depth of field.

 

[LASR]

This seems a very confusing way to present things. Why not just say that a 50 mm lens in FF has the same field of view as 50/1.5 = 33.3 mm (35 mm) lens in APS-C (if the crop factor is 1.5x).

Since the amount of light, depth of field, depends on the physical aperture (as BF said) the aperture of a 50 mm lens at f2.0 is 50/2.0 = 25 mm, the same aperture on a 35 mm lens corresponds to f1.4, 35/25 = 1.4, therefore, 50 mm f2.0 in FF has the same field of view and depth of field as 35 mm f1.4 in APS-C.

 

[mostlyboringphotog]

This seems a very confusing way to present things. Why not just say that a 50 mm lens in FF has the same field of view as 50/1.5 = 33.3 mm (35 mm) lens in APS-C (if the crop factor is 1.5x).

Since the amount of light, depth of field, depends on the physical aperture (as BF said) the aperture of a 50 mm lens at f2.0 is 50/2.0 = 25 mm, the same aperture on a 35 mm lens corresponds to f1.4, 35/25 = 1.4, therefore, 50 mm f2.0 in FF has the same field of view and depth of field as 35 mm f1.4 in APS-C.

To be a bit more stickler -

50mm f/2.0 and 35mm f/1.4 may have the same ~25mm aperture diameter but 50mm f/2 has 1.5x more (deeper) DOF than 35mm f/1.4. However, Fuji APS-C is enlarged by 1.5x crop factor relative to the FF sensor, making the APS-C sensor 1.5x shallower DOF, which cancels out the deeper DOF of 50mm f/2 lens.

Same aperture (entrance pupil) diameter does not keep the DOF same (THE TRUTH) ;-)

 

[Bill Ferris]

This seems a very confusing way to present things. Why not just say that a 50 mm lens in FF has the same field of view as 50/1.5 = 33.3 mm (35 mm) lens in APS-C (if the crop factor is 1.5x).

Since the amount of light, depth of field, depends on the physical aperture (as BF said) the aperture of a 50 mm lens at f2.0 is 50/2.0 = 25 mm, the same aperture on a 35 mm lens corresponds to f1.4, 35/25 = 1.4, therefore, 50 mm f2.0 in FF has the same field of view and depth of field as 35 mm f1.4 in APS-C.

To be a bit more stickler -

50mm f/2.0 and 35mm f/1.4 may have the same ~25mm aperture diameter but 50mm f/2 has 1.5x more (deeper) DOF than 35mm f/1.4. However, Fuji APS-C is enlarged by 1.5x crop factor relative to the FF sensor, making the APS-C sensor 1.5x shallower DOF, which cancels out the deeper DOF of 50mm f/2 lens.

Same aperture (entrance pupil) diameter does not keep the DOF same (THE TRUTH) ;-)

With respect to LASR's post, both field of view and entrance pupil are stated as being the same. In that scenario, both the geometric and perceived depth of field (DOF) will be the same.

If we're focusing solely on a geometric depth of field - a range of physical distances within which the image appears to be acceptably in focus - you are correct that two different focal lengths operating with the same diameter entrance pupils at the same distance from the subject can produce images having different geometric depths of field.

However, there is an element of subjective perception involved in DOF. If we examine DOF as it is perceived by the viewer, then two photos made at the same distance from the subject, using lenses having the same entrance pupil diameters but different focal lengths can be perceived as having the same DOF.

Here's a post from last year in which this is demonstrated: https://www.dpreview.com/forums/post/64536294

--
Bill Ferris Photography
Flagstaff, AZ
http://www.billferris.photoshelter.com

 

[PhotonBeam]

This seems a very confusing way to present things. Why not just say that a 50 mm lens in FF has the same field of view as 50/1.5 = 33.3 mm (35 mm) lens in APS-C (if the crop factor is 1.5x).

Since the amount of light, depth of field, depends on the physical aperture (as BF said) the aperture of a 50 mm lens at f2.0 is 50/2.0 = 25 mm, the same aperture on a 35 mm lens corresponds to f1.4, 35/25 = 1.4, therefore, 50 mm f2.0 in FF has the same field of view and depth of field as 35 mm f1.4 in APS-C.

To be a bit more stickler -

50mm f/2.0 and 35mm f/1.4 may have the same ~25mm aperture diameter but 50mm f/2 has 1.5x more (deeper) DOF than 35mm f/1.4. However, Fuji APS-C is enlarged by 1.5x crop factor relative to the FF sensor, making the APS-C sensor 1.5x shallower DOF, which cancels out the deeper DOF of 50mm f/2 lens.

Same aperture (entrance pupil) diameter does not keep the DOF same (THE TRUTH) ;-)

The circle of confusion strikes again.

 

[mostlyboringphotog]

This seems a very confusing way to present things. Why not just say that a 50 mm lens in FF has the same field of view as 50/1.5 = 33.3 mm (35 mm) lens in APS-C (if the crop factor is 1.5x).

Since the amount of light, depth of field, depends on the physical aperture (as BF said) the aperture of a 50 mm lens at f2.0 is 50/2.0 = 25 mm, the same aperture on a 35 mm lens corresponds to f1.4, 35/25 = 1.4, therefore, 50 mm f2.0 in FF has the same field of view and depth of field as 35 mm f1.4 in APS-C.

To be a bit more stickler -

50mm f/2.0 and 35mm f/1.4 may have the same ~25mm aperture diameter but 50mm f/2 has 1.5x more (deeper) DOF than 35mm f/1.4. However, Fuji APS-C is enlarged by 1.5x crop factor relative to the FF sensor, making the APS-C sensor 1.5x shallower DOF, which cancels out the deeper DOF of 50mm f/2 lens.

Same aperture (entrance pupil) diameter does not keep the DOF same (THE TRUTH) ;-)

The circle of confusion strikes again.

It was late...

50mm f/2.0 and 35mm f/1.4 may have the same ~25mm aperture diameter but 35mm f/1.4 has 1.5x more (deeper) DOF than 50mm f/2. However, Fuji APS-C is enlarged by 1.5x crop factor relative to the FF sensor, making the APS-C sensor 1.5x shallower DOF, which cancels out the deeper DOF of 35mm f/1.4 lens.

 

[mostlyboringphotog]

I checked the video ... I forgot to specify that the 53mm focal length of the Fujifilm is EQUIVALENT but not its REAL focal length which is actually 35mm, on the Fujifilm I kept its 35mm 1.4. I'll try to add something on the video if I can ...

Thanks for your comment!

It's better to think of APS-C as the 1.5x center crop relative to FF (which is exactly what it is) rather than APS-C having an equivalent FL.

Lens FL is what is stamped in front of the lens, even if there is no sensor ;-)

Moreover, if FL/F# and the focus distance are the same, APS-C will have shallower DOF and narrower FOV and FF will have deeper DOF and wider FOV.