Fuzzy edges-- does stopping down help?

[chris jordan]

Hi guys, please forgive a dumb question that I should know the answer to by now. I'm doing a shoot this afternoon that requires a lot of front lens tilt that pushes the limits of the image circle on my Nikkor 450M. The image is visibly fuzzy at the corners, and I'm wondering if this fuzziness gets any better, or goes away completely, by stopping down the lens? I have tried looking at the grould glass and stopping down the lens, but it is quite dark in here and I can't see anything when I stop then lens down. Any thoughts would be much appreciated.

~cj

www.chrisjordan.com

[Jorge Gasteazoro]

I cant imagine you getting to the soft part of the 450 with an 8x10. I use it on the 12x20 and at f/64 everything is sharp even when I use rise to the limits of the lens. IMO you are tilting so much that your plane of focus is just too great for the lens to handle.
If I was in your situation I would focus half way, apply the tilt, refocus until you get the best possible image and stop down to f/90. I have used the lens all the way to f/124 but it was for contact printing. I imagine since you are going to scan your transparency that f/90 and a bit of sharpening in PS might so the trick for you.

[Chris Gittins]

Ignoring tilt and plane of focus issues for the moment, stopping down should help sharpen the corners. The softness is probably due to coma. For a given off-axis angle, coma scales as 1/(f-number)^2, i.e., if you stop down by a factor of two the blur spot size should drop by a factor of four.

NB: The preceding is true for simple lenses. The details are probably a bit different - potentially significantly different - for the 450 M. (Sorry, I don't remember my optics that well.) That said, stopping down should help you out.

Chris

[QT Luong]

Compared to wide open, stopping down the lens will definitively improve the sharpness of corners up to a certain point (anything between f16 to f32). After that, there will be no more improvement, and then degradation due to diffraction effects.

[Emmanuel BIGLER]

In a lens affected by field curvature, stopping down does not cancel field curvature hence does not allow the edges to be sharp at the same time as the centre of the field. This an academic point of view, real lens situations being always more complex.
Now whether field curvature is visible or hidden beyond diffraction effects in a good "tessar-type" 450mm Nikon M, when stopped down generously, is another story.
Before blaming the lens, may be you could you check that the blur has a symmetrical shape around the optical axis. A small camera-induced residual unwanted tilt or swing can have a dramatic effet but will not be symetreical around the optical axis.