Need explaination about DOF

[Dan McCain]

I read a very technical white paper on why a 1/3 chip cant reproduce the same lens charateristics and DOF even with a 35mm lens, and I got confused. For instance just attaching a 35mm lens directly to the XL1 doesnt give you the same DOF as the mini. And I understand that the mini35 system solves this problem. However I need a very pratical explaintion from Guy or other expert about how this system works in terms of depth of field and reproducing depth of field on the chip. I understand how the adapter works by photographing the image, however i need to know the technical specifics about how this system is different from just attaching the 35mm lens to an xl1. Thanks in advance.

[Greg Boston]

Dan,

It's due to the fact that with the P&S unit, the camera is photographing a projected image onto a ground glass. The ground glass has the characteristics that maintain 35mm depth of field.

With the EF adapter, you are connecting a 35mm lense directly to the camera and the chips are receiving the initial image from the lense and therefore DOF is based on the chip size.

Another way to look at it is like this. Set your camera up in front of the TV set. Now, put a movie shot on 35mm film on the TV screen. Your camera sees only the TV screen and records that image. However, the DOF will be whatever it was when the film camera/lense captured the images.

Or, if you close one eye and look through the camera VF, your vision is now what the camera sees, not what your eye would see.

I'm trying to stay simple in my explanation. Hope this helps you Dan.

=gb=

[Dan McCain]

Thanks for the reply, however lets get more technical please. If a CCD is just recording the information that comes through the lens, why should the DOF be any different? I dont understand why various ccd sizes makes any differnce with DOF. I understand the proceedure through the mini (i have one), however I am trying to understand why the mini is necessary. Thanks

[Mizell Wilson]

Dan,

If you search through the P+S forum you will find a couple different threads where we discussed DoF in it's relation to the XL series and Mini35 including conversion factors.

In the end, for the purposes of this question, the major factor that effects DoF is the size of the capture frame and not the lens itself. This is where we also get into the question of effective AoV for a given lens based on it's capture frame.

It's not that you couldn't, theoretically, achieve similar DoFs with a 1/3" CCD but that the conditions necessary to do so are not easily achieved with today's DV cameras, no matter the lens. The size of the 35mm frame is so large compared to the 1/3" CCD that the physics line up much easier.

There is also Jeff Donald's ultimate DoF skinny available in the articles section of DVinfo.

Mizell

[Jean-Philippe Archibald]

Dan,

Please report you to this article written by by our great Jeff Donald if you want to understand all the concepts behing depth of field.

Hope this help,

[Guy Genin]

Hello Dan,

You say:

”If a CCD is just recording the information that comes through the lens, why should the DOF be any different? I dont understand why various ccd sizes makes any differnce with DOF. I understand the proceedure through the mini (i have one), however I am trying to understand why the mini is necessary. Thanks”

I sense that you are thinking abstractly about using one and same lens in front of CCD of different sizes. If the distance to the subject is the same and the aperture of the lens is the same then indeed the DOF will be the same as long as the size of the individual pixels is similar on each CCD.
However the angle of view will be very different so if you want to cover the subject with the same angle of view from a fixed distance, you will need to change to a much shorter focal length with the smaller CCD and thus get a greater depth of field.

[Dan McCain]

Thanks to Guy and to the article I have a grasp on the depth of field issue. Ok, besides DOF what about the other "charateristics" are preserved by photographing the image on ground glass? In other words if we attached a 100mm lens to the xl1 would it have a different look (minus the DOF) than if we went through the mini. And if so, exactly what are the different charateristics, and what are some terms we can use to describe them?

Also, where do we get the term "100mm" in a 35mm lens? What physically does that measure?

[Mizell Wilson]

Dan,

The other major factor preserved by the Mini35 is the effective AoV (Angle of View) of the lens.

If you do a search under my user name you will find some rather in depth discussions that we've had concerning these effects.

Mizell

[Jeff Donald]

Dan, it's real simple. Cut a 4" by 5" hole in a piece of paper. Now cut an 1" by 1 1/2" (size of 35mm film) hole in a piece of paper. Frame a scene in the 4x5 cutout. Overlay the smaller cutout. Notice how much less the smaller cutout shows of the scene? Walk backwards until the view through the smaller hole matches the original scene you chose. How many feet did you have to walk backwards to get the same view? Now imagine these cutouts were actual film frames. The change in distance (moving backwards from the original scene) increased the DOF.

See my article referenced above for all the factors that affect DOF.

The focal length of a lens is the distance in mm from the rear nodal plane to the film plane when the lens is focused at infinity and the subject is in best axial focus. The rear node is a mathematical point near the rear element of the lens. Set the lens to infinity and measure the distance between the rear node and the chip or film. The measured distance, in mm is the focal length.