DOF question 35mm vs. 4x5

[stradibarrius]

Is is correct that LF lenses and 35mm lense of the same focal length and set to the same f/stop would produce images with different DOF's??? If possible can someone give an "Explanation for Dummies" type answer as to what the difference and why?

[Heroique]

I'll try to keep this very simple for starters...

If you're using, say, a 100mm lens on a 35mm camera, and a 100mm lens on a 4x5 camera, each with the same f/stop, and at the same distance from the subject, it's the same DOF.

Of course, if you want the same framing for the two formats, you'd move the 35mm camera back from the subject, and that would provide greater DOF. Or you might move the 4x5 camera closer, which would provide less DOF, or "greater isolation," as some people say.

[Mark Woods]

Excellent description!

[duff photographer]

Is is correct that LF lenses and 35mm lense of the same focal length and set to the same f/stop would produce images with different DOF's??? If possible can someone give an "Explanation for Dummies" type answer as to what the difference and why?

I'll have a go but I stand to be shot down for being too simplistic.

Put simply, in the case you give the depth of field will actually be the same. If you took a 35mm (24x36mm) segment of that 4"x5", the depth of field will be the same. Another way of looking at is if I used a 35mm camera with a 35mm lens I would get exactly the same DOF as a 35mm digital back (covering the same format) shooting through a large format lens of the same focal length.

It is the larger format that effectively reduces the depth of field because it is covering more of the scene at the same focal length. Conversely a lens on an APS format will cover a more limited area of the scene so will give the impression of a greater DOF.

However, the difference in DOF shows itself best when comparing the same image coverage/angle of view between the systems. To get the same image coverage shown by a 35mm DSLR on a 4"x5" you'd have to use a lens which has a focal length of, in this case, about 3.3 times that of the 35mm format. Therefore a lens with a focal length of 50mm in 35mm format will need to be a 165mm for 4"x5" format. In other words, the 165mm large format lens on a 4"x5" will have the same angle of view as the 50mm on a 35mm DSLR (full frame).

The DOF of a 165mm lens is inherently less than that on a 50mm lens so you would then see a real world difference in DOF when comparing the images of the same dimensions and angle of view.

I think I've got that right. :-\

DP

[stradibarrius]

Thanks that makes perfect sense!!!

I'll try to keep this very simple for starters...

If you're using, say, a 100mm lens on a 35mm camera, and a 100mm lens on a 4x5 camera, each with the same f/stop, and at the same distance from the subject, it's the same DOF.

Of course, if you want the same framing for the two formats, you'd move the 35mm camera back from the subject, and that would provide greater DOF. Or you might move the 4x5 camera closer, which would provide less DOF, or "greater isolation," as some people say.

[BetterSense]

It's confusing because f-stops are a terrible unit of measure.

For a given composition, the only thing that determines DOF is the diameter of the lens aperture. If lens apertures were expressed in something sensible, like say mm or inches, we wouldn't have this confusion. An aperture of 5mm will give the same DOF for any camera format. Very simple. Obvious.

A long time ago, someone decided it was a great idea to express lens apertures as the diamters of the aperture divided by the focal length. So now, when I set up a still-life composition, I have to use 4 different f-stop settings depending whether I'm using a 35mm, 6x7, 4x5, or 8x10, to get the same aperture and thus DOF.

[Leigh]

Basic point...
The lens has no idea what film is behind it.

So obviously the DoF at the image plane is a fixed value.

Take a shot with a particular lens (300mm??) on 8x10 film.
Now cut a 24mmx36mm rectangle out of that film.
Has the DoF of that portion of the subject changed?

- Leigh

[Dan Fromm]

It's confusing because f-stops are a terrible unit of measure.

For a given composition, the only thing that determines DOF is the diameter of the lens aperture. If lens apertures were expressed in something sensible, like say mm or inches, we wouldn't have this confusion. An aperture of 5mm will give the same DOF for any camera format. Very simple. Obvious.

A long time ago, someone decided it was a great idea to express lens apertures as the diamters of the aperture divided by the focal length. So now, when I set up a still-life composition, I have to use 4 different f-stop settings depending whether I'm using a 35mm, 6x7, 4x5, or 8x10, to get the same aperture and thus DOF.

Very interesting. Why do you believe all this, BS?

Ror everyone else, which DoF concept are you talking about? DoF on film or in the final print? And are you talking about the same final print when you say that a short lens gives more DoF than a long lens?

[Leigh]

It's confusing because f-stops are a terrible unit of measure.

Why?

f/stops progress at a ratio of 1.414:1.

Since the f/stop (aperture diameter) determines the area of the pupil, each stop differs from its neighbors
by a factor of 2 (in square millimeters or square inches or square whatever).

- Leigh

[Patrick13]

Go any deeper down this rabbit hole and you'll have to introduce "Circle of Confusion" into this beginner's introduction and that may not be appropriate.

f/stops are a great unit, without conversion an f/stop of F11 on a 50mm lens yields exactly the same light transmission as an f/stop of F11 on a 300mm lens. This is perfect for the end user taking a picture and well worth the hidden engineering to make it happen via diameters and much less important than DOF changes and all that when taking a picture.