Still confused about FOV

**GUB** · 06-23-2016, 01:33 AM - **1 Like**

Originally posted by wombat2go

Here is a ray trace of a simple double gauss lens
(In this case about 80mm f/4 with the aperture opened fairly wide.)

The light arriving from a distant point at infinity has parallel rays in this example.
Rays passing through every "element" on the surface of the FRONT section of the lens are
converged to a "pixel" on the film plane.
Provided that pixel was inside both frame sizes under consideration,
that effect does not depend on the film plane dimensions.

However, when the film plane is smaller, tha maximum angle of view is reduced,
so the diameters of the glass lenses on the REAR section do not need to be so large.

Thanks for that diagram and explanation Wombat2go. Now I can see where Clackers is coming from. In your example you clearly show the peripheries of the rear elements are less used by the smaller sensor. I had not taken into account the physical depth of a compound lens nor the fact the image is partially focussed by the time it hits the rear elements.
It worried me though that maybe in practise this was not a significant difference. And I had a feeling that it applied less at closer distances rather than infinity.
So ------ I happened to have some self adhesive black dots. (Like on specials in shops). They are 14mm in diameter. That is 153sq mm area. A rear lens with a 20mm diameter would be half covered by this dot. That would halve the light transmitted. A M series 50mm f2 has a rear lens the right size. If the apsc sensor used all the rear lens then it would get half the light. If it didn't use all the lens then the dot would have a greater effect. You can draw your own conclusions from my chart but I think in the case of the close up the sensor is pretty well using all the glass. Isn't it amazing how invisible that dot is!!!
I would love to have feedback from this. I am a learner trying to gain knowledge – PF has been fantastic for that.

Attached Images

Last edited by GUB; 06-23-2016 at 02:30 AM.

**GUB** · 06-23-2016, 01:51 AM

I managed to get this dot off centre but you get the idea

wombat2go · 06-23-2016, 05:31 AM - **1 Like**

Originally posted by GUB

Isn't it amazing how invisible that dot is!!!

Maybe you should do a patent application for your "toroidal lens" !
Think of the glass we will save!

**MJSfoto1956** · 06-23-2016, 05:43 AM

Originally posted by k22

I have a DAL 50-200 lens. If I buy a 200mm prime FF lens, does it have the same field of view as the 50-200 zoomed to 200?

well there is an exception: lenses that focus breathe when focused up close to a subject do not have the same field of view as a lens that doesn't focus breathe. Rather than get into the details suffice it to say that a 200mm lens focuses at infinity will have the same FOV as any other 200mm lens. However when focused up close, it "depends".

Michael

**GUB** · 06-23-2016, 02:01 PM

Originally posted by wombat2go

Maybe you should do a patent application for your "toroidal lens" !
Think of the glass we will save!

Or an inside out iris for soft focus lenses!! getting rid of that annoying sharp bit in the middle.!!

**GUB** · 06-23-2016, 06:58 PM

Chinon 50mm1.4 @ 1.4. A 25mm velcro dot on a 29mm rear lens. I think it is pretty clear a crop sensor uses all the glass that a FF does. What say you Clackers?

MarkJerling · 06-23-2016, 07:21 PM

That is remarkable! Unless you showed this, I would have never thought it possible to still get an image!

**GUB** · 06-23-2016, 07:34 PM - **1 Like**

and its own special bokeh!!

MarkJerling · 06-23-2016, 07:46 PM

I like it!

**GUB** · 06-23-2016, 07:48 PM

Originally posted by MarkJerling

I like it!

Caption -- bad shedding day?

MarkJerling · 06-23-2016, 08:11 PM

Hairy McLary from Donaldson Dairy?

ScooterMaxi Jim · 06-23-2016, 08:12 PM - **1 Like**

Oh my, someone is going to come up with the bright idea to start a Kickstarter campaign for weird special effects lenses - and they won't have to contract a Chinese machine shop to launch it!

---------- Post added 06-23-2016 at 10:18 PM ----------

Originally posted by GUB

Chinon 50mm1.4 @ 1.4. A 25mm velcro dot on a 29mm rear lens. I think it is pretty clear a crop sensor uses all the glass that a FF does. What say you Clackers?

Hey, if you had put that dot on a Pentax limited lens (name your favorite), the resulting image would have been near perfect! I'm pretty sure that the lens is the problem, not that little dot obstructing a mere 84% of the rear objective surface.

**GUB** · 06-23-2016, 08:26 PM

Originally posted by ScooterMaxi Jim

Oh my, someone is going to come up with the bright idea to start a Kickstarter campaign for weird special effects lenses - and they won't have to contract a Chinese machine shop to launch it!

---------- Post added 06-23-2016 at 10:18 PM ----------

Hey, if you had put that dot on a Pentax limited lens (name your favorite), the resulting image would have been near perfect! I'm pretty sure that the lens is the problem, not that little dot obstructing a mere 84% of the rear objective surface.

I can just see someone putting a dirty great adhesive sticker on their limited ---not!!

ScooterMaxi Jim · 06-23-2016, 08:45 PM

Hey, then, if no one is willing to do that, then we will never know if my sarcasm happens to be right. Does the tree in the forest make a sound, etc.

**UncleVanya** · 06-23-2016, 09:09 PM

If you want to do this - use some soft lens cleaning material or something like that. Glue it to a strip of thin ribbon and put let it set. Lay the disc against the rear element gently and then grasp the loose ribbon ends around the lens body and mount - taking care to avoid the areas like the lens data pin and the screw drive etc. Make sure the ribbon stays taunt and take a shot. No damage should occur. No sticky nasty residue etc.

06-23-2016, 01:51 AM	#32
GUB Loyal Site Supporter Join Date: Aug 2012 Location: Wanganui Photos: Gallery \| Albums Posts: 5,751	I managed to get this dot off centre but you get the idea Attached Images

06-23-2016, 06:58 PM	#36
GUB Loyal Site Supporter Join Date: Aug 2012 Location: Wanganui Photos: Gallery \| Albums Posts: 5,751	Chinon 50mm1.4 @ 1.4. A 25mm velcro dot on a 29mm rear lens. I think it is pretty clear a crop sensor uses all the glass that a FF does. What say you Clackers? Attached Images

06-23-2016, 07:34 PM - 1 Like	#38
GUB Loyal Site Supporter Join Date: Aug 2012 Location: Wanganui Photos: Gallery \| Albums Posts: 5,751	and its own special bokeh!! Attached Images

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06-23-2016, 01:33 AM - 1 Like	#31
GUB Loyal Site Supporter Join Date: Aug 2012 Location: Wanganui Photos: Gallery \| Albums Posts: 5,751	Originally posted by wombat2go Here is a ray trace of a simple double gauss lens (In this case about 80mm f/4 with the aperture opened fairly wide.) The light arriving from a distant point at infinity has parallel rays in this example. Rays passing through every "element" on the surface of the FRONT section of the lens are converged to a "pixel" on the film plane. Provided that pixel was inside both frame sizes under consideration, that effect does not depend on the film plane dimensions. However, when the film plane is smaller, tha maximum angle of view is reduced, so the diameters of the glass lenses on the REAR section do not need to be so large. Thanks for that diagram and explanation Wombat2go. Now I can see where Clackers is coming from. In your example you clearly show the peripheries of the rear elements are less used by the smaller sensor. I had not taken into account the physical depth of a compound lens nor the fact the image is partially focussed by the time it hits the rear elements. It worried me though that maybe in practise this was not a significant difference. And I had a feeling that it applied less at closer distances rather than infinity. So ------ I happened to have some self adhesive black dots. (Like on specials in shops). They are 14mm in diameter. That is 153sq mm area. A rear lens with a 20mm diameter would be half covered by this dot. That would halve the light transmitted. A M series 50mm f2 has a rear lens the right size. If the apsc sensor used all the rear lens then it would get half the light. If it didn't use all the lens then the dot would have a greater effect. You can draw your own conclusions from my chart but I think in the case of the close up the sensor is pretty well using all the glass. Isn't it amazing how invisible that dot is!!! I would love to have feedback from this. I am a learner trying to gain knowledge – PF has been fantastic for that. Attached Images Last edited by GUB; 06-23-2016 at 02:30 AM.

06-23-2016, 05:31 AM - 1 Like	#33
wombat2go Veteran Member Join Date: Jan 2010 Location: Michigan Photos: Gallery Posts: 2,207	Originally posted by GUB Isn't it amazing how invisible that dot is!!! Maybe you should do a patent application for your "toroidal lens" ! Think of the glass we will save!

06-23-2016, 05:43 AM	#34
MJSfoto1956 Site Supporter Join Date: Dec 2013 Location: Boston Posts: 1,305	Originally posted by k22 I have a DAL 50-200 lens. If I buy a 200mm prime FF lens, does it have the same field of view as the 50-200 zoomed to 200? well there is an exception: lenses that focus breathe when focused up close to a subject do not have the same field of view as a lens that doesn't focus breathe. Rather than get into the details suffice it to say that a 200mm lens focuses at infinity will have the same FOV as any other 200mm lens. However when focused up close, it "depends". Michael

06-23-2016, 02:01 PM	#35
GUB Loyal Site Supporter Join Date: Aug 2012 Location: Wanganui Photos: Gallery \| Albums Posts: 5,751	Originally posted by wombat2go Maybe you should do a patent application for your "toroidal lens" ! Think of the glass we will save! Or an inside out iris for soft focus lenses!! getting rid of that annoying sharp bit in the middle.!!

06-23-2016, 07:21 PM	#37
MarkJerling Moderator Site Supporter Join Date: May 2012 Location: Wairarapa, New Zealand Photos: Gallery \| Albums Posts: 20,402	That is remarkable! Unless you showed this, I would have never thought it possible to still get an image!

06-23-2016, 07:46 PM	#39
MarkJerling Moderator Site Supporter Join Date: May 2012 Location: Wairarapa, New Zealand Photos: Gallery \| Albums Posts: 20,402	I like it!

06-23-2016, 07:48 PM	#40
GUB Loyal Site Supporter Join Date: Aug 2012 Location: Wanganui Photos: Gallery \| Albums Posts: 5,751	Originally posted by MarkJerling I like it! Caption -- bad shedding day?

06-23-2016, 08:11 PM	#41
MarkJerling Moderator Site Supporter Join Date: May 2012 Location: Wairarapa, New Zealand Photos: Gallery \| Albums Posts: 20,402	Hairy McLary from Donaldson Dairy?

06-23-2016, 08:12 PM - 1 Like	#42
ScooterMaxi Jim Veteran Member Join Date: Jul 2009 Location: Minnesota Posts: 1,520	Oh my, someone is going to come up with the bright idea to start a Kickstarter campaign for weird special effects lenses - and they won't have to contract a Chinese machine shop to launch it! ---------- Post added 06-23-2016 at 10:18 PM ---------- Originally posted by GUB Chinon 50mm1.4 @ 1.4. A 25mm velcro dot on a 29mm rear lens. I think it is pretty clear a crop sensor uses all the glass that a FF does. What say you Clackers? Hey, if you had put that dot on a Pentax limited lens (name your favorite), the resulting image would have been near perfect! I'm pretty sure that the lens is the problem, not that little dot obstructing a mere 84% of the rear objective surface.

06-23-2016, 08:26 PM	#43
GUB Loyal Site Supporter Join Date: Aug 2012 Location: Wanganui Photos: Gallery \| Albums Posts: 5,751	Originally posted by ScooterMaxi Jim Oh my, someone is going to come up with the bright idea to start a Kickstarter campaign for weird special effects lenses - and they won't have to contract a Chinese machine shop to launch it! ---------- Post added 06-23-2016 at 10:18 PM ---------- Hey, if you had put that dot on a Pentax limited lens (name your favorite), the resulting image would have been near perfect! I'm pretty sure that the lens is the problem, not that little dot obstructing a mere 84% of the rear objective surface. I can just see someone putting a dirty great adhesive sticker on their limited ---not!!

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06-23-2016, 08:45 PM	#44
ScooterMaxi Jim Veteran Member Join Date: Jul 2009 Location: Minnesota Posts: 1,520	Hey, then, if no one is willing to do that, then we will never know if my sarcasm happens to be right. Does the tree in the forest make a sound, etc.

06-23-2016, 09:09 PM	#45
UncleVanya Loyal Site Supporter Join Date: Jul 2014 Photos: Gallery \| Albums Posts: 28,383	If you want to do this - use some soft lens cleaning material or something like that. Glue it to a strip of thin ribbon and put let it set. Lay the disc against the rear element gently and then grasp the loose ribbon ends around the lens body and mount - taking care to avoid the areas like the lens data pin and the screw drive etc. Make sure the ribbon stays taunt and take a shot. No damage should occur. No sticky nasty residue etc.