# DOF and Cropping take 2

Started Feb 11, 2014 | Discussions
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DOF and Cropping take 2

http://www.dpreview.com/forums/post/53084843

continued from the post above

Just a highlight key points I would like to address

red is Ian

Green is GossCTP

and Black awaldram

But I see it as equivalent please see below

Do you really believe this website above , I mean really ?

How accurate is the information they provide ?

I haven’t read over the whole page so I will trust you when you say with everything you have provided here as true. But what if I can prove that they are wrong (if I rely on what you have said above that cropping has no effect on DOF)

What if I crop my k5 shot with 300mmF2.8 by a factor of 1.8

K5 shot with the 300mm F2.8 Cropped by 1.8 will have a resulting image that would share the same DOF and FOV as a k5 shot with a 540mm lens at F5

Below is a screen grab using the DOF calculator of a trusted site that I used more than once & that I believe to be true

As you can see that it calculates the same equivalent of lens and F stop as I had calc’d

Here is a quote from the site

“As sensor size increases, the depth of field will decrease for a given aperture (when filling the frame with a subject of the same size and distance). This is because larger sensors require one to get closer to their subject, or to use a longer focal length in order to fill the frame with that subject. This means that one has to use progressively smaller aperture sizes in order to maintain the same depth of field on larger sensors. The following calculator predicts the required aperture and focal length in order to achieve the same depth of field (while maintaining perspective).

As an example calculation, if one wanted to reproduce the same perspective and depth of field on a full frame sensor as that attained using a 10 mm lens at f/11 on a camera with a 1.6X crop factor, one would need to use a 16 mm lens and an aperture of roughly f/18. Alternatively, if one used a 50 mm f/1.4 lens on a full frame sensor, this would produce a depth of field so shallow it would require an aperture of 0.9 on a camera with a 1.6X crop factor — not possible with consumer lenses!”

And a reminder that whether you crop an image in the camera or in post with PS you are altering the size of the format, just as many do with a D7100 or a D800 in camera or during post

Now which site do I believe yours or mine?

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Re: DOF and Cropping take 2
4

“As sensor size increases, the depth of field will decrease for a given aperture (when filling the frame with a subject of the same size and distance). This is because larger sensors require one to get closer to their subject, or to use a longer focal length in order to fill the frame with that subject. This means that one has to use progressively smaller aperture sizes in order to maintain the same depth of field on larger sensors. The following calculator predicts the required aperture and focal length in order to achieve the same depth of field (while maintaining perspective).

This nothing different to what I've been saying or Cambridge in color for that matter if your using the longer focal length to deliver greater magnification on larger sensors the the DoF will alter i.e you will need to shrink the aperture.

But doing this is not giving you any advantage and will come at a cost to ISO and/or shutter speed.

Sensor size is nothing but the expected viewing size in reverse.

If you concern is to maintain perceived DoF at he same viewing angle then of cause you need to alter focal length and aperture and take the hit in ISO and shutter.

Most people myself included could not give to hoots about DoF as its to narrow on aps-C and even worse on FF, So I care about shutter speed and target size not DoF.

i.e Fast shutter, plenty of reach and bags of sharp area controllable noise

they are mutually exclusive and you can do very little to gain anything

I could use a 5.5 crop camera this would give me loads of reach, massive DoF (shorter focal length for same target size) but I'd still be shooting in the iso1600 range and the sensor noise can't cope.

So how about FF, this would be me issues with DoF , focal lengths requiring faster shutter than the target required leading to higher ISO than the ambient light indicated driving up ISO beyond the possible stop gained by the format.

APS-C allows me to shoot at subject stopping speeds which are also-in the ball park 1/x for camera induced blur whilst maintain an ISO in 1600-3200 range giving noise that Ir can cope with.

exmples

q = 50mm (f-stop irrelevant) have to shoot >1/250 to freeze subject so in in 800-1600 range noise uncontrollable.

k3 = 300mm F4 shutter 1/250ish iso 1600-3200

FF = 450mm (have to shoot F5.6 for cost and DoF) shutter has to be >1/400 for consistent non camera blur this pushes iso into the 128,000 -256,000 range resulting in a poorer IQ than the aps-C will deliver.

Everything is a balancing act and what works for one may not work for another, In very bright light I can see Q type sensor delivering better Images than either FF or Aps-C due to its reach.

Yes if could have FF with a 200-400 f4 or the 400 f2.8 then yes I could realize the FF Dream but I don't have £10,000 to cover that aspect of my hobby.!

Of cause of Blurred backgrounds are all you care about then MF is the better option.

To achieve the same blur a portrait can deliver in MF on a FF camera if you were shooting a 300 f2.8 on MF

135mm f1.2

and if you were using a 135 f4 land camera (8x10) to get the same shots you need a 14mm f.4 on a Nikon FF body.

Hence my opinion that basing purchasing decisions on DoF equivalence is doomed to despair.

And even considering DoF equivalence outside academia is fruitless.

The only equivalence that makes some sense today is FoV where it can be used to roughly understand what magnification factor you might perceive based off something you know.

Given proliferation of small sensor image taking devices even that is becoming pointless.

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Re: DOF and Cropping take 2
1

Ian Stuart Forsyth wrote:

http://www.dpreview.com/forums/post/53084843

continued from the post above

Just a highlight key points I would like to address

red is Ian

Green is GossCTP

and Black awaldram

But I see it as equivalent please see below

Do you really believe this website above , I mean really ?

How accurate is the information they provide ?

.

That Cambridge Tutorials page is not very accurate, and that site has been debunked in the Nikon fora before.

Some folks are just not careful with their googling, and they still try to source it, adding to their confusion

.

As you can see that it calculates the same equivalent of lens and F stop as I had calc’d

Here is a quote from the site

“As sensor size increases, the depth of field will decrease for a given aperture (when filling the frame with a subject of the same size and distance). This is because larger sensors require one to get closer to their subject, or to use a longer focal length in order to fill the frame with that subject. This means that one has to use progressively smaller aperture sizes in order to maintain the same depth of field on larger sensors. The following calculator predicts the required aperture and focal length in order to achieve the same depth of field (while maintaining perspective).

As an example calculation, if one wanted to reproduce the same perspective and depth of field on a full frame sensor as that attained using a 10 mm lens at f/11 on a camera with a 1.6X crop factor, one would need to use a 16 mm lens and an aperture of roughly f/18. Alternatively, if one used a 50 mm f/1.4 lens on a full frame sensor, this would produce a depth of field so shallow it would require an aperture of 0.9 on a camera with a 1.6X crop factor — not possible with consumer lenses!”

And a reminder that whether you crop an image in the camera or in post with PS you are altering the size of the format, just as many do with a D7100 or a D800 in camera or during post

Now which site do I believe yours or mine?

Yours is of course correct.

.

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Back to basics

Here is a quote from the site

“As sensor size increases, the depth of field will decrease for a given aperture (when filling the frame with a subject of the same size and distance). This is because larger sensors require one to get closer to their subject, or to use a longer focal length in order to fill the frame with that subject. This means that one has to use progressively smaller aperture sizes in order to maintain the same depth of field on larger sensors. The following calculator predicts the required aperture and focal length in order to achieve the same depth of field (while maintaining perspective).

As an example calculation, if one wanted to reproduce the same perspective and depth of field on a full frame sensor as that attained using a 10 mm lens at f/11 on a camera with a 1.6X crop factor, one would need to use a 16 mm lens and an aperture of roughly f/18. Alternatively, if one used a 50 mm f/1.4 lens on a full frame sensor, this would produce a depth of field so shallow it would require an aperture of 0.9 on a camera with a 1.6X crop factor — not possible with consumer lenses!”

Back to basics.

1. If you draw a ray diagram, you have two triangles with their bases at the lens aperture, and their apexes at the subject and the image. If the lens is focused at infinity the height of the triangle on the on image side is the focal length (f) We always write the aperture (a) in terms of f - we don't say we have a 50mm lens with a 20mm aperture we say the aperture is f / 2.

If the image is formed in front of or behind the sensor / film, instead of a point we get a circle. So if we move the lens so it is slightly further from the sensor than f we can calculate the distance D where it will be focused, and if we know the aperture size and we can calculate the size of the circle c we get for something at it infinity.

The formula given in all books I've seen and many websites and which I sat down and proved for myself years ago is

c = f^2/(a*D)

Now there is some size of c which is so small it is indistinguishable from a point. This size depends on the how much you magnify a film negative, or how much of the sensor it take up. AFTER ALL CROPPING HAS BEEN DONE. It's normally taken as longest side of the used image / 1000 rounded down to 1 significant figure. c= 0.03 mm for 35mm film or FF digital and c=0.02 for APSc

We can rearrange the equation above so that we calculate the distance where something at infinity just appears to still be in focus

D = f^2/a*c  - this distance is called the hyperfocal distance H

c changes when you change sensor size.
If you want to keep the same angle of view so does f, and the equation contains f SQUARED.

So if you go from a 100mm lens on APSc to a 150MM lens on FF, and change the circle size from 0.02 to 0.03 you still increase the hyperfocal distance by 50%.

So the same d.o.f has a smaller aperture on a big sensor than a small one. In the extreme case, the lens can't stop down to a small enough aperture to give the big sensor as Deep a d.o.f as a small one; and or can't open wide enough to get give the small as shallow a D.o.f as the big one.

When you work out the distances of some other focus distance D which is in the d.o.f zone H is used in the calculation (D*H)/(D+H), and (D*H)/(D-H)

Once you know the bits in bold you can do your own calculations.

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Re: DOF and Cropping take 2
3

moving_comfort wrote:

Ian Stuart Forsyth wrote:

http://www.dpreview.com/forums/post/53084843

continued from the post above

Just a highlight key points I would like to address

red is Ian

Green is GossCTP

and Black awaldram

But I see it as equivalent please see below

Do you really believe this website above , I mean really ?

How accurate is the information they provide ?

.

That Cambridge Tutorials page is not very accurate, and that site has been debunked in the Nikon fora before.

Some folks are just not careful with their googling, and they still try to source it, adding to their confusion

About the author of Cambridge in colour.

"As part of the Cambridge University Photographic Society, I conducted student workshops relating to general camera and digital SLR technique, photographic post-processing and low-light photography. These included participants ranging from those with their first camera, to more seasoned users wishing to refine their techniques. Time permitting,"

Well you could always sign up for a workshop and tell him he and everyone in the world doesn't know what he's talking about

.

As you can see that it calculates the same equivalent of lens and F stop as I had calc’d

Here is a quote from the site

“As sensor size increases, the depth of field will decrease for a given aperture (when filling the frame with a subject of the same size and distance). This is because larger sensors require one to get closer to their subject, or to use a longer focal length in order to fill the frame with that subject. This means that one has to use progressively smaller aperture sizes in order to maintain the same depth of field on larger sensors. The following calculator predicts the required aperture and focal length in order to achieve the same depth of field (while maintaining perspective).

As an example calculation, if one wanted to reproduce the same perspective and depth of field on a full frame sensor as that attained using a 10 mm lens at f/11 on a camera with a 1.6X crop factor, one would need to use a 16 mm lens and an aperture of roughly f/18. Alternatively, if one used a 50 mm f/1.4 lens on a full frame sensor, this would produce a depth of field so shallow it would require an aperture of 0.9 on a camera with a 1.6X crop factor — not possible with consumer lenses!”

And a reminder that whether you crop an image in the camera or in post with PS you are altering the size of the format, just as many do with a D7100 or a D800 in camera or during post

Now which site do I believe yours or mine?

Yours is of course correct.

mm lets try another 'wrong source'

http://www.luminous-landscape.com/tutorials/dof2.shtml

Most photographers accept the common belief that short focal length lenses have greater depth of field than do long lenses. A wide angle lens, in other words, will give greater depth of field than will a telephoto. Right?

Sounds about right, but it's not the case. In fact, if the subject image size remains the same, then at any given aperture all lenses will give the same depth of field.

Has the super Nikon Fora debunked that one as well because I have many more thousands even , en-fact every body who has bothered to work it out besides it seems the Nikon Fora

.

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This is so obvious to me I'm astounded you fail to see the error in your opinion and even more attempt to ridicule those who do.

whether you use common sense or indeed apply the algorithms given above you will hit the right answer so how could you be so wrong for so long ?

Unless you change both the CoC and Focal length then for any constant image size DoF remains constant ..... simpulz.!

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Re: Back to basics
1

Hi James

This site provides pretty pictures to further expand on what your saying

http://www.normankoren.com/Tutorials/MTF6.html

And fro MC an

And for MC if he hasn't quite got it yet with his Nikon fora buddies.......

It is well known that short focal length lenses have large apparent depths of field and long telephoto lenses have small apparent depths of field. There are some very practical reasons for this conception, but it isn't quite true. DOF is much more closely related to magnification and f-stop; DOF expressed in distance is nearly independent of focal length. It appears smaller with telephoto lenses because it is smaller when expressed as a fraction of the lens-to-subject distance, s.

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Re: DOF and Cropping take 2

I think confusion usually results from the one part of the DOF calculation which is slippery and subjective: the definition of how much blur is unnoticeable. Someone looking at a landscape on a monitor might define it as a given fraction of picture width... someone printing the landscape might define it as a small distance on the print, so it shrinks as you print larger... and someone like me who crops telephoto shots a lot might use a really strict definition that asks whether a focus error is undetectable at any magnification. You have to get everyone agreeing on those criteria before the equations actually clear things up in a discussion.

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Re: DOF and Cropping take 2
2

awaldram wrote:

moving_comfort wrote:

Ian Stuart Forsyth wrote:

http://www.dpreview.com/forums/post/53084843

continued from the post above

Just a highlight key points I would like to address

red is Ian

Green is GossCTP

and Black awaldram

But I see it as equivalent please see below

Do you really believe this website above , I mean really ?

How accurate is the information they provide ?

.

That Cambridge Tutorials page is not very accurate, and that site has been debunked in the Nikon fora before.

Some folks are just not careful with their googling, and they still try to source it, adding to their confusion

About the author of Cambridge in colour.

"As part of the Cambridge University Photographic Society, I conducted student workshops relating to general camera and digital SLR technique, photographic post-processing and low-light photography. These included participants ranging from those with their first camera, to more seasoned users wishing to refine their techniques. Time permitting,"

Well you could always sign up for a workshop and tell him he and everyone in the world doesn't know what he's talking about

On Pentaxforums there's a guy who claims to have 'taught photography for years', yet he continually makes mistakes about DOF and formats and doesn't understand some of the very basics of what ISO does.  He's always being called out for his mistakes and gets quite angry about it...  but he's often wrong.  And stubborn about it.

You, he, and this "Cambridge" fellow who wrote the misleading article seem to be three peas in a pod.  And yes, he's wrong on that point, and yes, it's been pointed out before by folks who know better.   If I get time later today I'll lay it out clearly for you.

You can stick with referencing misinformation if you like...   but you'lll get called out.

.

.

As you can see that it calculates the same equivalent of lens and F stop as I had calc’d

Here is a quote from the site

“As sensor size increases, the depth of field will decrease for a given aperture (when filling the frame with a subject of the same size and distance). This is because larger sensors require one to get closer to their subject, or to use a longer focal length in order to fill the frame with that subject. This means that one has to use progressively smaller aperture sizes in order to maintain the same depth of field on larger sensors. The following calculator predicts the required aperture and focal length in order to achieve the same depth of field (while maintaining perspective).

As an example calculation, if one wanted to reproduce the same perspective and depth of field on a full frame sensor as that attained using a 10 mm lens at f/11 on a camera with a 1.6X crop factor, one would need to use a 16 mm lens and an aperture of roughly f/18. Alternatively, if one used a 50 mm f/1.4 lens on a full frame sensor, this would produce a depth of field so shallow it would require an aperture of 0.9 on a camera with a 1.6X crop factor — not possible with consumer lenses!”

And a reminder that whether you crop an image in the camera or in post with PS you are altering the size of the format, just as many do with a D7100 or a D800 in camera or during post

Now which site do I believe yours or mine?

Yours is of course correct.

mm lets try another 'wrong source'

http://www.luminous-landscape.com/tutorials/dof2.shtml

Most photographers accept the common belief that short focal length lenses have greater depth of field than do long lenses. A wide angle lens, in other words, will give greater depth of field than will a telephoto. Right?

Sounds about right, but it's not the case. In fact, if the subject image size remains the same, then at any given aperture all lenses will give the same depth of field.

On the same formatDid you leave that part out on purpose?     Has anyone tried to say that's not the case?

By the way, are you still trying to claim that cropping FF and displaying at the same size as before does not change DOF, or have you moved on to something else?  I'm confused about what point you're trying to make with that link.

.

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Re: DOF and Cropping take 2
3

moving_comfort wrote:

.

On the same format.Did you leave that part out on purpose? Has anyone tried to say that's not the case?

By the way, are you still trying to claim that cropping FF and displaying at the same size as before does not change DOF, or have you moved on to something else? I'm confused about what point you're trying to make with that link.

I refer you to my first post in this thread, I think maybe it you that's skipping stuff as I clearly point out that magnification is relative to both sides , Standard vewing or sensor hence why Ians reference and Cambridge in color and Myself are saying essentially the same thing.

"This is nothing different to what I've been saying or Cambridge in color for that matter if your using the longer focal length to deliver greater magnification on larger sensors the the DoF will alter i.e you will need to shrink the aperture."

Hence why "at the same image size" is critical and why "cropping FF and displaying at the same size as before does not change DOF" is true

It makes no odds if your Zoom, Crop or use a magnifying class if your representing the target at the same size the DoF will also be the same, Look at James algorithms and see.

Ian's source says exactlly the same

"This is because larger sensors require one to get closer to their subject, or to use a longer focal length in order to fill the frame with that subject."

This is about maintaining image size to what it was before

.

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Re: DOF and Cropping take 2
1

Let break it down even further

DoF and Focal Length is an inverse Quadratic.

As you increase focal length so magnification increased therefore fairly obvious as we know the entry pupil is as viewed through the front of the lens that if we double focal length then for the same F number the physical aperture is doubled.

So for the same F number at the same Magnification you have the same DoF.

so if you shoot 50mm f2.8 at 10ft then walk 40ft away and shoot the same target with 200 F2.8 then DoF and image will be identical.

http://groups.csail.mit.edu/graphics/classes/CompPhoto06/html/lecturenotes/22_DepthDefocus_6.pdf

which is the lecture notes for courses 6.098 (Digital and computational Photography) and 6.882 (Advanced Computational Photography) MIT-EECs

or are you now going to say

Cambridge in colour are wrong

Luminous Landscapes are wrong

I'm wrong James is wrong and the Massachusetts Institute of Technology is wrong as well !!

Do you not think maybe they are not ?

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Re: DOF and Cropping take 2
1

I think the problem and source of most of the confusion comes in is what people are actually comparing...

Across format sizes, are they maintaining magnification (FOV) or are they maintaining perspective (distance to subject)

You've made good proofs for your side of the argument when maintaining magnification and their is good reason to compare systems that way. However if for whatever reason the person wants to know the comparison for what you get when you stay in one spot then you've not maintained FOV or magnification and therefore the DOF changes.

2 schools of comparison, both right, it all depends on what you're really comparing.

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Re: DOF and Cropping take 2

moving_comfort wrote:

On the same format.Did you leave that part out on purpose? Has anyone tried to say that's not the case?

By the way, are you still trying to claim that cropping FF and displaying at the same size as before does not change DOF, or have you moved on to something else? I'm confused about what point you're trying to make with that link.

The point is that a 300f2.8 lens is a 300mm f2.8 lens on any format camera it characteristics do not alter one jot it behavior is not altered by what subsection of its protected image the sensors captures the-fore cutting that same subsection out of a larger sample will deliver exactly the same image as the smaller sensor would have captured DoF and all.

You perceive them to change  because of the different viewing size (i.e magnification).

But your exposure will be the same and hence you have more reach on smaller sensors there is no magic FF formula .

Voff pointed out you were breaking the laws of physics to make your claims, I've simply presented the physics.! (repeatedly from multiple sources)

.

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Re: DOF and Cropping take 2
1

awaldram wrote:

Let break it down even further

DoF and Focal Length is an inverse Quadratic.

As you increase focal length so magnification increased therefore fairly obvious as we know the entry pupil is as viewed through the front of the lens that if we double focal length then for the same F number the physical aperture is doubled.

So for the same F number at the same Magnification you have the same DoF.

so if you shoot 50mm f2.8 at 10ft then walk 40ft away and shoot the same target with 200 F2.8 then DoF and image will be identical.

http://groups.csail.mit.edu/graphics/classes/CompPhoto06/html/lecturenotes/22_DepthDefocus_6.pdf

which is the lecture notes for courses 6.098 (Digital and computational Photography) and 6.882 (Advanced Computational Photography) MIT-EECs

or are you now going to say

Cambridge in colour are wrong

Luminous Landscapes are wrong

I'm wrong James is wrong and the Massachusetts Institute of Technology is wrong as well !!

Do you not think maybe they are not ?

I think the problem here is that you're throwing some good information in with the bad in order to support something you were saying earlier.  The appearance of the 'good' information does not suport all your previous statements, because the 'good' information is in conflict with them.

Let's summarize, try to be succinct so we can know the point you're trying to make:

Q:  Does cropping FF (to ap-sc, for example) change DOF when displayed at the same sizes?  Because I think you used to hold the position that it does not.**

** I'm referring to, for example, this post where you seemed to be getting it wrong

.

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Re: DOF and Cropping take 2

You are right Mike and I thought I'd acknowledged that very point in my first post.

where I stated

"And even considering DoF equivalence outside academia is fruitless.

The only equivalence that makes some sense today is FoV where it can be used to roughly understand what magnification factor you might perceive based off something you know.

Given proliferation of small sensor image taking devices even that is becoming pointless."

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Re: Back to basics

awaldram wrote:

Hi James

This site provides pretty pictures to further expand on what your saying

http://www.normankoren.com/Tutorials/MTF6.html

And fro MC an

And for MC if he hasn't quite got it yet with his Nikon fora buddies.......

It is well known that short focal length lenses have large apparent depths of field and long telephoto lenses have small apparent depths of field. There are some very practical reasons for this conception, but it isn't quite true. DOF is much more closely related to magnification and f-stop; DOF expressed in distance is nearly independent of focal length. It appears smaller with telephoto lenses because it is smaller when expressed as a fraction of the lens-to-subject distance, s.

Thanks for the diagram, I toyed with the idea of drawing something similar.

I'm not sure that the other bit is right though.

Lets say I focus on infinity and my hyperfocal distance is 10M. There is an object at 10 M which takes up a certain amount of the frame. And appears just about in focus. Or I can focus on the object and have infinity just in focus.

I change my to one of double the focal length. c hasn't changed, and I set the same aperture, so now my hyperfocal distance is 40M. To make the object the same size I double the distance between it and the camera. It is now 20M from me and 20M in-front of the hyperfocal distance, so if I focus on infinity my object is out of focus and if I focus at 20M D.o.F doesn't go all the way to infinity.

I have seen an awful lot of mis-explanation which seems to be work with
H = F/ca instead of F^2/ca
is that what is happening here ?

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Re: DOF and Cropping take 2

moving_comfort wrote:

Q: Does cropping FF (to ap-sc, for example) change DOF when displayed at the same sizes? Because I think you used to hold the position that it does not.

Simple question  how can it.?

The lens projects an image onto the film plane that image is the same whatever sensor is behind it

look at it this way assume we have a camera with interchangeable backs.

We put a 50mm f4 lens on the camera on a tripod with a FF back on it and take a picture.

we then do nothing but change the back to a sensor 1/2 the size and take another picture.

if we superimpose the two images on-top of each other will the be different in any way shape or form where they overlap?

I think your not totally mad so we'll assume you said yes at this point.

you now take the FF image cut that middle 50% out do you now have to identical image and is DoF the same ?

As I've tried in vain to point out it only because your viewing position alter across formats does the DoF appear different exactly the same as why a telephoto appears to have a shorter dof than wide angle.

.

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Re: DOF and Cropping take 2

awaldram wrote:

mm lets try another 'wrong source'

http://www.luminous-landscape.com/tutorials/dof2.shtml

Most photographers accept the common belief that short focal length lenses have greater depth of field than do long lenses. A wide angle lens, in other words, will give greater depth of field than will a telephoto. Right?

Sounds about right, but it's not the case. In fact, if the subject image size remains the same, then at any given aperture all lenses will give the same depth of field.

That's wrong . You only need to look at the tower in the background behind the doll he has in the foreground to see it looks less and less out of focus the shorter the focal length gets.

as said I most in another branch

Lets say I focus on infinity and my hyperfocal distance is 10M. There is an object at 10 M which takes up a certain amount of the frame. And appears just about in focus. Or I can focus on the object and have infinity just in focus.

I change my to one of double the focal length. c hasn't changed, and I set the same aperture, so [using the h = f^2/ca formula] my hyperfocal distance is 40M. To make the object the same size I double the distance between it and the camera. It is now 20M from me and 20M in-front of the hyperfocal distance, so if I focus on infinity my object is out of focus and if I focus at 20M D.o.F doesn't go all the way to infinity.

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Re: Back to basics

I think your right there are a lot of simplification in play in photography that break under some circumstances.

If you check that MIT slide deck I linked there is some interesting material on simplifications and covers off both distance shooting and indeed macro shooting where its also busted

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Re: DOF and Cropping take 2

James O'Neill wrote:

awaldram wrote:

mm lets try another 'wrong source'

http://www.luminous-landscape.com/tutorials/dof2.shtml

Most photographers accept the common belief that short focal length lenses have greater depth of field than do long lenses. A wide angle lens, in other words, will give greater depth of field than will a telephoto. Right?

Sounds about right, but it's not the case. In fact, if the subject image size remains the same, then at any given aperture all lenses will give the same depth of field.

That's wrong . You only need to look at the tower in the background behind the doll he has in the foreground to see it looks less and less out of focus the shorter the focal length gets.

as said I most in another branch

Lets say I focus on infinity and my hyperfocal distance is 10M. There is an object at 10 M which takes up a certain amount of the frame. And appears just about in focus. Or I can focus on the object and have infinity just in focus.

I change my to one of double the focal length. c hasn't changed, and I set the same aperture, so [using the h = f^2/ca formula] my hyperfocal distance is 40M. To make the object the same size I double the distance between it and the camera. It is now 20M from me and 20M in-front of the hyperfocal distance, so if I focus on infinity my object is out of focus and if I focus at 20M D.o.F doesn't go all the way to infinity.

Yep they forgot to put 'nearly' I think (could be wrong ) what happens is as you approach infinity the object compress hence giving fractional magnification changes  now they mean nothing but when multiplied by infinity can have a massive affect.

Either that or the formula is an approximation fro something considerably more complex

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Re: DOF and Cropping take 2

awaldram wrote:

moving_comfort wrote:

Q: Does cropping FF (to ap-sc, for example) change DOF when displayed at the same sizes? Because I think you used to hold the position that it does not.

You missed the point!

Does cropping FF (to ap-sc, for example) change DOF when displayed at the same sizes?

To crop and then display at the same size the magnification must change and therefore the appearance of DOF will change!

Simple question how can it.?

The lens projects an image onto the film plane that image is the same whatever sensor is behind it

look at it this way assume we have a camera with interchangeable backs.

We put a 50mm f4 lens on the camera on a tripod with a FF back on it and take a picture.

we then do nothing but change the back to a sensor 1/2 the size and take another picture.

if we superimpose the two images on-top of each other will the be different in any way shape or form where they overlap?

I think your not totally mad so we'll assume you said yes at this point.

you now take the FF image cut that middle 50% out do you now have to identical image and is DoF the same ?

As I've tried in vain to point out it only because your viewing position alter across formats does the DoF appear different exactly the same as why a telephoto appears to have a shorter dof than wide angle.

.

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Here are a few of my favorite things...
---> http://www.flickr.com/photos/95095968@N00/sets/72157626171532197/

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