Ephotozine Review: Z 100-400/4.5-5.6 S VR

Started Jan 25, 2022 | Discussions thread
JimKasson
JimKasson Forum Pro • Posts: 41,567
Re: Ode to Slanted Edges (vs Bench)
2

Jack Hogan wrote:

all good points, let me provide a different take on them. A Modulation Transfer Function is just a relevant way to interpret how an imaging system responds to points of light (Point Spread Functions) throughout the field of view. Since the image is a collection of very many such points of light it gives a good indication of what one can expect.

If obtained with proper technique, MTF from say a set of slanted edge captures can in practice give you much of the information useful to photographers that can be extracted from an optical bench - and vice versa. It nicely complements just judging what we see in published images. It allows us to quantify generic statements like 'the Zeiss 135 is better than the 100-400 at 135mm at infinity' (is it? by how much? will most people notice it? or do we have to trust the conflicted influencer du jour?). Which is why people like Jim Kasson show pictures *and* MTF information when evaluating critical lenses on a particular platform.

The main advantage of running slanted edge tests vs a bench is that if you are somewhat familiar with a command line interpreter you can do it for free at home, right now. On the other hand, you have to have a dedicated space not to mention a degree and a full time job in the optics industry to set up, keep calibrated and run an optical bench. Not to mention the cost of a bench: impractically unaffordable for lens review purposes (I wish I had a Hubble to replace the telescope in the yard but ...).

So we take a chunk of an afternoon to set up and take captures of slanted edges at various positions, orientations and typical usage set ups. A large utility knife works well, not perfect but often close enough. A flat chart has advantages and disadvantages. And, contrary to a bench, we get system results that also include the effect of the camera/sensor on resolution , which as photographers is useful. Want through-focus? Got it. Then we feed the captures to excellent open source MTF Mapper by DPR contributor Frans van den Bergh -and out come MTF curves and all sorts of fantastic information relevant to the spatial resolution performance of the camera and lens.

Z7+50mm/1.8S at f/4, G ch, from utility knife slanted about 4 degrees captured at 'infinity' (10m away).

As for testing at infinity: there is no such thing, since we are never able to test at actual infinity. Even optical benches are never *perfectly* collimated everywhere. So it's a question of diminishing returns and good enough. For most practical purposes 50x focal length or more is good enough, that would be about 22' for the 135mm above. I can do about twice that with minimal inconvenience in my house (I would have to hit the backyard for the 400mm; there is a gentleman who wrote a paper on this subject working in his condo's parking lot :).

Contrary to popular belief the slanted edge method does provide a full MTF curve, including *the higher (finer structure) spatial frequencies*, which however are often less than useful at working f-numbers because drowned in aliasing when pixel peeping a typical monitor at 100%.

Since most people would not know how to properly interpret an MTF curve, testers have tried to come up with a single figure that summarizes much of what the curve has to say as far as the spatial frequency response of a system is concerned. They have names like SQF, SQRI, CMT Acutance and, yes, MTF50. It turns out that in many practical cases MTF50 correlates well with the others (if you are interested in the reason why post a question in the PST forum or take a peak here ). They are quantitative and useful: for instance a 5% difference in MTF50 is hard to spot but by 10% most people notice it.

Perfect? Of course not, just another tool in the evaluation bag. But don't discount MTF from a slanted edge off hand just because it is not from an optical bench: you can perform the 135mm experiment up top yourself and get a decent quantitative answer useful to photographers - today.

Jack
PS On the other hand comparing results from different sites is not a good idea and a subject for another day.

Good summary, Jack. Mike touched on wanting MTFs at particular frequencies, rather than the frequencies for particular contrasts. Slanted edge MTFs can be reported that way. These days I usually compute a metric that I call microcontrast, which I define as contrast at 0.25 cy/px. The frequency could just as easily be in cy/mm, but I don't find that as useful.

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