image circles

[SimplyMo]

((i wasn't sure if this is considered basic or not..here goes--))

i read this following paragraph in a short article about perspective control lenses and have one question. how is the IMAGE CIRCLE formed?
from the rear lens opening? and if the image circle casted is larger, would that be because the rear lens opening is larger?
i know this is probably not of great importance.. im just curiously dissecting everything i read..:nerd:


"Perspective control and tilt-shift lenses cast larger image circles than the actual image area and can be shifted back and forth in one plane in relation to the camera. This lets you compensate for the converging vertical problem - you shift the lens back and forth, moving the position of the image area within the image circle."

 

[reg]

It's larger than the sensor/film in use.

large format lens, which can be used to make a ghetto tilt/shift:

Notice the large circle on the back of the lens.

A Nikon 50mm lens, for a 35mm film/digital camera:

Notice a rather small back end.

 

[davebmck]

Read the last post in this thread: http://www.thephotoforum.com/forum/showthread.php?t=131354

 

[Garbz]

Yeah that's basically right, although the physics of it are far more complicated than larger opening = larger image circle.

large format lens, which can be used to make a ghetto tilt/shift:.

Uah dear god. If I was spending my lotto winnings on a large format lens the last thing I would want is for it to look ghetto

 

[reg]

But OP mentioned tilt/shift, and while most people use medium format lenses for ghetto T/Ses, I just figured that I'd try to show a more... extreme difference.

Although I do admit to knowing little about what makes an image circle.

 

[SimplyMo]

ohh all right, i figured the physics would be just too deep than to simply say larger opening = larger image circle...

thanks for the lens photo-comaprisons reg. =)

would a PC lens that is made for smaller format cameras also have a larger end to connect to the camera? (compared to just a normal lens).... i guess that's where the deeper physics of it plays a role...

 

[Helen B]

Wide angle PC lenses for small format SLRs have the size and location of their rear elements constrained by the mirror and the lens throat (the hole in the body). That means that the rear elements can't be too close to the film/sensor, and they can't be too big.

For that reason, the lenses are usually of a design that is known as retrofocus or inverted telephoto. In simple terms that means that the focal length of the lens is shorter than the distance from the rear of the rear element (known as the rear vertex) to the film/sensor.

One of the features of retrofocus lenses is usually that the rear elements are small in comparison to the front elements. The lenses that are typically used for large format photography do not have to be retrofocus, because the equivalent of the throat is huge, and there is no mirror between the lens and the film/sensor. This means that the design is frequently more balanced in terms of bulk - the design is more of an hourglass shape rather than a funnel. The picture of the Schneider Super Symmar XL that reg has posted shows this hourglass shape.

This all means that the angle of view is not the only factor that determines how large the rear element must be.

Aperture comes into it as well. The usable image circle of a lens is usually greater at small apertures (large f-numbers) than it is at large apertures. This is usually because the front and rear elements may not be large enough to pass all the oblique rays that could pass through the aperture when it is wide open.

I'm not at home right now, so I can't show you a picture of the rear element of the Schneider PC Super Angulon that is available for 35 mm SLRs. It is quite small.

Best,
Helen