85mm vs. 50mm which is which (same subject, same aperture, same magnification)

Wheatfield · 06-25-2008, 01:39 PM

Originally posted by attack11

this thread hurts my head. focal length is used to determine a general area for the dof in calculations, but every lens model has it's own magnification which will affect the persective, distortion, etc.

this isn't even a valid test because it's different focal lengths, all you did was find generally a sweet spot of distance where the result looks similiar if you don't look too close.

fov crop does not turn a 50mm lens into a 75mm. it's still a 50mm lens, but what you see from it is similiar to the fov of a 75mm lens. you lose a lot of the border of the picture.

Send me some of what you are smoking!
Reproduction ratio is used to determine depth of field. Focal length is secondary, in that it affects subject to camera distance to get the desired reproduction ratio.
For the purposes of photographer, a 50mm lens (for example) is a 50mm lens. It matters not what model of 50mm lens it is, it is still a 50mm lens, and will adhere to fairly strict physics relating to reproduction ratio and aperture (and depth of field).
The exeption is those voodoo lenses that Nikon makes that can alter the front/back ratio of where the DOF falls, although not the actual depth of field.
Perspective and distortion have no effect on depth of field, perspective is decided by camera position, and is not related to focal length.
The test is perfectly valid, as it was meant to prove (and it did) that depth of field is determined by reproduction ratio and aperture, not focal length.
Now, does your head hurt even more?

attack11 · 06-25-2008, 01:45 PM

Originally posted by Wheatfield

Send me some of what you are smoking!
Reproduction ratio is used to determine depth of field. Focal length is secondary, in that it affects subject to camera distance to get the desired reproduction ratio.
For the purposes of photographer, a 50mm lens (for example) is a 50mm lens. It matters not what model of 50mm lens it is, it is still a 50mm lens, and will adhere to fairly strict physics relating to reproduction ratio and aperture (and depth of field).
The exeption is those voodoo lenses that Nikon makes that can alter the front/back ratio of where the DOF falls, although not the actual depth of field.
Perspective and distortion have no effect on depth of field, perspective is decided by camera position, and is not related to focal length.
The test is perfectly valid, as it was meant to prove (and it did) that depth of field is determined by reproduction ratio and aperture, not focal length.
Now, does your head hurt even more?

lol, so .. a razor thin dof will look the same at different angles on a subject that's stationary? my god, my own photos lie.

perspective means the lens... aka focal length. the dictionary is a powerful thing. my words still apply due to the laws of physics as we know em.

distortion is why those 2 photos look somewhat similar. the bokeh, dof and everything else are different. if he shot it with a wider lens it wouldn't look nearly as good.

attack11 · 06-25-2008, 01:55 PM

you know what a real test is? comparing the difference of dof of lenses with the same focal length.

stevebrot · 06-25-2008, 02:53 PM

Originally posted by attack11

you know what a real test is? comparing the difference of dof of lenses with the same focal length.

Good idea! Do the test and post the pics!

I have to grin regarding the recent activity on this thread. Reason? Because the original rational for the test was because I thought that the 50mm would have substantially better DoF. After looking at the results, I had to modify my view.

Wheatfield · 06-25-2008, 04:58 PM

Originally posted by stevebrot

Good idea! Do the test and post the pics!

I have to grin regarding the recent activity on this thread. Reason? Because the original rational for the test was because I thought that the 50mm would have substantially better DoF. After looking at the results, I had to modify my view.

When you think about it, it only makes sense. The differences you will see relate only to the rendering of the image. Some lenses have nicer bokeh than others, with the consequence being that it may appear that DOF is related to focal length in a side by side comparison.

Miserere · 06-25-2008, 09:44 PM

Whoa! I leave this thread for a day, and look what I find when I come back!

atack11, DoF does not depend on focal length, period. I don't know what lenses you have, or what photos, but it's geometry, not opinion, that establishes this fact. Go here and calculate the DoF you get from a 50mm and a 100mm lens when the reproduction ratio is the same. For the reproduction ratio to be the same, you'll have to be twice as far when using the 100mm lens than the 50mm.

The values you'll need to input for the calculations are:

Circle of confusion = 0.020mm (for the K10D)

Format (size of CCD)
v = 15.67mm
h = 23.50mm

Focus = 10m (for 50mm lens)
Focus = 20m (for 100mm lens)

Of course, you'll have to enter the focal length. It's easier if you open up the page in two different windows/tabs and look at the results side by side. Feel free to chance the focus distance, as long as the 100mm lens is twice that of the 50mm.

Go ahead, we'll wait for you to get back.

axl · 06-26-2008, 12:08 AM

will we get an answer for ORIGINAL question, or will we just keep arguing what DoF does or doesn't depend on...

stevebrot · 06-26-2008, 12:20 AM

Originally posted by Miserere

Whoa! I leave this thread for a day, and look what I find when I come back!

atack11, DoF does not depend on focal length, period. I don't know what lenses you have, or what photos, but it's geometry, not opinion, that establishes this fact. Go here and calculate the DoF you get from a 50mm and a 100mm lens when the reproduction ratio is the same. For the reproduction ratio to be the same, you'll have to be twice as far when using the 100mm lens than the 50mm.

The values you'll need to input for the calculations are:

Circle of confusion = 0.020mm (for the K10D)

Format (size of CCD)
v = 15.67mm
h = 23.50mm

Focus = 10m (for 50mm lens)
Focus = 20m (for 100mm lens)

Of course, you'll have to enter the focal length. It's easier if you open up the page in two different windows/tabs and look at the results side by side. Feel free to chance the focus distance, as long as the 100mm lens is twice that of the 50mm.

Go ahead, we'll wait for you to get back.

I followed the link. What I cool calculator!

I am going to bookmark the page as a reference for doing bellows macro work since it supports calculation of magnification ratio and affective aperture.

Thanks!

Steve

Miserere · 06-26-2008, 06:00 AM

Originally posted by stevebrot

I followed the link. What I cool calculator!

I am going to bookmark the page as a reference for doing bellows macro work since it supports calculation of magnification ratio and affective aperture.

Thanks!

Steve

Oh yeah, it's a great calculator; it calculates just about everything you might want to know, huh?

stevebrot · 06-26-2008, 10:37 AM

Originally posted by axl

will we get an answer for ORIGINAL question, or will we just keep arguing what DoF does or doesn't depend on...

I don't know if this question was addressed to me (started thread) or attack11...

As to the original question I asked, here is the answer:

The top picture was taken with a Jupiter-9 85/2

The bottom picture was taken with a Pentax-A 50/1.7

Steve

axl · 06-27-2008, 12:11 AM

Originally posted by stevebrot

I don't know if this question was addressed to me (started thread) or attack11...

As to the original question I asked, here is the answer:

The top picture was taken with a Jupiter-9 85/2

The bottom picture was taken with a Pentax-A 50/1.7

Steve

thanx...
I'm happy to find out that my observation skills were right

The question was addressed to the public. I understand the issues behind why and how the conversation steered the way it did. But I found the original question very simple (no need to make more complicated etc etc), so I was quite surprised that instead of people trying to answer it they engaged in discussion that followed....
BR

stevebrot · 06-27-2008, 07:40 AM

Originally posted by axl

thanx...
I'm happy to find out that my observation skills were right

The question was addressed to the public. I understand the issues behind why and how the conversation steered the way it did. But I found the original question very simple (no need to make more complicated etc etc), so I was quite surprised that instead of people trying to answer it they engaged in discussion that followed....
BR

Ha, Ha, Ha...Yes, the original question was pretty simple. It was a leading question, however. There is so much confusion about focal length, DoF, and perspective. I thought that since a picture is worth a thousand words, I would offer two pictures!

Steve

Myoptimism · 06-28-2008, 04:50 PM

I thought three things affected depth of field...
-aperture
-working distance from subject
-focal length

With this test, closer working distance decreased the dof of one lens (the 50) to the equivalent of the lens with the longer focal lenth (the 85). If you want to call this reproduction ratio fine...but focal length and working distance are still two different things that affect dof.

An analogy...
Exposure is affected by three things.
-ISO
-aperture
-shutterspeed

Is somebody going to do a test with different shutterspeeds and ask if anyone notices any difference in exposure? Is another person then going to claim this test is proof that shutterspeeds don't affect exposure?

stevebrot · 06-28-2008, 05:32 PM

Originally posted by Myoptimism

I thought three things affected depth of field...
-aperture
-working distance from subject
-focal length

With this test, closer working distance decreased the dof of one lens (the 50) to the equivalent of the lens with the longer focal lenth (the 85). If you want to call this reproduction ratio fine...but focal length and working distance are still two different things that affect dof.

An analogy...
Exposure is affected by three things.
-ISO
-aperture
-shutterspeed

Is somebody going to do a test with different shutterspeeds and ask if anyone notices any difference in exposure? Is another person then going to claim this test is proof that shutterspeeds don't affect exposure?

You are correct.

The purpose of this example was to show that focal length and aperture are not the only factors affecting DoF.

Steve

Miserere · 06-28-2008, 09:38 PM

Originally posted by Myoptimism

An analogy...
Exposure is affected by three things.
-ISO
-aperture
-shutterspeed

Is somebody going to do a test with different shutterspeeds and ask if anyone notices any difference in exposure? Is another person then going to claim this test is proof that shutterspeeds don't affect exposure?

Yes, I am going to claim that. For a given exposure value N, and given the three variables that are tied automatically to each other (or otherwise they wouldn't produce the same exposure N), then what shutter speed you choose doesn't affect the value of th exposure, that is, the number of photons you are collecting. However, how this affects the artistic value of the photograph is another question altogether, and has nothing to do with what I'm claiming.

In the same way, DoF is independent of focal length because DoF depends only on reproduction ratio and aperture (if the size of the sensor remains constant). You can achieve the reproduction ratio with a 50mm lens standing at 5m, or with a 1000mm lens standing 100mm from your subject, it doesn't matter. Because the distance from your subject is tied (and is directly proportional) to the focal length, then the focal length ceases to be a factor.

Many people have the idea that a 24mm lens produces a greater DoF than a 50mm lens, and that is the myth that Steve here set out to test. He's shown with pictures what every lens maker for centuries already knew.

06-27-2008, 07:40 AM	#42
stevebrot Otis Memorial Pentaxian Join Date: Mar 2007 Location: Vancouver (USA) Photos: Gallery \| Albums Posts: 42,007 Original Poster	Originally posted by axl thanx... I'm happy to find out that my observation skills were right The question was addressed to the public. I understand the issues behind why and how the conversation steered the way it did. But I found the original question very simple (no need to make more complicated etc etc), so I was quite surprised that instead of people trying to answer it they engaged in discussion that followed.... BR Ha, Ha, Ha...Yes, the original question was pretty simple. It was a leading question, however. There is so much confusion about focal length, DoF, and perspective. I thought that since a picture is worth a thousand words, I would offer two pictures! Steve Last edited by stevebrot; 06-28-2008 at 05:36 PM.

06-28-2008, 04:50 PM	#43
Myoptimism Forum Member Join Date: May 2008 Location: Utah Photos: Gallery Posts: 94	...and then there were 3 I thought three things affected depth of field... -aperture -working distance from subject -focal length With this test, closer working distance decreased the dof of one lens (the 50) to the equivalent of the lens with the longer focal lenth (the 85). If you want to call this reproduction ratio fine...but focal length and working distance are still two different things that affect dof. An analogy... Exposure is affected by three things. -ISO -aperture -shutterspeed Is somebody going to do a test with different shutterspeeds and ask if anyone notices any difference in exposure? Is another person then going to claim this test is proof that shutterspeeds don't affect exposure?

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06-25-2008, 01:39 PM	#31
Wheatfield Moderator Join Date: Apr 2008 Location: The wheatfields of Canada Posts: 15,981	Originally posted by attack11 this thread hurts my head. focal length is used to determine a general area for the dof in calculations, but every lens model has it's own magnification which will affect the persective, distortion, etc. this isn't even a valid test because it's different focal lengths, all you did was find generally a sweet spot of distance where the result looks similiar if you don't look too close. fov crop does not turn a 50mm lens into a 75mm. it's still a 50mm lens, but what you see from it is similiar to the fov of a 75mm lens. you lose a lot of the border of the picture. Send me some of what you are smoking! Reproduction ratio is used to determine depth of field. Focal length is secondary, in that it affects subject to camera distance to get the desired reproduction ratio. For the purposes of photographer, a 50mm lens (for example) is a 50mm lens. It matters not what model of 50mm lens it is, it is still a 50mm lens, and will adhere to fairly strict physics relating to reproduction ratio and aperture (and depth of field). The exeption is those voodoo lenses that Nikon makes that can alter the front/back ratio of where the DOF falls, although not the actual depth of field. Perspective and distortion have no effect on depth of field, perspective is decided by camera position, and is not related to focal length. The test is perfectly valid, as it was meant to prove (and it did) that depth of field is determined by reproduction ratio and aperture, not focal length. Now, does your head hurt even more?

06-25-2008, 01:45 PM	#32
attack11 Veteran Member Join Date: Apr 2007 Location: Ottawa, ON - Canada Photos: Gallery Posts: 658	Originally posted by Wheatfield Send me some of what you are smoking! Reproduction ratio is used to determine depth of field. Focal length is secondary, in that it affects subject to camera distance to get the desired reproduction ratio. For the purposes of photographer, a 50mm lens (for example) is a 50mm lens. It matters not what model of 50mm lens it is, it is still a 50mm lens, and will adhere to fairly strict physics relating to reproduction ratio and aperture (and depth of field). The exeption is those voodoo lenses that Nikon makes that can alter the front/back ratio of where the DOF falls, although not the actual depth of field. Perspective and distortion have no effect on depth of field, perspective is decided by camera position, and is not related to focal length. The test is perfectly valid, as it was meant to prove (and it did) that depth of field is determined by reproduction ratio and aperture, not focal length. Now, does your head hurt even more? lol, so .. a razor thin dof will look the same at different angles on a subject that's stationary? my god, my own photos lie. perspective means the lens... aka focal length. the dictionary is a powerful thing. my words still apply due to the laws of physics as we know em. distortion is why those 2 photos look somewhat similar. the bokeh, dof and everything else are different. if he shot it with a wider lens it wouldn't look nearly as good.

06-25-2008, 01:55 PM	#33
attack11 Veteran Member Join Date: Apr 2007 Location: Ottawa, ON - Canada Photos: Gallery Posts: 658	you know what a real test is? comparing the difference of dof of lenses with the same focal length.

06-25-2008, 02:53 PM	#34
stevebrot Otis Memorial Pentaxian Join Date: Mar 2007 Location: Vancouver (USA) Photos: Gallery \| Albums Posts: 42,007 Original Poster	Originally posted by attack11 you know what a real test is? comparing the difference of dof of lenses with the same focal length. Good idea! Do the test and post the pics! I have to grin regarding the recent activity on this thread. Reason? Because the original rational for the test was because I thought that the 50mm would have substantially better DoF. After looking at the results, I had to modify my view.

06-25-2008, 04:58 PM	#35
Wheatfield Moderator Join Date: Apr 2008 Location: The wheatfields of Canada Posts: 15,981	Originally posted by stevebrot Good idea! Do the test and post the pics! I have to grin regarding the recent activity on this thread. Reason? Because the original rational for the test was because I thought that the 50mm would have substantially better DoF. After looking at the results, I had to modify my view. When you think about it, it only makes sense. The differences you will see relate only to the rendering of the image. Some lenses have nicer bokeh than others, with the consequence being that it may appear that DOF is related to focal length in a side by side comparison.

06-25-2008, 09:44 PM	#36
Miserere Veteran Member Join Date: Sep 2007 Location: Boston Photos: Gallery Posts: 1,993	Whoa! I leave this thread for a day, and look what I find when I come back! atack11, DoF does not depend on focal length, period. I don't know what lenses you have, or what photos, but it's geometry, not opinion, that establishes this fact. Go here and calculate the DoF you get from a 50mm and a 100mm lens when the reproduction ratio is the same. For the reproduction ratio to be the same, you'll have to be twice as far when using the 100mm lens than the 50mm. The values you'll need to input for the calculations are: Circle of confusion = 0.020mm (for the K10D) Format (size of CCD) v = 15.67mm h = 23.50mm Focus = 10m (for 50mm lens) Focus = 20m (for 100mm lens) Of course, you'll have to enter the focal length. It's easier if you open up the page in two different windows/tabs and look at the results side by side. Feel free to chance the focus distance, as long as the 100mm lens is twice that of the 50mm. Go ahead, we'll wait for you to get back.

06-26-2008, 12:08 AM	#37
axl Veteran Member Join Date: Mar 2007 Location: Nove Zamky, Slovakia Photos: Gallery \| Albums Posts: 7,183	will we get an answer for ORIGINAL question, or will we just keep arguing what DoF does or doesn't depend on...

06-26-2008, 12:20 AM	#38
stevebrot Otis Memorial Pentaxian Join Date: Mar 2007 Location: Vancouver (USA) Photos: Gallery \| Albums Posts: 42,007 Original Poster	Originally posted by Miserere Whoa! I leave this thread for a day, and look what I find when I come back! atack11, DoF does not depend on focal length, period. I don't know what lenses you have, or what photos, but it's geometry, not opinion, that establishes this fact. Go here and calculate the DoF you get from a 50mm and a 100mm lens when the reproduction ratio is the same. For the reproduction ratio to be the same, you'll have to be twice as far when using the 100mm lens than the 50mm. The values you'll need to input for the calculations are: Circle of confusion = 0.020mm (for the K10D) Format (size of CCD) v = 15.67mm h = 23.50mm Focus = 10m (for 50mm lens) Focus = 20m (for 100mm lens) Of course, you'll have to enter the focal length. It's easier if you open up the page in two different windows/tabs and look at the results side by side. Feel free to chance the focus distance, as long as the 100mm lens is twice that of the 50mm. Go ahead, we'll wait for you to get back. I followed the link. What I cool calculator! I am going to bookmark the page as a reference for doing bellows macro work since it supports calculation of magnification ratio and affective aperture. Thanks! Steve

06-26-2008, 06:00 AM	#39
Miserere Veteran Member Join Date: Sep 2007 Location: Boston Photos: Gallery Posts: 1,993	Originally posted by stevebrot I followed the link. What I cool calculator! I am going to bookmark the page as a reference for doing bellows macro work since it supports calculation of magnification ratio and affective aperture. Thanks! Steve Oh yeah, it's a great calculator; it calculates just about everything you might want to know, huh?

06-26-2008, 10:37 AM	#40
stevebrot Otis Memorial Pentaxian Join Date: Mar 2007 Location: Vancouver (USA) Photos: Gallery \| Albums Posts: 42,007 Original Poster	Originally posted by axl will we get an answer for ORIGINAL question, or will we just keep arguing what DoF does or doesn't depend on... I don't know if this question was addressed to me (started thread) or attack11... As to the original question I asked, here is the answer: The top picture was taken with a Jupiter-9 85/2 The bottom picture was taken with a Pentax-A 50/1.7 Steve

06-27-2008, 12:11 AM	#41
axl Veteran Member Join Date: Mar 2007 Location: Nove Zamky, Slovakia Photos: Gallery \| Albums Posts: 7,183	Originally posted by stevebrot I don't know if this question was addressed to me (started thread) or attack11... As to the original question I asked, here is the answer: The top picture was taken with a Jupiter-9 85/2 The bottom picture was taken with a Pentax-A 50/1.7 Steve thanx... I'm happy to find out that my observation skills were right The question was addressed to the public. I understand the issues behind why and how the conversation steered the way it did. But I found the original question very simple (no need to make more complicated etc etc), so I was quite surprised that instead of people trying to answer it they engaged in discussion that followed.... BR

06-28-2008, 05:32 PM	#44
stevebrot Otis Memorial Pentaxian Join Date: Mar 2007 Location: Vancouver (USA) Photos: Gallery \| Albums Posts: 42,007 Original Poster	Originally posted by Myoptimism I thought three things affected depth of field... -aperture -working distance from subject -focal length With this test, closer working distance decreased the dof of one lens (the 50) to the equivalent of the lens with the longer focal lenth (the 85). If you want to call this reproduction ratio fine...but focal length and working distance are still two different things that affect dof. An analogy... Exposure is affected by three things. -ISO -aperture -shutterspeed Is somebody going to do a test with different shutterspeeds and ask if anyone notices any difference in exposure? Is another person then going to claim this test is proof that shutterspeeds don't affect exposure? You are correct. The purpose of this example was to show that focal length and aperture are not the only factors affecting DoF. Steve

06-28-2008, 09:38 PM	#45
Miserere Veteran Member Join Date: Sep 2007 Location: Boston Photos: Gallery Posts: 1,993	Originally posted by Myoptimism An analogy... Exposure is affected by three things. -ISO -aperture -shutterspeed Is somebody going to do a test with different shutterspeeds and ask if anyone notices any difference in exposure? Is another person then going to claim this test is proof that shutterspeeds don't affect exposure? Yes, I am going to claim that. For a given exposure value N, and given the three variables that are tied automatically to each other (or otherwise they wouldn't produce the same exposure N), then what shutter speed you choose doesn't affect the value of th exposure, that is, the number of photons you are collecting. However, how this affects the artistic value of the photograph is another question altogether, and has nothing to do with what I'm claiming. In the same way, DoF is independent of focal length because DoF depends only on reproduction ratio and aperture (if the size of the sensor remains constant). You can achieve the reproduction ratio with a 50mm lens standing at 5m, or with a 1000mm lens standing 100mm from your subject, it doesn't matter. Because the distance from your subject is tied (and is directly proportional) to the focal length, then the focal length ceases to be a factor. Many people have the idea that a 24mm lens produces a greater DoF than a 50mm lens, and that is the myth that Steve here set out to test. He's shown with pictures what every lens maker for centuries already knew.