Advice please on Canon lens

[Psychobilly]

Using front "circular" aperture mask stops you getting multiple diffraction spikes on brighter stars off the aperture blades inside the lens which is left "wide open"... If you like the effect then you can just stop the lens down "internally"

Peter...

[ollypenrice]

Carole, the F ratio of your scope is fixed because you always use it at its maximum aperture, so its focal ratio never changes. With a camera lens you have a diaphragm inside which can reduce the aperture by contracting to 'stop down' the lens by blocking the outer parts of it. So if it is 'wide open' it uses all its aperture and has a fast F ratio. If you limit its aperture by stopping down you slow down its F ratio. This would be an odd thing for an astronomer to do with a scope because we are always starved of light and want more, not less, and we are always shooting at infinity and we always want all of our images sharp. Daytime photographers can sometimes have too much light and frequently don't want all of their images sharp. They may want a face sharp but the background blurred - so they use a big aperture lens with ultra fast F ratio giving a very shallow depth of field. This means they can get the face and only the face in focus. The people who make these portrait lenses know that, wide open, they will not be sharp at the edge. In a sense that is the whole point of them. When we use them we want them to be sharp at the edge but they aren't, so we block off the extreme edges of the lens where the distortions are produced.

Olly

[carastro]

So is stepping down and stopping down the same thing?

What is it called if you do it the opposite way and make it more open and faster?

The only visible problem in that image is bad focus.

I thought that was the case.  

Carole 

[Psychobilly]

Opening up is the term I have always used... and Stopping down...

Peter...

[ollypenrice]

Stepping down is something that politicians should do more often. It isn't a photographic term, though we do talk about step down rings to fit smaller filters to larger lenses.

Olly

[Viewmaster]

Carole, photographers sometimes choose to shoot at full aperture in order to get a shallow depth of field...

....when taking a portrait for example,  it is useful to have the background out of focus. So a large aperture is used (low f number)

At other times if needing the foreground and the background both to be in focus, say a long street scene,

a smaller aperture is chosen. (high f number)

 Of course the exposure has to be adjusted in each case. 

f numbers normally run..........f22  f16  f11  f8  f5.6  f4  f2.8  f2  f1.4

Each one gives x2  exposure.  So f2.8 gives you twice the exposure of f4 and so on.

[carastro]

Thanks for all this useful information, all being noted down.

Carole 

[carastro]

Just checked my kit lens.  Those focussing rings are not going to work with it,  the main part of the barrel of the lens is the zoom and the manual focus part is just a small bit on the end of the lens and I think it is too far to spread the rings apart.  

Looks like I will have to get myself a non zoom lens.

Carole 

[Viewmaster]

Looks like I will have to get myself a non zoom lens.

If you search on eBay and ABS etc for a non zoom lens you will not find any.

But search for a prime lens and hey presto.

[carastro]

If you search on eBay and ABS etc for a non zoom lens you will not find any.

 

But search for a prime lens and hey presto.  :grin:If you search on eBay and ABS etc for a non zoom lens you will not find any.

 

Thanks, I didn't mean that literally as a title lol.

I'll look for a prime lens being the correct name, thanks.

Carole

[Knight of Clear Skies]

The 50mm lens you linked to in the OP is the model I have, it's very good for AP. Here's one of my recent images taken with it.

There are a few more in this thread if you are interested.

It's much easier to find focus with it than with the kit lens because it's much faster, so more stars show up on the liveview. This is true even when the f-number is set the same value (e.g. f3.5) as light transmittance is higher. When focussing and taking framing shots I bump the ISO up to 3200 or 6400 and open the aperture up a bit more. The focus ring is quite loose and will easily creep out of position - my low tech solution is a blob of blu tack.

It will need stopping down to get reasonable star shapes, f3.5 is useable but it is better at f4.0 or f4.5. It has 5 internal aperture blades which somehow gives 10 spoked stars. It does tend to dew up quite quickly, a lens hood helps a bit.

An alternative would be to buy an old M42 50mm lens off eBay, they can be fitted to Canon bodies using an M42 adaptor. The main advantage is that the focus ring will probably stay where you put it.

Hope that's some help, lens imaging can be a lot of fun.

[carastro]

Well that sounds like a good recommendation.

Thanks Knight

[carastro]

It does tend to dew up quite quickly

Assuming you can fix your focus ring so it doesn't move, can you use dew heaters on a DSLR lens?

[Psychobilly]

Yes..

Sent from my GT-I9505 using Tapatalk

[carastro]

Thanks

[Psychobilly]

With the plastic nifty fiifty it takes a while for the heat to get through to the glass so prevention is better than cure so turn the dew heater on early.

Sent from my GT-I9505 using Tapatalk

[JamesF]

I like the nifty fifty.  I was going to post one of my images using it, but since something rather better has already been done, I'll pass on that  For comparison however, this is one of my first images with the kit 18-55 lens on my unmodded 450D.  It's far from perfect, but hopefully it should give you an idea of what's possible.

James

[glappkaeft]

Just checked my kit lens.  Those focussing rings are not going to work with it,  the main part of the barrel of the lens is the zoom and the manual focus part is just a small bit on the end of the lens and I think it is too far to spread the rings apart.  

 

Looks like I will have to get myself a non zoom lens.

A quality zoom lens will also have a much better manual focus ring and will work fine for astrophotography (I use a quite nice Sigma IF EX DG HSM F/2.8 24-70 mm) but you will have to pay a premium for the added flexibility. In the focal length range of around 35-70mm a prime lenses can be both cheap, fast and good. If you want a good ultra wide lens or a long focal length lens you only get to choose two out of three...

 

Assuming you can fix your focus ring so it doesn't move, can you use dew heaters on a DSLR lens?

Yes, but if electrical power (or cost, a dew band, controller and battery is a non-trivial cost) is hard to find a chemical hand warmer or two (available at outdoors stores) fixed with a couple of stout rubber bands can work just as well.

The best advice I can give you is to practise. Practice using our camera and lens, learn its features and characteristics. Then do the same in the dark, it will be much harder. If you have little experience with telescope mounts get the Astrotrac (or similar) well in advance of the trip and learn to set it up. Again start indoors in good lighting and then move out and do the same in the dark. Practice until you are confident.

A good astrophoto book is a life saver, it will teach you things you don't even know you should ask about. If there is a local astronomy club nearby that is even better, there is no substitute to face-to-face discussions or hands on help.

Me and a couple of friends went to Tenerife. The main purpose was to observe with a airportable 38 sm Dobson but we had some tripods a Astrotrac so we made this

video:

[carastro]

What a fabulous video, and in particular I want to get that last image of the Antares region.  

 If you have little experience with telescope mounts

I'm OK with Equatorial mounts as I've been telescope imaging for a few years now (see my website), but never made any progress with imaging with the camera lens.  However it has always frustrated me that i can never take any images when abroad.  Now that they have brought out some lightweight tracking mounts that will sit on a sturdy camera tripod and the fact i am booked to go to some dark sites over the next 2 - 3 years, I thought it was time I got the camera lens imaging aspect sorted.

Yes forgot about needing power for the dew bands on holiday, will be OK from a campsite or home as I already have dew heaters for my imaging set up.  Will look into those chemical hand warmers you mentioned.

Nice image James, better than anything I have achieved with the lens. 

Carole 

[bogdog]

you could buy one of these

http://srb-photographic.co.uk/canon-eos-to-m42-lens-adaptor-5995-p.asp

then pick up older m42 prime lenses  for around £10 to £30

[carastro]

By the way, I have ordered that lens on Ebay after the good recommendation.  Then would you Adam and Eve it, a kind soul messaged me just afterwards and offered me one for free !!!!!!!!

So for the time being no need to suggest lenses, but any other tips would be welcome.

Carole 

[RichM63]

It is the physical diameter of the lens body, not the filter size.

For the 50mm you mention, have a look at KnightofClearSkies posts in the widefield sections.

(Stopping Down = Decreasing the size of the apperture. The higher the f number the smaller the aperture, so f1.4 = large aperture, and f22 = very small aperture ie "stopped down". Not to be confused with telescope "f's" where we talk about speed of the telescope although the theory is related).

The 'Stopping Down' term is very old and came from the days when literally the photographer would have to insert a metal plate with a hole to reduce the aperture or increase the exposure time. IE smaller aperture higher f number generally = longer exposure time for the same film speed. ie less light gets through so we need to keep it open for longer.

Why, well keeping it open for longer will allow the darker bits to have an effect on the film and thus give a much better 'depth of field', rather than just the 'bright' tree being sharp and all the background being blurred, you can now have enough light for the tree and the background to show detail.

I'm only just getting used to modern DSLR kit, having spent 7 years of my teens working in a traditional photographic shop with dark cupboards and anything from minature Minox to Hasselblad ( how I now wish I had accepted the token buy out).

[carastro]

Thanks Richard, I've copied that into my useful info folder.

Carole 

[Gina]

I use the M42 Pentax/Praktica thread prime lenses for my widefield imaging.  The Asahi Super Takumar or later SMC Takumar lenses are of superb quality I used them when I had a Pentax Spotmatic SLR 35mm film camera and now buy more from ebay for my astro imaging.  I have posted various threads of my experiments with these lenses including the latest Widefield Triple Imaging Rig in the DIY Astronomer forum.  But this is somewhat more advanced.  I should add that I have been an amateur photographer most of my life so it's nothing new to me from the equipment point of view.  The new part is using it for astro imaging.

[michaelmorris]

Hi Carole

I've recently started using my nifty-fifty (Canon 50mm f1.8 lens) for wide field.  It's great.  Set the f ratio (aperture) to f3.5 and it should be sharp right to the edge of the field of view.  I recently took it up on holiday with us to Scotland and got easy 5 minutes subs when it was mounted on an Astrotrac.

I use a cheap intervalometer to control the camera exposures.  (£10 on EBay).

Focussing can be pain, but I've found a fix that works for me

• Get yourself an Android smart phone or Tablet with 'USB host' function - many don't have it, so check.
• Download an app called 'DSLR controller' (about $5 by recollection) http://dslrcontroller.com/
• Get a small cable called an 'OTG' cable ( £2 - £3 on Ebay)
• Plug your camera's USB lead into the OTG cable and thence to the phone.
• plug the intervalometer into your camera
• Set the camera lens to auto focus.
• On the Custom functions menu on the screen of your camera, set it to lock/AF - test by half pressing the shutter release, it shouldn't  activate the autofocus routine.
• You can now use the focus controller on DSLR controller to remotely focus your camera lens.  You can also take test shots for framing and exposure testing- all without touching your imaging rig.
• Set up the desired exposure plan on your intervalometer and press start. -alternatively DSLR controller has a basic intervalometer function, but it's pretty crude and will use up
• Your camera will now chug away taking subs. which you can review on your tablet/smartphone.

I tend to disconnect by phone/tablet whilst it's taking subs and just plug the phone back onto the OTG cable for occasionally checking the subs for focus and exposure.

The other great advantage of this technique is that the autofocus motor holds the focus mechanism pretty rigid, thus reducing the risk of the focus slip problem associated with lenses in manual focus mode.

Dew can be a problem so I always lug along a battery pack to power a dew heater.

I hope this helps.