If this is your first visit, be sure to check out the FAQ by clicking the link above. You may have to register before you can post: click the register link above to proceed. To start viewing messages, select the forum that you want to visit from the selection below. |
|
|
Thread Tools | Display Modes |
#1
|
|||
|
|||
Hybrid use query about equipment purchase
"jjs" wrote in message
... "Bob Monaghan" wrote in message ... [... it's about mirror lenses ...] You get the worst of both worlds, a lens delivering only f/11 amount of light, but with fixed DOF of an f/8 rather than the greater f/11 DOF you might want ;-) If speed isn't an issue, you _can_ have variable DOF with a mirror lens. Just cover the front of the lens and place an aperture to one side of the centre mirror. Would a spare lens cap with a large hole in it, so it covered just part of the edge of the front mirror all the way around, be an equally good solution? A disk in the centre that was a bit bigger than the 'centre plug' ought to work too, but would be less convenient to attach. Peter |
#2
|
|||
|
|||
Hybrid use query about equipment purchase
ah, no, probably not. I have a 25cm telescope, and the manual also recommends an off-side circular/ellipsoidal cutouts. This is very different from a ring for various reasons. In effect, the circular off-center cutout is acting to create a smaller diameter circular mirror, hence the smaller f/stop equivalence. There was an article a number of years ago in Modern Photography, IIRC, by Herb Keppler showing a series of cutout layouts for various f/stops. You cut them out with cardboard, though a cutout lens cap would probably work. Some of the astro sites may have more info. here is a note by lens designer Brian C. on this from posting at http://medfmt.8k.com/mf/mirror.html From: (brian) Newsgroups: rec.photo.equipment.medium-format Subject: MIRROR LENS Special problems Date: 21 Jan 2003 .... Bob: When you put a cutout in front of a catadioptric lens the cutout *becomes* the aperture stop, and the DOF is increased accordingly. Essentially what you are doing is creating an unobscured system with a front stop. The main problem when you do this with a 500/8 is that the f/# gets really large, which will cut down the resolution. If you want to do this trick a 500/5 would be a better choice to start with. Brian www.caldwellphotographic.com ==== endquote hth bobm -- ************************************************** ********************* * Robert Monaghan POB 752182 Southern Methodist Univ. Dallas Tx 75275 * ********************Standard Disclaimers Apply************************* |
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
E6 Developing at Home - Is Jobo the right equipment? | Mehawitchi | In The Darkroom | 8 | April 13th 04 07:20 PM |
Fuji S2 and Metz 44 Mz-2 Flash | elchief | In The Darkroom | 3 | April 7th 04 10:20 AM |
Opinions on Jobo LPL C 7700 Color Enlarger & Other Equipment | Andrew McCall | In The Darkroom | 1 | February 23rd 04 08:34 AM |