Depth of field and size of lens

[Stephan Schuh]

In a description of a snorkle-lens like the revolution I´ve read that the small lenses they use offer a greater depth of field than regular size lenses. This sounds to me like complete bullshit, because DOF should only depend on the local lenght and the f-stop and not on the size of the construction of the lens. So what´s true?

 

"Vielen Dank" for your answer!

Stephan (Germany).

[Greg Gross]

Focal Length:

 

Focal length is the optical distance from the optical center of the lens to the

film plane where an image is formed when focused at infinity. Every lens has:

 

1. Angle of view(what the lens sees both vertically

and horizontally)

2. Depth of Field(distance between points nearest

and farthest from the camera that

look sharp)

For more depth of field-smaller format,smaller fstop,shorter focal length or move

the point of focus farther from camera.

 

For less depth of field-larger format,bigger fstop,longer focal length,or move the

point of focus closer to the camera.

 

I'm not sure I know what you mean about larger or smaller lens.

Greg

Edited November 5, 2004 by pd170user

[Greg Gross]

Hello again Schuh,

This snorkel lens thing you mentioned was blowing my mind,so I went

and researched it. Kenworthy/Nettmann Snorkel Camera System. They

are located in Hollywood. I don't know about you but I can't afford to rent

it. I'd love to play with it though for a weekend. Listen to this:You supply

supply the prime- 35mm,40mm,50mm,60mm. Divide by 4.1 for combined

focal length! You can add it to a film or video camera. The lens on the camera

will be 4.1 times net focal length of the combined optical system with the peri-

scope included.For crew you need Snorkel Specialist(technician),Snorkel Double

Arm Operator(usually a grip who has been trained). You can supply Zeiss primes

(jesus,you know I'm an older photographer so Zeiss really rings my bell but then

there's Cooke too) 85-180mm range. They say on their web site that you can dis-

cuss advantages and disadvantages with them. They make a water jacket for

this thing so you can shoot under water. I'm not sure if you can use a Cooke lens

though. With the double arm deal you get a scissors-like action that provides max.

maneurverability for positioning the end of the snorkel periscope precisely where

needed with movement in any direction,even circular. You have to have a skilled

person though, so the guy doing panning and tilting will not have trouble getting

shot. Can you imagine looking down through the sun roof of a car with this thing?

Action!! Greg

Edited November 5, 2004 by pd170user

[Stephan Schuh]

The description I´ve read compared lenses of the same focal lenght or angle of view. The difference was just the diameter of the lens itself. (In combination with snorkles, you often use smaller size lenses.) The question is: Can lenses with the same focal lenght, the same angle of view, on the same format, at the same f-stop and at the same distance have a different depth of field, only because their construction is different?

Thank you for your answer!

 

(I think it´s hardly possible. Could it be that a very small lens has a slighty different angle of view than a larger lens of the same focal lenght? This would mean that you could get more depth of field at the same angle of view. But this sounds a little strange to me as well.)

[Mitch Gross]

Depth of field is a mathematical relationship and has absolutely nothing to do with the physical dimensions of a given lens. But it is important to note that not all lenses perform identically, so if a given 50mm lens is not particularly sharp or not particularly contrasty then it can appear to have greater depth of field than a higher resolving or contrastier 50mm lens because the latter will see the differences in the optical characteristics earlier.

 

Another factor is a given lens' design for the bokeh, which is the shape of the cone of light as objects pass in and out of the plane of focus. To clarify, an object very much out of focus can take on certain shape characteristics depending on the lens design. Different lens designs produce different bokeh, which in turn can slightly alter the perceived depth of field as objects shift out of focus. This is one of the reasons so many DPs love the Cooke lenses, which have such a smooth and soft bokeh that objects do not appear to drop harshly out of focus but instead drift off gently. This is different from Zeiss glass, which generally snaps points in and out of focus with a quicker drop off. One is not better or worse, only different. Yet when measured on a technical test bench they can have the identical depth of field as measured using a distinct circle of confusion, but in perception they can look different.

 

I might add that these are incredibly subtle differences and even many professionals can be hard pressed to see the difference.

[Greg Gross]

Thanks Mitch,

Point well taken sir,Schuh was actually talking about the real physical dem-

ensions of the lens itself(obviously not relating to DOF). I had never heard

of a snorkel lens so I had to research that.

 

Greg

[Sam Wells]

I've worked with and shot with the Kenworthy-Nettmann.

 

The key to it is that it's not a "snorkel lens" but a System as its name implies; it's effectively an afocal converter looking down into the tilting mirror, so you're getting your image off something that's rather small; rather than have you're "prime" lens in the shooting area its up at the top of the rig, mounted to the camera.

 

There are limitations as to which wide angle primes; I shot w/ the Pentax 80-160 zoom (which becomes a 19-38mm zoom with this system).

 

DOF is whatever it would be with anything else at the chosen focal length/aperature/focal distance.

 

-Sam

[Sam Wells]

"Can you imagine looking down through the sun roof of a car with this thing?"

 

It's quite possible & I'm sure it's been done. In the "old days" with it's predecessor the Kenworthy "Type A" we bid a job for a VW Rabbit commercial which was to do just that.

Then, the agency decided the sunroof would restrict the camera travel too much (they wanted a bunch of moves across the seats, dash etc) so they decided the roof of the Rabbit should come off instead (we couldn't use a convertible because at the time the convertibles were made-in-Germany Golf's with different instrumentation etc).

 

Sounded like a good plan until someone pointed out removing the roof off a unibody car would cause it to sort of collapse !

 

I bet the grips we worked with could've shored the thing up somehow, but the Agency went to some plan B....

 

-Sam

[Greg Gross]

Thank you Mr. Wells,

Enjoyed your post very much on Kenworthy/Nettmann.

Appreciated!

 

Greg

Edited November 5, 2004 by pd170user