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CANON EF200L PRIME LENS


ollypenrice
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Hi All,

Although frustrated in my attempt to use it in anger on the Witch Head nebula tonight, I have at least got some useful info on how this lens performs. (Thin high haze did for me.)

Firstly here is a flat on a 15mm square CCD chip. I exposed to about half way up. I'd call that about as good as it gets. The FSQ is no more evenly illuminated than that. Pretty clean, too, given that it wasn't new!

1168915900_uBJ6Y-XL.jpg

Next a corner crop, 10 minute sub. All corners were the same. Again, that easily meets the house standard and to be fair to the lens my chip distance is still a couple of mm out. I am painfully grinding a few microns per half hour off part of my Geoptik adapter to get it correct. If you use a Canon DSLR you'd be laughing.

1168915647_rreKC-X2.jpg

Both the above were at f3.5, which may be conservative because some imagers use them even faster.

Edit based on Robertm's point in the replies: focus is incredibly sensitive. Just looking at enlarged stellar images would, I think, be a non starter. I used FWHM in Artemis capture. Though touchy, this gave a credible result but you make the smallest movements imaginable, that is for sure. Allow five to ten minutes, but then I always do.

So, pros and cons over a small apo? If you are a DSLR imager the only con is that you can't look through it, but you can use it by day on your camera. If you are a colour CCD user you will need an adapter like the Geoptik I use. Mine came from Altair Astro. And you will have to sort out the chip distance. You will also need a Canon camera in order to be able to set the f stop, which you do by pretending to take a pic in AVI mode and pulling out the lens while the camera is thinking about it.

If you are a mono CCD user you will have to persuade someone to make an adapter with built in filter slide, which would be perfectly possible. If, like me, you have mono and OSC cameras then you are a lucky so and so and can shoot Ha in the mono!!

Optically it seems to me to be pure gold and better than similarly priced telescopes. Anyone wanting to get into imaging with a DSLR should give it real thought because you get top quality refracting optics, a flat field and a superbly useful focal length. (The Horsehead and M42 in one, for instance, and 200 mm is so easy on the guiding.)

And let's just say this again; f3.5 is conservative. You can go half a stop down on that by all accounts.

New they are about 700 pounds but I found this for 500 from London Camera Exchange. An F3 ish, flat field apo for that does seem like a bargain...

Olly

Edited by ollypenrice
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hmm...food for thought! Especialy at the £500 mark!

what would be a few of the APO alternatives??

Great review Olly!

Michael

Cheers Michael. At such a short FL I can only think of the MiniBorg astrographs and they are more expensive. However, the Canon is a similar price to a small apo with flattener, which would give you a longer focal length. It would depend what you wanted to image. But 200mm gives you the Sword of Orion, say, or in two panels the Rosette up to and including the Cone. That sounds like a plan! Or in one, the whole Veil complex.

So for someone wanting a first imaging setup it has huge advantages and will never need upgrading because, at its FL, it is about as good as you can get.

It would be so much better to start with this than with some huge FL SCT as so many, including myself, find to their cost. I have come to this the other way round, already having good optics to offer at longer FLs.

Olly

Edited by ollypenrice
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Thanks for clearing that up, I was racking my brians trying to think of a scope even including a reducer that would give you as wide a FOV as your lens but couldn't think of one and certainly not with the quality and flat field as you are talking about...again like you say especialy for the price.

I think you have convinced me to invest in somthing like this, certainly in the future, saying that you had me convinced after seeing your wide Orion!! :)

Looking forward to seeing the 2 pannel rossette! :(

Thanks,

Michael

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I have the EF200L and also a Sigma 500/F4.5 APO in EF mount. The Ali plate to make the adaptor to T2 arrived yesterday so hopefully I'll be able to post some shots soon so it can be seen how flat the field is, what kind of other aberrations there are etc.

I made an adaptor out of a rear lens cap and a short piece of plastic pipe so a 1.25" eyepiece can be used for visual. Only drawback is that you can't use a diagonal as the focal plane is too far in.

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I use the Canon 200l with my two Starlight Xpress CCD cameras via an adapter that I designed that houses a 1.25" filter. When I designed this, I erred on the cautious side and made it a little short in length to ensure infinity focus but your comment on 'grinding off microns' made me think, "although I can easily achieve infinity focus, this is with the lens apparently focussed at about 15 feet - what are the ramifications for that in terms of image fidelity?" Any one know?

This is my adapter:-

canon_adapter_4.jpg

canon_adapter_5.jpg

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Thanks, Brian, I was hoping that would be the case but can't visualise what effect if any it would have.

Hang on, I'm getting confused here. I am well short of infinity too, when I focus. I thought this was because I was too far back from the chip. Have I got this the wrong way round? When I measure it I am certainly a couple of mm further back than the optimum based on this info from the Altair site;

Achieving the correct focal distance and spacing between the lens and CCD camera:

EOS format lenses have a back focus of 45mm from the back of the lens to the camera chip. The Geoptik adapter has a 19mm optical path. Subtract 19mm from 45mm and you have 26mm remaining. Then check the back focus of your CCD camera in the manual. If you subtract this value from 26mm you get the optimum spacing for the T thread adapter you need to bring the lens into focus. T Thread spacers are widely available for experimentation from most camera shops. Try to err in favour of a shorter length than that required rather than a longer one, as you can adjust the lens focus to compensate. (Click the detailed image tab below for more pictures)

Olly

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Great review of a fantastic lens Olly. I would say the only drawback, which is the same for all EF fitting lenses is the limited back focus.

Regarding the 135L f/2, all the MTF charts show that lens as being one of the sharpest even at f/2.

The problem with focusing at these low focal ratios is not to be understated. Just tapping the focus collar at f/2.8 is enough to take the lens in and out of focus. The use of Bhatinov grabber and mask is the only exact way of getting focus I've found. On the other hand once you have focus then it will happily keep focus for many many hours. Sometimes I've not even had to refocus from the previous night.

I'm now after something with a shorter focal length, the 135L is certainly tempting but maybe too close the the 200L. Perhaps the 85mm 1.8 but then that has no weather sealing - always something to consider in a damp climate.

Robert

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Great review of a fantastic lens Olly. I would say the only drawback, which is the same for all EF fitting lenses is the limited back focus.

Regarding the 135L f/2, all the MTF charts show that lens as being one of the sharpest even at f/2.

The problem with focusing at these low focal ratios is not to be understated. Just tapping the focus collar at f/2.8 is enough to take the lens in and out of focus. The use of Bhatinov grabber and mask is the only exact way of getting focus I've found. On the other hand once you have focus then it will happily keep focus for many many hours. Sometimes I've not even had to refocus from the previous night.

I'm now after something with a shorter focal length, the 135L is certainly tempting but maybe too close the the 200L. Perhaps the 85mm 1.8 but then that has no weather sealing - always something to consider in a damp climate.

Robert

Good point, Robert. I should have mentioned the critical, hypersensitive focus and will edit that in. Thanks. I used FWHM in Artemis Capture and this seemed to work. What is a Bahtinov Grabber??

Olly

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Hang on, I'm getting confused here. I am well short of infinity too, when I focus. I thought this was because I was too far back from the chip. Have I got this the wrong way round? When I measure it I am certainly a couple of mm further back than the optimum based on this info from the Altair site;

Achieving the correct focal distance and spacing between the lens and CCD camera:

EOS format lenses have a back focus of 45mm from the back of the lens to the camera chip. The Geoptik adapter has a 19mm optical path. Subtract 19mm from 45mm and you have 26mm remaining. Then check the back focus of your CCD camera in the manual. If you subtract this value from 26mm you get the optimum spacing for the T thread adapter you need to bring the lens into focus. T Thread spacers are widely available for experimentation from most camera shops. Try to err in favour of a shorter length than that required rather than a longer one, as you can adjust the lens focus to compensate. (Click the detailed image tab below for more pictures)

Olly

Think of the spacer like extension tubes Olly, the longer the tubes the closer the focus. Shorter will allow some compensation, too long could mean you will never reach infinity

Edited by Photosbykev
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I believe the chip distance is 44mm on an EOS.

I have read that camera lenses, unlike scopes, are not designed to be at their best at inf. Some macro lenses are. As Brian said.........

Dave.

Edited by davew
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Thanks guys, so no need to go on grinding to get the chip nearer to the lens? I get a focus at infinity, albeit showing less than that, and the stars look fine... so hunky dory. The Samyang 85 lens is not as clean to the edge at f4 even, but maybe that is just how it is. I still like it.

Olly

Edited by ollypenrice
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What Kev said!

Olly, my understanding of the lens mounting to sensor distance on an EOS is that it is 44.0mm, not 45.0mm. The 200L has rear focussing so you can see the effect of adjusting the focus mechanism by turning the focus ring and looking into the back of the lens.

I can confirm the difficulty in focussing this focal length on a star but a Bahtinov mask does a great job even on this focal length - in fact I also use a B. mask on my 135mm Olympus lens.

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What Kev said!

Olly, my understanding of the lens mounting to sensor distance on an EOS is that it is 44.0mm, not 45.0mm. The 200L has rear focussing so you can see the effect of adjusting the focus mechanism by turning the focus ring and looking into the back of the lens.

I can confirm the difficulty in focussing this focal length on a star but a Bahtinov mask does a great job even on this focal length - in fact I also use a B. mask on my 135mm Olympus lens.

What Steve said! 44mm is the backfocus distance for the EOS bodies

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