Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
06-27-2011, 12:37 AM   #1
New Member
multiweb's Avatar

Join Date: Jun 2011
Location: sydney
Posts: 11
200mm Lens stop

Hi there, first post here. I recently bought a SMC Pentax-M 200mm F4. I use it primarily for astro-photography. I was able to use this lens using 2" narrowband filters Ha 7nm and Sii 8nm with success. The field is flat with very little aberrations even with the lens fully open.

This week-end I tried without a filter and did straight RGB with an OSC cooled CCD. This time I shot with one stop down. The red channel was fine but the green and blue showed a lot of star bloating. Again no field curvature or coma, just very fat stars and some chromatic aberration.

My question is would stopping the lens further get rid of star bloat or am I best to use a UV/IR cut in front of the lens altogether?

My subs were 2min exposures guided.
Thanks for any tips.

PS: What is the thread size in front of the glass and where can I get a compression ring to M48 M/F?

Attached Images
 

Last edited by multiweb; 06-27-2011 at 01:07 AM. Reason: added link
06-27-2011, 01:50 AM   #2
Site Supporter
Stone G.'s Avatar

Join Date: Jun 2010
Location: North Zealand, Denmark
Photos: Gallery | Albums
Posts: 1,516
QuoteOriginally posted by multiweb Quote
Hi there, first post here. I recently bought a SMC Pentax-M 200mm F4. I use it primarily for astro-photography. .......

My question is would stopping the lens further get rid of star bloat or am I best to use a UV/IR cut in front of the lens altogether?

......
HI multiweb, stopping down your lens might well reduce bloat and chromatic aberration - however, I would advice aginst it:

Stars are point-like light sources and behave differently from exteded objects.

If you stop down your lens you will not only reduce the light gathering power of your lens; the Airy disk ("bloat") of a star will also grow in size propotional to the inverse of your lens opening diameter. In other words: You will collect less light and that light will be smeared out over more pixels and thus, become additionally dimmed.

All in all, as you stop down your lens, you may soon need excessively long exposure times to capture the weaker stars.
06-27-2011, 02:20 AM   #3
New Member
multiweb's Avatar

Join Date: Jun 2011
Location: sydney
Posts: 11
Original Poster
Thanks for the quick reply. I actually don't mind going for longer exposure. With the Sii filter my subs were 20min. The longer the better because I get less read-out noise. The only reason I did 2min subs is that 5min burnt all the details.

I realise the light gathering increases with aperture but I wasn't aware that star bloat was also related to aperture (?)
06-27-2011, 02:43 AM   #4
Site Supporter
Stone G.'s Avatar

Join Date: Jun 2010
Location: North Zealand, Denmark
Photos: Gallery | Albums
Posts: 1,516
QuoteOriginally posted by multiweb Quote
.....
I realise the light gathering increases with aperture but I wasn't aware that star bloat was also related to aperture (?)
Oh yes, large telescopes / lenses gather more light, they have a better resolution and the have better defined stellar / point-source images. There is a good interactive demo on this page:

Diffraction Limited Photography: Pixel Size, Aperture and Airy Disks

06-27-2011, 09:32 AM   #5
Site Supporter
smigol's Avatar

Join Date: Sep 2008
Location: Menlo Park, CA
Photos: Albums
Posts: 406
I highly advise using a UV/IR filter in the front of your lenses as it will make a big difference. I'm not surprised that the Blue showed bloat because of the near UV spilling over. I am surprised at the Green. Does this filter cut out UV -- what is the frequency response?

Also, the lens has a filter ring of 52mm size. To adapt to 48mm size, you'll need a step down ring. B&H has these for about $6.

Note that this will reduce the effective diameter of the lens so the light gathering ability will be reduced.

Last edited by smigol; 06-27-2011 at 11:56 AM. Reason: Added filter ring size info.
06-27-2011, 04:26 PM   #6
New Member
multiweb's Avatar

Join Date: Jun 2011
Location: sydney
Posts: 11
Original Poster
QuoteOriginally posted by Stone G. Quote
Oh yes, large telescopes / lenses gather more light, they have a better resolution and the have better defined stellar / point-source images. There is a good interactive demo on this page:

Diffraction Limited Photography: Pixel Size, Aperture and Airy Disks
Thanks for the link. Very cool and informative.

QuoteOriginally posted by smigol Quote
I highly advise using a UV/IR filter in the front of your lenses as it will make a big difference. I'm not surprised that the Blue showed bloat because of the near UV spilling over. I am surprised at the Green. Does this filter cut out UV -- what is the frequency response?

Also, the lens has a filter ring of 52mm size. To adapt to 48mm size, you'll need a step down ring. B&H has these for about $6.

Note that this will reduce the effective diameter of the lens so the light gathering ability will be reduced.
Thanks. I've later figured 52mm. Getting a stepdown ring from eBay now.

I used an OSC. Some of the blue/turquoise still makes it in the green pixels of the bayer matrix. The pattern is RGGB so 2Gs makes the sensor more sensitive in the green as well.

The UV/IR filter I plan to use is the 2" Baader UV/IR Blocking. Frequency is 420 to 680nm. I'll try again when I've got all the bits and pieces.
06-27-2011, 06:43 PM   #7
Site Supporter
smigol's Avatar

Join Date: Sep 2008
Location: Menlo Park, CA
Photos: Albums
Posts: 406
That Baader is the one I keep in my kit, too. I also use the Moon/Skyglow filter for good effect, too.

I posted this question to my local astronomy group to understand how the aperture effects the way star bloat happens.

Here are the interesting bits from the conversation:

QuoteQuote:
With a good quality camera lens, the sharpest image is usually obtained by stopping the lens down two stops from wide open. Beyond that, resolution will go down as the lens is stopped down. This "inverted U-shaped curve" is due to two competing effects: the residual aberrations present in even the most expensive camera lenses become less important as the focal ratio increases/lens shrinks but at the same time the effects of diffraction start to reduce resolution as the lens is stopped down.

Think in terms of a 6" f/10 newtonian telescope with a spherical mirror. AT F/10 it hardly matters optically that the mirror is not parabolic, the sphere is a good enough approximation of a paraboloid and images will be good. At F/5 the 6" sphere is a poor approximation of a paraboloid optically and the images would be very poor, with lots of spherical aberration. The principle is the almost the same with camera lenses used wide open--they are imperfect and need to be stopped down to get past these imperfections.

But, as you further stop down on a camera lens, the lens becomes very small, and as you know, a big lens has higher resolution than small lens (all other things being equal). As a result, the star images will start to bloat and resolution will go down (depends on pixel size, focal length too). This is all caused by diffraction.

This is shown remarkably well at this link: Lens Reviews: Digital Photography Review
Lets choose to view the data on the Canon 100 mm F2.8 older (non-L series) macro lens, which is a great wide field astro lens: dpreview.com - Lens Review - Fullscreen

You will see a chart that shows the resolution of the lens (in lines 50% resolved across the narrow side of the sensor, the metric is not important at this point) across the field of view, starting at the center on the left side and moving to the corners of the sensor on the right. The higher the line is, the higher the resolution.

Notice the F/stop "wheel" at the bottom of the page--move your cursor there, click and hold and you can "stop down" the lens by moving the mouse to the left. Watch what happens to the resolution line--first, when stopping down from F/2.8, the resolution rises, until about F/8, at which point the resolution will start to fall off with each increase in F/stop.

Then go to Canon 100mm F2.8 L IS USM Macro Lens Review: 3. Test results (APS-C): Digital Photography Review which is the new L-series 100 mm macro lens. Right off the bat is is clear that this lens has higher resolution wide open at F/2.8, resolution peaks at astro? >$1K to find out!

Just to further entertain yourself, go here Canon EF 50mm F1.8 II Lens Review: 4. Test results (Full Frame): Digital Photography Review and look at the data for the cheap Canon 50 mm F1.8 lens on a full frame camera. Note that resolution is very uneven at 1.8, very good in the center but worthless in the corners--this is indeed a cheap lens. Maximum corner resolution rises until >F/8, but by that point center resolution is falling quickly.

Here is another way to look at it. Use the tool at Diffraction Limited Photography: Pixel Size, Aperture and Airy Disks which allows you to see how many pixels a "perfect" star (Airy disk) would cover on different sensors and different f/stops. Click on a small sensor camera (Canon Powershot G6 is good) and then scroll down the F/stops in the left column. You can see the star bloat as you move to higher f/stops. It starts with all the light on one pixel and bloats up to dozens of pixels as you stop down.

Increasing the f/stop in a telescope via a barlow lens will also "bloat" the star size at the sensor, but the scale will increase proportionately, so unlike a camera lens, no resolution is lost.

The star test is the most rigorous test you can subject a lens to. No camera lens I have tested (up to $2000) works well wide open. All have to be stopped down to provide good astro images. If you start with a fast, high quality lens, you can usually get by with a 2 stop reduction, e.g. from F 2.0 to F 4. A poor lens will likely be slower to start with and will require 3 stops reduction, and may still produce inferior star images.

This is where high quality astrographs excel, they are designed to be used wide open and provide excellent images.
I understood already how diffraction effects images at the f16+ range, I didn't expect that star images would show diffraction problems lower. For my camera (K10D), that calculator is showing that I'll see effects at f12+. This is what I experience in everyday shooting.

Like is noted, the quality of the lens matters: I've noticed that my 100mm macro lens works very well nearly wide open where almost all of my other lenses require 1.5 to 2 stops down.

And a follow up, regarding a question about bloat:

QuoteQuote:
There are many causes of star bloat. You may also get bloat due to diffraction, other lens aberrations (spherical aberrations, coma, astigmatism), focusing errors, seeing, guiding errors, atmospheric refraction etc. So chromatic aberration is only part of the picture.

Lenses are corrected for the two types of chromatic aberration over a limited range of wavelengths, and anytime you start using light outside of the corrected range (for an achromat, violet and UV, deep red, IR) you will get bloat.

You can see this in this example plot, showing color focus error for an achromat as a function of wavelength. File:Achromatic focal curve.svg - Wikipedia, the free encyclopedia Only 2 colors (light red, blue) are accurately focused, although between those colors the error is modest. Beyond those two colors (violet, UV, deep red, IR) the focusing error gets very large, hence halos, bloat, colors.

Apochromats are corrected for three colors, which usually includes the IR. Superachromats are corrected for 4 colors, from UV through IR.

Stopping down a camera lens always reduces spherical aberration. Stopping down a camera lens typically reduces chromatic aberration, but the effect is not as simple as I though based on the DP review test results.

Lastly, I use an IDAS LPS filter on the front of my big lenses - the frequency curve really clamps down on the UV so the bloat is much better controlled.

Hope this helps!
06-28-2011, 12:22 AM   #8
New Member
multiweb's Avatar

Join Date: Jun 2011
Location: sydney
Posts: 11
Original Poster
Thanks for all the links and info. Got a bit of reading to do. I got a 52mm to 48mm step down ring from eBay for the 200mm. I've just received my 100mm today. I checked the front thread and this one's 49mm - LOL. Can't win.

I've also noticed the new lens is very dusty at the back. It's loose enough to come out with a cotton tip but not with a blow up air spray. So my question is do I clean it dry with a Qtip or wet the Qtip with acetone and wipe it this way. Is the back element (sensor side) coated?

06-28-2011, 08:01 AM   #9
Site Supporter
smigol's Avatar

Join Date: Sep 2008
Location: Menlo Park, CA
Photos: Albums
Posts: 406
All the elements should be coated, so the gentle wiping can be done. You can also use distilled water before the acetone if you're worried. If your observing site is anything like mine, the overnight wind and dust will make the front element grungy over time. Dew can also be an issue, but not so much for me here.

Regarding filters, there are 48-49 mm step down rings, too. There are a lot of 49mm lenses so that ring will be more likely to be used over the larger size.
06-28-2011, 03:24 PM   #10
New Member
multiweb's Avatar

Join Date: Jun 2011
Location: sydney
Posts: 11
Original Poster
QuoteOriginally posted by smigol Quote
All the elements should be coated, so the gentle wiping can be done. You can also use distilled water before the acetone if you're worried. If your observing site is anything like mine, the overnight wind and dust will make the front element grungy over time. Dew can also be an issue, but not so much for me here.
No worries. A gentle brush got rid of it. I didn't have to wet anything. I have a special pencil brush that I use for my eyepieces. I use dew heater straps over a sock and dew shields when necessary when I image. I always keep the lenses dry stored with dessicant. I treat them like my other scopes.


QuoteOriginally posted by smigol Quote
Regarding filters, there are 48-49 mm step down rings, too. There are a lot of 49mm lenses so that ring will be more likely to be used over the larger size.
Sounds good. I'll get a 49mm step down ring too then so I can use all my baader stuff.

Going to a dark site this new moon week-end. Will see what happens with the mods.
Reply

Bookmarks
  • Submit Thread to Facebook Facebook
  • Submit Thread to Twitter Twitter
  • Submit Thread to Digg Digg
Tags - Make this thread easier to find by adding keywords to it!
200mm, field, front, lens, pentax help, star
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
Manual lens does not stop down when metering or shooting. FH_le Pentax SLR Lens Discussion 5 01-21-2011 01:23 PM
Auto Lenses on Pentax Kx..stop down metering..HELP CLYNN0928 Pentax DSLR Discussion 7 11-10-2010 03:54 AM
Cannot stop down manual lens for some reason SamWarrick Troubleshooting and Beginner Help 28 09-06-2010 10:21 AM
Lens F/stop Limitation Question seachunk2 Troubleshooting and Beginner Help 10 02-26-2010 07:27 AM
Lens buying affliction... make it stop! JasonA Pentax SLR Lens Discussion 31 12-02-2009 12:15 AM



All times are GMT -7. The time now is 04:55 PM. | See also: NikonForums.com, CanonForums.com part of our network of photo forums!
  • Red (Default)
  • Green
  • Gray
  • Dark
  • Dark Yellow
  • Dark Blue
  • Old Red
  • Old Green
  • Old Gray
  • Dial-Up Style
Hello! It's great to see you back on the forum! Have you considered joining the community?
register
Creating a FREE ACCOUNT takes under a minute, removes ads, and lets you post! [Dismiss]
Top