Re: FM Lens test for Centering (Mount tilt / swing)
aman74 wrote:
Are figures one and three supposed to come out in vertical format? I’m guessing not by looking at the full [...]
I can’t tell ya how much someone helping or even reaching out on DM as well would be appreciated. I’ve already made one attempt in the frigid cold to two sites that didn’t work out.
I think you might be over-thinking this. It really isn't complicated. I can try to help, let's see if it works.
Here is an example:
Above: The whole picture. The black building in the center is, measured on map, about 550 meters away and it's a common apartment building. The image is taken a bright day using an A7CR, a 50mm lens at f/2, handheld, focus and exposure values are locked. A 100% crop looks like this:
Above: 100% crop of the center of the image, nothing strange.
Then I change one setting: Exposure compensation 1.5 stop up, or brighter, to compensate for vignetting. Now I take four more images now with the top of the building in each corner of the viewfinder. I don't turn the camera upside down or anything else.(That's not a critical extra finesse, we keep it simple and skip it for now.)
After getting the images i check the five images in the raw converter and enlarge the point of interest to either 100% or 200% depending on the size of the target I found. Here is the 100% crop of the lower left corner just to illustrate what the difference between center and corner can look like:
Above: 100% crop of the lower left of the image, nothing strange. What we are looking for is differences between the four corner crops.
In this case with the A7CR and a Sigma 50i (50/2 Contemporary DG DN) there was no problem.
So, I don't do it exactly the same way as Fred, keepcoding or hasenbein does it. The idea however is the same and in my experience it works well enoght to judge if the lens is good enough and to be kept or returned. If I may say so, just go ahead and don't be too worried about the size of the target. Just make sure it is distant enough, I would say not closer than 100 meters or so. I hope this is to some help and good luck with the lens!
Jan 30, 2025 at 04:19 AM
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