Lens testing

[KeithB]

Bob:
When performing your lens testing instructions with the resolution chart, should I just fill my 95% viewfinder, or should I take some test images to make sure that the chart really fills the frame?

[KeithB]

Bob:
I just figured out - via your FOV calculator - a clever relationship you might want to mention in the testing article.  For the 1.6x sensors, (I am not sure if it holds for the FF sensors) the focusing distance will be the focal length of the lens expressed in inches.  (I believe it has to do with the 26x magnification being close to 25.4.)  For example, a camera with a 50mm lens will be 50 inches away from the target, a 300 mm lens 300 inches and so on.

[Bob Atkins]

Keith - that's correct (in principle). In fact it doesn't really matter what the distance is since any numerical calculations will take magnification into account. For telephoto lenses like a 500mm telephoto the distance is easy to measure - 500", though exactly where you measure to may be uncertain! For example with short zooms like a 10-22mm, when you measure the 10" to you measure to the front of the lens, the camera body or the focal plane of the sensor and what do you do with a lens that has seperation of the principle planes (as all retrofocus and telphoto lenses do).

So as you can see, measuring distance is just a rough guide. As I explain in the article (http://www.bobatkins.com/photography/technical/lens_sharpness.html) you can measure the mahnification you shot at pretty easily

What I do is make sure the corner targets are tight into the corners of the frame, especially with cameras that don't have 100% viewfinders. That's usually good enough.