What length Petzval for 5x7 portraits?

[Ian Greenhalgh]

Hi folks

I've got a hankering for a Petzval to shoot head and shoulder portraits on 5x7, but have no idea how long the focal length would need to be. I read that only about 30 degrees of the coverage is sharp, but how big is the overall coverage?

You see, I think if the eyes are sharp, then the rest can be as soft and funky as it wants to be.

So if we assume the width of the eyes will not be more than 3 inches on the negative, how do I work out what length Petzval will be suitable? Vignetting of corners is not a problem and I don't care if the outer area is really swirly.

If my maths is correct, to cover a 3 inch circle with a 30 degrees lens, you would need a focal length of 5.6 inches. However, what would the diameter of the illuminated circle be for a 5.5" to 6" Petzval? Would it be big enough to illuminate 5x7?

[goamules]

No, a 5 or 6 inch will not cover 5x7 unless it's really slow. Like an f10 projection one. The camera Petzvals are usually F3.7 to F5. The coverage increases as the speed gets slower. But most people want a fast one. I'd look for about an 8 or 10 inch F3.8.

[Ian Greenhalgh]

Cheers Garrett. 8-10 inches sounds more logical.

[adelorenzo]

One guideline I have heard for portraits is to add the length and width of the format to get the desirable focal length. So that would be 12 inches for 5x7.

[E. von Hoegh]

One guideline I have heard for portraits is to add the length and width of the format to get the desirable focal length. So that would be 12 inches for 5x7.

That's what I was thinking. A good Petzval has a really nice smooth look when you get away from those dreadful nauseating swirlies.

[Jimi]

Why does the coverage increase with a slower lens?

[jnantz]

for head and shoulders portraits i have always used a "normal" lens for the next format up

for a 4x5 it would be a 10"
for 5x7 13.-14"
8x10 a 20"

its like adding the sides but adding a little bit more so you aren't in the face of your subject.

[goamules]

A 12" would be a good lens for 5x7 too. But we didn't discuss lensboard sizes. An F3.8 13" Petzval is as big around as a coffee can. Will your lensboard accept that flange, or your camera support several pounds of weight? I've never been able to get a fast Petzval on my 5x7s longer than about 8-10 inches.
Here is a 13" Vitax held up to a Eastman 2D 5x7 for comparison.

Jimi, As a general rule, slower lenses have wider coverage. A 6" F3.7 Petzval may cover 4x5, but a 6" F18 Protar may cover 11x14! Same with Petzvals, the F5 and F6 ones cover more of the same focal length F3 ones. I'm not going to try to explain why.

[Mark Sawyer]

I've got a hankering for a Petzval to shoot head and shoulder portraits on 5x7...

...and I don't care if the outer area is really swirly.

These are two of your variables. With the bellows being extended to focus closer, the coverage will be bigger. And more than any other lens I can think of, the Petzval has a huge difference between "illumination" and "coverage".

If you get a longer Petzval, you'll have a sharp image throughout the plate, and the result won't be noticeably different from a "normal" modern lens. Shorter would show the Petzval signature, but where that signature, (swirling, field curvature, loss of resolution), sets in will vary considerably with different Petzvals, depending on speed and design corrections.

Choice of focal length with Petzvals is a complex matter. In their 1916 catalog, Wollensak notes about their Petzval-design Vitax lens: "We do not list (plate) sizes with the Vitax, as the different numbers are used on different plates, from 5x7 to 11x14, according to the class of work, and selection depends upon this and the size of the studio generally must govern focal length."

My recommendation would be to get two or three in the 6-inch to 10-inch range. But honestly, with Petzvals you could get a half-dozen 8-inch Petzvals from different makers, and each would have a different coverage and signature.

Why does the coverage increase with a slower lens?

There are a couple of interpretations of that question. If you're talking "useful coverage" in the traditional sense, the outside area may be softer than is considered useful, and this will sharpen up at smaller apertures.

If you're talking "full illumination" regardless of aberrations, then you're talking about mechanical vignetting, or where the lens barrel or hood eclipses the light path through the lens. As you move off-axis, the vignetting starts at the periphery of the full aperture and moves inward, so a small aperture in the middle will start vignetting later than the edges. A wide aperture actually allow a small amount of illumination farther out, but it's to dim to be useful exposure out there. It just makes for a softer transition to no exposure.

I'll add that on some Petzvals, the hood can cause vignetting top occur sooner, and it can be removed to get more of the outside "signature" area.

[brandon13]

For petzvals I personally prefer 8-9 inch for 4x5 12-13 inch for 5x7, and 16 inch for 8x10. I also think the swirl is a bit over rated.