Coverage of 400mm Fujinon-T

[sanking]

I just bought a 400mm Fujinon-T for use on my old Nagaoka 5X7 which does not have enough bellows draw for a regular 400mm lens. Before buying the lens I checked the lens specification chart on the home page of the Large Format Forum and it indicated that the circle of coverage would be 220mm at f/22. That would of course be more than enough for 5X7 since the exact amount needed is around 204mm-208mm.

The lens arrived this morning and immediately put it on my camera to check out the coverage. First look suggested that there would be plenty of coverage since there was light over all of the ground glass. However, when I looked at the iris of the lens through the corners of the ground glass I discovered that the lens has to be stopped down to f/64 or more before one can see the entire iris. My understanding of coverage was that in order to avoid vignetting at the corners it is necessary to close down the lens until the ellipse was gone and one can see the entire iris. Is my thinking wrong on this? If not, I am curious as to how it was determined that the coverage of this lens was 220mm when in fact it appears to be slightly less than 200mm?

Sandy

[Michael S. Briggs]

A Fuji paper brochure that I have lists 220 mm diameter at f22 as the coverage.

If you have the lens stopped down to f22 and look through the lens, directly on axis, the aperture should look circularly (or approximately so, from the finite number of blades of the aperture diaphragm). As you move your head off-axis, the aperture will look elliptical because you are viewing a circle at an angle. This is normal and is one reason that all lenses (with normal rendering of perspective) have reduced illumination off-axis. As you move your head further off-axis, eventually some part of the lens barrel will block some light rays and cut off part of the elliptical shape. The aperture won't be defined entirely by the iris blades, but also by for some portion of the shape by the barrel. It sounds like this is happening with your Fuji-T at f22 at 110 mm radius. If the area of the ellipse that is cutoff is small, probably Fuji considered it acceptable reduction of illumination in the effected corner. What fraction of the ellipse is cutoff at f22 at radius 110 mm? Avoiding this would have required larger glass elements and a much heavier lens. Because of the cutoff, you are losing some illumination to the corner beyond the simple mathematical model for an ideal lens.

[Larry H-L]

I use one on 5x7, covers just fine at infinity.

[sanking]

The aperture won't be defined entirely by the iris blades, but also by for some portion of the shape by the barrel. It sounds like this is happening with your Fuji-T at f22 at 110 mm radius. If the area of the ellipse that is cutoff is small, probably Fuji considered it acceptable reduction of illumination in the effected corner. What fraction of the ellipse is cutoff at f22 at radius 110 mm? Avoiding this would have required larger glass elements and a much heavier lens. Because of the cutoff, you are losing some illumination to the corner beyond the simple mathematical model for an ideal lens.

Michael,

Thanks for your response. At an aperture of f/22, and with the lens focused on an object about 100 yards away, approximately 50% of the ellipse is cut off by the lens barrel.

Sandy

[sanking]

I use one on 5x7, covers just fine at infinity.

Just curious, have you looked at the ellipse of the aperture with the lens focused at infinity? If so, are you getting as much cut off as I am, i.e. about 50%?

I am fairly certain that the lens can be used for 5X7, but the amount of cut off I am getting at a circle of about 104mm radius is much greater than I would have expected. I understand that light fall off on the corners is not always undesirable, but it does surprise me that it is this great at 104mm radium when the lens is said to cover a circle of 220mm!

Sandy

[Capocheny]

Hi Sandy,

If I may... I'd suggest that you take some pictures with the lens and critcally look at the end result.

After all, that's where the rubber hits the road!

Let us know what the end results are.

Cheers