Schneider 355/620mm Symmar Convertible

[Paul Ewins]

Pere, the rear of the front cell of the 360/5.6 is only 54mm in diameter and the front of the rear cell is smaller again at 52mm, so a #4 shutter with a 52mm throat will not cause any mechanical vignetting.

The front cell for the f6.8 version is physically different with a rear diameter of 50mm while the rear cell is unchanged apart from the location of the threads. The diameters of the Symmar-S are smaller again at 46mm front and 39mm rear in part due to a design that partly encloses the end of the cell.

[Pere Casals]

You are right, mine also has those 52mm in the front of the rear cell... no vignet...

[Dan Fromm]

Also it would be interesting (to me) to understand how "aperture" is calculated when mechanical vigneting is there, and if this is a factor to consider with the Symmar 360mm in IV and V shutters.

Papi, mechnical vignetting usually refers to vignetting by a physical barrier outside of the lens. For example, when a 60/14 Perigraphe is stuffed into the front of an unmodified Ilex #3 the shutter's rear tube vignettes severely, greatly restricts coverage.

What do you mean by mechanical vignetting?

[Pere Casals]

Papi, mechnical vignetting usually refers to vignetting by a physical barrier outside of the lens. For example, when a is stuffed into the front of an unmodified Ilex #3 the shutter's rear tube vignettes severely, greatly restricts coverage.

What do you mean by mechanical vignetting?

Dan, yes... usually it refers to obstacles that are outside of the lens: filters, a too tight compendium shade... because designers don't place much obstacles inside the lens.

But I guess that Mechanical Vigneting concept is not limited to the outside, a physical obstacle may be also inside, in a way that's not (necessarily) limiting the image circle but generating additional fall-off, so adding fall-off that's not explained by the lens formula or to the (size of elements related) optical vigneting.

This was my understanding, but I don't know for sure if I'm right...

For example, when a 60/14 Perigraphe is stuffed into the front of an unmodified Ilex #3 the shutter's rear tube vignettes severely, greatly restricts coverage.

In that case, if the Perigraphe had an additional cell in the rear of the (unmodified Ilex #3) shutter, then we still would have Mechanical Vigneting, produced inside the lens...

So... a too small shutter may provocate internal Mechanical Vigneting, isn't it ? ...because what's optical vigneting this is from too small elements...

The question is if the vignet from a too small shutter (in a plasmat, for example) is Mechanical Vigneting or if it isn't... Perhaps it's possible to generate fall-off in this way while not limiting the circle, or the effective transmission for the image center, mostly when wide open...

[Dan Fromm]

Papi, I'm not sure that what you're thinking of -- diaphragm that doesn't open wide enough to allow a lens to reach full aperture -- has a name. At least in English, it is not called mechanical vignetting.

[Pere Casals]

Dan, diafragm is (usually) placed in a node point, this is a point in which we can restrict the light amount without vignetting, this is without mechanically vignetting, so diafragm is an special case in what the obstacle does not produce a vignet.

If the diafragm was not in the right place then we would have a mechanical vignet, IIRC...