7artisans BMPCC4k...?

I see in the forum not many have been talking about these lenses options.

http://www.7artisans.com/en/

They are cheap but the image that they give is not bad at all :

I'm mostly interested in 2 lenses :

the 7.5mm f2.8 fish eye

the 12mm f2.8.

I have three doubts :

1- what the actual focal lenght will be on the BMPCC 4k sensor?

2- will the 7.5mm have a significant distorsion on the BMPCC4k sensor? I saw this video and it looks like it won't :

3- will these lenses vignette at low apertures?


If any of you gentlemen can help me with this, I would be grateful. Thanks.

Since I don't own them, I can answer only two of your questions:

– No, they will not change their focal length. Never. Not on any camera.

– They will not vignette if you don't use a focal reducer. If you do, maybe.

the 7.5mm will be a 14mm in the BMPCC 4k?

I have the 25mm 7artisans lens. It's nice and sharp, and with a full-frame equivalent view of about a 50mm, depending on if you're shooting UHD or 4kDCI.

I know some people don't like discussing focal lengths in terms of full-frame equivalent, but it is a useful measure that most people understand. It would be great to be able to make everyone discuss lenses in terms of angle of view, but most people can't do that kind of math in their heads while they typically understand and can visualize a full-frame field of view equivalent.

the 7.5mm will be a 14mm in the BMPCC 4k?

Something between 14mm and 15mm, depending on if you're shooting UHD or 4kDCI.

Relative to the descriptions on the APS-C lens your effective focal length will be.

7.5mm * 1.32 = 9.9mm on BMPCC 4K (at 4096x2160 4K DCI)
12mm * 1.32 = 15.84mm on BMPCC 4K

Given

APS-C Fujifilm (one of the largest) 23.6x15.6mm sensor with a diameter of 28.23mm
& BMPCC 4K diameter is 21.44mm
28.23/21.44=1.32

.

Relative to the 35mm full frame 36x24mm (diameter of 43.27mm).

then

7.5mm APS-C is equivalent to 4.893mm; or 4.9mm 35mm FF.
12mm APS-C is 7.8289; or 7.8mm 35mm FF.

Using; 28.23/43.27=0.652415068 as the multiplier.

.

Relative to the micro 4/3 which is 1/2 the diameter of 35mm full frame.
21.6333 * 2 = 43.2666
Note that m43 21.6333 is a fraction bigger that the 21.44 for BMPCC 4K, with a crop of 1.00901.

So just double 4.9 and 7.8 to get the size relative for m43.

.

Therefore the APS-C lens which are accredited to cover the largest APS-C sensor are effectively:

APS-C 7.5mm … with a smaller m43 image circle … is (4.9*2) 9.8mm m4/3 lens
APS-C 12mm … with a smaller m43 image circle … is (7.8*2) 15.6mm m4/3 lens



A 4.9mm 35FF lens will give the same FOV as a 7.5mm APS-C lens on a micro 4/3 sensor; which is the same FOV as a 19.6mm 35FF lens. (9.8*2).

A 7.8mm 35FF lens will give the same FOV as a 12mm APS-C lens on a micro 4/3 sensor; which is the same FOV as a 31.2mm 35FF lens on a 35FF sensor. (15.6*2)



If my maths is wrong Im gona drink a beer, if my maths is right - Im gona drink a beer.
Either way Im gona drink 2 beers.

Nice lens thanks for the link.

EDIT: added "on a 35FF sensor"

Relative to the descriptions on the APS-C lens your effective focal length will be.

7.5mm * 1.32 = 9.9mm on BMPCC 4K (at 4096x2160 4K DCI)
12mm * 1.32 = 15.84mm on BMPCC 4K

Given

APS-C Fujifilm (one of the largest) 23.6x15.6mm sensor with a diameter of 28.23mm
& BMPCC 4K diameter is 21.44mm
28.23/21.44=1.32

.

Relative to the 35mm full frame 36x24mm (diameter of 43.27mm).

then

7.5mm APS-C is equivalent to 4.893mm; or 4.9mm 35mm FF.
12mm APS-C is 7.8289; or 7.8mm 35mm FF.

Using; 28.23/43.27=0.652415068 as the multiplier.

.

Relative to the micro 4/3 which is 1/2 the diameter of 35mm full frame.
21.6333 * 2 = 43.2666
Note that m43 21.6333 is a fraction bigger that the 21.44 for BMPCC 4K, with a crop of 1.00901.

So just double 4.9 and 7.8 to get the size relative for m43.

.

Therefore the APS-C lens which are accredited to cover the largest APS-C sensor are effectively:

APS-C 7.5mm … with a smaller m43 image circle … is (4.9*2) 9.8mm m4/3 lens
APS-C 12mm … with a smaller m43 image circle … is (7.8*2) 15.6mm m4/3 lens



A 4.9mm 35FF lens will give the same FOV as a 7.5mm APS-C lens on a micro 4/3 sensor; which is the same FOV as a 19.6mm 35FF lens. (9.8*2).

A 7.8mm 35FF lens will give the same FOV as a 12mm APS-C lens on a micro 4/3 sensor; which is the same FOV as a 31.2mm 35FF lens on a 35FF sensor. (15.6*2)



If my maths is wrong Im gona drink a beer, if my maths is right - Im gona drink a beer.
Either way Im gona drink 2 beers.

Nice lens thanks for the link.

EDIT: added "on a 35FF sensor"

A beer is well deserved after this math work !

So why people constantly multiplicate lenses * 1.9-2 when calculating the focal length for the BMPCC4k sensor?

I'm pretty sure that math is not accurate. Or at least not meaningful. I'm going to take a stab at this.

First, the focal length of a lens is is physical property of the lens. It is not at all related to the sensor or the camera that lens gets put on. A 50mm lens has (theoretically) the same optical properties as any other 50mm lens. Now, some lenses are physically larger than others of the same focal lengths in order to prevent vignetting/cropping on a larger sensor.

For example, if you take a 50mm lens designed for a full frame mount, and a 50mm lens designed for a mft mount and put them both on a mft camera you will get exactly the same image. If you went the other way and (somehow) mounted them both on a full frame camera you would also get exactly the same image, except that the mft mount lens would vignette very badly (probably all the way to black) towards the outside of the image. The 35mm sensor is larger so it can "see" wider/more of what the lens projects. That's why full frame lenses are bigger. To project a wider circle on the sensor. The mft sensor is effectively a crop of what the full frame sensor sees from the same lens.

When people are comparing focal lengths of lenses between systems they are usually using it as a shorthand for Field of View. You have lots of people with deep experience with one system or another. When they put a 50mm lens on a 35mm camera they know exactly what FoV to expect as compared to, say a 35mm lens on the same camera.

When they move to a different system they want to translate that understanding. So when someone asks for a "50mm equivalent lens" what they are asking is "What lens can I put on this camera to give me a the same field of view that I would get from a 50mm lens on a full frame camera".

Now, this relates only to FoV. There are other properties such as depth of field that will also change. That tends to be a very different and more complex discussion.

So, for example the 7.5mm you are looking at would roughly give the same field of view as a 15mm on a full frame camera (roughly a 2x crop factor). Or the same FoV as a 10mm lens on an APSC sensor (roughly 1.3x crop factor).

Or flipped the other way, if you want the same "reach" as a 400mm lens on a full frame camera, you only need a 200mm lens on a mft camera.

That does not mean you will get the exactly the same image. Just that the FoV will be the same.

A beer is well deserved after this math work !

thanks; take one for yourself, for showing me some new lens.

So why people constantly multiplicate lenses * 1.9-2 when calculating the focal length for the BMPCC4k sensor?

I think? The 2x crop you are highlighting is the ratio between the 35mm Full Frame and Micro 4/3.
The image circle of the 35FF is double that of m4/3.

The FOV for a 50mm m4/3 lens on a m4/3 sensor is EQ to a 100mm 35FF lens on a 35FF sensor.
A m4/3 lens has a shorter focal length for the same FOV over that of 35FF lens. So the lens is lighter & smaller.
But a wide 35FF lens is more accessible; its easier to go wide with 35FF system.

APS-C is not well defined. The sensor diameter is between (approx) 26.82mm and 28.4mm.
You would hope that a lens will cover the largest.

I tried to show the relative 35FF lens that would give the same FOV as an APS-C lens; both on a m4/3 sensor.
e.g. 4.9mm 35FF … 7.5mm APS-C … 9.8mm m4/3 system
I than use the crop 2x to get the 35FF equivalent. 9.8*2 = 19.6mm 35FF system.
e.g. A passive 4.9mm 35FF lens on a m4/3 sensor has a FOV equal to a 19.6mm 35FF lens on a 35FF sensor. (Which is the same field of view as a passive 7.5mm APS-C lens on a m4/3 sensor).

If you know; square(width) + square(height) = square(diameter)
then you can get drunk and have fun working it out.

I'm pretty sure that math is not accurate. Or at least not meaningful. I'm going to take a stab at this.

Cool - We need more beers.

First, the focal length of a lens is is physical property of the lens. It is not at all related to the sensor or the camera that lens gets put on. ...

For example, if you take a 50mm lens designed for a full frame mount, and a 50mm lens designed for a mft mount and put them both on a mft camera you will get exactly the same image.

A 50 is a 50 is a 50 its the golden lens rule.

But when you connect a 50mm 35FF lens to a m4/3 camera mount, you are capturing 1/2 the image circle that the 35FF lens is focusing. So the lens is 100mm in a m4/3 system. (50*2).

What people are interested in (I don't know why), is what 35FF lens do I need to get the same FOV as the m4/3. You need to scale up the m4/3 image circle to the surface area of a 35mm sensor. So a 100mm m4/3 lens is 200mm 35FF lens. (100*2).

If I am wrong; I'm gona insist we drink whisky.

So, for example the 7.5mm you are looking at would roughly give the same field of view as a 15mm on a full frame camera (roughly a 2x crop factor). Or the same FoV as a 10mm lens on an APSC sensor (roughly 1.3x crop factor).

I think we are on the same page. As I to use a 1.32 crop to translate APS-C to MFT (BMPCC 4K). Once you do this you can then apply the 2x crop from MFT to 35FF.

Don't worry, this is not unique to this forum, the discussion is about as common as asking what lens to get.

Hopefully no one starts talking about 'equivalence', as then the discussion will be in the toilet in no time.

I got so dizzy with all the discussion I wrongly bought a 600mm tele thinking it would become a 6 mm in the BMPCC 4k

2- will the 7.5mm have a significant distorsion on the BMPCC4k sensor? I saw this video and it looks like it won't :

Nobody answered to this. Do you get to see significant distorsion in the 7,5 mm shots of this video?

Don't worry, this is not unique to this forum, the discussion is about as common as asking what lens to get.

Hopefully no one starts talking about 'equivalence', as then the discussion will be in the toilet in no time.

I got so dizzy with all the discussion I wrongly bought a 600mm tele thinking it would become a 6 mm in the BMPCC 4k

Yeah; I made it too complicated. Ryan simplified it for me. But I don't understand APS-C; so I had to work it out (falling down the rabbit hole).

A camera system is like a system of slavery. You read so my photographic forums; where people only talk about the lens for the designated mount. But over here we talk about, and buy anything and everything. Freedom is C mount; m4/3; APS-C; EF; Nikon; PL. The possibility to shoot a multi cam setup; utilising any and all lens.

2- will the 7.5mm have a significant distorsion on the BMPCC4k sensor? I saw this video and it looks like it won't :

Nobody answered to this. Do you get to see significant distorsion in the 7,5 mm shots of this video?

Yes, you'll still have significant distortion.

I have both the 7.5mm f/2.8 fisheye, and the 25mm f/1.8 lenses. Both excellent value for money. The 25mm has quite a vintage look in the way it flares, and gets slightly soft/vignettes in the corners. A great character lens for portraits.