mp-e 65mm diffraction question

[Jon Ruyle]

Dann:


Those are great pics! Exactly the type that makes me want the lens. Of course, it is a common trap to see someone else's great pictures and want the same equipment... Still, for me, this is a hobby. It's all about having fun. It strikes me that if I'm spending a lot of time sticking extension tubes on my 100 macro, it is time for the mp-e... I was just a bit worried that due to diffraction, my images with the mpe would be only modestly improved over those with the 100. There's one way to find out, I suppose


Dann and Daniel:


I didn't know about this focus stacking stuff. I spoke to a friend who knows a bit about image processing and he's thinks he'll try implimenting this on his own. He's asked me to take some pics for him to process.


Another thing he suggested was deconvolving diffraction, which only requires a single exposure and is a much simpler process. Have either of you tried this?

[Daniel Browning]

Another thing he suggested was deconvolving diffraction, which only requires a single exposure and is a much simpler process. Have either of you tried this?

I've only done it for astrophotography, where noise makes the deconvolution less effective. If you can shoot low ISO, I would imagine that the kernels for deconvolving diffraction are more effective than a typical "sharpening" process. I don't know of what software to recommend for it, the astro stuff is probably the wrong tool.


I highly suggest you checkout the forums at http://www.photomacrography.net/ -- it's the single biggest concentration of macro expertise on the web.

[Jon Ruyle]

Thanks, Daniel. I just checked out photomacrography. Looks like another time sink...


Right. With astro you have noise, in which case (I'm told) something like maximum entropy should work better (I have no experience with this, though). And personally, my astro images have so many other problems that diffraction isn't even an issue.


Maybe for planetary photography deconvolution would help (f/ is high and light is plentiful.)

[Daniel Browning]

I didn't know about this focus stacking stuff. I spoke to a friend who knows a bit about image processing and he's thinks he'll try implimenting this on his own. He's asked me to take some pics for him to process.

There are some good guides for focus stacking on the web (GIYF). One pitfall to avoid is turning the focus ring on the lens: that is hard to move in the right increments and will also introduce breathing and other problems (apocryphal focus). It's better to use a focusing rail, bellows, or move the subject itself.

[Jon Ruyle]

Interesting.


The guy I spoke to said he wanted me to change focus, not move the camera.


Could you explain what you mean by "breathing" and "apocryphal focus"?

[Daniel Browning]

The guy I spoke to said he wanted me to change focus, not move the camera.

That should be fine.

Could you explain what you mean by "breathing" and "apocryphal focus"?

Breathing is when turning the focus ring causes the focal length of the lens to change. In this way, prime lenses (even macro) "zoom" as they focus. It's possible to create a lens that compensates for the focal length change so that changing focus does not change focal length, but this is not usually done except in expensive cinema lenses. As you can imagine, changing focal length slightly as you focus will cause a slight difference in magnification between layers in the stack. CombineZ can compensate for that without introducing artifacts as long as the changes are slight enough.

Apocryphal focus is a term that I learned from Joseph S. Wisniewski, a talented macrophotographer. He informed me that it causes exaggerated perspective (i.e. the same effect you would get from moving in closer and using a wider focal length) and that many find the resulting perspective to be unnatural. I don't know why using the focus ring causes it, except that perhaps it's the same thing as breathing. (Obviously, I'm not an expert in macro.)

If you use a bellows that can separate the movements between just the rear standard by itself (camera without lens) and everything (rear + front standard together), then it's possible to take layers that don't each have a different perspective.

The first possibility is by moving just the rear standard. The perspective will correspond to whatever the focal length of the lens is, but at least it wont change like it does when using the focus ring.

The second possibility is to move everything together (rear and front standard both). This results in "orthographic" perspective, which is as if the object were at infinity (i.e. flat or no perspective at all). Many macrophotographers desire orthographic perspective the most.

Hope that helps.

By the way, I can't stress enough how important it is to try a dedicated stacking program. Compared to shooting the layers in the first place, learning a new software program is a piece of cake.

[Dann Thombs]

When using the MP-E and stacking, just move closer to the object. The perspective doesn't change all that much, and the actual movement is so little, that it makes little difference. Here's a quick example using my handy dead wasp:








Which then ends up like this:

[Jon Ruyle]

The deail on that pic is pretty impressive... (unless, of course, that dead wasp is unusually large [])


I'll have to try this as soon as I have a chance.


Daniel: Thanks for the explanations. WRT breathing, you don't mean that the focal length really changes, do you? I'm guessing you're talking about how the framing changes rapidly because magnification increases asymptotically as you approach focal distance, which sort of looks like zooming. Or *do* you mean that the focal length changes? (It wouldn't for an ideal lens, but I don't understand what goes on with complicated real life things)


Thanks both of you for turning me on to this.

[Daniel Browning]

WRT breathing, you don't mean that the focal focal length really changes, do you?

Yes, I do. The numbers printed on lenses are really more like guidelines (usually corresponding to infinity focus) than actual reality. When you change the focus ring, it literally changes the focal length. The lens elements physically move. A 100mm focused on infinity could be 110mm focused at 1 meter.