Depth of Field, Focal Length, and Sensor Sizes

[pnodrog]

I can't ever recall shooting one at F16 and cursing that it couldn't be F15.65 because that's what a calculator said would give me an ideal DoF either ...

Colin I see you are assuming you can still hold the camera steady with the slower shutter speed F16 will give.

[RobertsMx]

Sometimes, f/16 can come in handy though... even handheld under cloudy conditions with 3-stops of stabilization:

Sony A55 + Sony 16-50/2.8 SSM (ISO 100, 35mm, f/16, 1/6s)

And, of course the idea also being an extended DoF.

One thing I'd like to point out is the DOF calculator mentioned above (It happens to be the first google result when you do a search on "dof calculator"). I've used it many times too, but after all, I think the calculator is very misleading. At the same focal length (in absolute mm, not in 35mm equivalent), same aperture and same subject distance, the depth of field

Below is from Wikipedia on depth of field. It also says that if the focal length and subject distance are the same, the DOF in different sensor size will depend on the size of the final print. The two sensor will have the same DOF if the final images are not magnified. The smaller sensor will have LARGER DOF only if its final image is magnified to match that of a larger sensor (the second bullet point below). The DOF Master at the above link assumes that this will be the case (i.e., the final images will be magnified to result in the same size regardless of the sensor size) without saying as much on the web page. It took me a while to figure out their assumptions.

DoF calculators plug-in crop factor to account for the differences either with selection by model name or with such factors as circle of confusion.

And a lot of confusion seems to come from setting equivalences between sensor sizes. For example, a typical argument would be that a f/1.8 lens on APS-c would give a DoF of f/2.8 lens on Full Frame. However, that is true only when we also consider equivalence on field of view. A 35mm f/1.8 lens on APS-c would compare to a 52mm f/2.8 lens on FF, in terms of DoF and FoV, with similar subject size.

[Colin Southern]

Colin I see you are assuming you can still hold the camera steady with the slower shutter speed F16 will give.

I've taken hand-held shots at F32 - but usually I just use my tripod.

[xpatUSA]

Hello, Flamell and welcome,

I'm a great believer in formulae for a full understanding of something like DOF. Many folks are not.

Unfortunately, general statements have to be qualified with lots of ifs, buts and mebbes and so it is with general questions but I'll give it a shot:

That wasn't my question, really.

I asked:

Here we go . . . . .

Does focal length change depth of field if the subject occupies the same portion of the image? (ie. if you move forward/back)

The short answer is: No.

If you use the same exact focal length on a full frame camera as on a cropped sensor camera, would you get the same depth of field?

I'm assuming that all other values, including aperture, are not changed.

The short answer is: Yes.

What I don't get is this: assuming that a cropped sensor gives you the equivalent of cropping the final image (assuming cropped res. = res. of cropped sensor, pixel size remains constant), how would a larger sensor have a shallower depth of field?

Sensors do not have a depth of field, which is perhaps why you don't get it.

Depth of field is a function of the lens. A cropped sensor just uses a smaller portion of the image circle.

No, depth of field is not just a function of the lens.

So, to say that larger sensors have a smaller depth of field must mean: to achieve the same image as a full frame camera, an otherwise identical cropped sensor must use a wider focal length. Thus, the argument is that longer focal lengths give you a shallower depth of field.

So, the original question: Does focal length affect depth of field?

"wider focal length" is a peculiar term, haven't seen that before, but focal length does not by itself affect DOF. So, the answer is: No.

So: Does sensor size directly affect depth of field?

Yes or No, see below.

And now for a formula that clearly defines the parameters of DOF, for stuff that is close-ish to the camera:

DOF = 2*CoC*N*(1+m)/m^2 where:

CoC (unfortunately) depends on how the image is to viewed, i.e. the enlargement from the sensor to the viewing medium. This definition introduces the sensor size as a factor. Other folk use a multiple of the sensor pixel pitch which removes the viewing medium and hence sensor size from the equation.

N is the f-number that you set in-camera. (1+m) accounts for extension of the lens for closer subjects.

m is the magnification factor as in the size of a subject at the sensor divided by the actual size of the subject in the scene. m is what changes if you step back and forth to frame a scene and that is why the DOF changes by so doing.

m^2 means m squared.

You might want to read the above a time or two.

[Colin Southern]

Hello, Flamell and welcome,

Unfortunately ... Flammel hasn't been seen here for over 3years.

[New Daddy]

Unfortunately ... Flammel hasn't been seen here for over 3years.

Mea culpa.

I didn't want to start a new thread just to reiterate the discussion made in this outdated thread.

[xpatUSA]

Thanks Gents,

Must remember to check next time