Corfield England APO-Lumar 50mm F3.5 on A7R II Part I

[calvin83]

The lens:



This lens is one of the rare lens from Corfield. It is probably one of the earliest post-war enlarger with 'APO' designation although it is only a triplet.

Click to enlarge:

#1


#2


#3


#4


#5


#6


#7



P.S. The hood I used is a bit too long for this lens and the adapter seems not perfectly aligned. Part II will follow when I get better photos after I fix the two issues.

[Himself]

Looking good so far. I see no CA along the ridges . How about you at 100%?
What mount is?

[calvin83]

I think it may be too harsh to pixel peep to check the CA for a triplet... I would say the CA is not quiet visible even under harsh condition.

The mount is M39.

[Lightshow]

Looks quite good to me.

[kds315*]

wow what a rare lens!

(I honestly doubt that 6 lens surfaces are enough to allow for an apochromatic correction...
but it looks surprisingly well!!)

[iangreenhalgh1]

Actually, the lens may well be apochromatic, or damn close to it, certainly closer than many modern lenses marked APO.

I say this because Voigtlander recalculated their lenses after WW2 to use the then new Lanthanum glass - the Skopar became the Color-Skopar and it is damn close to being apochromatic, they also made triplets with Lanthanum, they had various names such as Color-Lanthar.

This Corfield is made by Enna, and I bet it, like the Voigtlanders, uses a Lanthanum glass element, so it may well, like the Voigtlanders, be close to apochromatic.

[visualopsins]

Zeiss indicates 3 groups is sufficient. http://lenspire.zeiss.com/en/apochromat/

[iangreenhalgh1]

Yes, there are a lot of older process lenses that are fully apochromatic but have simple 3 or 4 group designs - most are either 4e/3g Tessar/Xenar/Skopar types or 4e/4g Dialytes.

[calvin83]

I have tried Color-Lanthar in DKL too. The Color-Lanthar, which is also a triplet, hardly show any CA under back lit or high contrast scene. It is better than this lens for general photography.

Here is the 100% crop as requested. Do you agree it is a APO lens?



100% crop

[iangreenhalgh1]

I'd say it is APO, yes.

[Gerald]

In fact it is possible to make an APO lens with just 3 optical surfaces! Some examples here:

http://www.telescope-optics.net/semiapo_and_apo_examples.htm

The problem is that with only 3 optical surfaces it is not possible to properly correct the monochromatic aberrations (spherical, coma and astigmatism) for the apertures and angular coverage of a typical photography lens.

[calvin83]

[visualopsins]

[kds315*]

Yep, that's what I meant of course, an APO which not only corrects CA, but also the other aberrations to make it a suitable photographic lens ... sighh

[Palabrant]

These early Corfield lenses were not made by Enna but are all British by the Birmingham Optical Company [BOLCO] who partly financed Corfield .

[pdccameras]

Nice images - is this an enlarger lens?

P.

[Lightshow]

[pdccameras]

Yes it is, They(Enlarger Lenses) are typically written such that the front element is at the bottom of the lettering, because when mounted in an enlarger.... You can read it without standing on your head. I.e. It's printed from the observers position, which is why most camera lenses have their writing readable from behind the camera.[/quote]

Good tip - and a good example of ergonomic design. I never really noticed it before - just like i just recently found out about the little arrow on the fuel gage in cars that points to the side of the vehicle where the gas fill is located!

Cheers,

Paul

[Ernst Dinkla]

[simple.joy]

I've been curious about this lens for a while. I love triplets and while this one probably won't be among the sharpest ones wide open, the rendering is quite nice (as is often the case with lenses made in England from a certain era):










All of those are wide open.

[calvin83]

Good to know you got one too!