Depth of Field: 16x manual servo lens vs. 35mm adapter? at DVinfo.net

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Old January 15th, 2006, 07:24 PM   #1
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Depth of Field: 16x manual servo lens vs. 35mm adapter?

Hi Everyone,

I'm trying to determine what lens setup produces the best DOF control on the XL2. Basically, I'm looking for very shallow DOF control. I'm trying to wrap my head around all the information I've read.

I've done a fair bit of reading on the Canon 16x manual servo lens (http://tinyurl.com/8eqx3) and the various available 35mm lens adapters, such as the Letus35, G35, and the ones from RedRock, P+S, etc.

So I have a couple questions:

1) I own the stock Canon 20x OIS lens. From what I understand, the 16x lens is better for rack focus and DOF control. But I have yet been able to figure out why this is true. Is it only because the 16x has markings on the focus ring? Or is the 16x lens capable of producing a shallower DOF? If so, why? (Perhaps my rudimentary knowledge of optics is in the way of my understanding.)

2) With the availability of the 16x lens with rack focus, why are 35mm lens adapters such as the Letus35/micro35/etc are so popular with the XL2 for DOF control? I realize that 35mm lenses have a shallower DOF, hence their popularity on miniDV cameras. However, I've also heard that the 16x lens is capable of great DOF control -- when I asked a respected dealer about 35mm adapters, he recommended using the 16x lens instead. But is that not quite true? Do 35mm lenses on the XL2 still produce a much more shallow DOF than the 16x?

Thanks!
Adam
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Old January 16th, 2006, 02:12 AM   #2
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On the 20x lens, the focus is controlled by a servo motor. When you turn the focus ring, you're actually telling a little motor which way to focus the lens. The result is that its an infinite spinning ring. This makes focusing virtually unrepeatable.

On the other hand, the 16x lens has a true manual focus. When you turn the focus ring, you are directly changing the positions of the lens elements. The focus ring has hard stops and focusing is repeatedable.

I can't imagine why the 16x would be better at DOF control. The DOF of 35mm comes from the fact that the lens is projecting onto a relatively large patch and therefore doesn't have to be very long. In contrast, prosumer minidv cameras, with their tiny 1/3" CCDs, have very long lenses in order to project an image onto such a small receptive patch.

It is true that you can control DOF to a degree by using a larger aperture and zooming in, but no lens alone on a minidv camera will create any where as shallow a field as a 35 adapter.
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Old January 16th, 2006, 03:01 AM   #3
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35mm = Shallow DOF

I have the fujinon 14x manual lens. Whilst this is a great lens for all the manual features, it doesn't come anywhere near the SG35 35mm adapter & canon FD lenses for DOF. With an F1.4 lens attached to the adapter I get something like 3cm DOF on close shots. Check out the alternative imaging forum and view a few sample clips put up by DVINFO members. You won't be able to get these results with the Canon 20x, or 16x video lenses.

There is a tradeoff though, slight light loss and a softer picture, but whether that is a problem all depends on the end result you are looking for.
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Old January 16th, 2006, 07:32 AM   #4
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Without using a ground glass and intermediate for focussing, your depth of field will be best controlled by the largest possible aperture. Therefore a lens that will open to 1.2 or 1.4 will give you a shallower depth of field then one that will open to only 2.x The 16x lens is a bit FASTER than the 20x especially at the long end where the perception of the DOF effect is greatest. Hence the 16x APPEARS to give you a shallower depth of field due to the faster F stop.
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Old January 16th, 2006, 08:38 AM   #5
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Thanks, everyone, for your replies! The answers you have given me are what I thought was true, but was having a hard time pulling it all together.

One more thing, though. As I have always understood it, the term "rack focus" just meant changing focus from a foreground subject to a background subject.

However, one person I talked with described the 16x lens as having a rack focus capability that allows you to specify a near focus point distance (X) and an infinity focus point distance (Y), so that you can focus on only everything at distances between X and Y. For example, you could set X to be 45 feet and Y to be 55 feet, so everything inbetween would be in focus, and everything outside of those bounds would be out of focus.

Is this actually possible with the 16x? Does the 16x have manually adjustable dual focus points? All my reading so far hasn't exposed anything like this.

Thanks again!
Adam
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Old January 16th, 2006, 10:15 AM   #6
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Adam,

What you are describing, only makes sense in respect to the concept of "Depth of Field". The depth of field includes everything that's in focus between the nearpoint and farpoint of the focal range.

"Rack Focus" does in fact, refer to racking, or 'rolling' the focus between two specific points in the frame. For instance, one might 'rack focus' between the end of the gun barrel, and the eyes of the person HOLDING the gun.

The 16x focal ring, is a manual ring. That is, when you turn it, the lens elements actually move, as opposed to an electronic servo moving in the 20x lens.

When "Pulling Focus" using a mechanical focus puller mounted on rails next to the lens, it is easier to get accurate focus adjustments with a manual lens, as it can be repeatable. Not always so with a servo focussing lens. (Not impossible, just not always spot on.)

I don't think I understand your question about DUAL focus points. Just not possible. As I mentioned, when listing a specific Depth of Field, one might mention the near point, and the far point of the field while everything in between those points is in focus.... but that is not to say there are DUAL focus points. THis is true of all lenses with their respective depths of field.

You might go dig up a reference on basic photography, to come to a thourough understand of Depth of Field and how it is controlled and determined by TARGET SIZE, FOCAL LENGTH, and APERTURE. Changing any of these three elements, will affect the depth of field.
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Old January 16th, 2006, 10:26 AM   #7
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Thanks Richard. Like you, when I heard "dual focus points" in the description this other person sent me, I scratched my head, as it sounded impossible to focus on more than one area. I've asked them what they mean by that. Very odd indeed.
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Old January 16th, 2006, 03:04 PM   #8
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Focus Preset

Adam,

Reading your post on what you heard (focus on X, then focus on Y, etc), brought to mind one nice feature of the 20X lens. You can prefocus on point Y, program it in, then set the focus to point X. Begin shooting (point X is in focus), then hit the focus preset button and the lens automatically refocuses to point Y. So you get an automated rack focus feature which I've found to be pretty precise. This can also be set up for zoom instead of focus. I'm wondering if this is what you heard, and the speaker confused the 16X and 20X prefocus abilities?

The only drawback that I have with this is that the rate of change in focus is not adjustable. It's pretty quick and most times I'd like a slower change for a little more dramatic effect. The only solution I've found is to adjust the speed in post, but this only works well for stationary subjects or if a slow-motion effect is also desired. Has anyone found a way to modify the rate of focus on the camera?

Pat
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Old January 16th, 2006, 03:15 PM   #9
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Hi Pat,

I've used the feature you're referring to on the 20x lens before, it is quite nice. You can change the focus speed for the preset, but you only have a choice between "Low", "Middle", and "High", and I find "Low" to still be too fast!

To change the speed, use the menu button on the XL2, go to "Camera Setup", then go to "F Speed Pset". You can choose the speed there.

Adam
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Old January 16th, 2006, 03:16 PM   #10
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Patrick,

Thanks for pointing that out. Yeah, I'd say the previous speaker was referring to racking focus or zoom BETWEEN two points, that could be pre-set. Perhaps he mis-spoke, or Adam misunderstood, but that is no doubt what they meant.

This is not available obviously on the manual 16x. That becomes the job of the focus puller!
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Old January 16th, 2006, 04:18 PM   #11
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Aha!

Ahh. Thanks Adam. I'll try that out on my next shoot.

Pat
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Old January 16th, 2006, 04:26 PM   #12
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By the way, has anyone tried using the EF mount adapter for the XL2 (or one of the mount adapters from Quyen Le at http://www.letus35.com/XL.html) with either a very wide angle lens, or a wide angle lens and a wide angle adapter on top of that? This is described here: http://www.dvinfo.net/canonxl2/artic...cle04.php#ceos

The goal of that would be to get close to what a 50mm lens would looke like on a 35mm camera -- since you get a 7.8 magnification in the focal length with the EF (or other) adapters, you need to go really wide so that the magnification results in a "normal human" field of view.

If anyone has tried that out, let me know. Theoretically you should be able to get shallow 35mm-like DOF with this approach, but you loose some light due to the fact that you have to use a slower wide angle lens.

Thanks!
Adam
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Old January 16th, 2006, 07:10 PM   #13
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Adam,

I've tried the EF/XL adapter on my XL2 with some of my EOS zooms that I have for a 35mm SLR. After much experimentation, I've realized that the cropping (7.8X) factor makes most EOS lenses unusuable for practical shooting, unless you do nature or surveillance work. I don't think you can find a 6.4mm SLR lens, which is what you need for a 50mm (SLR) field of view. Besides that, a 6.4mm (ultra-ultra wide) lens will not have the shallow DOF that we're looking for.

I've now picked up a Redrock adapter, which projects the SLR-lens image circle onto ground glass, and uses the 20X lens as a relay lens that captures the entire image (with shallow depth of field, and full field of view). I'm still working on it, but now have settled on using a SLR prime lens (with the EF/XL adapter) as my relay lens, instead of the 20X. I haven't had a lot of time to work on this, so I don't have any captures yet. I'll post to this board when I have some results to share.

Here's a photo of the current setup. Note that the SLR lens is not attached at the front. The 50mm prime in this photo is a placeholder until I've purchased a wider 20mm prime lens which will capture the full image circle of the SLR lens.

http://static.flickr.com/37/87599587_d298149141_o.jpg

Pat
ps: BTW, there's a few more details on my setup and reasons why I've set it up this way in this thread:
http://www.dvinfo.net/conf/showthread.php?t=57789
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Old January 16th, 2006, 09:36 PM   #14
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Pat,

Thanks for the info from your own experiences, very helpful!

How are you attaching the Redrock to the 20mm relay lens -- is there another adapter in the middle that attaches the Redrock to the threads on the 20mm lens?

I've been looking at this and other 35mm adapters for a little bit now, such as the Letus35 and the G35. The G35 looks really interesting, and I'm really looking forward to seeing their press release on the product announcement this week. I *think* that the G35 might have it's own relay lens that can attach directly to the XL2, and I *think* the G35 has no motor. I read that on the cinemek.com forum, but I'll wait until the press release to see what the real deal is.

How much noise does the motor on the Redrock produce?

Adam
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Old January 18th, 2006, 02:43 PM   #15
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Adam,

Sorry for the delay in replying. The 20mm relay lens attaches directly to the EF-side of the EF/XL adapter, which I still have to order. To be totally honest, what is shown in the photo is a 50mm Nikon prime as a relay lens, with a Nikon f-mount to EF adapter (which is simply a machined aluminum ring). I didn't have an EF prime on hand to try out in its place.

The motor in the RR adapter does make a very soft whirring noise. I think I've read that it comes from a CD player, so perhaps you can listen to a portable CD or DVD player, and use that as a rough guideline. I don't think it'll be an issue for most shooting, particularly if you use a shotgun mike on your XL2. I'll try to quantify the noise level, when calibrated to a person speaking about 3 - 5 ft. from the mike. Can't do this for a few weeks, though.

One other thing you might want to consider in your comparison is the efficiency of the ground glass. RR claims that their ground glass is nearly lossless, meaning that very little light is scattered away from capture by the relay lens. Given the fact that you are already losing about 2-3 stops due to the use of a relay lens, you might find that a low-efficiency GG will mean that you need a lot of light in order to film. The tradeoff to the high-efficiency RR GG is that it will not work well for the combination of dim subjects in focus + bright backgrounds that should be out of focus. They do include a second GG in the package for these situations.

For an example of bright-background effects w/ the RR adapter, see
http://www.redrockmicro.com/zen_and_m2.htm
And scroll down to the section on "ghost image"

Pat
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