Best Technique to Test Corner Sharpness: All the talk about "soft corners" just wondering what is the best way to asses/test the corners? Assume you have a test chart or a nice big...

tinplater Loc: Scottsdale, AZ

All the talk about "soft corners" just wondering what is the best way to asses/test the corners? Assume you have a test chart or a nice big brick wall..something flat. Do you focus on the center then examine the corners? Or do you move flexible focus spot to the corner and shoot, or do you use the center focus but focus at corner, keep focus and then go back to center and shoot? Focusing at the center would seem to bring into play the fact that the corners are farther from the lens center than the center portion of the image.

John_F Loc: Minneapolis, MN

I would imagine your test shots would be affected by the f-stop in that that is your principle way to control depth of focus.

jcboy3

tinplater wrote:

All the talk about "soft corners" just w... (show quote)

A corner can be soft for two reasons: it is just plain soft, or the focus plane is not perpendicular. In the first case, you can never achieve sharp focus in the corner. In the latter case, you can achieve sharp focus but then the center will not be sharp.

You test corner sharpness first by focusing in the center of a target that is perpendicular to the camera lens. Brick walls work well; I use a mirror attached to the wall to ensure the line of sight is perpendicular (the reflection of the lens should be in the center of the image). Then, you can make small adjustments to focus, or small rotations of the camera, to assess corner sharpness.

tinplater Loc: Scottsdale, AZ

jcboy3 wrote:

A corner can be soft for two reasons: it is just p... (show quote)

Thanks but I don't understand the last sentence. If I want to take a SINGLE photograph and for the purpose of assessing corner sharpness, do I focus in the center of the target (meaning the corners are farther away than center) or do I keep camera centered on target but focus on corner (either manually or by moving the flexible focus spot)? It seems to me most people say corners are soft in images when in fact the corner may be in an entirely different focus plane than the center.

rehess Loc: South Bend, Indiana, USA

tinplater wrote:

All the talk about "soft corners" just w... (show quote)

Make F-stop at least f/4 and all distances would be within DOF in any case. Even wider might work, but I'm too lazy to do the trig right now.

In any case, focusing in center is replicating the field conditions you want to test.

RWR Loc: La Mesa, CA

tinplater wrote:

Thanks but I don't understand the last sentence. If I want to take a SINGLE photograph and for the purpose of assessing corner sharpness, do I focus in the center of the target (meaning the corners are farther away than center) or do I keep camera centered on target but focus on corner (either manually or by moving the flexible focus spot)? It seems to me most people say corners are soft in images when in fact the corner may be in an entirely different focus plane than the center.

You want a flat surface with fine detail parallel to the camera - meaning the center and edges will be of equal distance - focus wherever you wish.

RWR Loc: La Mesa, CA

John_F wrote:

I would imagine your test shots would be affected by the f-stop in that that is your principle way to control depth of focus.

The f-stop has no effect on depth of focus.

tinplater Loc: Scottsdale, AZ

RWR wrote:

You want a flat surface with fine detail parallel to the camera - meaning the center and edges will be of equal distance - focus wherever you wish.

I don't think that is supported by geometry. Depending upon the field of view (focal length of camera, sensor size) the edges and corners are definitely further away than center. That is the crux of my understanding of how to determine sharpness when the distance varies between center and edge/corner.

tinplater Loc: Scottsdale, AZ

RWR wrote:

The f-stop has no effect on depth of focus.

Really? I thought depth of focus/field was directly related to f stop.

tinplater Loc: Scottsdale, AZ

RWR wrote:

You want a flat surface with fine detail parallel to the camera - meaning the center and edges will be of equal distance - focus wherever you wish.

Here is a sketch, roughly to scale. Camera 8 feet from wall, FOV of lens is 45 Degrees. The edges in this example are half a foot further away from the camera lens than the center of the wall. Shooting wide open should give you best information about lens inherent sharpness...but again do you focus at the center to judge sharpness or on the edge?

(Download)

RWR Loc: La Mesa, CA

tinplater wrote:

Really? I thought depth of focus/field was directly related to f stop.

https://photo.stackexchange.com/questions/13191/what-is-the-difference-between-depth-of-field-and-depth-of-focus
https://www.videomaker.com/article/d2/13086-the-difference-between-depth-of-field-and-depth-of-focus

RWR Loc: La Mesa, CA

tinplater wrote:

Here is a sketch, roughly to scale. Camera 8 feet from wall, FOV of lens is 45 Degrees. The edges in this example are half a foot further away from the camera lens than the center of the wall. Shooting wide open should give you best information about lens inherent sharpness...but again do you focus at the center to judge sharpness or on the edge?

Good point. I always focus on the center, and had never heard or read about focusing on the edge. I do have a never used set of USAF lens test charts with instructions somewhere, should dig them out and do some studying.

Longshadow Loc: Audubon, PA, United States

tinplater wrote:

I don't think that is supported by geometry. Depending upon the field of view (focal length of camera, sensor size) the edges and corners are definitely further away than center. That is the crux of my understanding of how to determine sharpness when the distance varies between center and edge/corner.

You're over analyzing?

rgrenaderphoto Loc: Hollywood, CA

Here are some free lens calibration targets:

http://default.secure.media.ipcdigital.co.uk/11134/0000028af/b9e0/ResolutionCardDownload.jpg

tinplater Loc: Scottsdale, AZ

Longshadow wrote:

You're over analyzing?

All I really want to know is what technique is best to judge corner sharpness. Since the corners are farther away than the center, point of focus becomes critical. So does distance from the target. So does aperture. My thought is to shoot wide open which should expose maximum unsharpness.