# Diffraction Question

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• 08-24-2008, 05:49 PM
keithwms
Jason, I also wondered about motion blur but supposed that even if there was some finger or shutter impulse, the OP's rig probably settled down in a small fraction of 2 sec. But you are absolutely right, if there was any continuous subject motion then it might well show up.

I am not familiar with this particular taking lens, is it really optimal at close focus? Does one use a diopter to get that kind of magnification? Is the focus match precise enough with a tlr to do this kind of thing?

(N.b. I still say that the scan looks good as far as I can tell!)
• 08-24-2008, 06:07 PM
JBrunner
Quote:

Originally Posted by keithwms
Jason, I also wondered about motion blur but supposed that even if there was some finger or shutter impulse, the OP's rig probably settled down in a small fraction of 2 sec. But you are absolutely right, if there was any continuous subject motion then it might well show up.

Yes, I guess what I'm trying to communicate is that movement, focus, or some other part of the process is a far more likely culprit than diffraction, in my opinion.
• 08-24-2008, 06:08 PM
Q.G.
Quote:

Originally Posted by JBrunner
I don't believe it has anything to do with diffraction, nor do i think the effective stop due to bellows factor is doing anything to create increased diffraction. [...]

Perhaps not the culprit here, no.

Often ignored, but diffraction increases every time you stop a lens down, no matter what image scale it is set to.
So much so that with roughly every two stops you close a lens down, resolution* is halved.
That's quite something.

*maximum achievable resolution - there are more limiting factors, like motion blur, or lens aberrations, which often are (even) worse than diffraction.
• 08-24-2008, 06:20 PM
JBrunner
Quote:

Originally Posted by Q.G.
Perhaps not the culprit here, no.

Often ignored, but diffraction increases every time you stop a lens down, no matter what image scale it is set to.
So much so that with roughly every two stops you close a lens down, (maximum achievable - there are more limiting factors, like motion blur, or lens aberrations) resolution is halved.
That's quite something.

I would qualify that as starting at where the lens is made to be sharpest. Most real world sharpness limitations that I have encountered with quality optics are set by the technique, circumstance or equipment, not diffraction. I am not, of course denying the principle, or that it shouldn't be considered, I just find it unlikely in this case. In reviewing the OP, I have realized he is using a TLR and now suspect the calibration of the focusing lens, or focus issues as a side effect of parallax (taking lens closer than focusing lens) as my primary suspicion.
• 08-24-2008, 06:31 PM
keithwms
I think one simply has to meditate on the definition of circle of confusion and realize that with a larger format neg, the enlargement factor for the final print is much smaller than what is required for a print from a 35mm neg.

There simply is no free lunch with optics. Either you enlarge at capture time or you enlarge at print time. And before somebody says "yes but you can always drumscan the smaller neg thus bypassing print enlargement..." well, sure! you can! But the starting smaller-format neg contains far less tonal information and rougher transitions from in focus and OOF elements.... these are issues quite separate from ultimate resolution of high-frequency detail.

So, again, one simply must assess which tool is right for the particular job. If I thought that shot I posted could have been done on 35mm, well of course I would've done it on 35mm. I don't have any particular compulsion to spend extra time and money on LF... ;)

Again, I worry about focus match with a tlr for macro. I mean, how do you know that it's right on the money.
• 08-25-2008, 02:56 AM
Jack Lusted
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
• 08-25-2008, 12:25 PM
Q.G.
Quote:

Originally Posted by JBrunner
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.
• 08-25-2008, 04:30 PM
ic-racer
Quote:

Originally Posted by Q.G.
Diffraction always increases with stopping down.

Good point; always, always always...
• 05-20-2009, 02:19 AM
Galah
Keithwms,

Just checked out your gallery: nice touch! :)
• 05-24-2009, 04:23 PM
Ed Sukach
Quote:

Originally Posted by JBrunner
... 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.
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