Controlling bokeh on 500mm mirror lenses?

[Markster]

I've been reading reviews and comments on 500mm mirror lenses today. There is, of course, mention of the donut-shaped bokeh for out-of-focus light reflections. However, some folks seem to have mastered the bokeh to the point that backgrounds simply look out of focus as with any other lens.

It's not something I've thought about too much, so I'll ask: With such a lens, what would you do to minimize it? What are some of the basic tricks?

 

[polyglot]

For the donuts to be invisible, you need to make sure that the smallest spatial frequency in your image (i.e. detail) for an out-of-focus area is larger than the bokeh-size for that out-of-focus region.

In other words, if you have point-lights (e.g. light glittering off rippled water, streetlights, fairylights, etc), then you will get unavoidable donuts. If the background is all fairly uniform in tone and has no fine features (points or lines) with strongly separated tones, then it will all tend to blend together despite the donut bokeh.

If you're shooting birds against the sky, you get no issues. If you shoot birds in front of shrubbery, the shrubbery can look bad because all the lines get doubled.

 

[Markster]

Interesting... Thanks!

 

[nicholai]

Sometimes, i feel like i'm the only one that likes the mirror's bokeh. As a means of pictorial expression, i like the it. Some real cool tricks can be done with it. (eg: THIS)

 

[RattyMouse]

I loathe mirror lens bokeh. By far, to my eyes, one of the ugliest things you can ever put into a photograph.

 

[Matthew Wagg]

Sometimes, i feel like i'm the only one that likes the mirror's bokeh. As a means of pictorial expression, i like the it. Some real cool tricks can be done with it. (eg: THIS)

That is very cool. What lens is it you used for it. I know there's a lot of variance in the quality of catadioptric lenses.

 

[dorff]

I recall a discussion with a photographer way back (20+ yrs) who maintained that using a 1.4x TC reduces or removes the donut shaped highlights. I can't understand why it would work, and haven't got a mirror lens to try it with, but maybe someone here can try it and give feedback. As for the effect in nicolai's example, that's about as good as it gets and even then I find it very distracting. In fact, it almost makes me motion sick looking at that pic. Sorry, just my humble take on it - no intention to offend.

 

[vpwphoto]

Nothing wrong with the doughnut other than if you use it a lot it can be repetitive... I like the "boat" photo referred to, I shot rugby with one once and got some neat images. There will likely be somebody soon that will make it a "thing" and everyone will want one. I think I will start stocking up on them now.

 

[Maris]

I get rid of the donut bokeh of mirror lenses by fitting an off-centre circular aperture on the front of the lens. The circular aperture has got to be small enough to avoid the central obstruction. Off-centre apertures can be made in smaller and smaller sizes thus stopping down a mirror lens; a procedure popularly thought to be impossible. Downside? The viewfinder image gets dim. Upside? Better bokeh and more depth of field.

 

[Markster]

I wonder how such a lense would do with astronomy shots... Anybody tried it, or know?

 

[Diapositivo]

I wonder how such a lense would do with astronomy shots... Anybody tried it, or know?

As far as know many telescopes are based on mirrors (Newton, Cassegrain, that stuff) and catadioptric lenses in photography are basically based on the same concepts.
In astrophotography everything is in focus so one should never get rings.

When I'm rich I'll certainly buy a Minolta RF 250/5.6 such a nice portable addition to my walk-around bag.
At the moment they are a bit too expensive:

http://www.ebay.it/itm/Minolta-RF-5..._Foto_Camcorder_Objektive&hash=item4850e73dbe (this probably is exaggerately expensive).

For most subjects (those without shining out-of-focus points) the circular effect is noticeable but not really disturbing.

 

[Barry S]

I wonder how such a lense would do with astronomy shots... Anybody tried it, or know?

If you're experiencing bokeh issues in the midst of astrophotography--I suggest you dive for cover! Seriously, these are a similar design to mirror telescopes and there are no issues with photographing the moon. I don't think a 500mm lens will be of much use with other astronomical objects.

 

[hgernhardt]

I wonder how such a lense would do with astronomy shots... Anybody tried it, or know?

They work absolutely amazingly. Especially when your mirror lens is an 8-inch Celestron with a mirrored Mylar solar filter over the aperture. My father took some astounding sunspot images in his time with that ol' Celestron.

Of course, the closest I personally got to astrophotography was taking images of the Orion constellation, and a couple of the moon—all with standard camera lenses.

 

[Markster]

I was thinking more specifically of the 500mm, rather than attaching the camera directly to a telescope.

I imagine it would work well with the moon (as mentioned).

As far as cheap (or, cheapER) conventional solutions, if you put a 1.4x or 2x converter on it and you can bring that up to 1000mm. Further, I don't think there's anything stopping you from using 2 convertors on the 500mm.

The problem then becomes light sensitivity and shutter speeds required, I suppose.

It does pique the imagination, does it not?

 

[lxdude]

I have the Tamron SP 500mm f/8 mirror lens (55B), and I thoroughly enjoy it. Inexpensive these days, it was always reasonably priced. Sharp and with very good contrast for a mirror. I have the Tamron SP 2X teleconverter, (01F) a good performer, which makes it a 1000mm f/16. I bought it mainly for long shots of distant objects, most at infinity, and birds and aircraft against the sky- bokeh issues are nonexistent for those. The whole setup with cases, filters, everything, cost me about $120.

As polyglot says, avoiding specular highlights, or any distinct highlights, will minimize the donut bokeh. Having the background in shadow, or shooting against a very uniform background will minimize it, too. I have found that a very busy background, like dense grasses, will also work, because the doubling of lines is not so noticeable, and if the background is really busy, the doubling actually has the effect of blending, reducing the effect.
One thing I like about the Tamron is the very close focusing ability. It goes to 1:3, and 1:1.5 with the 2X converter. I can get nice shots close up in which the depth of field is so shallow that the background just blurs into a smooth mass. I have been able to take flower shots in which objects which were out of focus did not display objectionable bokeh.

A mirror can be a lot of fun. Avoiding the donuts means choosing shots carefully, and simply avoiding certain types of shots, but to me that's part of the fun. There is no way I would be carrying a regular 500mm, so the trade-offs mean getting shots with a mirror and dealing with its characteristics, or not getting the shots at all.