Testing lenses with grass 29 December 2020

RF 50mm f1.8 at f1.8. Find edges filter. The lens has focussed right where I told it to, on the leaf. The  distribution of sharpness in front of and behind the focal plane is fairly even. The zone of sharpness is slightly tilted anti-clockwise in the photo.  In front of the camera the plane of sharpness is, relative to the focus point,  further from the camera on the right side and closer on the left side.  There is some detectable curvature of the focal plane away from the camera towards the edges of the frame.  In the field this lens performs well with no substantial problems although the corners are soft at the wider apertures.

In October 2020 Roger Cicala published  in D P Review  a method for testing field curvature of lenses using grass as a test subject. Really. Here is the link:

https://www.dpreview.com/opinion/4042117089/roger-cicala-why-i-dont-use-an-mtf-bench-to-test-my-own-lenses

I have for several years been aware that testing lenses by photographing a flat test chart of some kind does provide considerable useful information but cannot tell us about the three dimensional distribution of sharpness including any field curvature.

It turns out there is a very simple way to display this.

All I need to do is photograph a field of grass then apply Photoshop [find edges] filter.

Easy-peasy.

Method

I select an area of grass about 15x8 meters and place a large yellow leaf on the grass about 2.5 meters from my feet so grass fills the frame, hand holding the camera at eye level with the focus area in the center of the frame. I focus on the leaf and make shots at the widest available aperture for the lens and each stop down to f16.

In Photoshop I go to Filter>Stylise>Find edges. This produces the kind of result you see in this post with the sharpest parts of the image represented by the darkest parts of the filtered image.

I can easily see how sharpness is distributed both side to side and from near to far.

So I can obtain information about 

* Depth of field

* Field curvature 

* Field tilt

Example

For this post I show results with the RF 50mm f1.8 lens.

RF 50mm f1.8 at f16  Everything is decently sharp at f16 but nothing is tack-sharp. There is no part of the frame showing a very dark tone.

RF 50mm f1.8 at f4.  The tilt in the sharp zone is a bit easier to see here than at f1.8. The distribution of sharpness has a moustache/W type shape.   Notwithstanding this minor technical imperfection the lens gives a very good account of itself in real world use.