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Corfield England APO-Lumar 50mm F3.5 on A7R II Part I
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PostPosted: Fri Feb 16, 2018 1:13 pm    Post subject: Corfield England APO-Lumar 50mm F3.5 on A7R II Part I Reply with quote

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. Wink

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.


PostPosted: Fri Feb 16, 2018 2:27 pm    Post subject: Reply with quote

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


PostPosted: Fri Feb 16, 2018 3:08 pm    Post subject: Reply with quote

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.


PostPosted: Fri Feb 16, 2018 4:21 pm    Post subject: Reply with quote

Looks quite good to me.


PostPosted: Fri Feb 16, 2018 4:23 pm    Post subject: Reply with quote

Like 1 Like 1 Like 1 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!!)


PostPosted: Fri Feb 16, 2018 9:36 pm    Post subject: Reply with quote

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.


PostPosted: Fri Feb 16, 2018 11:50 pm    Post subject: Reply with quote

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


PostPosted: Sat Feb 17, 2018 12:10 am    Post subject: Reply with quote

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.


PostPosted: Sat Feb 17, 2018 2:01 am    Post subject: Reply with quote

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? Wink



100% crop


PostPosted: Sat Feb 17, 2018 5:41 am    Post subject: Reply with quote

I'd say it is APO, yes.


PostPosted: Sat Feb 17, 2018 4:49 pm    Post subject: Reply with quote

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.


PostPosted: Sat Feb 17, 2018 5:07 pm    Post subject: Reply with quote

Gerald wrote:
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.

Like 1 Thank you!


PostPosted: Sat Feb 17, 2018 5:46 pm    Post subject: Reply with quote

Gerald wrote:
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.


6 optical surfaces...



I think Dr Schmidt is thinking about IR&UV.

Thanks Gerald for the highly technical reference!


PostPosted: Sat Feb 17, 2018 10:49 pm    Post subject: Reply with quote

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