Dumb Question About Image Circle

[tommygdawg]

All of the numbers are still a bit over my head, but let me see if I get this straight in regards to focal length...

Focal length is the literal measure of how far the light travels before it's sharply in focus on the rear elements, correct? So a shorter focal length, ie 20mm, travels a shorter difference in the lens, yes?

Here's a question, how then can you have these huge (in literal size) cine lenses that are, say, 50mm? The light obviously has to travel a much longer path to get to the rear elements than it does in my tiny FD lens.

*bump*

Anyone?

 

[j1clark@ucsd.edu]

*bump*

Anyone?

The largeness of cine lenses vs 'ordinary' still lenses has to do with how much 'correction', and how many additional elements are required for such correction, of 'defects', as well as allowing for a 'large f-stop'.

For many lenses, the 'optimal' f-stop is 2-3 stops down from 'wide open', because at 'wide open', there were be a number of imperfections, wavelengths of light refract differently, and result the 'sharp' focus for a particular 'color' to be somewhere other than exactly at the focal (sensor) plane... called chromatic aberration... is one such defect, and with a given lens and a smaller aperture, 'Depth of Field' covers those errors in focus...

For cine lenses where it is desired to shoot more nearly 'wide open', because in Film one is pretty much locked to shooting a 1/48 second, the other 'adjustments' are 'lighting levels'... or 'f-stop'... ok for a given fixed ISO value... upping the ISO value has the negative effect of 'more grain' in Film film, and more digital noise in Digital film...

Another type of error a lens may have is the 'bowing' of parallel lines. The fish eye lens is used to exaggerate this effect, but normally it is an effect to be minimized... add more lens elements...

For the 'still' 35mm camera user, there are lenses in the $2-3K region that deal better with these sorts of issues, but that is still much lower than cine lenses in price. Another issue with cine lenses is matching across a series of lenses from a given manufacturer. Most 35mm still shooters do not worry about the change in 'color' because of the type of glass in the lens, or even differences in DoF because the 'f-stop' number is not an absolutely calibrated indication for still lenses. They may even shoot across several different manufacturers, without to much concern for 'fine detail consistency'.

With 'white balance' and in-camera autoexposure/metering most differences between lenses can be 'easily' compensated for.

 

[tommygdawg]

Good points. I was just curios, because it had been said that the "focal length" (ie a 50mm lens) is a measurement of how far the light travels within the lens (I assumed that this is from front element to rear element), and maybe this is just my simpleton's way of looking at it, but wouldn't light travel farther in a cine lens? I get that there are more elements in the lens housing to correct for errors in the image, but doesn't the light still have to travel way farther than 50mm?

 

[j1clark@ucsd.edu]

Good points. I was just curios, because it had been said that the "focal length" (ie a 50mm lens) is a measurement of how far the light travels within the lens (I assumed that this is from front element to rear element), and maybe this is just my simpleton's way of looking at it, but wouldn't light travel farther in a cine lens? I get that there are more elements in the lens housing to correct for errors in the image, but doesn't the light still have to travel way farther than 50mm?

The focal point of a complex lens... is complex calculate... the calculations are 'simplified' by using an ideal equivalent lens.

Such simplifications are used throughout 'engineering' based on the physics of the situation.

 

[tommygdawg]

So it's calculated based on it's smaller, ff35 counterpart?

 

[Mitch Gross]

Focal length of a lens is defined as the distance between the nodal point and the imaging plane. In plain English, that is the distance between the point where all of the light converges and the surface of the camera's sensor.

 

[tommygdawg]

Focal length of a lens is defined as the distance between the nodal point and the imaging plane. In plain English, that is the distance between the point where all of the light converges and the surface of the camera's sensor.

That's what I thought! But wasn't Barry saying that it's the distance that light travels in the lens?

 

[FilmDude]

This saying needs to be repeated again... seems like every few years this comes up. A 50mm is a 50mm is a 50mm. Nothing changes. There is no "magnification" of a lens. Also in cinematography there is NO need to talk about FF35. People need to get out of their heads about FF35. It was never the right fov and never will be for cinema, so thats that.

When in film we say we're shooting with a 50mm, we talk about the fov for an S35mm sensor or gate. So people who say "my 50 on my Canon mark 7893876 is going to be different" ... No its not different. Its a 50mm lens in cinema world.

I really hate the DSLR spill into cinema for this, there is no mixing these two.


So I guess point is this... Get your head out of thinking about Full Frame. It has no meaning for s35mm cinematography. Learn what 50mm fov is on a 7D, thats about the right size. NOT 5D. You're favorite lenses for s35 are going to change. If you loved a 50mm on FF35 you're more than likely going to then move to a 35 for your favorite lens. Thats all there is to it.

All of these other cameras with 4/3 and all that nonsense is just... I dont know, I think the market is flooded. I'm too old for this. Also EPIC with a FF35 is annoying as well. No lenses cover it. I dont see the point for this use other than shooting 4:3 for anamorphic 2:1 squeeze.

 

[j1clark@ucsd.edu]

That's what I thought! But wasn't Barry saying that it's the distance that light travels in the lens?

My 'guess' is that he was using casual terms... I think if you want more technical explanations, a search of the web for 'optics' would be beneficial to understand the principles of lenses in general, and in particular 'complex lenses', since all 35mm and cine lenses that I've seen in 'recent use' are complex lenses. Many people who make films, may not have had a formal class in optics. In college physics programs its usually given at the second year level.

I doubt most photographers have put their lenses on an optical bench... or even sent them out for calibration... unless they believe the lens is broken..

 

[j1clark@ucsd.edu]

When in film we say we're shooting with a 50mm, we talk about the fov for an S35mm sensor or gate. So people who say "my 50 on my Canon mark 7893876 is going to be different" ... No its not different. Its a 50mm lens in cinema world.

I really hate the DSLR spill into cinema for this, there is no mixing these two.

This phenomenon did not start with the influx of DSLR shooters into things 'cinema'. The real transition was between SLR and DSLR, when the sensors of the DSLRs did not match the size of standard 35mm Film film cameras.

For things cine, the stability, relatively speaking, of the 35mm format, and the several 'format's that could be used, allowed such books as the American Cinematographer Manual to list tables of lens, format, FOV/AOV/DOF, etc. and have those pretty much fixed.

In the pro digital cine camera, some amount of effort has been to make the sensor size match up better with these traditional Film film sizes, than has been the case in the still DSLR world. One would have the same problem with people moving from small sensor video cameras to the larger... but that would often be sort of 'buried' because many video cameras did not have the ability to change lenses, had zooms, and the shooter would be more focused on zooming to frame rather than using a prime, and needed to know if the FOV was adequate for the shot.

I've recommend that people by one of two apps that calculate these parameters for a given camera and lens combo... some people may not have the 30 bux or so, but there are 'free' online calculators that they could use... I just don't have a handy set any more now that I have the two apps... much more convenient when I need to understand what the 'camera will see'...

In reality all of this boils down to a few simple trig formulas, but since trig is not typically taught... or remembered... there is a certain amount of difficulty at these sorts of points.