FAQ? using large format lens on smaller format

[andrew_moore1]

I'm a little confused about lenses and their circles of coverage. I plan to use a 6x6 roll film back on a view camera. Since I want to use movements I need a lens that covers slightly more than the 80mm (or so) diagonal of the film. However, if a lens covers more than the diagonal, won't part of the image be cropped off, effectively making my standard lens more like a telephoto? Or are the differences so small relative to the film area that they aren't worth worrying about?

In order to simplify my lens selection, are there any "rules of thumb" governing how much image circle coverage beyond the film area I'll need in order to allow movements? If the camera allows, for example,

a 50mm shift do I simply look for a lens that covers 50mm beyond the film area?

[pat_krentz]

You can use any size lens you wish if you are willing to move your

tripod so that your subject fills the viewing screen in the manner

you desire. Pat

[michael_mahoney1]

Andrew - yes, the same lens will produce different angles of view

with different film formats, so you're correct in that on 6X6mm

you'll see a "longer" view than with 4X5in. Or more correctly your

film is capturing a smaller middle part of the image.

As to movements and the lens coverage required, it depends on the

amount of movements you plan on using, and which ones. The circle of

good definition is expressed in the diameter, and you must compare

this circle with a square or rectangular film shape.

Try what I did - get an old exposed slide or neg, and place it in the

middle of a circle drawn to the diameter of the circle of good

definition of the lens you're considering - then move the film around

to simulate camera movements. Because of the linear shape of the film

and the circular shape of the lens image, you'll find that you'll

need more coverage than you think.

[paul_owen]

Andrew, Many of the major manufacturers produce info on their web

sites relating to image circle and suitable formats,most also list

amount of movement possible, I find them very useful!

As a matter of interest, why have you decided on 6x6 as a format?

Regards Paul

[cesar_barreto]

Andrew,

You should be careful about using such small format, for bellows and

focusing considerations. Normal and WA lenses for this format will

leave you with pretty short movement possibilities, unless deep

boards are used. And not every lens feels confortable on some small

and recessed boards. So the matter of angle covering shouldn't be

your greatest worry. Good luck.

 

<p>

 

Cesar B.

[pete_andrews]

Andrew. The image circle of a lens is exactly that, a circle. You

can't just say that because a lens has a 100mm circle that your 6x6

frame will allow you 40mm of movement. You have to imagine the 6x6

square (closer to 56 x 56 mm actually), centred inside a circle. This

will allow only 10 mm of diagonal movement either way, or about 13.5mm

of vertical or lateral movement.<br>You can calculate the amount of

movement by using Pythagoras' theorem of the square on the

hypotenuse...etc.<br>Take the <i>radius</i> of the image circle as the

hypotenuse, square it, subtract the square of <i>half</i> the height

of the frame (28mm in this case), and take the square root of the

remainder. This gives you the distance from the centre of the image

circle where the corner of the frame just hits the edge of the circle.

Take away half the frame <i>width</i> (again 28mm) from this distance,

and you have the amount of movement available.<br>A square frame makes

all this calculation a bit easier, but the same method can be used

with any frame dimensions.<p>A large image circle doesn't necessarily

mean that you get a 'telephoto' effect. A lens can be designed to have

a large image circle while still having a short focal length.<br>It's

only the focal length which determines the image size, and not the

diameter of the image circle.