Hopper/unknown/ladybird macros

[rjlittlefield]

One caution about stopping down farther: diffraction blur.

That lady beetle is maybe 2X onto the sensor. At 2X, marked f/13 is more like effective f/39. Marked f/16 would be effective f/48, and so on -- way too small an aperture to be sharp. When I take a look at the "full size" version at http://farm4.static.flickr.com/3071/291 ... 85f7_o.jpg , it already looks a bit soft, and it's only 814 x 1024.

I think you made a good choice, P_T. Stopping down farther would not have exposed much more detail in areas that are now OOF, and it would have softened even farther areas that are now fairly sharp.

--Rik

[Ken Ramos]

Well the math is over my head but the images aren't. Those are really nice there P_T Especially the second and third!

[Harold Gough]

I would normally use a high quality x2 teleconverter to get the extra stop without closing down and thus doubling the magnification at the same working distance.

Harold

[rjlittlefield]

Yes, this is a win for working distance. But inserting a 2X teleconverter also doubles the effective f-number, very much like extending the lens to get the same magnification. There's no free lunch -- the tradeoff between DOF and diffraction blur is the same no matter how you get the magnification.

--Rik

[Harold Gough]

Rik,

That's why I use a converter!

On the matter of diffraction in lenses in general, I have always understood that the concern is about that arising from the edges of the diaphragm. As you close down to a smaller and smaller aperture, that part of the image with the worst of the diffraction effects contributes an incrementally increasing proportion of the image. I don't see how that can be increased by using a teleconverter, which uses the part of the image furthest from the diaphragm.

Harold

[rjlittlefield]

On the matter of diffraction in lenses in general, I have always understood that the concern is about that arising from the edges of the diaphragm.

That understanding is not correct.

The edges of the diaphragm really have no significant effect.

What matters is that the aperture restricts the maximum angle between light rays, which in turn restricts their ability to interfere with each other to produce differences in intensity that the sensor can detect.

Graham Stabler and I had a long discussion about this some months ago. The discussion is hard to follow, and then takes off on another point, but take a look at the illustration at http://www.photomacrography.net/forum/v ... 0&start=47 .

The point is that light rays striking the sensor at relatively steep angles with respect to each other can interact to form fine patterns of intensity, while light rays striking the sensor at narrower angles can only form coarser patterns.

DOF depends on the same angles, hence my comment that there is no free lunch -- the tradeoff between DOF and sharpness is the same no matter how you get the magnification. The nominal setting of the lens to get a particular DOF/sharpness will vary depending on whether you use extension or a teleconverter, but the combination of DOF and sharpness will not.

--Rik

[Harold Gough]

I'm not sure that all this information about interference has much to do with diffraction, the latter being about change of direction, in which scattering would be included.

Harold

[rjlittlefield]

Harold, I started a separate topic to continue this discussion.

See What really causes "diffraction blur"?, in the Technical Discussions forum.

--Rik