Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
05-11-2010, 04:45 AM   #1
Veteran Member
chromo's Avatar

Join Date: Feb 2010
Location: North Coast NSW
Photos: Gallery | Albums
Posts: 571
Lens light path diagram ?

Howdy all,

Has anyone seen a cutaway diagram of a lens mounted to a Pentax camera, and showing the light path ?.... I'd like to see it too.

It's the way the light travels through the lens to the sensor I'm interested in.

I'm sure it wouldn't matter what lens it is. Just some standard or tele or whatever.

Chromo

05-11-2010, 05:22 AM   #2
Pentaxian
bdery's Avatar

Join Date: Mar 2008
Location: Quebec city, Canada
Photos: Gallery
Posts: 5,831
I could make you a generic diagram out of a light processing soft (I use OSLO at work). It wouldn't be a Pentax lens, but I'm sure there are a few fast fifties built-in the program, so it would be really close to the FA 50 f1.4. Would that work?
05-11-2010, 05:27 AM   #3
Veteran Member
chromo's Avatar

Join Date: Feb 2010
Location: North Coast NSW
Photos: Gallery | Albums
Posts: 571
Original Poster
Yes Thank you

That would be good to look at.
I'd like to see how if a lens is moved away from the sensor, what effect the focus is on the a particular lens' focus range(focus ring adjustment). Hope you understand what I mean.

And now you have me curious, what's OSLO ?

Chromo

Last edited by chromo; 05-11-2010 at 05:34 AM.
05-11-2010, 07:07 AM   #4
Veteran Member




Join Date: Jan 2010
Posts: 773
According to GOOGLE, Oslo is the Capital of Norway.

(Also Optical Design Software - see Lambda Research)

05-11-2010, 11:44 AM   #5
Senior Member
wavecurrent's Avatar

Join Date: Jul 2007
Location: Kiruna, Sweden
Posts: 147
QuoteOriginally posted by chromo Quote
Howdy all,

Has anyone seen a cutaway diagram of a lens mounted to a Pentax camera, and showing the light path ?.... I'd like to see it too.

It's the way the light travels through the lens to the sensor I'm interested in.

I'm sure it wouldn't matter what lens it is. Just some standard or tele or whatever.

Chromo
There is a free program called WinLens by Linos, you can make your own lenses
whit that

Mike

WinLens: Install
05-11-2010, 04:33 PM   #6
Veteran Member
chromo's Avatar

Join Date: Feb 2010
Location: North Coast NSW
Photos: Gallery | Albums
Posts: 571
Original Poster
What I mean is, for example :

The distance from the sensor to the mount is 45.46mm, If I made an extension tube 45.46mm long, would the image then be upside-down but still in focus and the same size on the sensor?

Last edited by chromo; 05-11-2010 at 04:46 PM.
05-11-2010, 06:47 PM   #7
Pentaxian
bdery's Avatar

Join Date: Mar 2008
Location: Quebec city, Canada
Photos: Gallery
Posts: 5,831
QuoteOriginally posted by chromo Quote
What I mean is, for example :

The distance from the sensor to the mount is 45.46mm, If I made an extension tube 45.46mm long, would the image then be upside-down but still in focus and the same size on the sensor?
That can get a bit more tricky. You'd have to actually USE the program to experiment with various things like this. I can place the image plane at one particular value, and the program will focus for me (the way I'm describing it, it will determine the object distance). But to change values, you will need to use the program, I cannot really do all the permutations. And anything I do with a standard 50 mm design will not apply directly to another lens.

OSLO also has an education version, which is free (and yeah, it's Lambda Research's lens design program...) The manual also is, in itself, a lens design course. If you haven't done any college physics, though, it might be a little steep.

Maybe someone near you could meet you and assist?
05-11-2010, 07:28 PM   #8
Veteran Member




Join Date: Dec 2009
Location: Deep Forest
Posts: 630
QuoteOriginally posted by chromo Quote
What I mean is, for example :

The distance from the sensor to the mount is 45.46mm, If I made an extension tube 45.46mm long, would the image then be upside-down but still in focus and the same size on the sensor?
The image projected by the lens onto the sensor is already upside-down. Adding a 45.46mm extension tube simply moves the lens farther away from the sensor; the lens focuses closer.

A lens bends light rays. If a lens diagram is drawn showing rays from an object meeting at a point behind the lens, that point is where those light rays from the object are in focus. Continuing the rays beyond the focus point does not indicate a flipped image, but rather the rays are out of focus.

05-11-2010, 08:51 PM   #9
Veteran Member
chromo's Avatar

Join Date: Feb 2010
Location: North Coast NSW
Photos: Gallery | Albums
Posts: 571
Original Poster
Sounds like I'm getting confused between focus and convergence.
05-11-2010, 09:06 PM   #10
Loyal Site Supporter
Canada_Rockies's Avatar

Join Date: Nov 2007
Location: Sparwood, BC, Canada
Photos: Gallery | Albums
Posts: 9,548
QuoteOriginally posted by chromo Quote
Sounds like I'm getting confused between focus and convergence.
Perhaps, and perhaps not. If we take a simple convex lens shaped like this () it has a focal length. When it is situated at exactly that focal length from the sensor/film/wall/whatever, it will bend the light rays in a manner that will exactly focus parallel rays on the sensor as a dot. Think magnifying glass and sun and lighting a fire with it.

That means that for a Pentax camera, a lens with a focal length of 45.46 mm will focus the light from the sun as a single point on your sensor when mounted flush with the lens mount. To allow for helicoid focusing and mounts on the lens and so forth, it works out that about an extra 10 mm of stuff goes between the optical centre of the lens and the film/sensor. That means that our theoretical 45.46mm lens will not focus to infinity. It seems that you need a 55.45 mm lens ... hmm ... I wonder why my first Takumar preset lens was a 55 mm f/2.2? And why 58mm lenses were quite common in the pre-history when I was learning about SLR cameras?
05-12-2010, 04:34 AM   #11
Pentaxian
Class A's Avatar

Join Date: Aug 2008
Location: Wellington, New Zealand
Posts: 9,201
QuoteOriginally posted by Canada_Rockies Quote
That means that for a Pentax camera, a lens with a focal length of 45.46 mm will focus the light from the sun as a single point on your sensor when mounted flush with the lens mount.
Are you saying you cannot focus on the sun with any other focal length?

QuoteOriginally posted by Canada_Rockies Quote
I wonder why my first Takumar preset lens was a 55 mm f/2.2? And why 58mm lenses were quite common in the pre-history when I was learning about SLR cameras?
The real focal lengths for a "normal" lens is 43.3mm but at the time it was easier to make a lens sharp that had a larger focal length.
05-12-2010, 04:37 AM   #12
Veteran Member
falconeye's Avatar

Join Date: Jan 2008
Location: Munich, Alps, Germany
Photos: Gallery
Posts: 6,863
QuoteOriginally posted by chromo Quote
What I mean is, for example :

The distance from the sensor to the mount is 45.46mm, If I made an extension tube 45.46mm long, would the image then be upside-down but still in focus and the same size on the sensor?
You are confused.

What you need is an understanding of the Thin lens equation. It will answer all your questions.

An online calculator is here: Thin Lens Equation

If you make an extension tube 45.46mm long, you will get a sharp image of a subject a few millimeters near the front lens (and everything else being fuzzy as hell). And the size will change as well because the field of view does.

You may also like this: http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=48
(just drag the lens around! )

Last edited by falconeye; 05-12-2010 at 04:49 AM.
05-12-2010, 06:28 AM   #13
Veteran Member
chromo's Avatar

Join Date: Feb 2010
Location: North Coast NSW
Photos: Gallery | Albums
Posts: 571
Original Poster
OK then, while designing my tilt-shift adapter I'll need to think about the following. Please let me know if you think I'm wrong.

Since the Pentax registry is 45.46mm, and I need room for the mechanics, I'll have to use a lens with a longer registry.

Pentacon 6 lens is 74.1mm.

Pentax 67 lens is 84.95mm.

So if I use a pentacon 6 then my adapter should be 27.64mm long.
or if I use a pentax 6x7 then my adapter should be 39.49mm long.

Chromo.

PS ... Is there another way without buying another lens ?
05-12-2010, 07:46 AM   #14
Veteran Member




Join Date: Dec 2009
Location: Deep Forest
Posts: 630
QuoteOriginally posted by chromo Quote
OK then, while designing my tilt-shift adapter I'll need to think about the following. Please let me know if you think I'm wrong.

Since the Pentax registry is 45.46mm, and I need room for the mechanics, I'll have to use a lens with a longer registry.

Pentacon 6 lens is 74.1mm.

Pentax 67 lens is 84.95mm.

So if I use a pentacon 6 then my adapter should be 27.64mm long.
or if I use a pentax 6x7 then my adapter should be 39.49mm long.

Chromo.

PS ... Is there another way without buying another lens ?
Not only is longer register lens needed for tilt-shift mechanics, the lens needs to project a larger image circle to prevent vignetting as lens position is tilted/shifted; medium format lenses project a larger image circle than APS-C/35mm lens, perfect for tilt-shift lens for a APS-C/35mm camera.

btw, 74.1 - 45.46 = 28.64
05-12-2010, 07:55 AM   #15
Loyal Site Supporter
Canada_Rockies's Avatar

Join Date: Nov 2007
Location: Sparwood, BC, Canada
Photos: Gallery | Albums
Posts: 9,548
QuoteOriginally posted by Class A Quote
Are you saying you cannot focus on the sun with any other focal length?
Not when the lens is in the mount.
QuoteQuote:
The real focal lengths for a "normal" lens is 43.3mm but at the time it was easier to make a lens sharp that had a larger focal length.
I know this theory about the "normal" lens being the diagonal of the format, but my personal take is that the lens + finder optics should make the image appear the same size as the other eye sees. With my early, early, early Asahi Pentax the 55mm did just that. There is a third theory that the lens should approximate the field of view - but that appears to be my 12mm lens, and seems a tad wide.
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!
diagram, k-mount, lens, light, path, pentax lens, slr lens
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
Accurate dimensions of Pentax K-7 - Light-Path karmazyn Pentax DSLR Discussion 0 02-28-2010 02:20 PM
Need exploded assembly diagram K200Duser Pentax SLR Lens Discussion 12 12-15-2009 03:29 PM
Sticky lens diagram MXP Pentax SLR Lens Discussion 4 10-11-2009 10:36 AM
Garden Path JMR Post Your Photos! 12 09-10-2008 07:47 AM
two-path lukastrika Post Your Photos! 2 02-23-2007 02:39 AM



All times are GMT -7. The time now is 07:03 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