• site home
  • blog home
  • galleries
  • contact
  • underwater
  • the bleeding edge

the last word

Photography meets digital computer technology. Photography wins -- most of the time.

You are here: Home / GFX 100 / Radial and tangential edges and blur

Radial and tangential edges and blur

July 16, 2021 JimK Leave a Comment

This is one in a series of posts on the Fujifilm GFX 100S. You should be able to find all the posts about that camera in the Category List on the right sidebar, below the Articles widget. There’s a drop-down menu there that you can use to get to all the posts in this series; just look for “GFX 100S”. Since it’s more about the lenses than the camera, I’m also tagging it with the other Fuji GFX tags.

There has been some confusion about the directions that I’ve been using in my off-axis testing of the Fuji GF lenses. I sympathize; the topic is inherently confusing on several levels. I’ll try to peel that onion for you.

The first step is to understand what I mean by radial and tangential directions. I am using those terms because, in a well-constructed lens, lens aberrations have radial symmetry, or symmetry in which the aberrations exhibit correspondence or regularity  around a central axis in the center of the image. I’ve drawn lines in the radial direction on the image below (made with the 23 mm f/4 GF lens).

Radial directions

Now here are lines in the tangential direction:

Tangential directions

If we refer to the edges in the slanted edge figure in the target, we can see that one of them is more-or-less in the tangential direction, and the other is mostly in the radial direction (they’re not exactly in those directions because they need to be a little off to make the slanted edge technique work). I’ve labeled the edges on the drawing below:

Radial and Tangential Edges

And that’s all you need to understand the testing I’ve reported on so far. However, there is one more layer of complexity to peel away. The slanted edge method measures the blur perpendicular to the edge being analyzed. So the tangential edge is used to measure radial blur, and the radial edge is used to measure tangential blur. I’ve marked the edges appropriately below:

 

Blur directions

In lens testing, the usual convention is to talk about the direction of the blur rather than the direction of the edge. Here is an Imatest analysis of the radial edge in the above image:

Note that it says the edge profile is tangential. And, if you understand what they’re doing, it makes sense. They are analyzing the profile of the edge perpendicular to the direction of the edge itself. In the nomenclature that I’m describing, radial is perpendicular to tangential.

Here’s what Imatest has to say about the tangential edge:

 

Wait, what? It’s calling that profile sagittal! In this context, sagittal is another name for radial.

GFX 100, GFX 100S, GFX 50S

← Transfocal bokeh — 32-64/4 on GFX 100S Off-axis contrast of Fuji 45-100/4 on GFX 100s →

Leave a Reply Cancel reply

Your email address will not be published. Required fields are marked *

May 2025
S M T W T F S
 123
45678910
11121314151617
18192021222324
25262728293031
« Apr    

Articles

  • About
    • Patents and papers about color
    • Who am I?
  • How to…
    • Backing up photographic images
    • How to change email providers
    • How to shoot slanted edge images for me
  • Lens screening testing
    • Equipment and Software
    • Examples
      • Bad and OK 200-600 at 600
      • Excellent 180-400 zoom
      • Fair 14-30mm zoom
      • Good 100-200 mm MF zoom
      • Good 100-400 zoom
      • Good 100mm lens on P1 P45+
      • Good 120mm MF lens
      • Good 18mm FF lens
      • Good 24-105 mm FF lens
      • Good 24-70 FF zoom
      • Good 35 mm FF lens
      • Good 35-70 MF lens
      • Good 60 mm lens on IQ3-100
      • Good 63 mm MF lens
      • Good 65 mm FF lens
      • Good 85 mm FF lens
      • Good and bad 25mm FF lenses
      • Good zoom at 24 mm
      • Marginal 18mm lens
      • Marginal 35mm FF lens
      • Mildly problematic 55 mm FF lens
      • OK 16-35mm zoom
      • OK 60mm lens on P1 P45+
      • OK Sony 600mm f/4
      • Pretty good 16-35 FF zoom
      • Pretty good 90mm FF lens
      • Problematic 400 mm FF lens
      • Tilted 20 mm f/1.8 FF lens
      • Tilted 30 mm MF lens
      • Tilted 50 mm FF lens
      • Two 15mm FF lenses
    • Found a problem – now what?
    • Goals for this test
    • Minimum target distances
      • MFT
      • APS-C
      • Full frame
      • Small medium format
    • Printable Siemens Star targets
    • Target size on sensor
      • MFT
      • APS-C
      • Full frame
      • Small medium format
    • Test instructions — postproduction
    • Test instructions — reading the images
    • Test instructions – capture
    • Theory of the test
    • What’s wrong with conventional lens screening?
  • Previsualization heresy
  • Privacy Policy
  • Recommended photographic web sites
  • Using in-camera histograms for ETTR
    • Acknowledgments
    • Why ETTR?
    • Normal in-camera histograms
    • Image processing for in-camera histograms
    • Making the in-camera histogram closely represent the raw histogram
    • Shortcuts to UniWB
    • Preparing for monitor-based UniWB
    • A one-step UniWB procedure
    • The math behind the one-step method
    • Iteration using Newton’s Method

Category List

Recent Comments

  • JimK on How Sensor Noise Scales with Exposure Time
  • Štěpán Kaňa on Calculating reach for wildlife photography
  • Štěpán Kaňa on How Sensor Noise Scales with Exposure Time
  • JimK on Calculating reach for wildlife photography
  • Geofrey on Calculating reach for wildlife photography
  • JimK on Calculating reach for wildlife photography
  • Geofrey on Calculating reach for wildlife photography
  • Javier Sanchez on The 16-Bit Fallacy: Why More Isn’t Always Better in Medium Format Cameras
  • Mike MacDonald on Your photograph looks like a painting?
  • Mike MacDonald on Your photograph looks like a painting?

Archives

Copyright © 2025 · Daily Dish Pro On Genesis Framework · WordPress · Log in

Unless otherwise noted, all images copyright Jim Kasson.