The theory of foucs and zoom

[JIMUSA]

Hey,

My name is Jimmy I'm a new member and just register today. I'm an Art school student and taking photography class and video production class at the same time.

Is the application theory of lens identical for digital camera like Nikon Digital SLR Camera D60 and digital video camera like Panasonic AG-DVC30 ?

If it is. Can anybody help me to find the answer for the following questions:


1: I have a Nikon D60 SLR Digital Camera and know by pressing the shutter button
halfway to lock focus, reframe the picture and press the shutter button all the way
down to take a picture.

Since the Panasonic AG-DVC30 is a zoom lens miniDV video camera, In Manual
Focus mode, when I want to get subject in focus, I zoom in all the way tight on the
subject and turn the focus ring to get subject in focus, and then I zoom out all the way
back to the distance I want to fit in the frame composition, Right ?

Theoretically, when I use zoom barrel to zoom in, the focal length of lens increased,
the size of subject increased, the view of angle and DOF decreased. The focus ring
is only to adjust the lens distance to get the enough light in order to get the contrast
and sharpness of that focus point or range ; therefore, the image will get sharp and in
focus. Right ?

Mechanically, When you use the focus ring, it won't affect the focal length of lens and
DOF and they will not get change, Right ? Is it because the zoom barrel and focus
ring are two separate stuff and different barrel. zoom barrel is to adjust the focus
length of lens either tight or wide. The focus ring is to adjust the different lens to get
enough light and get contrast and sharp for that focus range or point, Right ?

Any recommended websites or paper related to this issue ?


2: Since AG-DVC 30 is 16x optical zoom lens, What is the focus length of AG-DVC 30,
4.1mm to 65.6mm F1.6, 35mm= 39.5 to 632mm, meaning the wide angle is 39.5 mm
and telephoto is 632mm ?

Thanks !

[daft_biker]

Hello Jimmy

When you change focal length on a zoom lens the focus distance shouldn't change. If the focus does change when you zoom it's not a zoom lens but a varifocal lens.

When you change the focus distance focal length tends to change slightly (the focal length marked on a lens tends to be measured at infinity focus). Focussing nearer to the camera decreases DoF.

[ravncat]

Originally Posted by JIMUSA

1: Since the Panasonic AG-DVC30 is a zoom lens miniDV video camera, In Manual
Focus mode, when I want to get subject in focus, I zoom in all the way tight on the
subject and turn the focus ring to get subject in focus, and then I zoom out all the way
back to the distance I want to fit in the frame composition, Right ?

I don't know specifically about the Panasonic Ag-dvc... however, Nikon zoom lenses vary in design, In general Nikon lenses are not par-focal (see link). So you want to zoom to your desired framing and then focus. Focus will not be maintained as you zoom in most Nikon zoom lens designs.

Parfocal lens - Wikipedia, the free encyclopedia

Originally Posted by JIMUSA

Mechanically, When you use the focus ring, it won't affect the focal length of lens and
DOF and they will not get change, Right ?

The focus ring on Nikon lenses does affect focal length of the lens. This is called focus breathing. The Focal length of the nikon lenses is specified at a focus of infinity. In general there is a shortening of focal length as you focus closer. This is usually a small percent, but is sometimes quite noticable - for example Nikon's 18-200 lens at 200mm is more like 120mm or so at very close focus...

Originally Posted by JIMUSA

zoom barrel is to adjust the focus
length of lens either tight or wide. The focus ring is to adjust the different lens to get
enough light and get contrast and sharp for that focus range or point, Right ?

Zoom barrel adjusts the focal length - ie the angle of view.
Focus ring adjusts the point where your plane of focus is.

Originally Posted by JIMUSA

2: Since AG-DVC 30 is 16x optical zoom lens, What is the focus length of AG-DVC 30,
4.1mm to 65.6mm F1.6, 35mm= 39.5 to 632mm, meaning the wide angle is 39.5 mm
and telephoto is 632mm ?

Yes. This is right. Although - 39.5mm is not a very wide angle, it is closer to normal length. in 35mm terms, 24 - 40 is wide angle and less than 24 is ultra wide angle. The definitions of these points change a little bit depending on person.

[drr0b]

Do you know if the canon 15-85 IS USM is parfocal? It is my first lens that doesn't change length when focusing

[ravncat]

I don't know, I don't use canon, maybe inkista will come around, i did a quick google and found alot of people don't know, but think it behaves like one.

[daft_biker]

Originally Posted by drr0b

Do you know if the canon 15-85 IS USM is parfocal? It is my first lens that doesn't change length when focusing

It should be - AFAIK Canon zooms are proper zooms. Try it? The only varifocal I've used was a Sigma.

The doesn't change length when you focus is internal focus. Normally you can expect internal focus lenses to get shorter as you focus closer but external focus get longer. However internal or external focus doesn't tell you what type of zoom it is....you can get varifocals with internal focus too.