Dagor as convertible, cont'd

[Sanders McNew]

I want to mount a ca.1910 Goerz Dagor Series III No. 2, 7-inch lens (Compur dial shutter) on a 5x7 Deardorff. I want to use it as a convertible lens. I know this has been discussed in earlier threads, but I have additional questions that I hope someone can address before I go to the expense of a new lensboard for the lens.

I know that, using only one element, the focal length grows 1.75 times, to just over 12 inches. This is good.

I gather that the maximum aperture shrinks from f/6.8 to f/13.5. This one is a shocker to me.

Question one: How does one compute the aperture values at less than wide-open? I see the maximum aperture is basically multiplied by two. Is that true for the other apertures? For example, does f/11 become f/22, f/16 become f/32, and so on? If not, how does one compute aperture sizes? Basically, am I losing two stops across the board?

Question two: How does shooting with one element alter depth of field? If I'm shrinking the effective aperture, am I getting more DOF in the bargain?

Question three: Does shooting with one element affect lens coverage? A 7-inch Dagor should handle 5x7 work. Will a 12-inch demi-Dagor do as well?

Question four: In portraiture, I often shoot with this Dagor wide open at f/6.8, to get some slight diffusion in the highlights. Will I see the same effect when shooting the demi-Dagor wide open at f/13.5? Are there issues presented by shooting the single element wide-open, that the complete lens does not present? (I am shooting entirely in B+W, so color issues are of no concern to me.)

Question five: Is there a "sweet spot" for the aperture when shooting with a demi-Dagor?

I am grateful for your wisdom.

Sanders McNew

www.mcnew.net

[N Dhananjay]

1) The f-stop is given by aperture/focal length. So, if your focal length increases by 1.75, you can take all the marked f-stops and multiply by 1.75 to get the f-stops with the converted lens.

2) I'm not sure what you are asking here. DOF depends upon both the f-stop as well as the focal length. Since the focal length has increased, you should expect DOF to decrease and will have to stop down more for the same amount of DOF.

3) This is best answered by testing. Especially because you may find that the edges of the circle of illumination (especially when using the converted lens) are not sharp enough to be usable. In general, the converted lens has less angular coverage than the unconverted lens. But since the focal length is longer (i.e., the smaller cone of light has a longer distance to reach the ground glass), you will find that the circle of coverage is about the same. That is, when using the lens converted, you should be able to use it on the same format without too much problem.

4) Yes, the converted Dagor actually should work nicely for portraiture. The Dagor has some amount of zonal spherical aberration. In my experience, when using the Dagor converted, this spherical aberration does provide a somewhat soft focus look at large apertures. The only downside of course is the fact that the largest aperture is still farly small (i.e., f/12.5) so it is not quite like using a true soft focus but.. again, trying it is the best way to see if you like it.

5) The Dagor cells (unlike the Protar) are not corrected for coma. If you want sharp results, expect to stop well down - performance can be quite grim short of f/45.

Cheers, DJ