Zooming VS F/stop; How to Lower DOF for Portraits?

[mjhoward]

Several factors determine the DoF:

• Image sensor size - The bigger the image sensor, the shallower the DoF with everything else equal.

I believe that with all other things being equal, you'll have the same DoF. The advantage is that you can get shallower depth of field with a larger sensor because you are able to use a longer focal length to achieve the same angle of view (everything else like subject distance/background distance being equal). I believe if you use the same focal length (along with everything else the same), the DoF will be the same. I could be wrong but that was my understanding.

 

[KmH]

Image sensor size and the associated crop factor have to be defined for DoF calculators, because the crop factor affects the FoV a focal length delivers.

Using an online DoF calculator, compare the DoF delivered by a 100 mm lens, set to f/4 and a focus distance of 20 feet. Only change the crop factor (or camera used), and see how the DoF changes.
Even the difference between Canon APS-C 1.6x crop factor, and Nikon's 1.5x crop factor will alter the DoF at least a little bit. (Canon's APS-C size image sensors are slightly smaller than Nikon's APS-C size image sensors, which is why they have slightly different crop factors.)
Online Depth of Field Calculator

 

[DGMPhotography]

Sorry, I keep using bokeh in the wrong context. I know it means the quality of the blur, and not the blur itself but for some reason I keep saying it.. oops. So you're telling me that I *can't* achieve a shallow DoF with my lens?

 

[Buckster]

Sorry, I keep using bokeh in the wrong context. I know it means the quality of the blur, and not the blur itself but for some reason I keep saying it.. oops. So you're telling me that I *can't* achieve a shallow DoF with my lens?

Refer to the online DOF calculator that's been posted a couple of times. Plug in your information, and you'll see just exactly how much DOF you can achieve at different focal lengths and distances.

 

[mjhoward]

Image sensor size and the associated crop factor have to be defined for DoF calculators, because the crop factor affects the FoV a focal length delivers.

Using an online DoF calculator, compare the DoF delivered by a 100 mm lens, set to f/4 and a focus distance of 20 feet. Only change the crop factor (or camera used), and see how the DoF changes.
Even the difference between Canon APS-C 1.6x crop factor, and Nikon's 1.5x crop factor will alter the DoF at least a little bit. (Canon's APS-C size image sensors are slightly smaller than Nikon's APS-C size image sensors, which is why they have slightly different crop factors.)
Online Depth of Field Calculator

I can see that the calculator gives a little different DoF between say D800/D7000 but it just doesn't make sense to me as to why. If you take your FF body and place it into DX mode, does the DoF change??? Doing that doesn't seem any different than simply cropping out the same portion of the image in post (which obviously won't change DoF).

 

[o hey tyler]

I can see that the calculator gives a little different DoF between say D800/D7000 but it just doesn't make sense to me as to why. If you take your FF body and place it into DX mode, does the DoF change???

No, your DoF doesn't change just by putting a Dx lens on... But the distance between you and your subject would have to change (getting farther away) to frame the same shot as you would have with an fx lens on the camera. Thus making your DoF a bit extended.

 

[christop]

The sensor size does affect the depth of field because while the size of the circle of confusion does not change, its size relative to the size of the sensor decreases with larger sensors. Smaller circles of confusion (relative to the format) means that more is in focus. So the depth of field is actually greater on FF with the same lens and aperture than on crop.

If you increased the focal length, or moved closer, on FF to get a comparable framing on crop but left the aperture the same you'll get a shallower DoF on FF.

Edit: I should actually say that more is acceptably sharp, not that more is in focus, due to the smaller circles of confusion. Only what intersects the focal plane is really in focus.

 

[o hey tyler]

The sensor size does affect the depth of field because while the size of the circle of confusion does not change, its size relative to the size of the sensor decreases with larger sensors. Smaller circles of confusion (relative to the format) means that more is in focus. So the depth of field is actually greater on FF with the same lens and aperture than on crop.

The DoF is larger on a full frame camera? You said the opposite in your second paragraph. With the same lens, aperture, and subject framing, the full frame camera will have a shallower DoF. Just trying to figure out what you mean.

 

[christop]

I didn't contradict myself.

With the same focal length, aperture, and distance (I didn't mention that part), the DoF is larger on full frame than on crop, only because the circle of confusion diameter limit is larger on full frame that it is on crop.

With the same focal length, aperture, and subject framing, the DoF is smaller on full frame.

 

[o hey tyler]

I didn't contradict myself.

With the same focal length, aperture, and distance (I didn't mention that part), the DoF is larger on full frame than on crop, only because the circle of confusion diameter limit is larger on full frame that it is on crop.

With the same focal length, aperture, and subject framing, the DoF is smaller on full frame.

Oh I see. That was a pretty key part you left out.

 

[christop]

I didn't contradict myself.

With the same focal length, aperture, and distance (I didn't mention that part), the DoF is larger on full frame than on crop, only because the circle of confusion diameter limit is larger on full frame that it is on crop.

With the same focal length, aperture, and subject framing, the DoF is smaller on full frame.

Oh I see. That was a pretty key part you left out.

Yeah, it really was. Mea culpa! (Though to be fair to myself, I interpreted mjhoward's question to mean that the distance was constant, while you read it differently.)