Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
04-03-2009, 08:41 PM   #16
Veteran Member
Sean Nelson's Avatar

Join Date: Sep 2008
Location: Vancouver BC Canada
Posts: 353
QuoteOriginally posted by brkl Quote
DOF is measured in centimeters. How many centimeters deep the in-focus area of your photo is. If you take a photo and crop half of it away, the "circles of confusion" become larger, but the DOF remains exactly the same.
No, I'm afraid you're mistaken. "Circles of confusion" is techspeak for "blurriness". There's only one distance which is in perfect focus, everything else is varying amounts of blurriness - very little for objects close to the actual focus distance and more as you move away from it. "Depth of field" is the range of distances for which the blurriness is small enough to be unnoticeable.

If you crop a picture, you have to enlarge it more to make a viewable image of a given size (8x10", for instance). When you do this, some parts whose blurriness weren't noticeable before become noticeable. This means you have less depth of field.

04-05-2009, 05:59 PM   #17
Pentaxian
falconeye's Avatar

Join Date: Jan 2008
Location: Munich, Alps, Germany
Photos: Gallery
Posts: 6,862
QuoteOriginally posted by Sean Nelson Quote
No, I'm afraid you're mistaken.
Thanks for taking the job of explaining.
04-06-2009, 12:55 AM   #18
Veteran Member




Join Date: Oct 2008
Posts: 499
Just seems wrong to me that DOF should depend on how closely you look at the image or print size.
04-06-2009, 03:30 AM   #19
Pentaxian
falconeye's Avatar

Join Date: Jan 2008
Location: Munich, Alps, Germany
Photos: Gallery
Posts: 6,862
QuoteOriginally posted by brkl Quote
Just seems wrong to me that DOF should depend on how closely you look at the image or print size.
You are not alone. And still, it is exactly so.

Otherwise, DoF would always be zero thin. Don't forget that DoF is mathematically defined in a way which is dependent on the capabilities of the human eye only, i.e., independent from finite resolution possibly found in the camera lens or the recording medium.

04-06-2009, 09:44 AM   #20
Veteran Member
Sean Nelson's Avatar

Join Date: Sep 2008
Location: Vancouver BC Canada
Posts: 353
QuoteOriginally posted by brkl Quote
Just seems wrong to me that DOF should depend on how closely you look at the image or print size.
Believe me, if you had ever used slides or negatives you'd understand this implicitly. You can look at a slide or negative and everything looks just dandy. But as soon as you project it on a wall or blow it up into a print you get an entirely different perception of what's in focus and what's not.
04-06-2009, 02:10 PM   #21
Veteran Member




Join Date: Oct 2008
Posts: 499
OK then, glad to be corrected by experts!
04-07-2009, 03:48 AM   #22
Veteran Member




Join Date: Sep 2007
Photos: Gallery
Posts: 975
clarifications

while "everything has already been said", i'd like to try and sum it up somehow.

first of all, while falconeye's statement "fov and aperture size is all, and that's final" might sound arrogant to some (and while it probably is ), it is also true. however, based on his own words as well (related to understanding), i dare to try and point out some details.

- while the statement is true in the sense that it is sufficient for defining the "phenomenon" mathematically (it is probably the most elegant mathematical way to express it), understanding it, intuitively, is also important imho. as i stated in another thread (and stirred it up a bit, hope it won't be the case again); first i will point out what was already said, but maybe not clear enough yet for some people:

-unless i am mistaking, the concept of circle of confusion is actually based on a convention, it is mainly based on the intended size of the final result (print), the viewing distance for that print and the average human beings visual capability to separate details. unfortunately, this means this is a conventional choice rather than a mathematical one, and, as noted, dof does depend on how big a print you are looking at. in other words, the circle of confusion is intended to define how much detail we are capable of seeing at the end of the day, and is only incidently connected to media size and so on, it has more to do with the viewer than with the physical means involved in producing the image (camera, sensor/film, lens, paper and so on). one might say that dof is just an illusion.
-focal length is a physical property of a lens, it does not change when using the lens on a different system (which effectively only means using more or less of the image projected by the lens). field of view should never be confused with focal length (and "equivalent focal length" is a practice which now confuses many people in this respect).

dof: while falconeye's statement is indeed true, there are some things to clarify maybe. for most intents and purposes, he is right, it is worth noting though that dof does depend on the circle of confusion (which is only something we choose, not a given, in the general sense). understanding this helps make the next step: when using a lens of focal length x (let's say 200mm), regardless what format it was designed for (see above why), to determine the dof we need chosen circle of confusion for our particular application, focal length (200mm), and aperture. now it is becoming obvious why the same lens seems to have less dof on, say, a k20d sensor compared to a 6x7 film frame: in most cases, we will chose the circle of confusion for the intended final display size (print), for the k20d sensor, we will need to "enlarge" more to get the same final size, meaning we need "more details per square millimeter" on the sensor/film to have the same amount on the final print, compared to the bigger film frame. note that this does not contradict in any way the "fov & aperture is all you need to know" statement, it just (i hope) sheds some light on it. by now, it should also be obvious why the same focal length at the same aperture will not provide different dof depending on "where the lens comes from", used on the same "media" (same chosen circle of confusion), but it will certainly be different if the circle of confusion will be different (same lens used on a different sized sensor/film, with the same intended print size in mind);

cheat-sheet:
-same focal length will behave in the same way on a given sensor, as far as dof is concerned, all else being equal (aperture, intended print, etc)
-same focal length will effectively have less dof on a smaller sensor/film (while the same fov will have slightly more dof at the same aperture on a smaller sensor, very roughly, the fov "crop" of the smaller sensor outweighs the circle of confusion change, in general, or, in other words, the change of focal length (shortening) outweighs the decreaso in circle of confusion, or the change in effective size of aperture outweighs the change in circle of confusion. i would like to see the actual calculation, and result, for the same intended print size though, on this one)
-(unrelated perhaps to the thread, but essential to practical dof matters): effective dof, all else being equal, will decrease when: the distance to the subject decreases, and, (apparently), when the distance between subject and background increases (obvious, but often overlooked, helps a lot in the real world)

and finally: slightly offtopic: it would be interesting to try and put together very small and clear explanations about often discussed topics (with links to discussion threads when apropriate), with the help of very knowledgeable people here it might be easy to do, and it could increase the signal to noise ration on the forum (people asking an faq can be directed to the knowledge base, and come back with questions if more explanations are needed, rather than explain the same thing over and over again, from scratch?
04-07-2009, 11:02 AM   #23
Site Supporter




Join Date: Feb 2008
Location: MT
Posts: 1,075
Sorry for the impolite tone--was in the final prep stages with 11 wrestlers heading to nationals and was perhaps more ready to fight than reason. When you are coaching athletes to "beat people down" it's hard to turn it off. Not an excuse, just an explanation of where I was at that date...and being gone for nats. is my excuse for the delayed response as I'm not one to shy away from "discussion" of photography issues.

Both quantitative and qualitative analysis methods have merit and yield a level of understanding. I agree with the previous post that mathematical explanations are far more elegant. My comments are only posted in the hopes that it might help someone shoot across formats which is something I have experience doing. And this commentary only arose as a side issue linking into the original post.

I'll put it this way for those seeking to shoot better images and not wanting to have the learning curve I had...When shooting medium format, I've found that when shooting landscapes and macro I've needed to stop down farther than I expect to get sufficient depth of field. When I say stop down farther, I mean stop down farther than I need to for sufficient depth of field with 35mm or digital (based on printing at 16"x20" and larger). A nice factor in this is that medium format is less susceptible to diffraction which allows me to stop down farther without the negative effects that might occur with smaller formats.

And what about John Shaw's claim that the longer the distance from the film or sensor to the blades, the greater the risk of diffraction? This brings in a mathematical variable that I'm too dense to discern in the previous explanations. Or would it be easier to just shrug and move on?

There, that's the info that may be pertinent to someone looking to shoot across various formats. How that works with using medium format lenses on smaller format bodies--I don't know. I'll let the other posts solve that one.


Last edited by Ron Boggs; 04-07-2009 at 11:10 AM.
04-07-2009, 02:03 PM   #24
Veteran Member




Join Date: Sep 2007
Photos: Gallery
Posts: 975
QuoteOriginally posted by Ron Boggs Quote
Sorry for the impolite tone--was in the final prep stages with 11 wrestlers heading to nationals and was perhaps more ready to fight than reason. When you are coaching athletes to "beat people down" it's hard to turn it off. Not an excuse, just an explanation of where I was at that date...and being gone for nats. is my excuse for the delayed response as I'm not one to shy away from "discussion" of photography issues.
note to self: never argue with ron. if argument unavoidable, avoid physical proximity to him/his pupils at ALL costs.


i meant to say something about ron's statement too, but mine was too long already. now, that his day job is revealed.. i might want to pass.. :-P. seriously, as falconeye said, there is a difference between experimental observation and explaining stuff, there is also a tight connection though: one can not live without the other, while ron was a bit hasty, i wouldn't be hard on him (i was sure it was a misunderstanding anyway), the "observer" role is important too.
04-07-2009, 02:58 PM   #25
Site Supporter




Join Date: Feb 2008
Location: MT
Posts: 1,075
Pretty easy going...haven't ever let a mental discussion devolve into a physical one .
I don't really have a single day job, I'm a multi-hat guy with wrestling, writing and photography all sharing time with owning a marine industry rep group--wholesale boat sales and training type stuff.

Please note that this isn't a brag paragraph, but a "prove it" regarding my observation skills. I've sold perhaps a thousand images to magazines, and for corporate use in company brochures and such, plus a few to different states for use in state parks brochures etc. My fine art chromes have sold for as much as $2500 for a 30x40" framed print. So my observations aren't just the observations of an idiot.

My point is that having shot more than one format, it's impossible not to notice the shallow depth of field inherent in medium format. That's my observation and it's a generally accepted fact in the working world of photography. When people try to prove that isn't so, it bugs me because the types of images I sell require deep depth of field. I guess I'll continue to do it my way and continue to have moderate "amateur level" success with this hobby. If those who compete for photo sales just listen to the math and shoot shallow depth of field images, I guess I'll continue to attract a photo buyer every now and again.

My reasons for posting in these forums is to help myself get a grasp on this detailed hobby and also to help others figure out ways to enjoy the hobby too. My intent is not to shout anyone down or deride them. Mostly, I'm trying to pass my qualitatively determined ideas on to other forum members.

So if you think I'm way off base, shoot it your way, I'll shoot it my way. The final image is the real determination--not my theoretical perception or the mathematical truisms. It isn't about either qualitative or quantitative explanation. It's about the final image--ALWAYS.
04-07-2009, 04:27 PM   #26
Veteran Member




Join Date: Sep 2007
Photos: Gallery
Posts: 975
QuoteOriginally posted by Ron Boggs Quote
Pretty easy going...haven't ever let a mental discussion devolve into a physical one .
I don't really have a single day job, I'm a multi-hat guy with wrestling, writing and photography all sharing time with owning a marine industry rep group--wholesale boat sales and training type stuff.

Please note that this isn't a brag paragraph, but a "prove it" regarding my observation skills. I've sold perhaps a thousand images to magazines, and for corporate use in company brochures and such, plus a few to different states for use in state parks brochures etc. My fine art chromes have sold for as much as $2500 for a 30x40" framed print. So my observations aren't just the observations of an idiot.
i'd be interested to know how you do it, i'm sick of unix, networking and all the related junk and am thinking of switching seriously, good for you!

QuoteQuote:
My point is that having shot more than one format, it's impossible not to notice the shallow depth of field inherent in medium format. That's my observation and it's a generally accepted fact in the working world of photography. When people try to prove that isn't so, it bugs me because the types of images I sell require deep depth of field. I guess I'll continue to do it my way and continue to have moderate "amateur level" success with this hobby. If those who compete for photo sales just listen to the math and shoot shallow depth of field images, I guess I'll continue to attract a photo buyer every now and again.
wait a minute, easy there. i think this is a continuing misunderstanding: if you look closer at what flaconeye (and most others making a statement rather than a question) said, including me (i was just explaining what falconeye stated, and others, just in other words), nobody is trying to disaprove that point (as i said shortly, same fov on bigger media will result in smaller dof at the same relative aperture).

QuoteQuote:
My reasons for posting in these forums is to help myself get a grasp on this detailed hobby and also to help others figure out ways to enjoy the hobby too. My intent is not to shout anyone down or deride them. Mostly, I'm trying to pass my qualitatively determined ideas on to other forum members.
as far as i can tell since i've been here, that's the general attitude around here, that's the main reason i hang around (i usually avoid "forums"). i'd dare to say you are in good company (as am i)

QuoteQuote:
So if you think I'm way off base, shoot it your way, I'll shoot it my way. The final image is the real determination--not my theoretical perception or the mathematical truisms. It isn't about either qualitative or quantitative explanation. It's about the final image--ALWAYS.
in no way off base, a bit trigger happy, maybe . i do agree the final result is what matters, but understanding what happens is supposed to help us (all) get the results we want, in a predictable way (as you put it, without such a steep learning curve, typical of trial and error), that's why i value these discussions.

that being said: i feel it is still not obvious to you from what was written where "we" stand, this means to me that my explanations, in connection with flaconeyes and others, are not clear enough for at least one reader here (and a rather knowledgeable one as it seems), and i stand by my statement that i would like to get to something which is clear and not confusing to anybody. could you please help with that? (as in, point out what could use rephrasing/explanations, what is still unclear, seems to lead to a conclusion you know from experience to be wrong, etc)?
04-07-2009, 10:56 PM   #27
Veteran Member
Sean Nelson's Avatar

Join Date: Sep 2008
Location: Vancouver BC Canada
Posts: 353
QuoteOriginally posted by Ron Boggs Quote
My point is that having shot more than one format, it's impossible not to notice the shallow depth of field inherent in medium format.
...and of course it's not just medium format vs. APS-C, it's also APS-C vs. compact cameras with their tiny 1/2.5" sensors. Just tell any doubter to go out and shoot the same scene (from the same position with the same field of view) at the same aperture (f/number) with a compact and a DSLR. If there's a reasonable range of depths in the scene and they can't see the difference in DOF then they're blind enough to have their photography license revoked!
04-07-2009, 11:41 PM   #28
Veteran Member
Jewelltrail's Avatar

Join Date: May 2008
Location: Rhode Island
Photos: Albums
Posts: 4,180
Original Poster
QuoteQuote:
Sean Nelson: ...and of course it's not just medium format vs. APS-C, it's also APS-C vs. compact cameras with their tiny 1/2.5" sensors. Just tell any doubter to go out and shoot the same scene (from the same position with the same field of view) at the same aperture (f/number) with a compact and a DSLR. If there's a reasonable range of depths in the scene and they can't see the difference in DOF then they're blind enough to have their photography license revoked!
Yes, absolutely. Ron speaks of diffraction setting in earlier on APS-c cameras than on Medium Format. In the same way, diffraction sets in earlier on Point & Shoots, than it does on APS-c cameras. In fact, the smallest aperture on my FZ Panasonic is f8, and that is not accessible in the Auto-mode. The Auto mode system almost never selects an aperture over 5.6. And the "Intelligent Auto" system on the Panny is very, very good at maximizing image quality.
04-08-2009, 03:01 AM   #29
Pentaxian
falconeye's Avatar

Join Date: Jan 2008
Location: Munich, Alps, Germany
Photos: Gallery
Posts: 6,862
@nanok: Thanks for trying to make things clearer.

Because the topic about DoF tends to confuse many people and answers tend to be long, I established my take at it:

My short "FoV and physical aperture is all" statement.

I try to make this a rock in an ocean of incertitude to help people out. A certain level of arrogance helps in making it a rock. Sometimes. One simply cannot argue everything and forever. But I agree to your statement about my arrogance


@Ron: Nobody called you an idiot (I am not that arrogant). I assumed You have made observations and wanted to learn more about it.

That DoF is shallower for larger formats was never put into doubt by anybody in this thread. As nanok explained.

It may help to realize that the term "aperture" can have two meanings:
- the absolute (or physical) aperture (in mm)
- the relative aperture (or f-stop value, which is the quotient of focal-length / physical aperture -- the reason why you write f/16)

You may have missed this distinction and if you read this thread again, you may find that nobody is in contradiction with what you observed.

Because glass for medium format is bigger (thicker and longer) than for 35mm. So, the physical aperture are bigger even for the same f-stop.


But you asked another question: What if I mount medium format glass to 35mm? As it is a simple one, this question had been answered as well. Many times.


QuoteOriginally posted by Ron Boggs Quote
And what about John Shaw's claim that the longer the distance from the film or sensor to the blades, the greater the risk of diffraction? This brings in a mathematical variable that I'm too dense to discern in the previous explanations.
This was the one serious question you asked. My response may have been hidden in the noise about answers to the simple question.

The position of aperture blades doesn't matter. Relative aperture, DoF and diffraction all depend in the same way on their position. So this dependency cancels out. Read my post #12 about the entrance pupil for further details.

And anyway, medium size doesn't call for a particularly different position of aperture blades.


@nanak:

About the circle of confusion CoC...

In DoF calculations, the circle of confusion is mathematically defined to be

"diagonal size of medium" / CONSTANT

where the CONSTANT is constant reflecting an observer's biology (the human eye).

The only convention is which exact value is chosen for the constant.

The most common convention is the "Zeiss formula" which is CONSTANT=1730.



Obviously, with 1920x1080 HDTV, the constant ought to be 2200
04-08-2009, 12:01 PM   #30
Pentaxian
Wheatfield's Avatar

Join Date: Apr 2008
Location: The wheatfields of Canada
Posts: 10,042
QuoteOriginally posted by brkl Quote
Just seems wrong to me that DOF should depend on how closely you look at the image or print size.
You can actually pretty much forget about the circle of confusion thing that people like to bat around.
Depth of field is controled by two things and two things only:
1: Image magnification on the sensor/film
2: Aperture being used.

Thats it.

The depth of field doesn't change depending on the size of the print, it's just that a bigger print makes it easier to see.
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!
camera, crop, dof, forum, k20, k20d, knowledge, lenses, medium format, sensor, shot
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
Sensor size vs. DOF future_retro Photographic Technique 24 09-16-2010 04:30 PM
Do you shoot and crop in camera or shoot loose and crop in photoshop? hockmasm Pentax DSLR Discussion 23 05-02-2010 06:10 PM
DOF and Crop Sensors edl Pentax SLR Lens Discussion 16 11-28-2009 01:11 PM
DOF & Crop factor octavmandru Pentax DSLR Discussion 14 09-01-2009 08:44 AM
DOF and sensor size simons-photography Photographic Technique 60 04-23-2008 10:55 AM



All times are GMT -7. The time now is 04:12 PM. | See also: NikonForums.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