Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
03-01-2010, 01:38 AM   #1
Veteran Member
octavmandru's Avatar

Join Date: Jan 2009
Location: somewhere around
Posts: 615
focal length discrepancy

I recently bought a Pentax 70-200/f4.
the lens is fine for it's price, but I was surprised to notice a HUGE discrepancy when comparing the magnification with my Tamron 28-200.

I am using an ME (can't beat that viewfinder), one tripod and two light stands.
With the ME on the tripod and the Tamron at 200 mm, I placed the lightstands over the the edges of the image in the viewfinder.
Measured the distance between the lighstands: 400 mm @ 1750 mm (1750 mm is the distance from the film to the lightstands.)

Mounted now the Pentax 70-200 @ 200 mm.
If I check the equivalent focal lenghts on the scale results that 200 mm on the Tamron is equal to 110 mm on the Pentax.
OK, I put now the Pentax at 200 mm. Adjusted again the lightstands over the edges of the image in the viewfinder:
Measured the distance between the lighstands: 220 mm @ 1750 mm


Hmmm. 220 V 400. That's a big difference, isn't it?
Ok, I make some calculations now on the paper:
1750-200=1550 mm (distance from the optical center of the lenses to the object)

a1 = angle of Pentax lens @ 200 mm
a2 = angle of Tamron lens @ 200 mm

a1 = 2 arctg 110/1550 = 8,1 degrees
a2 = 2 arctg 200/1550 = 14,7 degrees

So.... What about that?
Octav

03-01-2010, 03:11 AM   #2
Veteran Member




Join Date: Jan 2008
Location: Ste-Anne des Plaines, Qc., Canada
Posts: 2,014
The focal length of lenses with internal focus changes as they focus closer. That is likely your problem. The focal length is only accurate at infinity.
03-01-2010, 03:52 AM   #3
Veteran Member
Ben_Edict's Avatar

Join Date: Jul 2007
Location: SouthWest "Regio"
Photos: Albums
Posts: 3,303
QuoteOriginally posted by flyer Quote
The focal length of lenses with internal focus changes as they focus closer. That is likely your problem. The focal length is only accurate at infinity.
Exactly… Older lenses without IF and generally a simpler construction, will not exhibit such a severe discrepancy of the focal lengthes at near and infinite distances.

Ben
03-01-2010, 05:26 AM   #4
Pentaxian
Lowell Goudge's Avatar

Join Date: Jan 2007
Location: Toronto
Photos: Gallery | Albums
Posts: 15,400
aside from what others have posted, I think it is a mistake to use 200 mm as the optical center of the lens.

What I do is set the distance from the front element not the film plane,

once the light has begun to be bent around it is really hard to tell the true optical center,

03-01-2010, 06:07 AM   #5
Veteran Member
octavmandru's Avatar

Join Date: Jan 2009
Location: somewhere around
Posts: 615
Original Poster
Interesting... So according my calculations, the Pentax 70-200 is in fact 300 mm at that distance!
200 mm corresponds to an angle-of view of about 12,34 degrees.

I feel like I did a great deal, even if it's just me. Well, better this than tricking myself with the crop factor

Octav
03-01-2010, 06:45 AM   #6
Pentaxian
jimH's Avatar

Join Date: Apr 2009
Location: South Central Nebraska - USA
Photos: Gallery | Albums
Posts: 1,422
What I did didn't involve any measuring but I've noticed that the image size that I produced from the same distance with a Pentax FA 50mm f/1.4 was noticeably larger than the same image taken with a Tamron AF 17-50 f/2.8 lens set at 50 mm. I was amazed by this since they were both supposedly 50mm lenses at that setting.

Anyone have any ideas what's going on here?
03-01-2010, 08:13 AM   #7
Veteran Member




Join Date: Jan 2008
Location: Ste-Anne des Plaines, Qc., Canada
Posts: 2,014
QuoteOriginally posted by jimH Quote
What I did didn't involve any measuring but I've noticed that the image size that I produced from the same distance with a Pentax FA 50mm f/1.4 was noticeably larger than the same image taken with a Tamron AF 17-50 f/2.8 lens set at 50 mm. I was amazed by this since they were both supposedly 50mm lenses at that setting.

Anyone have any ideas what's going on here?
Generally speaking, most lenses (not withstanding Pentax Ltd's) focal length are not exactly as indicated. A manufacturer's 50mm might actually be 47 while another one might be 53. That could explain part of the discrepancies. And lenses with internal focus will have a longer focal length at close focusing than at infinity. Put those two factors together and you end up with a lot of latitude as to what you call 50mm.
03-01-2010, 08:32 AM   #8
Pentaxian
jimH's Avatar

Join Date: Apr 2009
Location: South Central Nebraska - USA
Photos: Gallery | Albums
Posts: 1,422
QuoteOriginally posted by flyer Quote
Generally speaking, most lenses (not withstanding Pentax Ltd's) focal length are not exactly as indicated. A manufacturer's 50mm might actually be 47 while another one might be 53. That could explain part of the discrepancies. And lenses with internal focus will have a longer focal length at close focusing than at infinity. Put those two factors together and you end up with a lot of latitude as to what you call 50mm.
Thanks for the insight Yves and I believe that probably is the case with my two lenses.

03-01-2010, 10:26 AM   #9
Veteran Member
Ben_Edict's Avatar

Join Date: Jul 2007
Location: SouthWest "Regio"
Photos: Albums
Posts: 3,303
QuoteOriginally posted by octavmandru Quote
I recently bought a Pentax 70-200/f4.
the lens is fine for it's price, but I was surprised to notice a HUGE discrepancy when comparing the magnification with my Tamron 28-200.

I am using an ME (can't beat that viewfinder), one tripod and two light stands.
With the ME on the tripod and the Tamron at 200 mm, I placed the lightstands over the the edges of the image in the viewfinder.
Measured the distance between the lighstands: 400 mm @ 1750 mm (1750 mm is the distance from the film to the lightstands.)

Mounted now the Pentax 70-200 @ 200 mm.
If I check the equivalent focal lenghts on the scale results that 200 mm on the Tamron is equal to 110 mm on the Pentax.
OK, I put now the Pentax at 200 mm. Adjusted again the lightstands over the edges of the image in the viewfinder:
Measured the distance between the lighstands: 220 mm @ 1750 mm

Hmmm. 220 V 400. That's a big difference, isn't it?
Ok, I make some calculations now on the paper:
1750-200=1550 mm (distance from the optical center of the lenses to the object)

a1 = angle of Pentax lens @ 200 mm
a2 = angle of Tamron lens @ 200 mm

a1 = 2 arctg 110/1550 = 8,1 degrees
a2 = 2 arctg 200/1550 = 14,7 degrees

So.... What about that?
Octav

You did a lot of work. But basically your measurements are way too far off, to be useful. You simply cannot use the viewfinder for placing the poles as targets. The only really valid way to achieve repeatable and comparable results is to take real images and measure the negs. Simply leave your camera at a fixed distance from a target, which could be a simple ruler. position the ruler parallel to the film plain and near the center of the field of view. Than you can take test shots with both lenses at different focal settings and measure the real angle of view.

Why am I so outspoken? Because while the Pentax A 70-200/4 is a fine lens, it is certainly nowhere near 300mm in focal length. A simple look through the zoom and a 300mm prime will confirm that, the 300mm lens offering a much smaller angle of view! So, if your measurements lead you to showing 300mm fl for the Pentax A zoom, there must be something fundamentaly flawed with your way of mesuring this.

Ben
03-01-2010, 10:54 AM   #10
Veteran Member
octavmandru's Avatar

Join Date: Jan 2009
Location: somewhere around
Posts: 615
Original Poster
Dear Ben,

My measurements were nowhere even close to difficult. Lightstands are perfectly vertical and easy to move since they are voice activated.
The viewfinder is a constant in the equation.
My measurements/calculations could be flawed, but how?

For confirmation of my calculations I used this:
Lens Focal Length and Field of View

What I am certain about is this: the horizontal FOV on Pentax is close to half of the Tamron's at the same setting.
Octav

Last edited by octavmandru; 03-01-2010 at 11:06 AM.
03-01-2010, 12:56 PM   #11
Veteran Member
Ben_Edict's Avatar

Join Date: Jul 2007
Location: SouthWest "Regio"
Photos: Albums
Posts: 3,303
QuoteOriginally posted by octavmandru Quote
Interesting... So according my calculations, the Pentax 70-200 is in fact 300 mm at that distance!
200 mm corresponds to an angle-of view of about 12,34 degrees.

I feel like I did a great deal, even if it's just me. Well, better this than tricking myself with the crop factor

Octav
No, that's wrong. The Dimitrov calculator gives the angle of view diagonally(!), whereas you are measuring along the longer side of the image. So your measurement needs to be smaller. It does not indicate a focal length increase.

Ben
03-01-2010, 12:59 PM   #12
Veteran Member
Ben_Edict's Avatar

Join Date: Jul 2007
Location: SouthWest "Regio"
Photos: Albums
Posts: 3,303
QuoteOriginally posted by octavmandru Quote
What I am certain about is this: the horizontal FOV on Pentax is close to half of the Tamron's at the same setting.
Octav
That would be the conclusion, which I can follow, based on your measurements. But the "translation" into real focal lengthes needs to be more thinking (not meant as criticism, but more meant as a challenge to myself!)

Ben
03-01-2010, 01:54 PM   #13
Veteran Member
octavmandru's Avatar

Join Date: Jan 2009
Location: somewhere around
Posts: 615
Original Poster
QuoteOriginally posted by Ben_Edict Quote
No, that's wrong. The Dimitrov calculator gives the angle of view diagonally(!), whereas you are measuring along the longer side of the image. So your measurement needs to be smaller. It does not indicate a focal length increase.

Ben
Ok, I was hasty posting B. Dimitrov link... I like this more: Depth of Field & Field of View Calculator Wildframe
Acc. to this, HFOV is 7 dgr. with a 300 mm lens and a FF camera. I've got 8 dgr.
For the theory check this link: Field of view and focal length
Take into account I calculated it first using my method and only after searched for more info. I admit I was a bit crude, but I didn't want to make rocket science out of a simple observation.
The math is very simple. I just wonder how such discrepancies occur.

But at least, according to this: http://www.rmassa.com/specsheets/BOOKLET-200704%2033.pdf you calculate the focal lenght of the lens from the optical centre of the lens called here secondary principal point, so I wasn't wrong in the first place.

Octav
03-01-2010, 03:34 PM   #14
Veteran Member
Ben_Edict's Avatar

Join Date: Jul 2007
Location: SouthWest "Regio"
Photos: Albums
Posts: 3,303
QuoteOriginally posted by octavmandru Quote
But at least, according to this: http://www.rmassa.com/specsheets/BOOKLET-200704%2033.pdf you calculate the focal lenght of the lens from the optical centre of the lens called here secondary principal point, so I wasn't wrong in the first place.

Octav
The problem is, that we do not know, where the rear nodal point is located in these complex zoom lenses, unless the manufacturer released the data, which I don't know.

At the short distances, which obvíously are unavoidable, to see and measure the decrease in fl at near distances, the the placing of the rear nodal point is critical, as it can amount to app. 10% of the whole object distance and thus influence any subsequent calculations significantly.

Ben
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!
a1, angle, degrees, distance, k-mount, lens, mm, pentax, pentax lens, slr lens, tamron, viewfinder
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
focal length ewig Troubleshooting and Beginner Help 11 07-23-2010 09:32 PM
K7 is 1.5 x focal length? rdrum76 Pentax DSLR Discussion 18 06-30-2009 08:30 AM
What is Focal Length Ole Photography Articles 2 05-15-2009 12:20 PM
Focal Length ? deludel Photographic Technique 14 01-30-2008 06:58 PM
Focal Length Jimsi777 Pentax SLR Lens Discussion 2 11-09-2007 06:46 PM



All times are GMT -7. The time now is 09:58 AM. | 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