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02-01-2009, 01:53 AM   #1
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Using catadioptric lenses - any hints?

Hello folks,

I recently bought a second hand Opteka catadioptric (mirror) lens because I have been wondering about them and it was cheap. I know I know, cheap for a reason, but I figured it might be interesting.
Can anyone give me any pointers on the use of these things? With the sensor crop factor it's effectively something like 800mm or 1600mm with the TC and I'm finding it utterly impossible to focus at that focal length. I appreciate this is probably my manual focussing technique, but is there a knack or something? I was taking pictures of the moon and not one was in focus - the focus indicator wasn't showing up either for some reason. Mind you, for something that bright against pure black there wasn't a trace of CA, so that was quite impressive - my Tamron 70-300 normally paints purple all over it when I take moon shots.
Also, as it uses an interchangeable T-mount, is there any way to tell the K10D that the lens is using a fixed aperture of f8?

Cheers!

02-01-2009, 05:51 AM   #2
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I'm not sure how the K10D menu is set up, but with the K100D you go to the custom menu and set "FI with S lens used" to "Available".

However, I doubt you are doing anything wrong technique-wise, and I don't think there are any pointers that anyone can give you that will help.

You are unlikely to obtain any useable results using any lens that has the name Opteka on it, regardless of the type of lens.

Seriously.

Opteka markets the worst crap on the planet. How it manages to remain in the marketplace is one of the universe's great mysteries.

That's my experience with Opteka, and I've never heard anyone say anything more positive about Opteka than what I just wrote.
02-01-2009, 06:07 AM   #3
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Hmm... I could be off the mark here, but from your post I'm sensing a slight lack of confidence in Opteka kit
Fair enough, I knew it was cheapish, but didn't realise the brand was so roundly reviled.

Can't find anything like that in the custom menu, seems a strange item to have removed for the K10D.

Thanks for your help, I think maybe I'll perservere with this lens for a while to get used to the method in case I encounter a better one for sale at a reasonable price, I do like having such a long lens in such a small package. Do AF versions of catadioptric lenses exist?

Cheers!
02-01-2009, 06:45 AM   #4
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Hello, I have a Rokinon 500mm f/6.3 and I get some passable results with it.

First off, get a hood or shoot from the shade. My lens quickly loses contrast if the front is exposed to direct sunlight, even from an angle.

Two, forget about the teleconverter.

Three, focus confirmation barely works on mine (f/6.3), it certainly won´t work on an f/8 lens. That has to do with aperture and how the light rays hit the sensors.

Focusing really takes practice. Try rocking the ring back and forth around the focus point to get a feel of the point where the image "pops" into focus. Also if the subject is on grass/pavemente/etc try to follow the focus in the grass to the point where it meets the subject.

If you're really serious about mirror lenses the rokinon is usable. See the reviews in the lens database and these threads:

https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/39219-rokinon-...on+mirror+lens

https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/44979-mirror-l...on+mirror+lens

some of my shots taken with it

http://flickr.com/photos/cdallacosta/sets/72157610342970934/

02-01-2009, 02:09 PM   #5
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Hi there,

I have been experimenting as you say and I think I'm slowly getting a handle on it. Took some reasonable shots of the moon from hand-held and got some spot on pics of my little boys playing. I think I might get on with cat lenses after all
Looked at your Flickr thread, all fabulous, but that one with the reflection in the rider's visor - stunning!
02-01-2009, 02:53 PM   #6
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QuoteQuote:

You are unlikely to obtain any useable results using any lens that has the name Opteka on it, regardless of the type of lens.

Seriously.

Opteka markets the worst crap on the planet. How it manages to remain in the marketplace is one of the universe's great mysteries.

That's my experience with Opteka, and I've never heard anyone say anything more positive about Opteka than what I just wrote.
Apparently you missed my review elsewhere. They make excellent paperweights.
02-02-2009, 03:29 PM   #7
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QuoteOriginally posted by SpecialK Quote
Apparently you missed my review elsewhere. They make excellent paperweights.
You're right. I need to be more diligent in future with my research.
02-02-2009, 03:31 PM   #8
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QuoteOriginally posted by Vormulac Quote
Fair enough, I knew it was cheapish, but didn't realise the brand was so roundly reviled.
Neither did I until I foolishly bought one of their teleconverters.

02-02-2009, 04:29 PM   #9
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QuoteOriginally posted by Vormulac Quote
Hi there,

I have been experimenting as you say and I think I'm slowly getting a handle on it. Took some reasonable shots of the moon from hand-held and got some spot on pics of my little boys playing. I think I might get on with cat lenses after all
Looked at your Flickr thread, all fabulous, but that one with the reflection in the rider's visor - stunning!
For Moon shots there is no way to obtain good results without using a sturdy tripod and very short expsoure times with mirror prefire and remote control. Even if you go to 1/500s you will blurr a handheld image with such a lens. The tiny size and low weight is deceiving! After all the focal length is 500mm and with a refracting lens of say f/4.5 (the old Pentax 500 for instance) you would not even think to use it handheld. So don't do it with a lens, that is smaller, but quite as demanding.

For focusing: just take your time. It is easier, when the lens rests firmly on a tripod, handheld it is very hard. Take the advice for using a matching (aka long) lens hood seriously. Catadioptric lens, due to their construction, are very prone to flaring, which gives a severe loss of contrast. Nevertheless, if handles properly, they can produce very good results. If you search the forum, you will some recent discussion threads on the topic, by the way.

Ben
02-02-2009, 07:57 PM   #10
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spend some time checking out your tripod and mount stability.

I have a celestron C90 and it took some time to get the shakes out. these things amplify tripod shake like you wouldn't believe

Also make up a bag to hold some rocks under your tripod the extra weight helps
02-03-2009, 01:22 PM   #11
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I have a 800 mm f 10 Bauch&Lomb and my wife has a 500 mm f 8 Quantaray mirror lens. Neither is usable as a lens. I bought mine as a guide scope so it got some use for a few years. Her lens just collects dust. I'm sure some camera companies make a usable mirror lens but I have not seen one.

I use a Catadioptric telescope (old Meade, 2000mm f 10) all the time with good results. The difference in my opinion is telescope companies know how to make them right. All Cat scopes I've dealt with have adjustment screws for the secondary mirror. The Cat lenses are set once in the factory, before long they are out of collimation permanently.
02-03-2009, 07:26 PM   #12
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