Can anyone recommend me a nifty fifty for AP?

[WilliamAstro]

I was just thinking of acquiring a nifty fifty for taking widefield images of nebulae and etc even. (e.g. Barnards Loop) Does anyone have any recommendations of any high quality yet affordable nifty fifties? Cheers.

[vlaiv]

Best recommendation that I can give is that you don't get one.

Look instead for fast lens that is around 100mm.

Lenses, in general, are not diffraction limited. Especially fast lens that you'll use at F/2 - F/2.8. Most lenses you are not going to use at F/1.4-F/2 although they are capable of this - because they have really bad aberrations for astrophotography (softness in daytime photography = horrible stars in astrophotography).

What does this have to do with getting 100mm vs 50mm?

Well, it turns out that level of aberration of the lens is tied to linear units - all lenses define their sharpness with MFT diagrams that are in lines per mm - not arc seconds on the sky. Further more - conversion between arc seconds and linear units depends solely on focal length of the system (we often call that "zoom" or "magnification").

Aberrations that are not in nature due to diffraction of the light but rather due to construction of the lens are going to be smaller on longer FL lens when converted into arc seconds.

Since you need 50mm FOV, here is what I'm actually proposing.

Get 100mm lens, shoot 2x2 mosaic and bin your data x2. Result will be the same as using 50mm lens - both in FOV and in speed, but sharpness of the image will be greater with 100mm lens.

[Louis D]

I haven't tried it for astrophotography, but I used the heck out my Sigma 50mm f/1.4 EX DG HSM for indoor dance performance photography for years.  Autofocus was always fast and on target despite hundreds of thousands of exposures, and it was very sharp to the edge stopped down to f/2.4 or f/2.8 on an APS sensor Canon DSLR.  It was a tiny bit soft at the edges below that, but nowhere near as bad as the Canon 50mm f/1.8 offering.  It can be picked up used for quite a reasonable price.

My old Olympus Zuiko 50mm f/1.8 and f/1.4 were terrible in comparison, and they were manual focus only.  I don't know if there are some better manual focus contenders out there for cheap.

[Gfamily]

My experience with using telephoto lenses is that the variable aperture iris gives multiple diffraction spikes, which a nifty fifty will avoid.

The only scope in that range that I've any second hand experience of, is a RedCat 51 that a friend has used. It makes a very compact until, and being a Petzval design doesn't need a separate Field Flattener. 

[vlaiv]

2 minutes ago, Gfamily said:

My experience with using telephoto lenses is that the variable aperture iris gives multiple diffraction spikes, which a nifty fifty will avoid.

Any lens that has variable aperture iris (and that is 99.99% lens out there) will give you diffraction spikes.

There is very simple way to avoid this however. It is simply DIY project involving some cardboard (or something more fancy - like 3d print).

Lens have filter threads, and there are step down rings, or even very cheap filter for given thread can be used. We want just metal part that screws in thread and not optics of filter.

We then take piece of cardboard and make aperture mask of wanted aperture. We make it circular.

Say we have 56mm lens and we want to use it at F/2.4 (we concluded that F/2.4 gives us good sharpness for our sensor). We then make cardboard piece with hole in the center that is 56/2.4 = 23.3333mm in diameter. We glue or otherwise secure cardboard in that filter ring so we can screw on our fixed circular iris whenever we want to shoot at F/2.4

We can make several of these for different F/ratios and just use one that is wanted for given application (narrowband might need slower aperture due to NB filters and so on).

Just a final note - we open up actual iris on the lens all the way so it does not interfere with our new iris.

[Louis D]

1 hour ago, vlaiv said:

Say we have 56mm lens and we want to use it at F/2.4 (we concluded that F/2.4 gives us good sharpness for our sensor). We then make cardboard piece with hole in the center that is 56/2.4 = 23.3333mm in diameter. We glue or otherwise secure cardboard in that filter ring so we can screw on our fixed circular iris whenever we want to shoot at F/2.4

Is 56mm the clear aperture of the objective lens?  My 50mm Sigma lens has a 77mm filter thread, and the front lens is a bit smaller.  I don't have it handy to measure it exactly, so let's presume for these discussions it is 77mm.

Wouldn't the calculation involve a ratio between the wide open f-ratio and the desired f-ratio?  For my lens, being f/1.4, wouldn't you want the reduction to be 1.4/2.4=0.58 or 77*0.58 = 44.9mm opening?  Just dividing 77mm by 2.4 yields 20.8mm which seems way off.

As a check, f/2.8 would correspond to half the diameter (1/4th the light gathering), so I'd want a 77*0.5 = 38.5mm opening.  Does 1.4/2.8 equal 0.5?  Yes it does.

To get to 38.5mm, I could use a series of step-down rings until I got to a ~37mm opening since 37mm is a pretty common filter size.  For f/2.4, I could look for step rings ending up at 45mm.  They're not quite as common, but still available.

I hadn't thought of using step-down rings for this purpose before.  Thanks for the idea!

Using this method, you could use the lens with any camera since you don't need an electronic connection to control the actual lens iris anymore, and it defaults to wide open.  The focus ring is still manually coupled as I recall.

[vlaiv]

1 hour ago, Louis D said:

Is 56mm the clear aperture of the objective lens?

No, I was referring to the focal length. I was not clear about it.

1 hour ago, Louis D said:

As a check, f/2.8 would correspond to half the diameter (1/4th the light gathering), so I'd want a 77*0.5 = 38.5mm opening.  Does 1.4/2.8 equal 0.5?  Yes it does.

You are quite right in your calculation - it is ratio of aperture size and focal length - that is why I used 56 and F/2.4 in my example

F/ratio = aperture / focal length

1/2.4 = X / 56

56 = X * 2.4

X = 56 / 2.4 = 23.333mmm

Simple as that, but yes, you can use "stops" to help in calculation.

1 hour ago, Louis D said:

To get to 38.5mm, I could use a series of step-down rings until I got to a ~37mm opening since 37mm is a pretty common filter size.  For f/2.4, I could look for step rings ending up at 45mm.  They're not quite as common, but still available.

I hadn't thought of using step-down rings for this purpose before.  Thanks for the idea!

Yes, you can "chain" step down rings until you hit desired aperture (does not need to be exactly F/2.4, I think F/2.53 is fine as well) or if you have 3d printer or know someone that has one - you end up with something like this:

1 hour ago, Louis D said:

Using this method, you could use the lens with any camera since you don't need an electronic connection to control the actual lens iris anymore, and it defaults to wide open.  The focus ring is still manually coupled as I recall.

I haven't had luck with electronic lenses in that regard. On all mine - focusing ring is actually just electronic control/sensor for focusing motor. No power - no focusing. For that reason I just look at fully manual lens to use with astronomy cameras.

[Louis D]

5 hours ago, vlaiv said:

I haven't had luck with electronic lenses in that regard. On all mine - focusing ring is actually just electronic control/sensor for focusing motor. No power - no focusing. For that reason I just look at fully manual lens to use with astronomy cameras.

I just checked my Sigma 50mm I mentioned above, and I was right.  The manual focus ring moves the internal focusing group back and forth even when not attached to a camera.  When it reaches either end of travel, the ring just slips.  When focusing on a camera body, the focusing group moves and the ring stays stationary, so the slip is completely normal.  It's kind of a neat manual/electronic hybrid design.

 

5 hours ago, vlaiv said:

F/ratio = aperture / focal length

The front element is nearly the entire 77mm filter thread opening, so does 77/50=1.54 indicate mild vignetting wide open on a full frame 35mm sensor?

[vlaiv]

14 minutes ago, Louis D said:

The front element is nearly the entire 77mm filter thread opening, so does 77/50=1.54 indicate mild vignetting wide open on a full frame 35mm sensor?

I think you are going at it from "wrong side".

If your lens is 50mm of FL and it is F/1.4 lens - that means it has 50mm / 1.4 = 35.7mm of aperture.

If it actually had 77mm of aperture as front lens suggests, then it would be 77/50 = F/0.65 lens.

Ok, so here we have two "confusing" things here. First is F/ratio notation where ratio is being used as number dividing (below fractional mark not above it) - which can cause confusion.

200mm / 1200mm scope is F/6 instrument, right?

Note position of numbers

200 / 1200 = 1/6 = F/6 instrument

This can be also read as "focal length is x6 the aperture".

If focal length is 50mm and aperture is 77mm, then above sentence would read "50mm focal length is ~ x0.65 the aperture of 77mm" - so actual F/number in that case would be F/0.65

Second thing that causes confusion is - how can lens that has such large opening operate at effective 35.7mm of aperture?

Lens design is complicated and it has many elements (like 8 or 9). Mechanical aperture / iris is inside the lens and it acts as aperture stop. It is never seen in photos even when stopped down because it is effectively "at infinity" optically speaking. Much like secondary of certain telescope designs. It sits in collimated light beam and acts as aperture stop.

But why put large front lens then? It has to do with correction of different optical aberrations. First - lens must have a flat field. Shorter the focal length, stronger the curvature, yet lens have flat field to at least 35mm diameter. It is also rather wide field (with that short FL) instrument - so it needs to be able to gather light from all directions and then focus them on sensor.  You will notice that shorter the FL - more bulged and curved front lens is because of that.

That is 12mm lens that is F/2.8. From the image it would look like F/~0.2 lens with around ~50mm front opening - but it is of course not.

In any case - not all of the front lens is used for all incoming light. Depending on incident angle of light - it will use just certain part of the front lens (one "facing" that direction") - to minimize problems that arise from glass being too curved with respect to incident light rays.

In the end - things that block light when light is collimated beam do not cause vignetting or shadows. Only things that block light when it is converging do that and more light is "concentrated" - or closer to focal plane - more concentrated the shadow is. Vignetting happens when converging beam is somehow partially blocked - like too narrow connection or maybe filters of small diameter where cell is blocking the light - or too small secondary.

 

[Paul2019]

Here’s to comparisons for you first we have a Canon 50mm stm f2.8

next we have the Samyang 135mm f2 

both imaging the Cygnus loop, using a eos2000d. Around half hour intergration each using PI and the same processing workflow (although the Samyang is probably slightly better processed as my skills improve albeit slowly).

But I’m super new so couldn’t offer any advice but know myself when considering kit direct comparisons are always helpful. So hope this helps 😊

[Adamar98]

I don’t think you could go wrong with the Canon EF 50mm f/1.8 STM . Yes it’s cheap and yes it’s plastic. But it does exactly what it says on the tin. I think it’s a fantastic little lens for AP. Excuse the stars towards the edge. My fault for not stopping the lens down.