Saturday, 3 February 2018

Samyang 500mm 1:6.3 DX mirror lens

Long focal length lenses for SLR cameras are mostly one of two types - refractors (true lenses) or "mirror" lenses (actually Cassegrain telescopes). Large aperture long lenses are big, heavy, and expensive, but they can perform really well. Mirror lenses are much smaller, lighter, and usually a lot cheaper. However they are fixed aperture, usually manual focus, and have notable optical limitations.

500mm f/6.3 mirror lens

Three years ago I bought a Samyang 500mm f/6.3 mirror lens. (What appears to be the same lens is sold under several other brand names. Mine is clearly labelled "made in Korea" though.) It was sold as "ex demo" but I can't see any sign it wasn't brand new. It comes with a screw-on lens hood not shown in the above photo. The lens has a T-thread (M42x0.75) mount, so I've used a T-thread to M42x1 adapter and an M42x1 to EOS adapter (which I already had) to attach it to my Canon 100D. The EOS adapter has a "focus chip" - this tells the camera it's got a Canon 50mm f/1.8 lens attached and allows the focus confirm system to work on well lit subjects. f/6.3 is about the smallest aperture the EOS 100D's focus system can cope with.

Stedi Stock camera shoulder brace

In another blog post I described how I've mounted the camera plus lens on a shoulder brace. I've recently been having some fun using this set up to photograph some birds in my garden. All the following pictures are taken through a double glazed window, which is sub optimal, so better pictures should be possible.

Fieldfare in my garden

This first picture is of a fieldfare (Turdus pilaris). This unprocessed picture is a bit dull, and if you zoom in (click on the picture to see it on Flickr, then click on it again to zoom in) you can see it's really not very sharp.

Fieldfare in my garden

I'm no expert in processing photographs but I thought I'd have a go using GIMP, an open source programme available on many operating systems. All I've done here is to boost the contrast a little, adjust the brightness, and use an "unsharp mask" filter to make it a bit less soft. I think it's a significant improvement.

On this more recent picture (a green woodpecker (Picus viridis)) I actually managed to get the bird in focus. I've done similar processing and am pleased with the glint in the eye and the detail in the feathers.

Woodpecker in my garden

This picture also shows one of the optical defects of mirror lenses, the doughnut shapes formed around out of focus highlights. The ethereal ring shape in front of the bird's shoulder is caused by a berry on a plant stem in the foreground. Some people find this sort of aberration very annoying. I don't think I mind it too much.

The most severe problem with this particular lens design is the difficulty in focusing. Its focal distance goes from 2m to infinity (and beyond) in a quarter turn of the lens body. If they'd only made this nearer a full turn it would be much easier to use. Some people have remedied this by attaching long levers or gearing systems. I've yet to try anything like this.

Tuesday, 30 January 2018

Sussex pond pudding

Last weekend I made a Sussex pond pudding and, as I occasionally do, I posted a picture of it on Facebook. The ensuing discussion included both a request for my recipe and a link to a recent Guardian article "the perfect... Sussex pond pudding".

Here's my recipe, fleshed out with a few pictures. I've taken it from a slim booklet of traditional Sussex recipes I bought a few years ago in a branch of "Sussex Stationers", a now defunct budget bookshop chain.

1/ Make suet pastry from 8 oz self raising flour, 4 oz suet, 2 oz breadcrumbs, pinch of salt, and enough water to make a soft dough. I use a food processor as I don't have the knack of making pastry by hand. The Guardian's recipe omits breadcrumbs, so won't have the brown flecks (from the crust) you can see in my picture.

Sussex pond pudding

2/ Roll out the pastry and line a buttered 2 pint pudding bowl. To make a better fit to the steep sided bowl I cut out three triangles so the remainder is like a radiation warning symbol shape. I then join these cut edges (after dampening with water) inside the bowl. Trim around the top edge then combine all the pastry offcuts and roll out to make a circular lid.

Sussex pond pudding

3/ Wash an unwaxed lemon and cut up 4 oz of butter and allow to soften. Stab the lemon many times with a skewer, then wrap in the butter, then roll in 4 oz of dark brown sugar. Place in the lined bowl with any left over sugar. The Guardian's version slashes the lemon with a knife and simply surrounds it with cubed butter and sugar which looks a lot easier.

Sussex pond pudding Sussex pond pudding

4/ Attach the lid and trim the edges, then cover with a circle of parchment and an aluminium foil lid with room for expansion. It domes right up as it cooks.

Sussex pond pudding Sussex pond pudding

5/ Steam for 3 to 4 hours (or a bit more), turn out on to a plate and eat.

Sussex pond pudding

Tuesday, 21 November 2017

AstroMedia Copernican Orrery kit - part 2

In part 1 I described the first half (approximately) of the construction of a cardboard Orrery kit from AstroMedia. In the second half things get more interesting, and more complicated, as the majority of the pulleys and shafts are assembled.

AstroMedia Copernican Orrery kit

The first part to be constructed is the "planetary gearing". This will eventually roll around the ecliptic disc, held in place by drive bands above and below the ecliptic disc.

AstroMedia Copernican Orrery kit

As you know, Earth's rotational axis is tilted and the Orrery has quite a clever system to keep the axis pointing in (roughly) the same direction as the Earth orbits the sun. The Earth rotates on a steel shaft which runs through a brass tube attached to a small bracket. The bracket is attached to a hollow shaft that passes through the planetary gearing to a drive wheel below.

The Earth's rotation is driven by another pulley and shaft that passes through the planetary gearing. A round magnet attached to the top drives the Earth's rotation via a small piece of silicone tubing. Unfortunately I didn't get the magnet exactly centred on the shaft and so the drive occasionally slips.

AstroMedia Copernican Orrery kit

Above the ecliptic disc are two large discs supporting Mercury and Venus. The orbit of Venus is driven by a belt around its large support disc, and Mercury is driven from below by a hollow shaft through the Venus disc. After fitting all the drive belts the moon and remaining planets are attached with steel support pins.

AstroMedia Copernican Orrery kit

There are five drive belts holding the planetary gearing in place. The topmost one drives the orbit of Venus, the next drives Mercury's orbit. The bottommost one drives Earth's rotation, the next keeps Earth's axis pointing in a constant direction and the last drives the planetary gearing to roll around the ecliptic disc. Does it all work? Watch the video and see for yourself.

AstroMedia Copernican Orrery kit

PS More pictures of its construction are available on Flickr.