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09-19-2009, 02:04 AM   #31
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QuoteOriginally posted by nanok Quote
dof:

sigh: Depth of field - Wikipedia, the free encyclopedia

it's one thing if you don't care about it, fine. however it doesn't justify stating it wrong. the way i see it, if you don't care about it, don't talk about it, if you do, make sure you get it right when stating something about it. one reason is that some poeple might care about it and could be confused about conflicting and fuzzy statements, when trying to understand it
I just read "DOF vs format size" of the link that you provided, and it is inline to what I said. I do not see how it supports what you said. Am I reading it wrong?

09-19-2009, 02:18 AM   #32
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QuoteOriginally posted by wlachan Quote
I just read "DOF vs format size" of the link that you provided, and it is inline to what I said. I do not see how it supports what you said. Am I reading it wrong?
Depends the way one see it:

1/ If you see APS-C as a crop of 24x36 then of course the DOF is the same.

2/ If you compare both format on a FOV base (using a 14mm APS-C lens and e.g. a 21mm FF lens, getting about the same FOV), then DOF is different.

Technically 2/ is stupid because it is misleading to compare different focal lenses. But practicaly, this is what users need most of the time.
09-19-2009, 02:52 AM   #33
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I know what you mean Thibs and both 1 & 2 are true. However, if I understand nanok correctly, he's saying the same lens with the same aperture being used will yield different DOF on different formats. This contradicts what you and the link that he provided say.
09-19-2009, 04:11 AM   #34
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QuoteOriginally posted by wlachan Quote
I know what you mean Thibs and both 1 & 2 are true. However, if I understand nanok correctly, he's saying the same lens with the same aperture being used will yield different DOF on different formats. This contradicts what you and the link that he provided say.
Yep, but I know it is difficult notion to understand, just wanted to put it differently

09-19-2009, 07:08 AM   #35
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we have a new rumour on Optyczne.pl - testy aparatów, testy obiektywów, testy lornetek, inne testy ,aparaty cyfrowe, obiektywy, lornetki, artyku?y, nowo?ci, opinie komentarze - Optyczne.pl website, that Samyang is planning to release new lens Q1 2010
full frame 35mm f/1.4 for Pentax, Nikon, Sony and 35mm f/1.2 for Canon (due to bigger mount diameter)
09-19-2009, 01:05 PM   #36
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QuoteOriginally posted by wlachan Quote
I just read "DOF vs format size" of the link that you provided, and it is inline to what I said. I do not see how it supports what you said. Am I reading it wrong?
you read that and it is inline to "dof is the same with the same lens on different formats" and contradicting what i said (shortly: less dof with the same lens at same aperture on different format)?

"To a first approximation, DOF is inversely proportional to format size. More precisely, if photographs with the same final-image size are taken in two different camera formats at the same subject distance with the same field of view and f-number, the DOF is, to a first approximation, inversely proportional to the format size. Strictly speaking, this is true only when the subject distance is large in comparison with the focal length and small in comparison with the hyperfocal distance, for both formats, but it nonetheless is generally useful for comparing results obtained from different formats."

further in the (small) paragraph there are examples and alternate explanations to the same effect.

so apart from the fact that, yes, it doesn't apply to macro and near-macro (which was not discussed here), the above reads as "format size has no influence on dof"? if the answer is yes, i am not sure i am capable of arguing any further. if there's any doubt, i's be glad to try and explain further

/me walks away, scratching head
09-19-2009, 01:15 PM   #37
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QuoteOriginally posted by thibs Quote
Depends the way one see it:

1/ If you see APS-C as a crop of 24x36 then of course the DOF is the same.

2/ If you compare both format on a FOV base (using a 14mm APS-C lens and e.g. a 21mm FF lens, getting about the same FOV), then DOF is different.

Technically 2/ is stupid because it is misleading to compare different focal lenses. But practicaly, this is what users need most of the time.
no. 2 is not stupid, it is just a practical way of looking at the same thing, which doesn't invalidate the result you get in any way. 1 is, however, simply wrong: when you crop, your dof changes, because, as i said, and as explained on the wikipedia article i pointed to, dof is tighlty connected with the format of the sensor/film. it is part of the way you DEFINE dof, so if we are talking about dof, we should agree on the same definition. if your definition is different, i respectfully suggest we call it different (not dof), to avoid confusion . furthermore, format size is not part of dof just incidentally, it is part of the dof definition because dof as we understand it today would be meaningless without it (if you think about it more in depth -- no pun intended -- it becomes obvious). if you still refuse to see that, take an extreme example: compare the dof on a compact digital camera, 16mm/4 with the dof of a 16mm at f4 on your dslr, and tell me which you think is greater: the dof on the dslr or on the compact? (gedanken experiment )
09-19-2009, 01:38 PM   #38
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QuoteOriginally posted by wlachan Quote
I know what you mean Thibs and both 1 & 2 are true. However, if I understand nanok correctly, he's saying the same lens with the same aperture being used will yield different DOF on different formats. This contradicts what you and the link that he provided say.
So you're saying that the DOF will be the same if one took an image with a 6x7 lens using an APS-C camera and then took one with the native 6x7 camera with the same lens, at the exact same settings; say 50mm at F4?

09-19-2009, 02:31 PM   #39
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QuoteOriginally posted by AM2 Quote
So you're saying that the DOF will be the same if one took an image with a 6x7 lens using an APS-C camera and then took one with the native 6x7 camera with the same lens, at the exact same settings; say 50mm at F4?

Yes. But angle of view would be totally different.
09-19-2009, 02:35 PM   #40
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QuoteOriginally posted by nanok Quote
you read that and it is inline to "dof is the same with the same lens on different formats" and contradicting what i said (shortly: less dof with the same lens at same aperture on different format)?

"To a first approximation, DOF is inversely proportional to format size. More precisely, if photographs with the same final-image size are taken in two different camera formats at the same subject distance with the same field of view and f-number, the DOF is, to a first approximation, inversely proportional to the format size. Strictly speaking, this is true only when the subject distance is large in comparison with the focal length and small in comparison with the hyperfocal distance, for both formats, but it nonetheless is generally useful for comparing results obtained from different formats."

further in the (small) paragraph there are examples and alternate explanations to the same effect.

so apart from the fact that, yes, it doesn't apply to macro and near-macro (which was not discussed here), the above reads as "format size has no influence on dof"? if the answer is yes, i am not sure i am capable of arguing any further. if there's any doubt, i's be glad to try and explain further

/me walks away, scratching head
I am not trying to argue, I am trying to understand what you have been suggesting since you insist you have the final say on DOF. I still fail to connect eveything you have said btw. If 2 different formats are shooting at the same distance with the same FOV, you can only achieve this by using different focal lengths. Since the actual focal lengths are different, DOF will be different too. This is what your quote says if I understand it correctly. However, DOF remains the same if focal length, distance & aperture are fixed regardless which formats they are (which you disagree if I understand your message correctly).

QuoteOriginally posted by AM2 Quote
So you're saying that the DOF will be the same if one took an image with a 6x7 lens using an APS-C camera and then took one with the native 6x7 camera with the same lens, at the exact same settings; say 50mm at F4?
This is my understanding, given the same distance & aperture. If you try to match the subject size, the shooting distances will be different, and so will the DOF.
09-19-2009, 03:34 PM   #41
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Interesting...

I always thought that larger negatives/sensors have the benefit of shallower DoF at the same aperture and focal length, compared to smaller film/sensors. Well it's clear now, especially considering the angle of view difference at the same focal length, on different formats... it all fits into place.

It's embarrassing to have been shooting with SLRs of various types for about 5 years now, yet I held that misconception in my head all that time. Well it's gone now.
09-20-2009, 02:19 AM   #42
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QuoteOriginally posted by wlachan Quote
I am not trying to argue, I am trying to understand what you have been suggesting since you insist you have the final say on DOF.
can't see where i stated such a thing. far from it, i probably wouldn't be here if i thought that, eh?

QuoteQuote:
I still fail to connect eveything you have said btw. If 2 different formats are shooting at the same distance with the same FOV, you can only achieve this by using different focal lengths. Since the actual focal lengths are different, DOF will be different too. This is what your quote says if I understand it correctly. However, DOF remains the same if focal length, distance & aperture are fixed regardless which formats they are (which you disagree if I understand your message correctly).
what i stated is very simple, i don't understand where is the confusion still. the quote from wikipedia above doesn't mention field of view, it talks about focal length. field of view is mentioned later in the paragraph, but i have only quoted the first part. so again:

if you take a lens and use it on two different formats, the dof you get on the smaller format at same aperture and focus point, for most intents and purposes, is less than on the bigger format. the same lens, not the same fov. try the experiment with the compact i mentioned above.

let me put it another way:

Online Depth of Field Calculator

i selected 6x4.5 film, 55mm, f8, 10m focus:
Subject distance 10 m

Depth of field
Near limit 4.58 m
Far limit Infinity
Total Infinite

In front of subject 5.4 m
Behind subject Infinite

Hyperfocal distance 8.46 m
Circle of confusion 0.045 mm

k20d, 55mm, f8, 10m focus

Subject distance 10 m

Depth of field
Near limit 6.55 m
Far limit 21.1 m
Total 14.5 m

In front of subject 3.4 m (24%)
Behind subject 11.1 m (76%)

Hyperfocal distance 19 m
Circle of confusion 0.02 mm


try to play with it yourself.

don't know how else i can explain it. does it make sense now?
09-25-2009, 08:36 AM   #43
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QuoteOriginally posted by nanok Quote
if you take a lens and use it on two different formats, the dof you get on the smaller format at same aperture and focus point, for most intents and purposes, is less than on the bigger format. the same lens, not the same fov. try the experiment with the compact i mentioned above.

don't know how else i can explain it. does it make sense now?
Not to butt in to a contentious discussions, butt....

Here's a thought experiment. Imagine we have a camera with a sensor that turn off the outer pixels, effectively changing from a 35mm to APS-C and back. This would be the same thing as changing the same lens into different formats.

As we turn the chip pixels off (shrinking it to APS-C), the DOF of the remaining image cannot possibly change. But here comes the tricky part:

The outside part of the image, the part that was turned off, actually had a greater DOF than the center part. That is because the focus plane is not flat, it is curved (representing all points a certain distance from the lens). It is like a thick inverted bowl just floating at the surface of water in a sink; the bowl is the focus plane, and the surface of the water is the sensor. The thickness of the bowl relative to the horizontal surface, (the DOF) is greater away from the center. This will have much less of an effect at longer focal lengths, because the wedge of focus falling on the sensor is more narrow. It can have an impact on really wide lenses, though, resulting in a higher AVERAGE DOF across a larger sensor.

So, both sides are sort-of correct on this one.

What has really be occupying my mind though is whether I should pick up a used 14mm Pentax lens here at the site, or if I should wait and possibly give the Samyang a chance....

Last edited by DavidWasch; 09-25-2009 at 08:51 AM. Reason: improved metaphor
09-26-2009, 01:45 AM   #44
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hmm, i think talking about field curvature at this point is the last thing we needed. we are still not over "dof is connected to final result (print of same size, for instance) not to the actual image on the sensor". dof is designed to be meaningfull for real world applications, for us, photographer, so YES, if you trun off the outer pixels, you will decrease the dof (same if you crop), because you will need to enalrge more to get the same print size, very simply put. you can try an experiment such as this (yeah, another one): take a picture shot with say a 50mm lens, resize it to fit your screen, and view it full screen, crop the original picture to what it would have looked like if you had taken it wit a 100mm lens, resize again to fit your screen, look at it full screen, do you get the same dof? no, you don't, you actually get the exact dof you would get with a 100mm lens at that same aperture, actually, if your pixel count would be infinite, there would be little use for anything else but a wide angle lens with infinite resolving power (except soem specialized application) -- of course it is impossible (at some point because of physics), but just to ilustrate the concept..
09-26-2009, 07:12 AM   #45
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The bottom line is the larger format has a more shallow DOF shooting an image of the same FOV. Here is an old photo taken with my 6x7 Pentax with 400mm F4 Pentax lens @ F4 and to get the same angle of view I would need a 135mm on a Pentax DSLR that would have a much deeper DOF @F4.


Look at the DOF on a FF Canon 5D MKII made HD video.
Canon Digital Learning Center - Sample EOS 5D Mark II Video: Reverie
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