[demp10]

The f-stop plays a dominant role in exposure, even for a point source of light. The light emitted by a star spreads uniformly in space and the total amount that gets into the camera is proportional to the actual open area of the aperture; which is proportional to 1/(f-stop)^2.

That's why large diameter telescopes are used for astronomy.

Dave in Iowa

For point sources the amount of light is proportional to the size of the front element of the lens and not the actual diaphragm. Light rays from a point source spread uniformly in space and then they hit the front element. After they pass the front element, they form a narrow beam that is far too small for the aperture blades to significantly affect it.

Large telescopes (with large diameter front elements or mirrors) gather more light, so do fast lenses which by necessity have larger front elements. If you stop down a fast lens while photographing a point object, the exposure for that object will not change much.

[Scootatheschool1990]

Not many options in the Pentax world. The closer might be the DA 14 f2.8, which is sort of slow compared to your criteria. There is the Samyang 14 f2,8 as well. If you aim for speed and can give up a bit of wideness, the Sigma 20 f1.8 can be an option. I suggest you read reviews of those lenses as they have their own characters.

Alright, I will look into these lenses. Thanks

Your not going to find a 15-17mm f/1.4 - f/2 lens in Pentaxland. The closest would be a Sigma 20mm 1/.8 otherwise go Canon or Nikon

YEAH RIGIIIGHT!

there was a prototype 20mm f/1.4 made in the 1980's - apparently there is a hood available for it, but the lens as of yet has not been seen..by anyone.

I can wait. . . jk, no I can't

Micro four thirds may be the way to go?
M.ZUIKO DIGITAL ED 12mm f2.0 | OLYMPUS E-SYSTEM | Olympus Imaging Asia
or amazing 17.5 f/0.95 (that's "equal" to 24mm on APS approx)
Four Thirds | Micro Four Thirds | Products(Lenses)

Or four thirds will let you get f/2 with a zoom.
ZUIKO DIGITAL ED 14-35mm F2.0 SWD | Digital SLR | Olympus Imaging Asia

I'm not sure I want a M4/3rds. But thank you for considering it for me.

Pentax generally is making fairly slow lenses. And at that focal length you will be hard pressed to find many fast lenses at all. You will have to choose between a bigger focal length and fast (up to 35mm, maybe even 50mm, with f1.4 or f1.7) and a wide lens but slower (like the 14mm f2.8)
Also, the newly announced K-30 camera will have an astrophotography mode where it moves the sensor according to the GPS in such a fashion that the stars stay dots. That would eliminate the need for a very wide angle (usually you want a wide angle so you can have a longer exposure without the stars turning into lines).
OR you can get a tripod head that moves with the sky (I forget what its called) and you can use a slow lens at whatever focal length. But that might be harder to come by.

Oh, and I don't think micro 4/3 is the answer, because it has smaller sensors and might have higher pixel density, focal length equivalence, and noise.

edit: Also, there are many astrophotgraphy tutorials on the net, some might give you more tips.

Hmmm... I just purchased a k-5. hope the k-3 has an astrophotography mode Yeah, I might have to go to 24-35 but look at the photo below, I still don't want to go narrower :/
Thanks for your response!

fact is the Canon 24mm f1.4 cost arpund 1.500$

the closest is the Samyang 24mm f1.4 (no AF) at 700$

in the less than 20mm range, you won't find anything very fast.

Anyway, you need to stop the lens a little bit to get the minimum sharpness required most of the time.


And you can do some sky shots with a 24-28-35 lens, and it will be nice too.

Alright, Good option. thanks

Why do you need a 1.4 or 2 to shoot astro anyways? I always thought astro was shot stopped down for sharpness.

Check out the photo below. I want to do photography that looks like Ben's. So I need a lens like Bens.

stop down and use astro-tracer mode? with the gps unit thing

I'll check into this, but just in thinking about this, I think the ground would be blurry. Maybe Use two pictures and PP them.

o you mean 35mm equivalent? And is a lens in that criteria even optically possible with reasonable IQ?

Yes, but sort of, I meant a 24mm in a 1.5 crop sensor- 16mm. So yes.

What Ben Canales is saying is to capture as much light as possible. With Canon you can go full frame with a relatively fast wide lens, along with the highest useable ISO. He is just using the most appropriate equipment - he choose Canon.

Now just because he is using a 24/f1.4 lens does not mean that you need to look at the exact equivalent field of view for the K5's cropped sensor - a 24mm lens is still a 24mm lens regardless of sensor size. Just about the best you can do (in terms of fast and wide) is the K 28/f2 lens (its the same lens as the Zeiss 28mm f2 Distagon that has a cult following). Remember that 24 to 28mm is about the widest you can go with the least amount of distortion.

Use what you have in terms of speed and width, then work up to where you want to go based on your equipment (or possibly move to another that has the lenses you desire - however, its expensive).

You can compensate and trade capabilities for what Pentax has available. The O-GPS unit with astrotracking will provide you with additional time, however since it stabilizes the moving stars (removing the trails), it will essentially blur the landscape (since it is not moving).

I guess you're right. I could budge a little bit on the wideness. Not sure if I want to go that narrow right now. I still have hope that I will find a wide angle.

When photographing stars, which are point objects represented by one or just a few pixels on the sensor, the f-stop does not play any role in the exposure for that star. It is only the shutter speed and ISO.

What do you mean?

The f-stop plays a dominant role in exposure, even for a point source of light. The light emitted by a star spreads uniformly in space and the total amount that gets into the camera is proportional to the actual open area of the aperture; which is proportional to 1/(f-stop)^2.

That's why large diameter telescopes are used for astronomy.

Dave in Iowa

That's what I thought too.

[Scootatheschool1990]

This is the photo I'm talking about. It's extremely wide and let's in a lot of light

[Scootatheschool1990]

For point sources the amount of light is proportional to the size of the front element of the lens and not the actual diaphragm. Light rays from a point source spread uniformly in space and then they hit the front element. After they pass the front element, they form a narrow beam that is far too small for the aperture blades to significantly affect it.

Large telescopes (with large diameter front elements or mirrors) gather more light, so do fast lenses which by necessity have larger front elements. If you stop down a fast lens while photographing a point object, the exposure for that object will not change much.

I see Ben's photo and I see the lens that he is using. I'm reverse engineering, trying to get the same set up (in pentax) so I can make create images similar to Ben's.
I want to create a landscape/ Astrophotography photo.

[demp10]

What do you mean?

What I mean is to get a fast lens and shoot wide open. Stopping down will not affect the exposure for the stars (point sources), but the sky (background) and the foreground being illuminated by scattered light will be affected of course as normal. The exposure of the stars will be affected only by the size of the front element.

In other words if you have 2 lenses (50mm f1.4 and 50mm F4) and stop them down to F8, the F1.4 lens with a much larger front element will let more light for a point source than the F4, even if both are stopped at F8.

[interested_observer]

Check out the photo below. I want to do photography that looks like Ben's. So I need a lens like Bens.

Here is some additional information (September 2011).....

• Interview with Ben Canales | RAW

What gear are you using?

Canon 5d Mkii and usually the 24-70 f/2.8

Whats your favourite lens and why?

Oh man… for me, lenses are like songs. Each one fits a different mood and style. Sometimes I need the uber-wide 15mm fisheye; other times the more normal view of the 16-35mm does the trick; since I have the 24-70 that lens gets used the most, but I really like that it has such little distortion. One of the sexiest night lenses is the 24 f/1.4 L – oh man, that’s a sexy piece of glass for star shooting… (anyone want to buy one for me?!)

So on the body, the 5DmkII has an ISO range of 100-25600 standard, 50-102,800 expanded and its full frame. The K5 matches on this, but not on sensor size.

Lenses -

• Canon 15mm fisheye and its f2.8
• Canon 16-35mm - is a f2.8 lens
• Canon 24-70mm - is a f2.8 lens
• Canon 24 f/1.4 L - and he is apparently looking to purchase this....

As of Sept 2011, the glass he has been using is essentially f2.8. I would venture to say that the vast body of his published work is using this f2.8 glass. Pentax glass on the wide side is f2 (one), f2.8 (a lot), f3.5 (a lot) and f4 (for the modern lenses) so you are going to loose a stop of light and or FoV width or at lease a bit of both. Also, when he is talking 15mm he is talking about a fisheye which is 180 wide corner to corner.

This type of photography does not use autofocus, so old glass is what you are after. Most wide angle glass from the film era is around f4 (+/- a stop) in the 15, 16, 18, 20-21mm focal length. The Pentax glass is also of wonderful quality for the most part.

• Pentax DA Prime Lenses - Reviews and Specifications - SLR and Interchangeable Lenses - Pentax Lens Reviews & Lens Database
• Pentax A Prime Lenses - Reviews and Specifications - SLR and Interchangeable Lenses - Pentax Lens Reviews & Lens Database
• Pentax M Prime Lenses - Reviews and Specifications - SLR and Interchangeable Lenses - Pentax Lens Reviews & Lens Database
• Pentax K Prime Lenses - Reviews and Specifications - SLR and Interchangeable Lenses - Pentax Lens Reviews & Lens Database
• Pentax M42 Screwmount Extreme Wide-Angle Primes - Lens Reviews and Specifications - SLR and Interchangeable Lenses - Pentax Lens Reviews & Lens Database
• Pentax M42 Screwmount Wide-Angle Primes - Lens Reviews and Specifications - SLR and Interchangeable Lenses - Pentax Lens Reviews & Lens Database
• Tokina Lenses for Pentax: Prime Lenses - Pentax Lens Review Database
• Sigma Lenses for Pentax: Legacy Prime Lenses - Pentax Lens Review Database
• Carl Zeiss Lenses for Pentax: Prime Lenses - Pentax Lens Review Database
• Miscellaneous Lenses for Pentax: Prime Lenses - Pentax Lens Review Database

So you have an inventory to select from:

• Pentax has the DA 10-17 Fisheye at f3.5 - so you get the 180 degrees corner to corner and give up a half a stop, at $500
• Pentax has the A 16mm f2.8 Fisheye - goes for around $500
• Zenitar 16mm f2.8 Fisheye for around $200
• -------------------------------
• Pentax has the DA 14 f2.8, but it is relatively expensive at around $550
• Pentax has a K 15 and 18mm lens at f3.5 so you are within a half of stop for about $400 to $800
• Pentax also has a M 28 f2 in the $270 area
• Tokina has a 17mm f3.5 again you are a half a stop down for $100 +/-
• Sigma has a 14 f2.8 or f3.5 - the f2.8 goes for about $550
• Zeiss Fleckron has a 20 f2.8
• Rokinon or Samyang (has a lot of brands) 14mm f/2.8 Lens For Pentax for $400
• and there are a few more if you paw through the lists

So by doing a bit more due diligence in terms of what he has done and used in the past, coupled with what is available for Pentax (if you don't change) makes the quest a bit more easier - obtainable.

You also have to recognize that he is shooting two different types of lenses:

• A fisheye - this is 180 degrees corner to corner
• A rectilinear - at 16 to 24 at the widest.

In terms of width, aperture and price - the Tokina and the Zenitar is probably going to be your best bet. The combination of these two would probably give you the widest and fast aperture coverage in lenses for Pentax. Together you could probably acquire them for a total of $300.

• Tokina SL-17 17mm f/3.5 Lens Reviews - Tokina Lenses - Pentax Lens Review Database
• Used Tokina Super Wide Angle 17mm f/3.5 Manual Focus SL17PK B&H
• -------------------------------
• https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/11928-opinions...8-fisheye.html
• Amazon.com: Fisheye Zenitar 2.8/16 MC Lens for Pentax K SLR DSLR Cameras: Camera & Photo

Also, you have to be somewhat practical. He was - he could not afford the f1.8 apparently until recently, so he went with what was available and affordable. I think that is the bottom line.

In terms of wide field of view, the sensor is going to be your greatest limiting factor here. Either until Pentax comes out with a FF body (and it will be relatively expensive - the rumors are saying $2,800 for the body) or you go to a different vendor's body - and lenses, there is always going to be somewhat of a limitation.

[calsan]

For what you want, aperture is everything.
If you want wide and fast, you're looking at buying a very expensive lens $2000-3000. Most of the expense will be in the lens, so starting with what camera you already have is back to front.
You need to go 'what is the fastest wide angle lens available' and then buy the matching camera. Canon/Nikon 24 f/1.4 seem obvious choices but you will not get sharp edges unless you stop down - this is the disadvantage of full frame - a very bent light path. Therefore a smaller format camera like m4/3 will give you sharpness wide open due to straighter light path and much faster lenses for less money. But still more than $1000. The camera also has the advantage of no mirror and smaller weight for less vibrations and sharper images and can be adapted to telescopes and old 'unwanted' manual focus lenses like a 400 2.8 for an "800 2.8" that's nearly portable.

[TER-OR]

Sigma makes a 28mm f1.8 lens. I rented it recently and was impressed. No, it's not super-wide but it's big. You can rent it from cameralensrentals.com and see what you think.

[Scootatheschool1990]

@interested_observer thank you sooo much, this is exactly the help I was looking for!