On 1/1/2022 6:40 AM, Wayne Shumaker wrote:
. . .
I recommend reading Jim Kasson https://blog.kasson.com/lens-screening-testing/
There are links to test targets you can download and print. I printed one on
ledger size paper (11x17inch) and use it for testing. For example of one of my
tests at 100%:
There is a minimum distance the subject needs to be for center focus and
shifting camera to the corners based on focal length. For instance following
this link in the article
he says (note the 4th point):
Here's my test target. <https://photos.app.goo.gl/UhCim2BaFKD1rYCy5>
I imagine I have more shots of it than any other subject. 😁 I added smaller bars in PS, which have proved useful. I've
yet to find a lens that clearly resolves the smallest.
I have tested FLs from 24 to 400 mm using it indoors and to 600 mm outdoors. I
think his objections are overstated:
Target alignment is critical.
I agree, but don't understand why that's a problem.
a. Hang/mount target on a flat wall.
b. Measure height of target center, set tripod so lens is at that height.
c. Measure distance from side wall to target center, set lens at same distance
d. Aim lens at center of target.
e. alignment is achieved.
Target flatness is equally so.
The cork board is pretty flat. Theory may say it isn't flat enough. Practice has shown it's sufficient to clearly show
differences in lenses, apertures, etc.
The target size gets to be impracticably large if the lenses are short.
This is trickier. What is his definition of correct lens to subject distance? I believe the standard for many years was
1:40. Certainly that's what Norman (whatshisname) at Modern used, as well as others. I assume, possibly erroneously,
that optical differences from 1:40 to infinity are minuscule. Otherwise, why did it become standard? For Astro, one
could test this assumption, then go ahead with simple tests if OK.
That is simple with my target. Set camera so that the target fills a 4:3 frame, or the height of a 3:2 frame. That's
1:40, easy, peasy, if you make the target the right size. I can test from very wide to 400 mm in my house.
It's hard to practically impossible to test short lenses at realistic distances.
It's hard to light the target evenly
That's just silly, as an objection to a test method. It's not hard, and it
doesn't matter much.
It's hard to find a space to set up the test with a big target.
Back to #3, again.
Short focal lengths are harder because of the distance to the subject required
to keep it in the plane of focus when you shift the camera.
I'm not sure I understand this, in Mike's tests and in the links. The distance from lens to center of a flat target is
shorter than that to the edge. A flat field lens must then be designed to focus both at the same time, i.e., to have a
slightly curved focal plane. If I then do this reframing, am I not defeating that design?
If I want to know how it does with stars, why not take pix of stars? When Ctein finally found his astro lens, he sent me
a full size JPEG of the night sky, not a test target, or a house. Point sources at the edges tell a lot.
Since you are looking for good Astro lens wide open, this is the most important
Given all that I don't think your test is going to tell you what you want to
know. Perhaps go out to one of the marshes, angle the horizon and take a photo
at infinity, which seems to be the way they do it on FM
I wonder what that may tell me that a 1:40 shot of my test target won't. I'm not saying that isn't possible, just that I
have a hard time imagining it.
Not a lens test expert - WayneS
I'm no expert, either, in any global sense. I have found that what I see in my tests relates well to what I get using
the lenses in the 'real' world. And that's all I really care about.
As mentioned in at least one of your links, atmospheric movement often trumps optical excellence. And DoF in a 3D world
often laughs at focus. 😁
Through a Glass Moose
What if the Hokey Pokey *IS* what it's all about?
Themed Olympus Photo Exhibition: http://www.tope.nl/