90mm f8 Super-Angulon as a convertible lens..?

[vickersdc]

I've just fitted my Schneider-Kreuznach Super-Angulon 90/1:8 lens to my quarter-plate camera - the 90mm focal length is *just* usable on it, but if I remove the rear lens element from the Compur 0 shutter then I can increase the focal length. I've not actually tried this yet, and I know that I will sacrifice some sharpness doing this, but my question relates to aperture / exposure.

If I do this what do I need to know about this will affect the f/stop at the new focal length? Presumably once I measure the focal length I can work out the new f/stop based on what it was as a 90mm? Is there anything else I need to know about the lens in this manner?

PS: I'm aiming to use this camera and lens for alpine photography.

 

[ic-racer]

You can try it 4 ways and see. That is front element only, front element on the back only, etc. Probably won't be very sharp, but not every one wants the sharpest lens.

 

[bdial]

If you measure the focal length of the lens in the new configuration, you can calculate what the new f/stops are for each marked setting.
It won't be precise unless you can accurately determine the lens's nodal point, but casual measurements will probably be close enough.

 

[Dan Fromm]

I just tried the experiment with my paperweight of a 90/8 SA. Short answer, as expected the cells pass light and form images. The rear cell has a shorter focal length. Lousy images wide open. I can't estimate the focal lengths very well, a poor estimate of the front's FL is ~ 150 mm. I won't stand by the estimate.

OP, measure your lens' cells' focal lengths. Focus on a ruler, adjust extension and camera or subject's position so that the image on the GG is life size. When you've done that, film plane to subject distance will be 4 * FL +internodal distance. Assume, possibly incorrectly, that a single cell's internodal distance is small. Both cells' entrance pupils' diameter are ~ 10 mm. And now you can divide FL by entrance pupil's diameter to get the cells' maximum apertures.

 

[ic-racer]

I use TTL exposure measurements (I use a Horseman meter but there are a number of meters that had been made) to answer many exposure questions like this. Funny how TTL exposure measurement is so popular with rollfilm users but rare with LF users.

 

[Dan Fromm]

ic, there aren't many TTL meters for LF. I can think of a couple of spot meters and the Horseman averaging meter, there are certainly more, all obscure. I don't know how obsolete and unsupported the spot meters are -- I wouldn't be surprised by Sinar still offering one -- but Horseman meters have no support at all.

 

[vickersdc]

I just tried the experiment with my paperweight of a 90/8 SA. Short answer, as expected the cells pass light and form images. The rear cell has a shorter focal length. Lousy images wide open. I can't estimate the focal lengths very well, a poor estimate of the front's FL is ~ 150 mm. I won't stand by the estimate.

OP, measure your lens' cells' focal lengths. Focus on a ruler, adjust extension and camera or subject's position so that the image on the GG is life size. When you've done that, film plane to subject distance will be 4 * FL +internodal distance. Assume, possibly incorrectly, that a single cell's internodal distance is small. Both cells' entrance pupils' diameter are ~ 10 mm. And now you can divide FL by entrance pupil's diameter to get the cells' maximum apertures.

Funnily enough I'd guessed on a 150mm focal length (based on nothing but the fact it was likely to be less than 180mm) - which left an aperture of f8 at 90mm to be roughly f13 with the rear cell removed... but I'll do some measuring at the weekend. Thanks for the other information too

 

[vickersdc]

Just to update on my findings - I removed the rear cell and focussed the quarter-plate camera on a house roof about 200m away... at this distance the 90/8 SA lens focussed approximately 220mm away (much further than I thought to be honest).

Based on those measurements f22 becomes f53, f32 becomes f78 and f64 becomes f156. To achieve f128 with the rear cell removed will mean I need to be setting the aperture to about f60(ish) - so somewhere between f45 and f64

With the rear cell removed and the aperture setting set to "f11" the image is focussed in the centre but that's about all... by the time we get to the f45 setting it seems to look okay on the ground glass. I'm also thinking I'm loosing about 6 stops over the lens in it's 90mm configuration.

 

[Dan Fromm]

Wrong way to estimate FL. When a lens is focused at infinity, its rear nodal point is one (1) focal length from the film plane. You don't know where the SA rear cell's rear nodal point is. There's no reason why it should be near the cell. Convertible lenses' rear cells are notorious for needing much more extension than their focal length to focus to infinity. There's a strong hint.

Do what I told you to do, take advantage of the fact that at 1:1 the film plane-to-subject distance is ~ 4 f.

Wrong way to estimate relative aperture. You know the diameter of the cell's exit pupil, you don't know its focal length. You can calculate the cell's f/number wide open only after you know its focal length. Until then you're guessing, and badly.

 

[vickersdc]

Wrong way to estimate FL. When a lens is focused at infinity, its rear nodal point is one (1) focal length from the film plane. You don't know where the SA rear cell's rear nodal point is. There's no reason why it should be near the cell. Convertible lenses' rear cells are notorious for needing much more extension than their focal length to focus to infinity. There's a strong hint.

Do what I told you to do, take advantage of the fact that at 1:1 the film plane-to-subject distance is ~ 4 f.

Wrong way to estimate relative aperture. You know the diameter of the cell's exit pupil, you don't know its focal length. You can calculate the cell's f/number wide open only after you know its focal length. Until then you're guessing, and badly.

I tried as you suggested but didn't have enough bellows extension, so instead I projected an LED torch beam through the camera lens and projected it on to a wall as per the photo below. That made it easier to measure too - so although these are ballpark figures, it would seem to achieve 1:1 (or pretty close to that) the film plane to subject distance was 82cm. That divided by 4 would give a focal length of ~200mm...

 

[vickersdc]

I then went one better and projected a ruler up on to the wall, actually I ended up using a piece of white card that had the same intervals marked off so that I could be fairly certain when it was 1:1... 82cm again. Not really a million miles out on my first estimate of a focal length of 220mm at an approx distance of 200m.

Using the new figures for focal length (820mm / 4) puts the f/128 - with rear cell removed - somewhere between the f/45 and f/64 markings as previously guesstimated.

 

[E. von Hoegh]

Is there a particular reason you want the Super Angulon? There's the plain f:6.8 Angulon, very small, and that might just work better as a converted lens due to it's "reverse" Dagor design. In fact Schneider once advertised them as triple convertible - http://www.cameraeccentric.com/html/info/schneider_3.html - see page 9 of the catalog, this is mid 1930s and I do not know if this will apply exactly to later versions of the lens. They're not expensive.

 

[vickersdc]

Is there a particular reason you want the Super Angulon?

For no other reason than I have one sitting on the shelf not being used!

 

[E. von Hoegh]

For no other reason than I have one sitting on the shelf not being used!

That's a good reason!
Based on a quick check, my ca. 1959 90mm Angulon behaves like the one in the catalog. You'd need to stop it down to F: 32 or 45 (single element) but it could be useable, the center is pretty sharp even at larger apertures.

 

[vickersdc]

Yes, the centre was pretty sharp even wide open but a centimetre or two away from that and it wasn't so pretty... stopping down to the marked f/32 - f/45 considerably improved things and I'm hoping that I'll be able to use this lens and camera for some alpine photography in the next couple of months.