Lens Magnifcation Factors

[Stephen Williams]

Thanks Sanjin,

Looking at Sanjin's images the Cooke (set at 50mm) records an image on the RED that is cropped/magnified compared to the Nikon/Leica 50mm.

How does one account for that difference mathematically if you wanted to replicate the same image (in terms of field of view, not depth of field) on a RED with two different lenses. Or is that simply a pointless exercise?

I keep being told "a 50 is a 50 is a 50" and I understand why, but still, the images the different lenses produced on the RED in Sanjin's test are not the same.

Is this because as Steven says:


or because


If it is because the circle of the image the lens produces is bigger, then is there a useful way to compare the circle of image produced by different lenses so that one could meaningfully compare the recorded FoV a given lens would produce when used on the RED?

I really appreciate all the help on this thread,
I just want to get the concepts nailed so that I don't feel confused about it in the future.

Hi Ed,

The Cooke 25-50 is a physically big lens so one has to take account of it's size.DOF & FOV CALCULATIONS are made from the front nodel point (or entrance pupil) of the lens (NOT FILM PLANE). With 2 'thin' lenses that can be ignored, but must be compensated with when using a big zoom. Details of this are explained in David Samuelsons Hands on Book an cinematography & in Blaine Brown's book. I don't have either with me know as I am not at home.

Stephen

 

[Ed Watkins]

Hi Ed,

The Cooke 25-50 is a physically big lens so one has to take account of it's size.DOF & FOV CALCULATIONS are made from the front nodel point (or entrance pupil) of the lens (NOT FILM PLANE). With 2 'thin' lenses that can be ignored, but must be compensated with when using a big zoom. Details of this are explained in David Samuelsons Hands on Book an cinematography & in Blaine Brown's book. I don't have either with me know as I am not at home.

Stephen

Thanks Stephen,

Your comments have been very useful.
And that clears up the last of my questions.
I'll check out those two books for further reading.

Ed.

 

[Stephen Williams]

Thanks Stephen,

Your comments have been very useful.
And that clears up the last of my questions.
I'll check out those two books for further reading.

Ed.

Hi Ed,

The Samuelsons book covers this in in some detail, IIRC it gives details of the front nodel point of some zoom lenses. The front nodel point will change as you zoom the lens, it's not fixed. The Blaine Brown book just mentions it briefly.

Stephen

EDIT A 50mm lens may not be exactly 50mm, it could be 48,95 or 50.87. I remember Zeiss Contax lenses coming with a piece of paper giving the exact focal length as tested and also MTF values. It's an issue when making motion control & 3D match, having played those games for 20 years I have a good idea of the variables to be changed. Lenses are often slightly wrong by a few percent.

 

[Sanjin Jukic]

Hi,

Moving the camera back about 20 cm would be better, then the front nodel points would match up. DOF & FOV would then be identical.

Stephen

Tomorrow I'll make test again and also in that direction of moving the camera 20 cm back. Right now is a sunrise time and lighting on my balcony is not good for the test.

 

[Stephen Williams]

Tomorrow I'll make test again and also in that direction of moving the camera 20 cm back. Right now is a sunrise time and lighting on my balcony is not good for the test.

Hi,

I am guessing exactly how far to move the camera back but it will be in that range. When the images line up exactly you can try a FOV & DOF test.
It took me some time to work out what was going on with my Cooke 20-100 @100mm and a Cooke 100mm S4. At first I believed there was less depth of field with the zoom, then I noticed we were zoomed in. Zooming back matched the frame but not FOV or DOF. Moving the Camera backwards everything matched.

If you look at a Samuelsons / Panavision DOF wheel you will see there are 2 scales, 1 for primes & 1 for Zooms.

Stephen

 

[Sanjin Jukic]

Hi,

I am guessing exactly how far to move the camera back but it will be in that range. When the images line up exactly you can try a FOV & DOF test.
It took me some time to work out what was going on with my Cooke 20-100 @100mm and a Cooke 100mm S4. At first I believed there was less depth of field with the zoom, then I noticed we were zoomed in. Zooming back matched the frame but not FOV or DOF. Moving the Camera backwards everything matched.

If you look at a Samuelsons / Panavision DOF wheel you will see there are 2 scales, 1 for primes & 1 for Zooms.

Stephen

I could use only Panavision DOF / FOV / AOV Image Calculator.

SamCine Mk III Depth of Field Calculator>>>LINK.

 

[Stephen Williams]

I could use only Panavision DOF / FOV / AOV Image Calculator.

SamCine Mk III Depth of Field Calculator>>>LINK.

Hi,

As you see the 2 scales.

Stephen

 

[Eddie]

i think much of the confusion comes from falsely regarding FOV as a property of lenses instead of cameras. A lens has a circle of confusion not a FOV, whereas cameras have different FOVs.

The reason many people rightfully dismiss the use of these cropfactor calculations is that they can be quite misguiding. for instance if you are shooting cropped 2K on the red, in a situation where you would normally use a 25mm (4K), it would be silly to bring out your pocket calculator, and then put on, say a 12mm. It might be better to back up and use the 25mm from farther away, to preserve the same perspectival properties of 25mm...

 

[Chad Terpstra]

Thanks for this discussion! I've been looking for a rough multiplication factor for a while now because I'm moving from 35mm adapter work using a 36x24mm sensor into the Red realm. Now at least I have a starting point to think about the relative FOV. So would you say a "Normal" lens on the Red would be around a 30mm? On certain shows I've been on the 25mm gets a lot more milage than anything else so this may be about right, especially when accounting for a wider aspect ratio.

Only question I have is about the zoom lens issue. When using a large zoom do you still measure focal distance from the film plane? I'm just amazed I haven't heard this fact before of their relative fov being different than that of primes so I'm trying to get a handle on it. I'm willing to bet illustrations on this matter would prove invaluable...

 

[Stephen Williams]

Only question I have is about the zoom lens issue. When using a large zoom do you still measure focal distance from the film plane? I'm just amazed I haven't heard this fact before of their relative fov being different than that of primes so I'm trying to get a handle on it. I'm willing to bet illustrations on this matter would prove invaluable...

Hi,

You measure focus to the film plane as the marks on the lens are so corrected. If you are using a DOF chart for the lens then also measure from the film plane. Many people are not aware of the difference as they have not used a "Long Zoom" it's actually the same issue with a physically thick wide angle prime lens as well. Simplified lens calculations assume a thin lens and ignore lens thickness. It's only noticeable at closer distances, nearer infinity you can ignore.
It's actually easier just to test what I am saying, rather than get involved learning about lens design & formulas, unless you want to design lenses.

Stephen