Sunday, November 22, 2009

F70EXR Macro mode ... an experiment

I did not perform this experiment ... it was performed by Fuji Talk Forum member Altruisto, in the best spirit of investigation. He shot an image of a ruler in order to get an exact measurement of the coverage area at each focal length's closest focus distance.

A bit of background ... with dSLRs, you shoot longer focal lengths (e.g. 300mm) with closer focus distances in order to isolate your subject by eliminating complex and cluttered backgrounds. In the macro world, shooting at 1:1 magnification (coverage area exactly the same as sensor size), you need special lenses. Small sensor cameras all have macro settings with some level of magnification, but usually the highest magnification is at the widest angle. This brings a lot of clutter with it and a lot of weird distortions. With the F70EXR, it also brings serious amounts of CA and purple fringing. And finally, it requires extremely short working distance ... with the subject often a centimeter or two away from the lense, cutting down light to the subject. Bad news all round ...

So back to Altruisto's experiment.

Shooting this target at every focal length the camera can select gives him an opportunity to see if there is a "sweet spot" with similar magnification at a longer focal length.

And he found it. Since the camera reports focus lock, one only has to select each possible focal length and then keep moving the camera forwards or backwards until focus lock is achieved. The resulting images tells you the coverage area (based on what you see of the ruler) and the focal length (in EXIF data) and aperture. Since the Fuji always uses its widest aperture in relatively dark circumstances, you see the actual widest aperture for each focal length. His table is here:

Why is this significant? Well, it turns out that at 55mm, you get the same magnification as you do at 27mm. Yet with a longer working distance (more light on subject, less chance of scaring subject away) and with a much nicer background ... much less angle of view to bring in clutter.

The F70EXR does not report focal lengths while shooting (Fuji, shame on you) ... but it does change the aperture slightly at each focal length, and the magic number is f4.9. So zoom in a bit, then one click at a time until you see f4.9, and you know you are at 55mm.

This is a very significant discovery and congratulations to Altruisto for discovering it.

See his original thread here.

My own confirmation of the technique worked exactly as advertised. I shot the closest possible image of the *copy function on the keyboard, then without moving the cam, I zoomed to f4.9 and backed up in tiny increments until I could again get focus lock. The resulting image has exactly the same magnification (as predicted by the table) and shows much less of the background. This will make for much better flower and insect shots.

Here are my samples ... two 800px images with a full sized crop of the word *copy in the bottom right hand corner. Shot in very dark circumstances, so I ran some NR in ACR and then ran Topaz Denoise 3 on them as the last step (when they were small) ... fading the amount of NR to get the right balance (in my opinion of course.) Bad lighting, so bear with it ...

One more time ... this is a major contribution to the body of knowledge on this camera.

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