Diffraction Question

[Jack Lusted]

Thank you all for your comments.
WRT vibration in the set up - I would be surprised if this were a significant factor as the photo was taken indoors, the subject and the camera were both well supported with the shutter tripped (naturally) by cable release. As noted, the camera is a TLR. Parallax was taken care of by a 'paramender' which puts the taking lens in the same position as the viewing lens. Generally the lack of sharpness seems constant - that is there does not appear to be a plane of sharpness 'behind' or 'in-front' of the plane I was intending to focus on.
On reflection it seems to be that there may be a diffraction effect, or the lens is not up to the job - it is not a bona fide macro lens.

Methinks that I'll do some tests, if only to find out if the lens has a 'sweet spot'.

Jack

 

[Q.G.]

I would qualify that as starting at where the lens is made to be sharpest.

That's O.K.
But only with the insertion of the words "to show". As in " starting to show at where the lens is made to be sharpest."

Diffraction always increases with stopping down. Whether you will notice any ill effects depends on what at the time is limiting performance. The effect of aberrations may be far worse than that of diffraction.
But diffractions starts to reduce the maximum possible resolution the moment you use anything but the largest aperture.

 

[ic-racer]

Diffraction always increases with stopping down.

Good point; always, always always...

 

[Galah]

Keithwms,

Just checked out your gallery: nice touch!

 

[Ed Sukach]

... Most real world sharpness limitations that I have encountered with quality optics are set by the technique, circumstance or equipment, not diffraction...

You are absolutely correct, JB.

"Diffraction limitation" is NOT linear, but much more like "falling off a cliff" when a certain ratio of aperture-to-focus lentgh is exceeded. That ratio (note that I am avoiding the idea of `f/stop`) is far greater with longer focal lengths - and that is why a 50mm lens will only have limit of f/16 and a 500mm may be stopped to f/64.

Somewhere in this mess, there is an empiracal formula for determining the location of the edge of that cliff - if I ever get my gluteus maximus sufficiently above the alligators.

 

[Q.G.]

In high magnification work - where depth of field is hard to find, hence small apertures are popular* - you frequently run into diffraction as a very real limiting factor.

(* Shouldn't be, since no matter how much you stop down, depth of field will be minimal.)

 

[ic-racer]

by using the small f:22 stop to increase depth of field did I inadvertently introduce a noticeable diffraction effect sufficient to to take the edge of the sharpness?

Yes

Would the image have been sharper had I used f:8?

In the center, Yes, but the edges will likely be better around f11.