Rescaling shutter

[Ian Greenhalgh]

Hi folks

I've been looking for a Compur #1 to put my Pullin Pulnar 2.8/100 in and the price for one on it's own was too much for me, but I found a mint working one with a Symmar 5.6/150 in it for a real bargain price.

Now what I need to do is recalculate the aperture scale for the Pulnar.

150/5.6 = 26.79

100/2.8 = 35.71

The aperture does open up a fair bit wider than the 5.6 mark on the scale so 2.8 should be possible.

I'm just not sure how to go about redoing the scale. The 5.6 mark doesn't correspond to an easy aperture with the 100mm lens - 100/26.79 = 3.73 so with the 100mm lens, the 5.6 mark corresponds to f3.73. The f8 mark would correspond to f5.3.

I would like to do this without spending any money, so I hoped I could just put a sticker over the existing aperture scale and mark on it 2.8, 4, 5.6, 8 etc by using the existing scale as a guide, but it seems it isn't going to be that simple.

Any advice as to how to go about doing this?

[Struan Gray]

Good enough for government work: mentally subtract a stop from the marked aperture. f8 is f5.6, f22 is f16 and so on.

Better: find a light meter which can measure at the film plane. Point the 150 at a broad, even light source (such as a light box or a well-diffused lamp housing) and focus. Note reading and aperture lever position. Change glass for 100 mm lens cells, adjust aperture until you get the same reading. Note offset and proceed as above.

If you want to measure holes, the 'correct' way is to measure the hole as seen through the front cell: this is the true definition of the entrance pupil, d, which you use in the d/F formula for the f-number. You need to look into the lens from far enough away that parallax though the front element doesn't muck up your measurement.

It is also possible to project the pupil forward for easy measurement. Focus the camera on infinity and then place a small bright light source at the centre of the film plane (a mini maglight bulb or a single LED does fine). Place a piece of paper over the front of the lens and you'll get a circle/hexagon/heptagon/whatever of light on it which matches the entrance pupil. Again, measure and compare to the 150.

Personally, I'd go with plan A, and adjust accordingly after a few test shots.


PS: I love the sapphire blue coatings you get on the post war British lenses. Yours is no exception.

[Ian Greenhalgh]

Cheers Struan, that's very useful. I agree, plan A then adjust a bit. I'll be shooting 90% BW with it so a slight discrepancy is no big deal really.

Funnily enough I was just thinking how nice the blue coating was earlier as I was putting the cells into the shutter. I have a few Wray lenses and they have the blue coating too, it's a good, hard coating and survives well. I think Voigtlander used a similar coating as there had a blue colour too and is hard.

[Bob Salomon]

Cheers Struan, that's very useful. I agree, plan A then adjust a bit. I'll be shooting 90% BW with it so a slight discrepancy is no big deal really.

Funnily enough I was just thinking how nice the blue coating was earlier as I was putting the cells into the shutter. I have a few Wray lenses and they have the blue coating too, it's a good, hard coating and survives well. I think Voigtlander used a similar coating as there had a blue colour too and is hard.

The material used for the coatings depends on the type of glass they are applied to. Since they are metallic coatings most all are more scratch resistant then the glass they are applied to.

[Ian Greenhalgh]

Aah, that's good to know Bob, cheers. I know that there are quite a few 1950s lenses that used soft glass, particularly the faster ones, for instance, Biotars had soft front glasses and you will see a lot of them looking like they were cleaned with sandpaper. I always thought this meant the coatings were soft too.

[Struan Gray]

I would guess the coating is quarter-wave magnesium fluoride. There weren't too many other options at the time, at least not for a front element.

I have most often seen the lovely sapphire colour on lenses from Wray and TTH, and regard it as something of a signature for British lenses of the era (a bit like TTH's engraving - very distinctive once you're tuned in). That said, postwar coated Heliars have a similar hue so it's not just a UK thing.

The colour of a coating when looked at obliquely is quite a complicated thing to predict (or reverse engineer). The underlying glass type has an effect, but also any surface prep that was done to improve coating adhesion.

If anyone is interested, there's an old thread with some good info here:

http://www.largeformatphotography.info/forum/archive/index.php/t-73740.html

[E. von Hoegh]

I would guess the coating is quarter-wave magnesium fluoride. There weren't too many other options at the time, at least not for a front element.

I have most often seen the lovely sapphire colour on lenses from Wray and TTH, and regard it as something of a signature for British lenses of the era (a bit like TTH's engraving - very distinctive once you're tuned in). That said, postwar coated Heliars have a similar hue so it's not just a UK thing.

The colour of a coating when looked at obliquely is quite a complicated thing to predict (or reverse engineer). The underlying glass type has an effect, but also any surface prep that was done to improve coating adhesion.

If anyone is interested, there's an old thread with some good info here:

http://www.largeformatphotography.info/forum/archive/index.php/t-73740.html

Goerz New York used a similar sapphire coating, and I have a pair of Spencer binoculars made in 1943 which exhibit a similar deep sapphire blue. AFIK all are MgFl, it was the about only option for a hard coating at the time.

[Ian Greenhalgh]

Cheers guys, most informative!

[Dave Gesell]

Ian, I've started looking for a Copal #1 shutter and in my Googlings I found this:

http://lensn2shutter.com/shutters.html

If you scroll to the chart at the bottom it says that the maximum iris of a Compur #1 is 29mm. In your first post it doesn't sound like you've mounted the Pulnar in the shutter yet, so I thought I'd pass that information along.