What effects Depth of Field

[Charles Papert]

Bill, this is making my head explode a little bit also.

I'm going to chat with an associate in the lens division at Panavision, because you do suggest an interesting theory, that the aperture has a different physical size on a 35mm format lens than a 1/3" format lens, to see if (given identical field of view) the circle of confusion is affected. Speaking of affected--this thread should properly read "What affects Depth of Field", eh?

Justin, your choice of a 35mm lens in your example has probably exploded a few other heads, as there may be a confusion between a 35mm focal length and the 35mm format...ever take a glance at your Zeiss speeds to check which lens you have up and catch your eye on the engraved "35mm" and get fooled into thinking that's the focal length when actually they all say that on the barrel? Hate that...!

Anyway, Bill, I'm not sure if we are all talking about the same thing--Justin's use of a 35mm focal length on his Mini-35 is yielded a true 35mm field of view and depth of field characteristic, which is very different than the same lens on an adaptor which turns it into a 245mm as you say. I would be interested in seeing a, say, 10mm lens on the Canon adaptor and comparing that to the 10mm setting on an the XL1 manual lens to see what the difference would be visually in terms of resolution, color, and depth of field etc.

My personal feeling about all this is that while it would be very nice to have the ability to control the depth of field in the 1/3" format, that characteristic is secondary to the choice of focal length. As you guys have pointed out it is not necessarily desirable or possible to be working at telephoto as it distances one from the action. There is a tremendous difference between a head and shoulders shot on a medium wide lens and a long lens. For an intimate scene between two people, shooting the closeups from 20 feet away may create more of a voyeuristic feel than being inside and privy to the conversation. Many filmmakers feel that comedy plays best on wider lenses, also. I would recommend thinking about blocking your action so that scenes play as far as possible from walls which will create that depth for you as well as make it easier to light the actors apart from the set.

[Bill Ravens]

Charles...

LOL....yeah, i understand...boggles the mind. Most of my info comes from KODAK. If I can find the site, I'll post the URL.
Let me know what you find out. I work with optical physicists and they have supported what I have said...that depth of field goes inversely with the max design aperture.

Agreed that 35mm FL lenses and lenses designed for 35mm image plane are VERY confusing terminology.

[Justin Chin]

Yes, the 35mm FL example is really confusing. But I wanted to illustrate the actual lens I used for the shot.

My lenses actually don't have their format etched in them. But so their is no confusion whatsoever I put yellow gaffers tape on the cap with the FL written on it. No need to read the small writing wrapped around the front of the lens AFTER you pull off the lens cap.

Charles, cool. I'd love to hear what the Panavision people have to say. The video lens numbers really start to confuse me. Math is not a strong asset of mine.

I always thought that the F stop (or the more accurate T stop) is the Focal Length divided by the size of the aperture. i.e. 100mm lens with a 50mm focal length is at f2. So a 25mm focal length (on a cinema 35mm format lens) is equivalent to 12mm focal length (on a 16mm format lens) the lens itself is smaller, hence the aperture is smaller... ahh... brain hurting...

[Bill Ravens]

Justin...

You're right about the f-stop definition. And, it's pretty common knowledge that the DOF goes directly with the numerical T-stop(f-stop)...ie the smaller the f-number the less the DOF.

The discussion above refers to the MAXIMUM aperture(iris open all the way). On a lens designed for 35mm film, the diameter of the glass is larger than the glass diameter on a lens designed for a 1/3 inch ccd. My contention is that this also effects DOF.As Charles points out, a larger diameter lens has a larger circle of confusion. Therefore it has less depth of field.

[Charles Papert]

Justin:

The t-stop is an adjusted version of the f-stop that measures the actual transmission (t) to take into account the design of the individual lens, to assure that it is delivering the correct amount of exposure to the imaging system (I was going to say film plane, but I am trying to keep in the spirit of where we are having this discussion!)

I think the more recent vintage of Speeds are the ones with the 35mm etched on them.

[Justin Chin]

Charles:

As always you are correct on both points, my lenses are older, and the description of the T stop.

Okay, I must stop my circle of confusion.

[John Steele]

Right OK, this is all hurting my head but I have a question. On an earlier post it was said a wide angle lens would increase the depth of field because it shortens the focal length. It was also said that to give a shallower DOF you would zoom in. So If I wanted to get a shallower DOF at a certain distance from a subject, but couldn't get far enough away to zoom in, could I use a wide angle lens that allowed full zoom, then zoom in and achieve a shallower DOF while being closer to the subject?????

I hope someone understood that question cause I don't think I did :-)

John.

[Josh Bass]

I'm not sure I did either. Why does how far the camera is from the subject matter to you so much? The main thing is getting what you want in frame, and achieving your desired depth of field, or lack thereof.

[Charles Papert]

Distance to the subject is one of the factors that affects depth of field. At a given focal length and aperture, focusing on a subject four feet away will result in a shallower image than if the subject was thirty feet away.

There's just one or two variables missing from the question which will help for us to provide an answer. Why exactly can't you "get far enough away to zoom in"? Is it because you are within the minimum distance that the lens you are using will focus (let's ignore the macro setting for the moment)? In other words, when you zoom in, you cannot get the lens to focus because the subject is too close?

If this is the case, it is possible that another lens may have a different close focus characteristic. On the manual lenses, it's easy to see what this is: turn the barrel until it stops, and look at the inscribed number (usually around 3-5 feet). That is the minimum distance that a subject can be from the camera to allow full use of the zoom and retain focus the whole way. I have not seen published stats on the various Canon lenses to see if the wide angle zoom offers a closer minimum focus, or if using the zoom-thru wide angle adaptors alter this range also.

There's a certain amount of academic discussion involved here that needs to be tempered with the fact that DV inherently has massive depth of field, and thus it is tricky to talk about how to minimize it at the wide angle. Suffice to say that if you are inside minimum focus (i.e. the subject is within that minimum distance that the lens can focus), it will appear soft at a telephoto setting but gradually come back into focus when zooming out. This essentially is because a wide angle lens setting has more depth of field than at telephoto, and is able to appear sharp enough even when the subject is too close (the wonderful phrase "circle of confusion" enters into the discussion, but I won't push it here). I would have to say that if you are too close for the standard lens to focus, there aren't any lenses out there that will cheat the minimum focus close enough to give you the effect you are looking for, considering the inherent depth of DV.

OK, so what am I saying? That a telephoto setting has less depth of field than wide angle, so it should produce a shallower image--but also that the closer that the subject gets to the camera, again the less depth of field you have and thus shouldn't the subject be able to get two inches away (where you can't use the telephoto end of the lenses any more) and result in a really shallow focus? Yes, and that's where you can use the macro setting on the lens as was pointed out in a recent thread. It will only work at the wide angle setting, but watch as the background goes out of soft as you focus on the subject's nose or whatever!

This makes all of our heads hurt. If it makes you feel better, I can tell you that in the film world the camera assistants carry around charts or Palm programs that calculate depth of field given all the variables, so that when the DP asks for a specific amount of the depth they can look it up (as well as figure out how much depth they have to play with to maintain focus, sometimes only a few inches).

Hope this helps--somehow!

[Rob Lohman]

Thanks Charles... you are really giving me a lot of information here
and I seem to understand it better and better. Great! Thanks a
lot.

I did some testing (finally) when partially and fully zoomed and
and I got some lovely shots with a shallow DOF... beautiful!
Only problem is that this will not work in a small space (that
zooming in part)... perhaps I should hunt down my macro
button.... didn't know I had one.

If I can find some time this week I will put up some results
of my DOF test. I'll include settings used.

[John Steele]

Hi Charles, excellent info thanks. You're right the reason I wanted the wide angle lense is if for example I wanted to get a shallow DOF in a room and couldn't back away far enough to zoom in. I've also been doing some playing around and the results I've been getting are good so I'm quite pleased, but I'm going to go to my local camera store to try out the wide angle lense to see if it can give a shallow DOF when zoomed allowing me to be a bit closer to the subject.

Thanks again guys, this is a really interesting thread.

John.

[Aaron Koolen]

Ok, now I've read these posts I really need a whiskey to recover. Can someone point me to a site that will explain how all this focal length, lens size and stuff really works. I don't understand what a focal length means for one thing. 5.5mm on the 16x lens? Is something in the lens 5.5mm away from something else? Why do you need to be 7x as far away when using a 35mm lens with an EOS adapter as opposed to something like the mini35 adapter?

I really would like to get this all understood, so I know what people mean and what the difference is when they say a 28mm lens, or 72mm lens or whatever. Once I have my camera I plan on doing many short movies and so I'd like to be reasonably up on the play as far as camera geek speak goes.

EEEK!

[Jeff Donald]

Ah, this could be a long one. First lets get our millemetres straight. MM can represent the focal length of the lens or it can represent the diamater of a filter or other attachment. Lens caps can be 49mm, 55mm, and filters can be 72mm etc.
Lets talk about the focal length of a lens. The strict definition is the distance between the rear nodal point of the lens and the best axial focus of an object, when the lens is focused at infinity. Huh? Ok. A nodal point is an imaginary point in the lens design. In simple terms it is the rear point at which the light at the back of the lens gets bent slightly to come to focus at a point on the CCD (or film). Measure the distance in mm from where the light bends to the chip. If its 50mm, it is a 50mm lens. That was easy.

What type of lenses are there? Normal, Telephoto, Wide-Angle,Ultra Wide-Angle and Fisheye. A normal lens makes the image appear to have the same perspective as the original scene when viewed from the normal viewing distance. It is roughly equal to the diagonal of the CCD. A telephoto, from the same position, will produce a larger image proportionate to the focal length of the normal lens. A 100mm lens will produce an image twice as large as a 50mm lens. A wide-angle has a greater angular field than a normal lens. shooting from the same position it will include more subject area, but with smaller images. It tends toexaggerate differences between near and far. An ultra wide-angle is a wide-angle have extreme angular coverage of up to 120¡. It maintains rectilinearity with no bowing of straight line. A fisheye is an extreme wide-angle that covers up to 180¡. it has noticeable barrel distortion. Lastly a zoom lens. A zoom lens is a complex lens system with variable focal lengths that are produced by moving (zooming) the spacing of some of the elements within the lens while maintaining the same back focus. The zoom lens allows the image size to change without moving the camera.

Zoom ratios are the last part for now. The 16x is the amount the zoom changes. The 5.5mm multiplied by 16 equals 88mm. The standard XL1 16x AF lens is a 16x zoom. There, not so hard. You'll be up to geek speed in no time.

Jeff

[Charles Papert]

Nice work Jeff. I am embarassed to say that after all these years as a cameraman I wouldn't have been able to define how the numerical formula for focal length is divined, but then again it doesn't come up in practice all that often--well, never, actually!

Aaron, the reason there is a 7.2x magnification between a 35mm lens and a DV lens is the target size (35mm negative vs a much smaller DV chip). Imagine projecting on your wall a 35mm slide of a head-to-toe image of a person. Now walk up to the wall and draw a frame around the person's head. This represents the DV chip. Using an EOS lens adaptor with a 35mm lens means that you are "projecting" a large image onto a small target, the DV chip, and it will only "see" a small area of the original frame. Thus a wide-angle lens like a 28mm will become a telephoto lens when placed on the XL1. To achieve that same telephoto field of view on your still camera you would have to multiple the focal length by 7.2, which would be around a 200mm lens.

Now, still thinking about your SLR camera, the image that you see when you look through the viewfinder is has a different quality than the viewfinder of a point and shoot; it is the actual image from the lens displayed on the surface of a ground glass. It has the depth of field characteristics that the final image will have, and will demonstrate focus and flares and other optical phenomenon. The Mini 35 incorporates such a ground glass, capturing the full frame of the 35mm motion picture lens and presenting it to the XL1 to be rephotographed. It's like a high end version of one of those devices you can get to transfer slides or 8mm movies to video.

[Rob Lohman]

Charles... I did not find any macro button/switch what-so-ever.
Perhaps I am not crazy at all? Where you perhaps referring to
another lens?