Monday, November 11, 2013

Tools of the Trade ~ Resolution and the Real World

Let's have a serious look at lenses and the ever-popular topics of resolution and IQ, shall we?

After reading my prior two posts (one and two) that set the stage for this series on Tools of the Trade, a reader should be able to easily follow details in this post.  I am about to make a potentially bold series of statements, and then will back them up with what I know from years of my own camera system testing.

1. Camera sensors currently limit image resolution.  Lenses do not. 

I know this is true from my own testing of optical resolution.  I looked at large and medium format lens performance on film, and, more recently, 35mm lens performance on digital sensors.  It took me over a decade of looking at this to understand what the results were clearly showing me from day one.  My understanding of optics, resolution, and true limiting factors to resolution were later confirmed by an optical physics professor who performs research at a university in the US.

It takes a terrible lens to see degradation in image resolution.  Are poor resolution lenses available?  It seem that there are not many, and those that exist tend to be priced accordingly.  Except if it has Zeiss or Leica label it.  Then you see the poor performers termed "quaint" or "having a certain look."  At the other end of cost, inexpensive kit lenses have had enough pressure from "pixel peepers" to force manufacturers to improve those optics (simply look at the number of 18-55mm kit lenses Canon has offered over the past decade).

Readers need to approach comments such as "...It is crop, not FF, that requires sharper lenses, since for photos displayed at the same size...", or any combination of "...this new big sensor requires sharper lenses..." with extreme caution.  The only factor of interest in terms of physics and resolution are the number of line pair per millimeter the sensor resolves.  The present limit of sensor resolution of any APS-C, micro 4/3rd's, Full Frame, Medium Format sensor remains less than 120 line pair per millimeter.

An optical physic effect called diffraction limit effects optical resolution only at very small apertures.  If your sensor resolves around 123 line pair per millimeter (center diffraction limit of any optic at f/11), you may begin to see resolution degradation start at f/16 and continue through to the end of your aperture range (to f/32 and beyond).  This leaves a very long aperture range available to you.  From wide open down through f/11, all these apertures will be available to you, and in terms of resolution, will out-perform your sensor.  This physics effect won't be seen on lower resolution sensors (including most APS-C and all Full Frame sensors in current production) until an optic is stopped down to f/16 and beyond.

Note: The obvious exceptions are "soft focus" optics that deliberately smudge a scene.  Nothing in over a hundred and fifty years of photography has changed.  

2. Modulation Transfer Function (MTF) charts do not tell us how sharp a lens is.

MTF charts only tell us the amount of scene contrast a lens is capable of passing along to the sensor at various low resolutions.  Look closely at any MTF chart and you will see lines that show the contrast given at, for instance, 10 line pair per millimeter resolution and another set of lines that show contrast retention at, for instance, 40 line pair per millimeter.  Given the physics of optics these are rather low resolution settings (scroll down to see the diffraction limit chart).

If MTF testing is not a measure of resolution, why then do lens manufacturers publish MTF charts?  It's because the human eye perceives resolution in most cases as contrast.  In practical terms, digital sensors will be able to capture a quick transition from black to white as long as a lens to provide it.  It's as simple as that.

When reading comments across the 'net, statements such as " you can see from the above MTF charts, now that you know how to read them, the difference that are seen can be easily be quantified..." need to be approached with extreme caution.  There is nothing in a MTF chart which correlates in any meaningful, direct way to other optical properties what you may find important.  This includes sensor resolution, field flatness, lens distortions, or chromatic aberrations.

Again, the only thing MTF is attempting to show is a lenses ability to pass contrast to the sensor.  And that, only on a flat two dimensional plane.  This last sentence has importance when we talk about field flatness.

3. Chromatic Aberrations (CA) can be measured and provide useful information about how a lens can perform at the edges of a scene at different apertures.

Many currently published lens tests measure a lenses CA.  It's worth the time it takes to review lens tests in this area as there is a real world and meaningful correlation between test results and real world camera system performance.

Let's take a look at three lenses: Canon 50mm f/1.4 USM, Zeiss 50mm f/1.4 Planar T* ZF, and Leica 50mm f/1.4 Summilux R.

What do we see?  Canon's CA, as measured at the edge of the image frame, is substantially less than one pixel width from f/1.4 all the way through f/11.  The Zeiss' CA is at least one pixel width, and varies according to aperture.  The Leica's CA also crosses over 1 pixel width at all apertures.

In the real world when using a Zeiss or Leica 50mm lens, a single pixel at the edge of a light to dark transition at the edge of the image frame may show purple or blue/green "fringing".  Is it enough to worry about?  That depends on your "pixel peeping" experiences.  A lenses inability to bring together the visible color spectrum to a common point may not be visible in a very large print.  You would need to decide.

Let's say you decide that a pixel's width of CA is important enough to you to avoid.  Taking this position, the currently priced 300USD new auto-focus Canon 50mm easily out-performs both the manual focus new 725USD Zeiss ZF and a used eBay'd 1100USD to 1600USD Leica Summulix R.  So understanding the level of CA a lens exhibits might be important in evaluating it's "performance" (using the subjective word).

Further, processing (either in-camera or on a computer) can eliminate CA effects.  Olympus and Panasonic are well known for providing this kind of processing in-camera.  In-camera CA correction by Canon, Nikon, and Sony should be catching up shortly (if they haven't already).

4. Different lens render the out of focus (OOF) areas in a scene differently.

The highly subjective phrase of "good" OOF attempts to define something called "bokeh".

You may read arguments on the 'net about OOF of one lens or other and which gives a better result than something else.  If "bokeh" is important to you, all that matters is that out of focus areas in an image give an even distribution of light across OOF highlight areas.

Out of focus area rendition testing is quite common.  On a practical level, any desired "bokeh" effect can be reviewed and compared between various lenses.  Note that there is nothing in a lens design nor in a MTF chart which would indicate how OOF will be rendered.

The exceptions, of course, are lenses that deliberately manipulate OOF areas.  Way back in the mid-1800's OOF effects were mathematically manipulated, starting with Petzval lenses.   The optical effects are in great demand today, if eBay auction results for Petzval lenses are any indication.

In the early part of the 20th century, Contax designed their lenses to produce a "creamy" OOF.  Leica lenses, on the other hand, were and are designed in a way that tend to give a "harsh" OOF.

In current times, Nikon offers two wonderful lenses, the 105 f/2 DC and 135mm f/2 DC.  Nikon's optical team used well understood optical principles that allow a user to change the lens element spacing which directly effects OOF. Twist the ring and change the OOF.

In reading comments across the 'net, statements such as "One of the areas of image quality that MTF can help determine is bokeh..." need to be approached with extreme caution.  There is absolutely nothing in a MTF chart that meaningfully relates to "bokeh".  A whitepaper from Zeiss confirms this.

5. Field flatness, or field curvature in lenses can be an important factor in determining optical performance.

Macro lenses are typically designed to ensure a flat field.  They are many times used in photographing documents, stamps, and other flat subjects.  On the other hand, many zoom and wide angle lenses suffer from varying degrees of field curvature.  Photographers using such lenses may feel, under certain circumstances, that a lens is not "good" (to use the subjective word).

If you photograph a flat two dimensional surface, such as a painting, and see that the edges are out of focus, but that the center is correctly sharp, you may be experiencing the effects of field curvature.  In this situation you could set the aperture to f/11 (which is at or above the limits of your sensor's resolution) and try again.  If the edges come into acceptable focus, your lens might suffer from only mild field curvature that is easily handled by selecting an aperture with sufficient depth of field to cover for the effect.

If you try to use MTF charts to fully evaluate a lenses performance, you can miss something important.  Take the MTF examples in this "test".  In noting the "drop off" of contrast toward the edge of the frame, the writer suggests the performance of Canon 400mm f/5.6L is superior to Canon's 100-400 f/4.5-5.6L.  It's important to realize that most MTF tests do not account for field flatness and will limit testing to a two dimensional surface.  In this case, if there is field curvature in the 100-400L the MTF results would not accurately illustrate the lenses contrast capturing abilities on the curved regions of focus.  To the MTF test, the edges of the frame would be less contrasty than the center by a fair amount.  I am making this particular point since there is a large community of photographers who claim their 100-400mm Canon L lenses are indeed quite sharp and contrasty across the field to dispute the Luminous Landscape writer's claims.

Before claiming that a lens is "bad", a user might want to check to see what the field curvature is before tossing the optic out.

6. Lens distortions (barrel or pincushion) can easily be seen and are a nuisance to correct when straight lines are important.

Lens distortions are easily measurable and many testers report their findings.

Back in my old film days, it was commonly accepted that 35mm wide angle, some "normal" and "short telephoto" lenses suffered from field distortions.  One of the most vivid examples came from a Canon SLR shooter who used an 85mm f/1.2L to photograph trains.  The photographer complained that the lenses barrel distortion was bad enough that straight lines were nearly always bent in his images.

Shooters of architecture are well aware of the issue of distortions.  I am convinced this is why companies like Sinar and Schneider continue to make cameras and lenses.  It's important to have an accurate and correct solution when you need it, and when cost is not the prime force in image generation, such solutions can provide a most direct solution..

From a lens design perspective, it is easier to control the broad range of design issues with a symmetrical lens than it is with a complex asymmetrical optic.  Look at a cross section diagram of a plasmat lens and compare it against a low cost kit lens.  What do you see?  Count the number of lens elements in each design.  Now imagine building one?  Which would be "easier"?

With the advent of software driven lens designs, manufacturers are able to build lenses of incredible complexity, while at the same time controlling and balancing trade-offs between resolution, contrast, chromatic aberration, field flatness and optical spatial distortions.

Which brings us back to resolution.  When a photographer "pixel peeps" and claims one lens is better than another, most of the time they woefully mis-understand the camera system's imaging system and it's capabilities and actual characteristics.  Further, readers of "tests" that share photos made with various lenses may be confused or under-educated by the lack of carefully gathered and properly understood and shared information.

While this blog entry has become much more complex than I originally intended, I remain interested in making sure the proper background is set for my making the claim that it does not matter what camera or which lens you use as long as you know how to use what you have.

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