Derogy Fab Eur lens question...

[seanwang]

I got a DEROGY lens, I'm sure it is a RR lens, but the the words on the barrel is "Eur. No.x" NOT "Apl. No.x", and also the aperture scale number on barrel is F6 NOT Apl's F8.
So, this is Derogy's RR portrait lens? Just like Voigtlander's Euryscope Serie IV?
Did any one have this "Eur F6" serie information about focus length? Thanks.:)

[jaytral]

Yes Derogy Euryscope are also known as Aplanetique Extra Rapide.
Same optical design than Aplananetique but faster

174526

[seanwang]

Yes Derogy Euryscope are also known as Aplanetique Extra Rapide.
Same optical design than Aplananetique but faster

174526

Thank you jaytral, that's very helpful! I have No.6, but I estmate the focus length more than 20 inch, I wil check it again.

[jaytral]

You're welcome.
According this data sheet, "foyer principal en centimetres" is measured from iris (or waterhouse slot).
And 46cm is nearby 18in

[seanwang]

You're welcome.
According this data sheet, "foyer principal en centimetres" is measured from iris (or waterhouse slot).
And 46cm is nearby 18in

I aim the sun, and measure the distance between the iris and the wall, focus lenth is 530mm, which is about 21in. Is there any chance of print mistake in Derogy's document?

[Steven Tribe]

Not really! I have a different source which has identical figures. Have you checked the lens diameter is indeed 95mm? Lens cells from the F8 series might fit the F6 barrel.

46/6 = just under 8cm max. aperture.

[seanwang]

Not really! I have a different source which has identical figures. Have you checked the lens diameter is indeed 95mm? Lens cells from the F8 series might fit the F6 barrel.

46/6 = just under 8cm max. aperture.

Hi, Steven

I double check the diameter of this lens, the glass cell dia. is 93-95mm, the iris wide open dia. is 82mm. So, the focus length is 82mm*F6/0.93=529mm(0.93 means, The diameter of the aperture in actual use, is smaller than the theoretical value.), just matched my measurement.
So, did the Derogy's documents mistake?

[seanwang]

The focus length scale. The focus point of the sun to the iris is about 520mm174906

[Steven Tribe]

Perhaps the focal lengths in the Derogy catalogue are "back" focal lengths - that is, measured from the rear lens / flange?
This was very common for UK lenses before about 1870.

Derogy's lens diameters are real glass diameters, rather than glass width after mounting.

I have a normal Derogy Aplanat size no. 7. I'll do a check on Monday to see if the same difference between published "focal length" and equivalent focal length exists with this one which is from the same period as yours.

It was also quite normal that the larger sizes in RR/Aplanat and even Petzval series were not quite as fast as the more common smaller sizes.