Sinar DB Question

[William Whitaker]

I just bought a lens in a Sinar DB mount. It's an f/4.5 lens. When the aperture control lever is fully open (maximum aperture) I noticed that the aperture blades were still visible in the lens and the clear aperture was the shape of a pentagon, not a circle as one would expect with the lens wide open. Then I saw that there is a small red anodized screw with a knurled handle. It's marked "5.6". Removing it allows the aperture control lever to more fully open, presumably to f/4.5.

Can anyone tell me why the aperture limit screw is there in the first place? Could it be because the Sinar shutter is calibrated to f/5.6?

I doubt that I would ever be shooting this lens wide open, but it would still be nice to have the aperture that I paid for available for composing.

[8x10 user]

There are two versions of the Sinar Auto Aperture shutter. The later one opens to F/4.5.

[William Whitaker]

The question is what is the purpose of the aperture limit screw?

[QCC]

The F5.6 version of the auto aperture shutter is far more common than the F4 version. The limit screw is there to insure proper coupling and function with the F5.6 version of the shutter. If you have the F4 version of the shutter remove the screw.

[8x10 user]

Well if it is set to F5.6 then its important that it is actually at 5.6... If not then the DOF system would not be accurate. Also it would cause issues if you want to spot meter wide open and then calculate your exposure for when you stopped it down.

The 5.6 version must predate the F/4.5 Grandagon's N/ Apo Grandagon's.

My recommendation is to sell it and look for the F4.5 version.

[William Whitaker]

My recommendation is to sell it and look for the F4.5 version.

Sell what?

[8x10 user]

If you have the F5.6 version of the auto aperture shutter and you want to use F4.5 lenses then I would sell the auto aperture shutter and look for the F4.5 version. The difference in cost is not likely to be that much. The F4.5 version is for the most part only used for 4x5, so for many Sinar users (5x7, 8x10) then F5.6 version is just as good.

[William Whitaker]

If you have the F5.6 version of the auto aperture shutter and you want to use F4.5 lenses then I would sell the auto aperture shutter and look for the F4.5 version. The difference in cost is not likely to be that much.

Not from what I've seen. Easier to return the lens and find one with a shutter.

[William Whitaker]

But I think it'll work fine.