Canon 200mm F2.8L vs. Fast Refractor

[Tommohawk]

If you're buying new then there are filters suitable for shooting at fast focal ratios available these days, such as these Baaders.

OK thanks for that - I had no idea about all this! Which reminds me of another thing I wanted to ask - but I'll post separately about that cos it's well off topic.

[Owmuchonomy]

Starting from scratch with filters, but hoping for NB. Interesting what you say about problems using NB at fast F's. Will have to bear that in mind.

Through the course of this I'm wondering if the F4 300mm might suit better, especially given the point about matching to CCD camera pixels. Would still be faster than a fast frac.

Is the general view that the image from say a 300mm F4 Canon lens is as good as a dedicated frac, say an F6 with 0.8 reducer?

I'm not quite in a position to contribute to that yet but not far off. I am stacking 3 hours of data today for the horse head taken using my 300 f4. However I have an issue with DSS with this data so am struggling. I get a weird zig zag gradient across the stacked tiff. I've never seen this before. Normal gradients are expected but this is weird.

[Tommohawk]

Look forward to seeing the final result, and envious that U have got any data at all in this weather.

Do the individual subs look ok?

[robertm]

An interesting topic, this is exactly what I went through a few years ago.

My images were taken with Baader 7nm filters and I feel they worked well at f/2.8. I think the shift in band pass was still well within the filter profile. You may also want to consider filter size and how large the sensor is as well as filter to sensor spacing.

As others have mentioned getting a filter wheel into the equation may be a challenge but I gave up with adding an oag. The light path is so narrow that any prism will affect the image even if you could fit it in.

An alternative is to go with larger format lenses but demand by people who think they need them has pushed up prices massively over the years.... sigh !

If you're worried about over sampling then consider dithering and drizzle stacking as that might help. Something else I've done is doubling the image size before stacking, that can work quite well.

Robert

[Tommohawk]

An interesting topic, this is exactly what I went through a few years ago.

My images were taken with Baader 7nm filters and I feel they worked well at f/2.8. I think the shift in band pass was still well within the filter profile. You may also want to consider filter size and how large the sensor is as well as filter to sensor spacing.

As others have mentioned getting a filter wheel into the equation may be a challenge but I gave up with adding an oag. The light path is so narrow that any prism will affect the image even if you could fit it in.

An alternative is to go with larger format lenses but demand by people who think they need them has pushed up prices massively over the years.... sigh !

If you're worried about over sampling then consider dithering and drizzle stacking as that might help. Something else I've done is doubling the image size before stacking, that can work quite well.

Robert

That's useful info - I had planned to be fast enough to work unguided where possible esp on holidays away, and then maybe use guide at home. Key point is OAG is problematic when working that far down the light cone - I hadn't though of that.

Which Baader filters do you have? Looks like Baader do standard type and F2 high speed version.

Still getting my head round the issue of matching Pixel size to FL, but looks like with 200 or 300mm lens I'm more at risk of undersampling, no? 

[Owmuchonomy]

Look forward to seeing the final result, and envious that U have got any data at all in this weather.

Do the individual subs look ok?

Yes the subs are great.  I think its a flats issue but I'm still experimenting.  Flats are important for the 300 f4 because of significant vignetting on my chip.

[Tommohawk]

Just a quick afterthought... Does anyone make filters which attach to the front of the camera lens? Would this work from a technical perspective?

[Macavity]

That is certainly the other (easier?) option! Baader make a variety of filter sizes...

http://www.teleskop-express.de/shop/advanced_search_result.php?keywords=baader+filters&suchdas=OK   (Idem more specialist optics firms?)

if you were content to limit yourself to 2" aperture, filter threads seem to be universal?

All you'd need is a stepper ring. Larger filter sizes seem to be square, are unmounted

and rather expensive! But, with a little DIY, a cheap UV filter (with the glass removed) 

could adapt the camera thread to something you have lashed together yourself?

Good question re. alignment. No doubt (to some limit) filter glasses will have to be

orthogonal to the optical axis, optically flat etc. And a question to optics experts?

Edited December 2, 2015 by Macavity

[Knight of Clear Skies]

Just a quick afterthought... Does anyone make filters which attach to the front of the camera lens? Would this work from a technical perspective?

Yes, that was what i was proposing in post #17. You'd need to use a step-down ring to attach the a 2" filter.

[Tommohawk]

I'm thinking of filters for the EF 300mm which looks like a 77mm thread. Doesnt look like anyone makes these, and even the 2" NB set is pretty expensive, and I'd probably want LRGB too.

The words "DIY" and "lashed together" are immensely appealing - but not sure how I could achieve this. Somewhere I have an old Cokin filter, though i think this is smaller dia. I see new 82mm Cokin filters on *bay are cheap as chips, but not sure where I'd get square NB filters that big. Baader do 65mm square for E845, so pretty stiff and still a bit small I think.

[Tommohawk]

Yes, that was what i was proposing in post #17. You'd need to use a step-down ring to attach the a 2" filter.

Yes, agreed, I went in a bit of a circle there, but 2" filters would lose me all that lovely aperture, no?

[Tommohawk]

... the other obvious (well not to me clearly) drawback with this approach is that when I go back to my 200PDS or whatever I'd have to start over. I cant see Baader making 200mm filters!!

[wxsatuser]

Starting from scratch with filters, but hoping for NB. Interesting what you say about problems using NB at fast F's. Will have to bear that in mind.

Through the course of this I'm wondering if the F4 300mm might suit better, especially given the point about matching to CCD camera pixels. Would still be faster than a fast frac.

Is the general view that the image from say a 300mm F4 Canon lens is as good as a dedicated frac, say an F6 with 0.8 reducer?

If you want to run wide open at f/2.8 your going to have to shell out big bucks.

The only Canon lens that can do this on full frame and APS-C is the EF 70-200mm f2.8 L IS II.

It's peak performance is still around f/4 but can be used through out it's zoom range with the same results.

For £1499 you get constant maximum aperture, 23 elements with a nice fluorite and 5 UD elements.

The 300mm F4 does'nt come close, still a nice lens but not close.

[Tommohawk]

If you want to run wide open at f/2.8 your going to have to shell out big bucks.

The only Canon lens that can do this on full frame and APS-C is the EF 70-200mm f2.8 L IS II.

It's peak performance is still around f/4 but can be used through out it's zoom range with the same results.

For £1499 you get constant maximum aperture, 23 elements with a nice fluorite and 5 UD elements.

The 300mm F4 does'nt come close, still a nice lens but not close.

Hi Mike. I'd have thought the zoom version would lose a little in performance, compared to the fixed focus, no? Certainly more versatile though.

BTW a non IS 70-200mm F2.8 went on *bay few days back for £599. Bargain!

Edited December 2, 2015 by Tommohawk

[Knight of Clear Skies]

Yes, agreed, I went in a bit of a circle there, but 2" filters would lose me all that lovely aperture, no?

See my posts above, I can't say whether this is worthwhile or not.

... the other obvious (well not to me clearly) drawback with this approach is that when I go back to my 200PDS or whatever I'd have to start over. I cant see Baader making 200mm filters!!

We need some kind of zoom filter suitable for all apertures, clearly a gap in the market there.

[wxsatuser]

Hi Mike. I'd have thought the zoom version would lose a little in performance, compared to the fixed focus, no? Certainly more versatile though.

BTW a non IS 70-200mm F2.8 went on *bay few days back for £599. Bargain!

One would have thought so but it's not the case.

If you want to check the performance of any lens go on Photozone and you will find plenty of test charts.

I looked at the EF 200mm L  at f/2.8 and of course it suffers a bit round the edges, it's ok at f/4 and even better at f/5.6.

The 70-200 mk2 is excellent across the board.

I have just purchased one myself but no clear skies to confirm this so far.

[Tommohawk]

One would have thought so but it's not the case.

If you want to check the performance of any lens go on Photozone and you will find plenty of test charts.

I looked at the EF 200mm L  at f/2.8 and of course it suffers a bit round the edges, it's ok at f/4 and even better at f/5.6.

The 70-200 mk2 is excellent across the board.

I have just purchased one myself but no clear skies to confirm this so far.

Fingers crossed for clear skies - maybe you can post results when you get some. Forecast only showing lots of  down here!  

[topgearuk]

I've just received my 70-200mm mk2 f/2.8 what a fanatic lens (also a 2 x converter for regular photos and experimenting),  tried a few test shots of the moon in the early hours of this morning, very happy with the shots although can see focusing is going to be super critical so have just ordered myself a TS Microfocuser.

I might get to see some stars before the end of this year if I'm lucky

[Tommohawk]

I've just received my 70-200mm mk2 f/2.8 what a fanatic lens (also a 2 x converter for regular photos and experimenting),  tried a few test shots of the moon in the early hours of this morning, very happy with the shots although can see focusing is going to be super critical so have just ordered myself a TS Microfocuser.

I might get to see some stars before the end of this year if I'm lucky

OK, well fingers crossed - be sure to post anything you shoot!

[Stub Mandrel]

If you stop down a lens, does it effectively reduce the diameter, so you lose resolution. or does it just reduce the light level?

[Tommohawk]

If you stop down a lens, does it effectively reduce the diameter, so you lose resolution. or does it just reduce the light level?

Hmm good question! Looks like when you stop down you reduce lens aberation but introduce diffractive effect. See here: http://www.kenrockwell.com/tech/diffraction.htm

Thats a bit of a random search return, and would be useful to see something specific to our field, but look like the answer is yes, on balance you lose resolution if you stop down.

One would have thought so but it's not the case.

If you want to check the performance of any lens go on Photozone and you will find plenty of test charts.

I looked at the EF 200mm L  at f/2.8 and of course it suffers a bit round the edges, it's ok at f/4 and even better at f/5.6.

The 70-200 mk2 is excellent across the board.

I have just purchased one myself but no clear skies to confirm this so far.

Mike I now realise there seem to be 3 variants of the 70-200. The pre IS, the IS , and the IS version II. So this probably accounts for the price variations, and looking at the review sites the latest seems to come out best. 

[wxsatuser]

Hmm good question! Looks like when you stop down you reduce lens aberation but introduce diffractive effect. See here: http://www.kenrockwell.com/tech/diffraction.htm

Thats a bit of a random search return, and would be useful to see something specific to our field, but look like the answer is yes, on balance you lose resolution if you stop down.

Mike I now realise there seem to be 3 variants of the 70-200. The pre IS, the IS , and the IS version II. So this probably accounts for the price variations, and looking at the review sites the latest seems to come out best. 

Tony Hallas recommends the IS II version as the best lens available.

The other lens he recommends is the Rokinon 35mm 1.4, for the price it's a fantastic lens.

I believe this is sold under the Samyang brand but stand to be corrected.

In Photozone this lens out performs all others including the most expensive.

As to stopping down.

Again Tony recommends f/4, of course diffraction spikes then start to appear but the lens

should be a bit sharper and most abberations tend to disappear.

Have'nt had a chance to use mine yet as it's been so bad on the cloud front.

[wxsatuser]

I've just received my 70-200mm mk2 f/2.8 what a fanatic lens (also a 2 x converter for regular photos and experimenting),  tried a few test shots of the moon in the early hours of this morning, very happy with the shots although can see focusing is going to be super critical so have just ordered myself a TS Microfocuser.

I might get to see some stars before the end of this year if I'm lucky

Be interested how yours performs, to cloudy to try mine.

[wxsatuser]

If you stop down a lens, does it effectively reduce the diameter, so you lose resolution. or does it just reduce the light level?

In theory the lens should get sharper as you stopdown.

For most lenses  f/4 to f/8 will be the sharpest.

Each full stop reduces the light gathering area by half.

[Stub Mandrel]

Hmm good question! Looks like when you stop down you reduce lens aberation but introduce diffractive effect. See here: http://www.kenrockwell.com/tech/diffraction.htm

That led me here: http://www.cambridgeincolour.com/tutorials/diffraction-photography.htm

the link has an interactive tool that lets you compare the airy disk for any given f-ratio with pixel size. It seems a scope's f-ratio is fundamentally more important than aperture for photography, though the link between f-ratio and magnification

Quite a bit of food for thought, but it reinforces the rule of thumb for using shorter camera lenses to drop down a couple of stops from full aperture.