25 January 2009

LBA - Rokinon 800mm f/8.0 Mirror Lens

This is not a hand-holdable lens.
Not because it is large (it isn't, especially) or heavy (also not; about 800 grammes) but because 800mm focal length combined with a razor-thin depth of field makes the vibration introduced by one's pulse significant.

Mounted on the K20D, itself on a tripod.

Not much view of the camera from the front...

Manual focus is fine—not up to the standards of the Pentax Limiteds, but for about 200 USD, that would be an expectation somewhere past unreasonable and deep in delusional—green button metering is fine, and focus confirmation works.

However, it requires a 30.5mm filter as part of the optical formula, and does not ship with one. Fortunately this is a common size; unfortunately you need fingers like an aye-aye to get it installed.

One side effect of the narrow depth of field is that things appear and disappear as the focus is adjusted. This is especially disconcerting when one is trying to find something like a bird feeder.

This is a downy woodpecker on the finch feeder. I have no idea what Mrs. Downy here thought she was doing, but none of the wee tiny holes have been swagged wider, so I think it ended well.
Goldfinch and a bright "sky" (mostly snow roof) background completely collapsed into blur.

Like the other bird pictures, this is from about four metres away, just at the inner limit of the focus distance. I suspect more distant subjects will not have the sky go uniformly bright like that.

Quite respectable for the cardinal's head; the background, on the other hand, is clearly bifurcating. So right up against the close focus limit has side effects.

This is a random branch, about 10 metres from the camera. It's respectably sharp; the branches behind are weirdly blurry, but I kinda like the effect. There's probably a way to get a distant snowy thicket to look the Gate of Horn and Ivory.

So, anyway; sharper than the Opteka, not hand-holdable, probably just the thing for waterfowl. Funky bokeh. Very tricky focus right in at the close focus distance, because the depth of field is razor-thin. Fixed f8 aperture means it's for sunny days.


brooksmoses said...

I bet part of the weirdness of the blurring is coming from the fact that the lens is blocked in the center by the mirror. An out-of-focus dot would (I think) thus get blurred into a fuzzy donut, rather than into a fuzzy circle, creating the bifurcation of sharp lines.

Graydon said...


This is called "iris blur" when what would be a point of light -- sparkles off the water, say -- turns into an annular shape rather than a point of light.

I also strongly suspect this is why the out of focus branches are sorta doubled, as well; darker outer edges, fainter middle.

Lots of people hate bokeh like that, but I find I don't mind it.

It might bother me for a portrait lens, admittedly, but if what I want to do is take pictures of birds far away it's not likely to be a major issue.

Harleyman said...

I'm thinking of buying one of these beasts for my Pentax K20D (it comes with a T-mount adapter), so I read your piece with interest. You mentioned the lens needed a 30.5mm filter (for the back end) but you didn't mention what filter it needed before it's usable. Could you fill me in please.

Graydon said...

Hi Harleyman --

It needs a plain glass skylight filter (unless you want to make it darker, which doesn't seem to be required much from F8, but if so, you want neutral density filters); I got a Tiffen filter sold as a "UV Protector". 30.5mm is a standard size for small video cams so they're readily available but generally not in the SLR part of the store.