Just MTF Charts

Just MTF Charts: Sigma Prime Lenses

Published April 4, 2019

This is the third post publishing all of our MTF results so that methodology is consistent and they are easy to find. No comparisons, no commentary, just the test results for you to use and abuse as you see fit. We’ll continue today with the Sigma prime lenses.

For those who wonder, the results presented are for EF (Canon) mount lenses, but we do test them in F and FE mount also (with appropriate glass for the mount), and there is no difference.

A Quick How to on Reading MTF Charts

If you’re new here, you’ll see we have a scientific methodology to our approach, and use MTF charts to measure lens resolution and sharpness. All of our MTF charts test ten of the same lenses, and then we average out the results. MTF (or (or Modulation Transfer Function) Charts measure the optical potential of a lens by plotting the contrast and resolution of the lens from the center to the outer corners of the frame. An MTF chart has two axis, the y-axis (vertical) and the x-axis (horizontal).

The y-axis (vertical) measures how accurately the lens reproduces the object (sharpness), where 1.0 would be the theoretical “perfect lens”. The x-axis (horizontal) measures the distance from the center of a lens to the edges (measured in millimeters where 0mm represents the center, and 20mm represents the corner point). Generally, a lens has the greatest theoretical sharpness in the center, with the sharpness being reduced in the corners.

Tangential & Sagittal Lines

The graph then plots two sets of five different ranges. These sets are broken down into Tangential lines (solid lines on our graphs) and Sagittal (dotted lines on our graphs). Sagittal lines are a pattern where the lines are oriented parallel to a line through the center of the image. Tangential (or Meridonial) lines are tested where the lines are aligned perpendicular to a line through the center of the image.

From there, the Sagittal and Tangential tests are done in 5 sets, started at 10 lines per millimeter (lp/mm), all the way up to 50 lines per millimeter (lp/mm). To put this in layman’s terms, the higher lp/mm measure how well the lens resolves fine detail. So, higher MTF is better than lower, and less separation of the sagittal and tangential lines are better than a lot of separation. Please keep in mind this is a simple introduction to MTF charts, for a more scientific explanation, feel free to read this article.

Sigma Prime Lenses

For a look at all the Just MTF Articles we’ve done so far, be sure to check them out here.

Roger Cicala, Brandon Dube, and Aaron Closz

Lensrentals.com

March, 2019

Addendum: The Digital Picture is hosting the MTF charts on their comparison tool, putting them up as we publish them here. It’s a great way to compare two lenses for the one or two of you who like to do that. The Canon and Zeiss primes are already up, the Sigma should be added by tomorrow.

Author: Roger Cicala

I’m Roger and I am the founder of Lensrentals.com. Hailed as one of the optic nerds here, I enjoy shooting collimated light through 30X microscope objectives in my spare time. When I do take real pictures I like using something different: a Medium format, or Pentax K1, or a Sony RX1R.

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Roger Cicala

Mattr, I simply don’t have the time right now. Perhaps in late summer or fall.
Roger Cicala

Yes, although they are several weeks away. I’m going to get all the primes out first.
Michael Hickey

Will there be any zoom tests?
xWidget

Except for the 28mm f/0.14 I’m guessing 🙂
Andreas Werle

Thanks for the Explanation, Roger and the very interesting link. You should write a book!
Mattr

Hi Roger,
thank you for the extremely informative charts – keep up the good work! Next will be Nikon primes, I hope.
Here is one suggestion: Can you perhaps on one or two lenses (e.g. the 35/1.4 and 105/1.4) post a series of MTF results at different apertures, all the way to the smallest aperture? Many photographers have pretty good experience on how diffraction softens their images and such a graph would be an extremely useful reference for how your wide open results will translate in real life photography. In addition, it would provide some idea how MTF changes when stopped down for perhaps a few lenses of different designs.
Roger Cicala

Correct.
Mike Aubrey

Are there no results from the Sigma 70mm Macro?
Brian Smith

What an awesome reply! Thanks Roger!
Roger Cicala

Brian,
It’s probably a couple of things. By dip, I’m thinking where at 12-18mm or so it gets low and then comes back at the very edge. This is probably field curvature; because we have focused at the center “M” shaped fields will be out of focus a bit at this area. Since depth of field is narrower at higher lp/mm it shows as a dip.

When the sudden at the very edges, especially in wide-angle lenses it’s probably real, but could occasionally be the extreme angle of the collimator makes a false-positive drop. It can also be the same as above, but the lens has a “U” shaped field curvature. (Oh, and remember the sagittal and tangential fields are different shapes usually, so it can affect one more than the other.)

Also, it can just be that the lenses are worse in that area.

If you’re interested in the field curvature stuff (I think it’s awesomely important to understand how your lens works) try https://www.lensrentals.com/blog/2016/09/fun-with-field-of-focus-part-1/
https://www.lensrentals.com/blog/2016/11/fun-with-field-of-focus-ii-copy-to-copy-variation-and-lens-testing/
Ben Rubinstein

I used one of our 70mm Macro’s to shoot some portraiture of one of the studio’s employees, head shot stuff. It had surprisingly pleasing rendition actually, I hadn’t expected that.
Brian Smith

Thanks for another great update!

I’ve noticed in a lot of MTF charts that lenses tend to have a “dip” in the saggital lines around 40-50 lp/mm near the edges. Can anyone explain the phenomenon to me or is this just a tendency of lenses?
M Anton

Roger, thanks for that. Yet another LR article to try and get my head around! 🙂

Mike
Roger Cicala

Roger, that’s correct, we’ve done basically FF lenses here. We have to modify the MTF bench programming to test APS-C; since we developed our rapid MTF methods for in-house testing, which doesn’t require much modification, we use just that for APS-C. It works great for quality control, but doesn’t create standard MTF graphs.
Roger Cicala

M Anton, where we talk about tilt we’re talking about ’tilt in the field of focus’ caused by the optics of the lens. It can be in any direction. Part of the problem comes from the internet habit of calling all optical problems ‘decentered’, when actually there are different issues: tilt, decentering, and spacing errors. This post https://www.lensrentals.com/blog/2013/09/there-is-no-perfect-lens/ has a section ‘the causes of optical imperfection’ that describes better.
Roger Cicala

No, I’m including the macro lenses where we’ve tested them but the Sigma 70mm I don’t have 10 copies for, @Ben Rubinstein. I also need to point out these tests are at infinity and not macro distances. Some macro lenses that are really good at Macro distances aren’t as good at infinity. These tests are worthwhile for when you use a macro for general photography, less so for how it behaves as a macro.
Someone

Roger, what about macro lenses? Is there going to be a separate post? (However, macros are included in the Canon post)
Ben Rubinstein

I’d be interested in seeing the 70mm Macro if you ever get the chance, thanks!
M Anton

Apologies for the possibly dumb question but what exactly is tilt?

In my mind I have a lens attached to a camera that’s horizontal and parallel to the ground. The tilted lens would then angled (up).

If that’s simplistically right, would tilt always be ‘up’ and are we talking about all lens elements or a particular group within a lens?
Roger

No Sigma 30mm 1.4 Art (is that because it’s APS-C)? It is the only one I own so I was curious about how it performs.
Andreas Werle

Thanks for sharing the data, Roger.
Nick Podrebarac

That’s an excellent question & explanation. I learned something today 🙂
geekyrocketguy

Thank you! This makes sense and answers my question.
Roger Cicala

If you ever get the chance to fix a camera on a tripod and get 10 copies of the same lens, and just change lenses, you’ll see that ‘optical center’ isn’t the same for each lens. If your camera is lined up on a test target when you change lenses each is aimed just a little bit away from where the other is. It was this thing that made Imatest so useless for testing multiple copies. Each copy had to go through the realignment process which was time consuming.

The longer the focal length of the lens, the more noticeable this is, which makes sense.

When we run the bench we center everything on the first lens. Then the process rotates the lens, takes multiple MTF slices, we change lenses, etc. The other 9 lenses have slightly different centers than that first one, not enough to make a difference in the lens measurement, but the ‘zero’ of the various copies might be at +0.5 or -0.5.

Some copies are also slightly tilted. So when you rotate them, the optical center doesn’t stay centered as it rotates, it actually rotates around the center. Again, the longer the focal length, the more apparent.

Now we take 40 MTF curves (4 for each lens) and average them. With a longer focal length lens, that means “0” is actually a summation of 40 measurements that actually might be from 0.75 to -0.75 mm from center. Hence you get some sag/tan separation. You’ll see it on most of the zooms at 200mm, and some lenses at 100mm and up, rarely on shorter focal lengths.
geekyrocketguy

I’m a bit confused that the tangential and sagittal lines are not the same at r=0 for the 135mm lens. What does this mean?
Roger Cicala

Thanks for the heads up; I’m working on 4 posts from now, I might have never noticed. Should be relabelled now.
Baconator

No worries and thanks again for the tremendous amount of work you guys do.
Roger Cicala

Oops, I left the chart maker on auto. I’ll get them fixed but until then, they are the correct charts for the post labels.
Wade Tregaskis

Someone’s got the 20/1.4 on their mind… ?

(multiple charts are mislabelled as being for the 20/1.4)
Baconator

Thank you Roger! The 105mm chart says it’s 20mm, probably just a wrong label (the chart is different from the first 20/1.4).