Is 4K really "more demanding" on still lenses ? at

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HD and UHD ( 2K+ ) Digital Cinema
Various topics: HD, UHD (2K / 4K) Digital Cinema acquisition to distribution.

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Old April 7th, 2014, 08:03 PM   #1
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Is 4K really "more demanding" on still lenses ?

Not sure if this is the right forum to post this on, but here goes. I hear a lot about "4K is extremely demanding on lenses!" Yes, I have zoom lenses made for still cameras that are amazing on 18MP sensor stills, and 4K is is only about 9MP. Is it really the case that these lenses are being stressed by 4K video acquisition? If so, why?

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Old April 11th, 2014, 12:28 AM   #2
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Re: Is 4K really "more demanding" on still lenses ?

There are optical techs who can explain the theory. My level of understanding is that of urban legend quality so please do not take to much notice of my comments.

Lenses as I understand things are tailored to the size of image they are intended to cover for an acceptable cost.

They are apparently intended to provide as near to uniform faithfulness of colour, contrast and sharpness across the entire image area.

Stills lenses are intended to yield this performance across an image about 32mm wide. To achieve acceptable cost/performance, they may not need to be as crisp as a lens designed to cover a much smaller image area to produce the same apparent sharpness in the final viewed product.

Many lenses tend to be sharper in the centre of the image than at the edges. It is proposed by some that this better centre performance enables stills lenses to be used effectively with smaller sensors or film formats.

Many digital cinema cameras with "35mm" sensors and 35mm motion picture film cameras use a smaller image area so take a smaller centre bite out of the image area thrown by a stills-camera lens.

For the same apparent sharpness of image across this smaller area to be achieved to the quality of the still-camera image at the same end-product display magnification, the resolution of the media ( film or digital sensor ) and the performance of the lens must be better.

On the SI2K digital cinema camera which uses an approx. Super16mm sized sensor, the sharpness numbers derived from a test chart via my best Nikon stills lens are lower, about 60% of what my Super16mm lenses yield to the same framing and lighting of the chart.

There are other optical theoretical reasons which I do not really comprehend at play in all of this.

Between the 32mm or so stills-image frame and the smaller 35mm digital motion picture frames which vary from about 22mm to about 28mm, there may be less apparent difference in comparative performance but the difference will be there.

A 2k digital rendition of a 28mm sensor size won't see the same difference as 4k will.

The best analogy may be the size 10 shoe on the size 7 foot. It will fit but which one will go ten miles making fewer blisters?
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Old April 11th, 2014, 01:23 AM   #3
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Re: Is 4K really "more demanding" on still lenses ?

Whilst I'm sure that modern computer designed lens are tailored to the frame size of the intended body, the fall-off of performance beyond the target area would not be a cliff-face as that would create other non-linear issues and would play havoc with any inadvertent reflections. I imagine that performance outside the wanted area would be allowed to decay gracefully.
Would it not be more about the demands of zoom and continuously variable aperture requirements of video? Video lens are expected to have smooth transition through the zoom range with minimum abberation. Zoom and Prime lens should also maintain constant aperture without stepping.
Zooms should have ratios greater than the 3:1 typical with high quality stills lens, normally up around 10:1 which is the territory of 'super zoom' for stills cameras that frequently have compromised performance in the pursuit of convenience.
Just an opinion, I'm no lens expert.
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Old April 28th, 2014, 09:05 AM   #4
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Re: Is 4K really "more demanding" on still lenses ?

The digital rebel (Rebel XT) from 2005 was 8 megapixels and that... is technically... 4K... It didn't do video though... uses EF lenses...
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Old April 28th, 2014, 11:21 AM   #5
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Re: Is 4K really "more demanding" on still lenses ?

Yes, that's my point. Still lenses are great for "greater than 4K" stills so why wouldn't they be the same for 4K video?
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Old April 28th, 2014, 03:39 PM   #6
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Re: Is 4K really "more demanding" on still lenses ?

Here is a quick overview of the relationship between lenses and camera sensors. There are two key measurements you need to understand about lenses. First, resolution is measured in terms of line pairs per millimeter, as projected on the sensor. A line pair consists of a black line and a white line of equal width with no space between them. Second, lenses have a modulation transfer function (MTF) which links the resolution with the contrast of the lens. As the lens attempts to pass finer detail (more line pairs per millimeter) the contrast decreases until the finest detail becomes grey. You cannot specify lens resolution without specifying the contrast because one company might specify the resolution at 80% MTF while another specifies at 50% MTF.

Lenses like the Canon EF series were designed for use with 36 mm still film cameras which have an image size of 36mm by 24mm. For illustration purposes, lets assume that an arbitrary EF lens has a minimum resolution of 40 lp/mm at some useful contrast level. When mounted a film camera, the lens will project an image that is 36mm in the longest dimension on the film. The resolution across the width of the projected image is 36 times 40 which is 1440 line pairs per picture width. In a full frame digital camera with a sensor width of 36 mm, the resolution across the picture width is the same 1440 line pairs.

Now assume that the same lens is mounted on camera with a smaller sensor. The Panasonic GH3 has a micro 4/3 sensor that is 17.3 mm across so there will be 692 line pairs across the width of the picture (17.3 x 40). The Canon C300 has a 24.6mm sensor width so the lens can project 984 line pairs/picture width (24.6 x 40) on that sensor. If you are shooting with a full frame DSLR, you can take full advantage of the resolving power of the lens across the whole 36 mm but if you use a camera with a smaller sensor then part of the projected image is 'wasted' because it does not illuminate the sensor.

In HD, the image size is 1920 by 1080 pixels for an image total of just over 2.1 megapixels. Ultra High Definition TV (UHD-1) is 3840 by 2160 pixels or about 8.3 megapixels. The correct use of '4K' applies to digital cinema which is slightly wider and has a higher pixel count. Professional still cameras are usually 16 megapixels or greater with a 3:2 sensor format. The useful pixel count is lower for wider HD and cinema aspect ratio images because the top and bottom are cropped off.

Now let's consider the digital image format. If the white line and the black line in a pair can be captured by two pixels, then an HD image requires 1920/2 or 960 line pairs across the image width. If a C300 is used to capture the image then the lens requires a resolution of 960/24.6 or 39 line pairs per millimeter. If a full frame sensor is used, the lens requires 960/36 which is 26.7 lp/mm. The UHD format is 3840 pixels across or 1920 line pairs. The C300 needs a lens with 1920/24.6 or 78 lp/mm while the full frame DSLR needs 1920/36 or 53.3 lp/mm.

So, to answer the question, UHD (4K) is much more demanding on lenses than HD. A lens may be able to project the number of line pairs needed for UHD but the contrast will be lower than at HD resolution. A large sensor camera with a given lens will be better at capturing UHD detail than a smaller sensor camera using the same lens.

There is more to the discussion that I have written but I hope it will provide a bit of background for anyone wanting to investigate the subject in greater depth.
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