Friday, November 26, 2010

A Primer On f-Stops and Apertures

handy f-stop chart
In this article I will explain f-stops as used in photography, on my way to a brief sketch of the history of aperture control in SLR lenses. This is a backgrounder so that when I come to present you with some old lenses in future articles I don't need to explain each and every term. I won't go into much detail here, just the basics. Any good photography reference can help you further.

I also present my handy f-stop guide, in the hopes it is useful as a reference. As usual you can click through that image to get to a full-size download from Flickr.
Friday, November 26, 2010

Random Links For November

Sorry I have been so quiet on the blog. This is not because I have been overwhelmingly busy, although I have finished two new pieces, a ninety minute live performance and other things I will document in due course. The main reason is that I tend to operate in terms of blocks of time and now I am ready for another "blog block"! To kick things off I will give you a random set of links of interest that I have stumbled across recently.
Wednesday, November 10, 2010

Gravity & Grace Tonight!

Jesse den DulkTonight, 10 November 2010, at 8pm, I hope you'll come down to Daghdha Space for the kick-off of this year's Gravity And Grace, "Four Evenings In Dance". I will be providing original music and sonic textures for the event, continuing my partnership with this distinctive contemporary dance group.

The structure will be informal, with several choreographed events happening over the two hours, following the theme of "An Opening". It's an opportunity to meet for the first time the new practitioners in the Daghdha Mentoring Programme. Jesse den Dulk is coordinating the event.
Thursday, November 04, 2010

Using A Macro Adapter (Bower Revisited)

The Face Of Bastt

In the last article I reviewed the Bower "Super Wide Macro" adapter, which gives a fish-eye effect when screwed onto the front of a wide-angle lens. I mentioned there that it was possible to use the macro portion of the lens on its own. Immediately after publishing that article I got curious... and so here is a follow-up in which I try out that functionality.

Using the included 49mm ring, I screwed the Bower onto the front of my FA 77mm Limited, which I have successfully used as a macro in the past. Then I went over to my bookshelf, which I typically use as a test subject. The setup required a tripod and long exposure. I used f/16 to increase depth of field as much as possible, without getting into bad diffraction territory. If I remember correctly, the exposure was 30 seconds. Here's the result (no processing, shot on the K20D).

Bower macro test

Well, that looks decent enough. There was insufficient light to get focus any better; even the slightest pressure on the camera threw it out of focus. So even though I used a remote release I gave up trying to get anything sharper. (I blame the tripod which is a cheap piece of junk. But heck, my good tripod broke in the first month! So I am left using my junk tripod.)

Why was shooting so difficult, even for someone used to macro? Well, that's a teeny tiny part of the book spine. For comparison, here is a "normal" shot of the entire shelf. See if you can pick out the portion in the magnified image!

K20D ISO 3200 processed

My curiosity now piqued, I began to wonder how strong this lens actually is. In case you are wondering, here's how you figure that out.

The APS-C sensor is 23.6 x 15.7mm in size. The portion of the book's spine visible here, measured vertically, is 9mm. Thus this shot has achieved greater than 1:1 magnification, since the object has been enlarged to greater than life-size. That amount can be calculated as 15.7/9, or 1.75x.

The magnification achieved with an add-on macro lens will differ with the focal length used. Thus it is meaningless to say the adapter itself has a particular magnification. Instead, the constant property one needs to refer to is the optical power. We calculate this starting with the following magnification formula. Here f is the focal length in mm, p the power in dioptres, and m the magnification factor:
m = (p * f) / 1000

Stating this in terms of p:
p = (1000 * m) / f

Substituting in our numbers:
p = (1000 * 1.75) / 77 = 22.7

For comparison we can look at two respected Raynox macro adapters. These are well-corrected 3-element achromats. The Raynox DCR-150 provides 4.8 dioptres while the DCR-250 is 8 dioptre. So we see that the Bower macro adapter is three times more powerful yet!

This is not an advantage to most shooters. Depth of field is incredibly thin at such high power -- now wonder I had difficulty focusing! Worse yet, working distance decreases with power. Indeed, I found the book had to be incredibly close to the front of the lens. Besides the physical difficulty of positioning the camera practically touching the subject, illumination becomes a problem. In such cases one would be well advised to use a ring flash. But if we are going to all that expense and bother, certainly we would also be using a proper macro lens?

I thought it might be fun to put the macro adapter on same lens I used last time, the Vivitar Series I 28mm f/1.9. Here's what I came up with, shooting one of my usual subjects, a statuette of Bastt, which measures 13.5cm high. This first photo is unprocessed, shot at 1/30s and f/16 (the smallest aperture available on the lens) with a on-camera flash providing light by bouncing off the ceiling. I measured the distance of subject to image plane as 14.5cm.

The Face Of Bastt [unprocessed]

Wow, yeah, I'm liking this look! So I processed the image by touching up some marks, cropping it square, adjusting exposure, removing noise and finally toning it monochrome... all subtle but important steps to get from the crude beginnings to a deliverable image. Here is the result.

The Face Of Bastt

In conclusion, next time I need an insanely powerful macro adapter I'll reach for the Bower. It may not be practical, but it can certainly be a useful artistic tool.
Thursday, November 04, 2010

Junky Fish-Eye Adapter On Trial

Thinking I might have use for a funky video effects lens, I decided to get a cheap wide-angle adapter. This gadget screws on the front of your existing lens, using the filter mount. To facilitate this it comes with a 49 and 58mm adapter rings. Presumably if you need different sizes you can use step-down thread adapters. Many of my lenses are happy with 49mm. But since it doesn't make much sense going wide based on anything but a wide-angle, I used instead the Vivitar Series I 28mm f/1.9, a legendary piece of glass in M42 mount. It has a 58mm filter thread, so I was sorted.