Lens resolution

[Baburam]

Dear Friends

 

I have tested lenses on century focus test chart. I am confused about the resolution of lens. For example. I found 15L/mm for okc 35mm at T2.3 . How can I know this is result is good or bad? What is basic measurement for resolution of lens? Where do I find this answer? Please help me.

Baburam Nepali

[David Mullen ASC]

You may have to do some research into MTF:

 

http://www.normankoren.com/Tutorials/MTF.html

http://www.luminous-landscape.com/tutorials/understanding-series/understanding-mtf.shtml

http://en.wikipedia.org/wiki/Optical_resolution

http://diglloyd.com/articles/UnderstandingOptics/understanding-MTF.html

 

 

Not that I understand all of that either. Truth is that for all practical purposes, I would just compare, on a lens projector, the lens against something considered to be the sharpest possible, like a ARRI Master Prime, for a frame of reference. I don't know if the numbers can really tell you how the image is going to look on the screen, you need some sort of frame of reference that you are familiar with.

[Neal Norton]

Hi Baburam:

 

What are you looking at the chart with? A digital camera? I have found all the digital cameras including the Epic to be limited to around 80 Lines per millimeter max. So, if you are following the directions and have set the camera at a distance of 50X the focal length of the lens and are using a very good monitor then you should be able to determine the resolution of the lens and camera system up to <80 L/mm.

Looking at the chart with an optical viewer (Unique Optics collimator) set at 50X the focal length of the lens, it is easy to see that at the center of the lens the resolution of most lenses will be greater than 112 L/mm. I have not seen any lens that is out resolved by a digital camera.

I do not know the OKC 35 lens but I would be very surprised at 15 L/mm. Make sure your flange focal distance is correct and that the chart is well lit.

 

Hope this was helpful.

 

Neal

 

[Dom Jaeger]

Measuring the perceived resolution of a lens is a much more complicated and subtle thing than a single number. David linked to various sites explaining MTF (modulation transfer frequency) because that is the best empirical measurement system we have, and the one lens manufacturers use when publishing specifications on their lenses. It factors in both resolution and contrast, as well as centre versus edge performance. There are even different graphs for sagittal and meridional MTF, because lenses often resolve a line that radiates out from the centre of the image differently to one that circles around the centre.

 

As a starting point, this webpage is a short but decent example of trying to extrapolate meaning from a test chart like yours using line pairs/mm and MTF:

http://www.creativeplanetnetwork.com/dv/feature/dv101-modulation-transfer-function-sizing-lens-resolution/60198

 

It's worth remembering that this kind of test is actually measuring the 'system' resolution of the camera/lens combination.

[Dom Jaeger]

Not that I understand all of that either. Truth is that for all practical purposes, I would just compare, on a lens projector, the lens against something considered to be the sharpest possible, like a ARRI Master Prime, for a frame of reference. I don't know if the numbers can really tell you how the image is going to look on the screen, you need some sort of frame of reference that you are familiar with.

 

I thoroughly agree, David. Still photographers seem to get quite hung up on MTF, but I imagine most professional cinematographers would be more familiar with lens projection. As a lens technician I certainly get much more information from projecting a lens and using my eyes than from using MTF readings.

Edited June 13, 2013 by Dom Jaeger