A Simple way to evaluate the focal plane curvature of a lens. August 2026

 

Understanding the characteristics of a lens can help us figure out the best approach to focussing and aperture selection for a subject like this

In an ideal world every camera lens would produce a perfectly flat focal plane  at all focal lengths (of a zoom) at any focussed distance and all apertures,  running exactly through the subject element on which we focus. 

In fact some lenses actually achieve this. True macro lenses are specifically formulated thus.

To clarify my use of the term focal plane in this post: I use this term for convenience. A lens actually produces a zone in which sharpness is greatest at some approximate mid point in the zone becoming less sharp both towards and away from the region of greatest sharpness. Textbooks say and in the real world it is often the case that this zone of sharpness extends a greater distance in actual measured units behind (away from) the sharpest  part than in front of the sharpest part.

Wide angle primes and the wide end of zooms often exhibit very strong curvature of the plane of focus which can vary with focal length (of a zoom)  aperture and focussed distance. 

I recently saw a review of a new ultra-wide zoom in which the test chart showed that if the lens is focussed at the center, it is out of focus at the edges and vice versa. The reviewer correctly deduced that the lens exhibits curvature of the focal plane but was unable to characterise this further due to the limitations of testing which uses a flat chart.

We also sometimes see that a sharp eyed tester has noticed that as a lens’ aperture is closed down the plane of focus moves towards or away from the camera. This is an important observation but only tells us what happens to the focus plane at one specific location in the frame. In fact focal plane shift is often complex and cannot be described as simply moving forward or back. What often happens is that the focal plane has a complex curvature which moves forward in one part of the frame and backward in a different part of the frame and this can vary with focal length, aperture and focussed distance.

Actual setup, on carpet in this case but any flat, level evenly lit textured surface will do

In this post I describe a simple procedure by which we can characterise the shape and curvature of the focal plane of any lens.

I first became aware of the basic principle involved by reading an article by Roger Cicala on the Digital Photography Review website several years ago. 

The task is to represent a complex three dimensional curved shape on a two dimensional communication medium, being a screen or page.

Please refer to the attached photos of the setup. 

The front face of the focus target is perpendicular to the optical axis and facing the camera. The slightly off-parallel vertical lines make it easy for the camera to focus sccurately with AF or MF.

I photograph a textured surface. This can be cut grass, a playing field, carpet, anything really,  as long as it has an evenly distributed  texture and is flat, preferably level and evenly lit.

I have figured out the details empirically which is a fancy way of saying that I try several different things and settle on the ones that work best in practice.

With the camera and test lens on a tripod, I focus on a straight stick ensuring accurate focus exactly on the stick with an angled focus target facing the camera which is elevated about 30 degrees from the flat plane. The camera needs to be far enough away from the straight stick to ensure we are well away from the close-up range. Many lenses produce a different focal plane curvature in the close-up range.

This shows distribution of the zone of sharpness for a premium, highly corrected zoom lens at the shortest focal length and widest aperture. There is no skewing of the sharpness zone indicating correct alignment of the optical elements.

Using the level gauge in the camera’s viewfinder I carefully align the camera with the straight stick. This is important. Some lenses which have not been assembled correctly will exhibit skewing of the focal plane which this procedure will reveal.

I make an exposure at each whole aperture step and a selection of focal lengths. 

The resulting files go into Photoshop where I go to Filter>Stylise>Find Edges. This produces the images you see in the attached photos. 

Note that in each case I focussed carefully on the focus target in the middle of the frame and exactly on the line of the front of the stick. 

Photo 6
The only change from the previous photo to this one is that the lens has been closed down from f1.8 to f4. The depth of the sharp zone has increased as expected but also the curvature of the sharpest part has increased.

So what do we see ?

The approximate middle of the zone of sharpness can in some cases be complex, sometimes curving both towards and away from the camera and often not passing exactly through the spot where we carefully focussed the lens.

This is not what the textbook would have us imagine at all.

Please note that this procedure tells us nothing about the actual amount of sharpness or resolution of a lens. It only tells us about the distribution of the zone of sharpness.

What is the point of this little exercise ?

Same lens, same test setup but now we are zoomed to 26mm. The distribution of sharpness is very different as you can see, in this case fairly close to perfection.

We may encounter a lens which is sharp in the middle of the frame but soft around the periphery or vice versa. Or a lens which is sharp in one part of the frame but not another part. Or a lens which appears to exhibit focus shift when the aperture is closed down. 

By running the test described in this post we can better understand what actually happens to the zone of sharpness produced by any lens at various focal lengths and apertures. This will often help us to better understand our initial observations and thus make more effective use of our optical devices.

Here is the result for a very good but less expensive and less highly corrected lens at minimum focal length and widest available aperture.  We can see that the zone of greates sharpness continues toward the camera at the periphery. Not bad at all but something to be aware of and not in the same class as the more expensive lens shown above. 

It might also explain why some camera makers are reluctant to allow third party makers to sell lenses for their proprietary mounts. 

For instance Canon typically focusses with the lens wide open then closes the aperture down to the set amount for the exposure. This means the lens maker has to ensure that focus shift is minimal and that the curvature of the focal plane is accurately taken into account when the focus motor in the lens drives the focus elements to their place. The calculations for this must be quite complex and different for each focal length, aperture and focus distance if we include close-ups.

Here is the result for a budget kit lens on the same test setup as those above. I have considerable experience with this lens which does a good job but we can see that the zone of greatest sharpness is not exactly on the focus target and deviates even further towards the periphery.  The relatively small maximum aperture allows this lens to give good results even though it is some way off optical perfection.

If, for instance we look at photo 6, the camera’s pre-programmed brain has to know to drive the lens focus motor so as to put the zone of greatest sharpness in the center of the frame back just the right amount behind the focus target so the average sharpness across the frame from left to right is optimised. That means the lens maker and the camera maker have to work very closely together on their focussing calculations.

Sony typically focusses at the set aperture so focus shift issues are minimised although focal plane curvature could still be problematic.

A. Concave towards the camera, like the inside of an umbrella
B. Concave away from the camera, like the outside of an umbrella. A difficult pattern to work with
C. Corrected type, seen at the wide end of better quality zooms or wide primes
D. Perfectly flat and aligned with the focus target, seen on macro lenses and good quality primes or zooms at longer focal lengths

Summary

This post describes a simple procedure by which we can discover information about focal plane curvature which is not available from general photography or flat test charts. This can help us to understand why some of our lenses appear to behave in unexpected ways. 

We might imagine that there has been a user error or focussing mistake when in some cases the problem is actually a characteristic of the lens.

I think it would be helpful if lens testers would routinely publish results of this procedure for every lens evaluated.