Is there any reason to think a lens designed to cover a 6x4.5
area would be unable to also cover a 6x6 area?

My thinking says that the common lenses produce a circular
image. If the resulting image is at least 6cm wide, it must
also be 6cm high. the camera may be masking it but the lens
still produces it.

Having never actually tried a lens from a 645 system I can't
say for sure. Am I mistaken?

John Thurston
Juneau, Alaska

I'm guessing that the corners are what you'd need to be concerned with.  The'yre obviously a a little further away from the center of a 6x6 area than a 6x4.5 area.
 
The math (if I'm right) shows that the corners of a 6x6 area are ~4.24cm from the center, while the corners of the 6x4.5 area are only 3.75cm from the center.  So, the circular images for a 645 lens don't have to be as big.
 
Ted

 

---

To: MF3D-group@yahoogroups.com
From: photo3d-list@thurstons.us
Date: Tue, 24 Jul 2007 20:30:41 -0600
Subject: [MF3D-group] Coverage of 645 lenses

Edwin Baskin wrote:

> I'm guessing that the corners are what you'd need to be concerned with.

I'm still dense enough to fail to understand. Why am i concerned
with the corners? The format is 6x4.5 not 4.5x4.5.

The lens must be able to produce an image 6cm wide or it couldn't make
the "6" part of 6x4.5. If it can make an image 6cm wide, why can it
not be expected to produce an image 6cm high?

John Thurston
Juneau, Alaska

John Thurston wrote:

> Edwin Baskin wrote:
> > I'm guessing that the corners are what you'd need to be concerned with.
>
> I'm still dense enough to fail to understand. Why am i concerned
> with the corners? The format is 6x4.5 not 4.5x4.5.
>
> The lens must be able to produce an image 6cm wide or it couldn't make
> the "6" part of 6x4.5. If it can make an image 6cm wide, why can it
> not be expected to produce an image 6cm high?

Lenses don't cover squares or rectangles. They cover circles.

From corner to corner a 6x6 frame (56x56) is about 80mm. A 6x4.5
frame is 70mm.

It is possible that someone would design a lens to only cover a 70mm
circle for a 6x4.5 camera. I no longer have a 6x4.5 camera so I can
check that out for you.

One way to test would be to mount a 6x4.5 lens on a large format
camera (at least 4x5) and examine the ground glass image, or take a
photo and examine the film. Note that the illuminated circle (largest
extent of light falling on the focal plane) is larger than the circle
of coverage. The circle of coverage is the area concidered sharp
enough to use for photographs. The image outside this area is
probably not sharp enough, depending on your purpose.

--
Brian Reynolds | "But in the new approach, as you know,
reynolds@panix.com | the important thing is to understand
http://www.panix.com/~reynolds/ | what you're doing rather than to get
NAR# 54438 | the right answer." -- Tom Lehrer

Brian's correct.
 
In other words, it would take a circle ~85mm in diameter to cover a 6x6 square whereas it would only take a circle 75mm in diameter to cover a 6x4.5 rectangle.
 
Ted

 

---

To: MF3D-group@yahoogroups.com
From: mf3d@reynolds.users.panix.com
Date: Wed, 25 Jul 2007 09:24:19 -0400
Subject: Re: [MF3D-group] Coverage of 645 lenses

> Brian's correct.
>
> In other words, it would take a circle ~85mm in diameter to cover a 6x6 square whereas it would only take a circle 75mm in diameter to cover a 6x4.5 rectangle.
>
>

You must test a lens to know the diam of its
projected image circle. They vary tremendously. The Seitz roundshot
needs ~ 65mm min. image circle diameter to cover its format with a small
amount of movement....and the most popular lenses used to cover this
image circle are the Nikon and Leica 35mm film lenses....