Tessar lens coverage

[Jennifer]

Hi,
I've seen mentioned that tessar type lens are not always sharp at the edge, unless stoped down. I realize this could be a many answer question, but if you were to crop out the softer edge of a 4x5 negative, what size would you might end up with ?. 3 1/4 x 4 1/4....3x4...etc.

Thanks,
Jennifer

 

[phfitz]

Hi Jennifer,

All the tessar types made would fill a phone book, so you would have to shoot and see. 127/4.7 Ektar will just cover to the corners, 135/4.5 Tessar will cover to the corners. Anything longer will cover with movements. f/6.3 have more coverage, f/3.5 have less.

Hope it's a help.

 

[MichaelBriggs]

It is true that the Tessar design has less coverage than some other designs. The current standard "normal" design for LF lenses is the Plastmat, which offers more coverage. But the Tessar design can be sufficient sharp over a sufficient large area, depending.....

There are a lot of factors that effect whether a Tessar lens will cover -- the first is the focal length compared to the format. Lenses project a cone of quality image. A given lens design will tend to have the same angle of coverage -- the angle of the cone -- independent of focal length. So as you increase the focal length, the apex of the cone moves farther from the film and the circle that is the intersection of the cone and the image plane grows larger. So even though the Tessar has "only" medium coverage, a Tessar lens that has a long focal length will over plenty of coverage -- "long" means longer than the format diagonal.

So a 300 mm Tessar will offer more than enough coverage for 4x5. A 210 mm will have plenty of coverage. A 127 mm will typically not quite cover 4x5.

It also depends on how critical you are. For contract prints you might rate the coverage as larger than for x10 enlargements. Some photographers are more accepting than others have a small reduction in sharpness in the corners.

Manufacturers typically rate Tessar designs as covering 55 to 65 degrees -- this is the full angle of the cone. Slow designs tend to have larger coverage. The diameter of the circle of coverage is then from 1.05 x focal length to 1.25 x focal length.