using the front element of a g-claron

[dave bulmer]

Hi all......I'm just in the process of finishing off a home-made lensboard for my new 240mm g-claron so I can use it with my V8 Deardorff. Whilst playing around, I fitted the lens with the front element only and the image looked okay on the ground glass somewhere just less than 480mm is my estimate. I don't remember reading anywhere that others have done this, so my questions are......am I missing something obvious, and I have an inkling that I've read that exposure compensation is required. Why? and how much? In easy to understand terms obviously. Thanks as always, Dave.

[Thilo Schmid]

Dave,

lens-speed or f-stop is by definition the ratio of the entrance pupil to the focal length. If you increase the focal length and do not increase the opening correspondingly, you will have less effective speed. Or in other words: if the amount of incoming light stays the same and you increase the area to illuminate (by increasing bellows and using larger film format), it naturally gets darker than before (BTW: you’ll have to apply bellows compensation below infinity for the same reason). I cannot tell you what focal length either the front or back element only has, but they are most likely different. A few tests should make clear what compensation you need to apply for which single element (and which one yields better results).

[Brian Ellis]

The G Claron can be used as a convertible lens but I doubt that you'll get particularly good images that way. It also will be difficult to figure out the exposure until you figure out what focal length it is with only one element (since the G Claron is a symetrical lens design it may very well be that with one element only a 240 becomes a 480, I'll let the lens experts comment on that). Frankly to me it wouldn't be worth the trouble. Even with lenses designed as convertibles the image isn't all that good when used in single element configuration and I'd guess it would be even worse with the G Claron because it wasn't intended to be used that way. But you've got the lens, there's no harm in trying.

[Ernest Purdum]

Results would be a little better if you used the rear cell rather than the front, but either way it's s very slow lens.



Although you don't have enough bellows to find out the exact focal length by measuring the difference in extension between infinity and 1:1 magnification, if you focus at infinity, measure the lensboard to grounglass distance and divide it by the diaphragm opening, you'll not only be in the ballpark but close to the pitcher's mound.



I like the analogy of a paint nozzle. If you hold the nozzle further from the wall, you have to hold it longer to deposit the same thickness of paint, though on a larger area of the wall. It works the same way with light.

[Tom Perkins]

I tried this with my 305 G-Claron about a year ago. The exposure compensation is exactly the same as it would be with the extension of the bellows in a close up. For the exposure I made it was at a distance and I just doubled it. Then I had to adjust for reciprocity. It gave the bushes in the foreground time to move around a little, but that can happen anyway. The problem is that the negative just isn't very sharp, even for contact printing. It is sharpest in the middle and falls off. I do not think, and have read various comments on this forum that seem to suggest, that the single element of the G-Claron is corrected very well for aberration. In fact, the G-Claron itself is not designed for general photography, but it works good for that purpose when stopped down. The use of a single element just compounds all of the problems. In spite of this I would do it again in a heartbeat, and will try mounting a yellow filter to correct some of the aberration next time around.

[dave bulmer]

Thanks for all those views. I intend using it for landscape and contact prints only, so I'll give it a go and see what happens. I like the paint spray analogy though, makes perfect sense to me now.

Dave