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Focal length conversion factor should apply to the camera not the lens.

I was asked in some post comments whether the a 50mm PL mount lens would give a wider picture than a 50mm DSLR lens. This confusion comes about I believe because of all the talk about focal length conversion factors. I don’t think this concept is well understood by some people as the implication is that somehow the lens is changing when its used on different cameras, when in fact it’s the camera that is different, not the lens.

It is important to understand that a 50mm lens will always be a 50mm lens. That is it’s focal length. It is determined by the shape of the glass elements and no matter what camera you put it on it will still be a 50mm lens. A 50mm DSLR lens has the same focal length as a 50mm PL mount and as a 50mm 2/3″ broadcast lens. In addition the lens focuses a set distance behind the rear element, agin the distance between the rear element and where it focuses does not change when it’s put on different cameras, so an adapter or spacer must be used to keep the designed distance between the lens and sensor, this distance is called the “flange back”.

The key thing is that it’s not the lens or it’s focal length that changes when you swap between different cameras. It is the size of the sensor that changes.

Imagine a projector shining an image on a screen so that the picture fills the screen. The projector is our “lens”. Without changing anything on the projector what happens if you move the screen closer or further away from the projector? The image projected on the screen will go in and out of focus, so that’s not good, we must keep the projector to screen distance constant, just like the lens to sensor distance (flange back) for any given lens remains constant.

What happens if we make the screen smaller? Well the image remains the same size but we see less of it as some of the image falls of the edge of the screen. If our projected picture was that of a wide landscape then on the reduced screen size what would now be seen would not appear less wide as we are now only seeing the middle part of the picture. The width of the view would be decreased, in other words the FIELD OF VIEW HAS NARROWED. The focal length has not changed.

This is what is happening inside cameras with different size sensors, the lens isn’t changing, just how much of the lenses projected image is falling on or off the sensor.

So the multiplication factor should be considered more accurately as being applied to the camera, not the lens and the multiplication factor changes the field of view, not the focal length.

So whether it is a PL mount lens, a Nikon or Canon DSLR lens or a Fujinon video lens, if it’s a 50mm lens then it’s a 50mm lens and the focal length is the same for all. However the field of view (width and height of the viewed image) will depend on the size of the sensor. So a 50mm PL lens will give the same field of view as a 50mm DSLR lens (no matter what camera the lens was designed for) on the same video camera.

The only other thing to consider is that lenses are designed to work with certain sizes of sensor. A lens designed for a full frame 35mm sensor will completely cover that size of sensor as well as any sensor smaller than that. On the other hand a 2/3? broadcast lens will only cover a 2/3? sensor, so if you try to use it on a larger sensor the image will not fill the frame.
The sensors in the Sony F3 and FS100 are “Super 35mm”. That is about the same size as APS-C. So lenses designed for Full frame 35mm can be used as well as lenses designed for 35mm cine film (35mm PL) and lenses designed for APS-C DSLR’s such as the Nikon DX series and Canon EF-S.

See also http://www.abelcine.com/fov/

 

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HD, SD and Depth of Field.


I was reminded of this by Perrone Ford on DVINFO.net. With HD cameras compared to SD cameras the depth of field appears shallower. Why is this and why is it important?

Visually depth of field is the loss of focus as you move away from the object that you have focussed on. If you have two cameras, one HD and one SD and they both have the same lens at the same aperture along with sensors of the same size then the change in focus with distance for both cameras will be exactly the same. However with the HD camera, because the image is sharper to start with, any small changes in focus will be more apparent than with the softer picture from the SD camera. So visually the HD camera will have a shallower depth of field. Now if you take that HD image and convert it to SD then the depth of field appears to increase again. This can be calculated and measured and is defined by the “circle of confusion”

So why is this important? Well lets look at what happens when you shoot an interview or face. The human brain is very good at looking at faces, we “read” faces day in and day out, taking in expressions, skin tone and subtle changes. We use these tiny visual cues to gauge emotion and see how someone is responding to the things that we do. Because of this any imperfection in the look of a face in a video tends to stand out (thats also why you normally expose for faces). With HD it’s quite possible to have a shot of a face where the tip of the persons nose or their ears are in sharp focus while the eyes are slightly soft. With an SD image we would be unlikely to notice this because of the greater depth of field, but HD with it’s visually shallower DoF can show up this small difference in focus and our brain flags it up. Very often you see the HD face and it looks OK, but something in your brain tells you it’s not quite right as the eyes are not quite as sharp as the nose or ears. So this apparently shallower DoF means that you can’t just focus on a face with HD but you must focus on the eyes, as that’s where we normally look when engaged in a conversation with someone.