Depth of Field calculations involving Letus 35XL, 50mm prime, & Canon XL2

[Henry Clayton]

I've been attempting to use the depth of field calculator at http://www.dofmaster.com/dofjs.html to make calculations for my camera rig: Canon XL2, Letus 35XL Enhanced (prototype version), & a Canon 1.4 50mm lens.

It is my understanding that the Letus adds magnification to the tune of 1.9x, giving the prime an effective focal length of 95mm or so. At the same time, account must be taken of the difference between the area of the 35mm film, on which the calculator is based, & the area of the XL2 CCD. Here I'm relying on numbers which may not be accurate, since I got them from a source discussing the XL1, & I'm unsure whether the CCD area is larger in the XL2 or not. So my first question is whether the numbers I'm using are correct. In any event, 35mm film exposure area is rounded 22 x 16mm, & the CCD area is 4.8 x 3.6 mm. So the CCD is smaller by a factor of about 4.6.

Now, in trying to get usable information out of the calculator, here is my attempted method. I use the film format choice of 35mm film (with a circle of confusion of 0.030). Multiply the prime focal length by the Letus magification factor of 1.9, getting 95mm. Then, to make up for the difference between 35mm film & the CCD, I divide the focal length by 4.6, giving 20.7 (rounded up). I enter this into the calculator as the lens focal length. I enter an f-stop of 1.4 (wide open on my lens), & an arbitrary subject distance, let's say seven for this discussion. Hit calculate & it spits out the following numbers: 3.06 ft total depth of field, 1.21 ft (40%) deep in front of subject & 1.85 ft (60%) behind, a near limit of 5.79 ft & a far limit of 8.85 ft, 33.2 ft hyperfocal distance, & a 0.03 mm circle of confusion.

So two questions:

1) Do my numbers check out?
2) Is this a viable method?

[Henry Clayton]

Hmm. No replies. I wonder if there is a more appropriate place for this question. Any hints?

H.

[Chad Terpstra]

Based on my relatively small knowledge of lenses, I would say that there is an error in assuming that you should enter 95mm as a focal length. Your lens is still a 50mm lens focusing on a similar plane that it was intended to focus on (a plane of film). The magnification comes in after it has been focused on the plane when the Letus (somewhere in the relay/prisms/whoknowswhat) crops the image to a section a little larger than half of the focused image that the lens created.

So my feeling is that you should enter 50mm as your lens and everything should fall into place like normal. It's just that you have a different field of view than you would normally, but that doesn't factor into the DOF. Your depth of field will be what it should be for any given lens. The only reason to multiply the lens focal length by 1.9 is to get an idea of what your FOV will be when shooting.

This is my take on it at least. It'd be great if Quyen could pop in and say yay or nay.

[Ben Winter]

I was going to type in my response, but realized Chad said it for me. The image projected onto the ground glass, as you can imagine, has absolutely nothing to do with what's focusing on it from behind and by how much.

Think of it this way: The SLR lens projects the same image onto a plane whether it's film, a ground glass, or the palm of your hand. It's just a lens after all. So none of your calculations are affected by what goes on after that.

Things would be much different if this was a field of view calculator.

[Henry Clayton]

Thanks for the information. I take your point about the irrelevance of the Letus magnification, so that eliminates that concern.

Another concern is that since the film format is different (in the case of the video camera it's actually the CCD) & that is a factor in the calculation, it would be necessary to compensate for that. I'd picked up somewhere on the web that a roundabout way to do it was via the focal length entry.

But if I understand you correctly, the CCD along with everything on the camera side of the ground glass are no longer a consideration (for the purposes of DOF calculation), & the relevant piece of information is the area of the gound glass. Is that right?

If that's the case, is the area of the ground glass the same as the area of 35mm film plane? Does anybody know the dimensions of the ground glass in the Letus 35XL Enhanced?

Thanks again for clarifying the issue for me.

H.

[Paul V Doherty]

Depth Of Field is a function of Lens focal length, lens apeture, and the size of the image circle. In the case of the Letus/XL2 combination you must account for the 1.9 x magnification factor on the 35mm frame size.
Simply perform your calculations based on a standard 35mm DOF calculator and multiply or divide by 1.9, whichever is appropriate. I don't know exactly how the DOF calculator works, but I suspect you would divide the percieved DOF by 1.9.

[Ben Winter]

Oy, this conversation is getting much more complicated than it should be. Yes, depth of field has to take into consideration the image plane size, but what you're referring to is the projected image plane size. He's using 35mm lenses, so 35mm calculations still stand, as a 35mm image plane is still being projected. However, he's getting a 1.9x cropped image off of that image plane, which does not affect DOF at all. Say I focus on the woods in my backyard and the table in front of is out of focus. If I crop the image 1.9x magnification and leave the table and the woods in the frame, the table will be just as out of focus as before! This 1.9x magnification business does nothing to affect DOF. Only field of view.

[Henry Clayton]

So to calculate depth of field with that calculator or some other method, act as though you're using 35mm film, the actual lens focal length you're using. No offsetting of calculations needed. For dealing with calculations pertaining to field of view, the 1.9x comes into play. A 50mm prime lens should be treated as a 95mm prime for the purposes of field of view, but not for depth of field.

Thanks everybody.

H.

[Steve Brady]

Originally Posted by Ben Winter

Say I focus on the woods in my backyard and the table in front of is out of focus. If I crop the image 1.9x magnification and leave the table and the woods in the frame, the table will be just as out of focus as before! This 1.9x magnification business does nothing to affect DOF. Only field of view.

But if you enlarge the cropped image so that it's the same size as the uncropped image, then details which appear acceptably sharp in the uncropped image might appear out of focus in the enlargement.

Mathematically, you should account for the 1.9 x magnification by changing the CoC (in the top drop-down box) to .016 mm.

Practically, you might decide that the relatively low resolution of video means that you can use a larger CoC.

[Ben Winter]

True. I have a feeling CoC won't get as small as .016 because of DV/HDV resolution.