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Thoughts on sharpness of B4 mount lenses?


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#1 Samuel Berger

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Posted 07 January 2018 - 12:15 AM

One of the reasons I don't use my Ursa Mini 4K very much is due to the PL mount. PL lenses are stupidly expensive. But I do know the UM4K has a B4 mount adapter available, and B4 lenses are cheap. There are some Canon and Fujinon zooms out there in good shape. Are those cinematic at all? Soft or sharp?

 

They might be what I need for some green screen work, if sharp enough. At the moment I only have a Helios 58mm and an Angenieux 25-250 in PL. The Angenieux is a moose. Some of those B4 zooms might be more manageable.

 

What I wish I had was a 35mm prime, but I don't see any B4 primes.


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#2 Stuart Brereton

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Posted 07 January 2018 - 12:24 AM

B4 mount lenses aren't made to cover a s35mm sensor, so they will almost certainly vignette, unless you are using the camera in crop mode (assuming it has one). The cheaper B4 glass will be SD rather than HD quality, and therefore fairly soft.


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#3 Samuel Berger

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Posted 07 January 2018 - 12:49 AM

Thanks Stuart. I confess I never looked into B4 lenses before, since I was never a video person.


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#4 Robin R Probyn

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Posted 07 January 2018 - 08:41 AM

also designed for 3 chip CCD sensors... when I very first got my F5 I used my Fujino B4 lens .. with a small adaptor for centre crop .. if works but its such a waste of sensor.. and I dont think you would ever want to project the image onto anything large.. probably better off with an actual 16mm zoom ?


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#5 Stuart Brereton

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Posted 07 January 2018 - 10:47 AM

Thanks Stuart. I confess I never looked into B4 lenses before, since I was never a video person.

You're far from the first person to be tempted by the large amount of cheap B4 glass available. Sadly, it's really not worth the trouble, I feel.


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#6 Phil Rhodes

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Posted 07 January 2018 - 11:05 AM

I think it can be.

 

The optical performance is not what you'd get from a DSLR lens, but then it wouldn't be.

 

The sheer performance of these lenses, the size, weight and flexibility, is enormous.

 

c224b199cc14230c1f8c24caff4fff1f_XL.jpg?

 

Ursa Mini 4.6K with Canon HJ11x4.6B

 

P


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#7 Stuart Brereton

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Posted 07 January 2018 - 11:12 AM

I assume you're cropping the sensor. Just how badly does it vignette (or rather porthole)?


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#8 Vladimir Cazacu

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Posted 07 January 2018 - 11:28 AM

There are B4 1.4x / 2x extenders out there that can be used to cover the whole sensor with a light loss of 1 and 2 stops respectively.


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#9 Brian Drysdale

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Posted 07 January 2018 - 11:46 AM

These 2/3 HD lenses are used by wildlife filmmakers on large sensor cameras. There are a number of adapters which allow you to so, some make use of the len's internal x 2 extender.while others don't.

 

 

https://www.abelcine...optical-adapter


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#10 Stuart Brereton

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Posted 07 January 2018 - 12:07 PM

Much as I respect Alastair Chapman, this adaptor really only seems to make sense if you already own expensive B4 lenses. A 2.5 stop light loss is considerable. Also, using the 2x extender to increase the image circle is going to soften the image no matter what sensor is behind it, even without the additional relay lens in the adaptor.


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#11 Phil Rhodes

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Posted 07 January 2018 - 12:33 PM

I assume you're cropping the sensor. Just how badly does it vignette (or rather porthole)?

 

This is a complicated question because various lenses project larger or smaller images and various cameras have various sizes of sensor.

 

There are a combination of approaches (which I'm sure you're aware of) which I will list here for completeness:

 

- Crop the sensor in the camera

- Engage the doubler on the lens

- Use some degree of optical expansion.

 

The depicted configuration uses the extender and a very small amount of optical expansion to fill the cropped area of the Ursa Mini 4.6K's sensor. The adaptor is made by MTF (Mike Tapa is the guy holding the camera.)

 

Optical correction is critical in any case, because B4 lenses do not perform well on single chip cameras without it. Below f/4 the glow and flare becomes unacceptable, because of their expectation of a 3-chip block. The optics need to correct for this.

 

Yes, it is probably only worthwhile if you own the B4 lens, although there may be some argument on this for specific circumstances. That lens goes for under £4500 on eBay. You cannot get anything which does even nearly what this setup does for even double the money.

 

P


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#12 Brian Drysdale

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Posted 07 January 2018 - 12:40 PM

At over $4k, the HDx35 Mark III B4 Optical Adapter (see Abelcine link above) may be one of the expensive methods described by Alastair Chapman in the video. I guess it depends on the purpose you wish to use the lens for.


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