Wednesday, August 19, 2009

Macro/micro and moth antenna

The camera club I belong to--CPM aka The Center for Photography in Madison--has moved downtown. After the last meeting in the old headquarters, I noticed that among the packing boxes there was one labeled 'Free.' Which happens to be my favorite price when it comes to photo equipment.

After checking with the club president that 'Free' really meant free, he told me "Take the whole box if you want " A strong hint since he was about to lock up. So the box and I went home.

Among the junk, and there was a lot of junk, I found a 2x neutral density filter and an Expodisk for setting a custom white balance. They will end up in my camera bag once the adapter rings I ordered from e-bay arrive. One thing I've learned about free stuff, you have to spend money to make it useful.

I also found a busted Minolta film camera with three lenses--a 28mm, 50mm and 135mm. Normally the lenses would have ended up in the busted junk pile. Minolta lenses won't mount on my Nikon and if they did they wouldn't focus properly since the sensors of the two brands aren't the same distance from the mounting flange. But since I'm now doing macro/ micro photography the 28mm lens might be an excellent find.

In an earlier post I mentioned that to closeup purist, macro photography meant your magnification went from 0.1 X (1:10) to 1X (1:1) and micro photography meant it went from 1X (1:1) to whatever magnification you could get away without using an electron scanning microscope.

{You may see the numbers in brackets on a macro lens's spec sheet. A run of the mill macro zoom should reach 1:4. Better and more expensive lens can reach 1:1. To go beyond that into the larger than life size micro mode you usually need extension tubes or other accessories.}

Until I read up on macro/micro photograph I believed the difference between macro and micro was more or less arbitrary. Even more arbitrary was the rule that the size of your bug image on the film or sensor determined whether it was life size or not. Maybe in the film age when you could hold up a slide and see the image the rule made sense. But in the digital age? My D60 sensor might be just under an inch long but I will be looking at the image on a 21 inch monitor .

But there is a bit more science behind that rule than I first thought.

In normal photography the cute girl you are photographing and the tree behind her are far larger than the image in the camera. But when as you move in closer and closer into the world of closeup and then macro photography the size difference become smaller and smaller. Finally you hit the magic 1:1 magnification where interesting things happen.

For starters your camera and your bug will never be closer. The distance between bug and sensor will be 4 times the focal length of your lens (plus or minus any fudge factor caused by principle planes). And that is where it makes major sense to flip the lens and mount it backwards.

Why? With anything except a totally symmetrical lens--identical glass on either side so the two principle planes overlap smack in the center of the lens assembly (not a very useful design)--there is always a good way and a not so good way to arrange your lens elements. While the focal length and image size might end up the same, the aberrations are different.

Anyone who has every looked at a cut out drawing of a modern camera lens's innards knows there is nothing symmetrical about them. Unless you are dealing with a specialize lens--a microscope objective for instance--lenses are always designed to image big things down on to little sensors. Once you start imaging things the opposite way the aberrations can go bad rapidly

So a reversed 28mm lens looked promising. It's 55mm filter threads screwed directly into the mount on my new bellow. For once I didn't have to spend money to use this free-be. And since the shorter the focal length of the lens the more magnification I get per mm of extension, I pulled out my trusty machinist rule to see what the lens gave me- 1/8 of an inch (3mm) visible in the viewfinder. A magnification of 8X (1:8)--now we are cooking.

But what quality of magnification? I had been using 3D flower samples, Queen Anne's Lace, because there is a field full of them behind the house. But because of the DOF and focusing accuracy problems these didn't work all that well at 4X. (See last post for details) At 8X the problems would be worse.

I needed a 2D target with precision detail. But what? In the past I'd used precision targets for testing microscope resolution limits but those babies went for big bucks. Needed to find myself a flat sample with a full range of detail

Then Charlotte came for a visit. We pulled out her new macro/micro equipment--precision optics by Mattel by way of Goodwill.

This jogged a few memory cells. Stashed away somewhere was a box of prepared microscope slides. Once found Charlotte and the dolls did their brand of science while I selected my sample-a section of a moth's antenna.

(to be continued)