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Author Topic: Sensor size and DoF  (Read 51620 times)
Ray
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« Reply #20 on: July 09, 2006, 05:22:38 AM »
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Historically, lenses for larger formats have typically had longer focal lengths and higher minimum f-stops, and have been most often used with higher f-stops in order to meet DOF needs, leading to the pattern of larger formats typically being used with lower shutter speeds, or alternatively higher ISO speeds to get the same DOF and shutter speed.
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What I say is, 'roll on the new technology currently under development to circumvent the laws of diffraction'.  
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AJSJones
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« Reply #21 on: September 03, 2006, 02:44:13 PM »
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....

Imagine that you took 35mm film and painted an opaque back ink frame around the film so that only the central 2/3 of the film frame is exposed to the light during exposure. Do you believe you get subject magnification or affect DOF? Well, you wouldn't.  Ask anyone who's had the film transport in a 35mm film camera go screwy.

A less than full-size sensor has no sensors around the periphery of a 35mm frame, just as if you blocked the extra sensors of a full-frame sensor with something opaque. That's the only difference in a 35mm DSLR. All you get is reduced FOV. You do not get a boost in magnification, subject or otherwise.   

Cheers,

Mitch
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Mitch,
To continue with your scenario and keep the "comparison" valid (i.e. not introduce new variables), you have to switch painted and unpainted frames in the same enlarger (and not change anything else in it) to make the prints.  Guess what, the cropped frame print is smaller.  Yup, same as cutting down the original print to leave the central 2/3.    It was the same image coming through the lens, so this is rational.

Now, from the same viewing distance, you are correct that each will have the same parts appear to be in focus and nothing will be different about the central 2/3 of the image.  So the "All you get is reduced FoV" comment must apply to the print as well. DoF as a physical thing doesn't exist until you view the image because HOW you view the image determines what will appear in focus aka acceptably sharp.

 "A beginning digital photographer" , perhaps someone coming from FF 35mm such as film, to cropped sensor such as digital needs to appreciate this consequence, don't you think?  Should there be a warning like "Attention, this camera makes smaller prints than you are used to"    

Andy
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Ray
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« Reply #22 on: September 03, 2006, 07:29:21 PM »
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It's odd that there still seems to be confusion over the effects of a cropped sensor after all this time. It's probably due to the unfortunate terminology of 'crop factor' which has resulted from the fact that APS-C format cameras all use 35mm lenses.

If you could fit Medium Format lenses to 35mm bodies, then we could call the 35mm format a 'cropped MF format' and we'd all be confused about which lens to use.  
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Chris_T
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« Reply #23 on: September 04, 2006, 07:19:13 AM »
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What do I have to lose?  I'll jump in to the fray  

2) However, to obtain a similar FRAMING from cameras with different sensor sizes but using the same focal length lens, we must change our shooting position -- which in turn changes the magnification factor, which in turn changes the DoF.  So it is ALWAYS true that smaller sensor cameras will have MORE DoF when generating framing comparable to a larger-sensor camera, all else equal.

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I'm no DOF expert and don't even own a digital camera. So I'm qualified to jump in as well. <g>

Your statement makes most common sense to me, and is what I would expect from digital cameras with small or full frame sensors.

Lets leave small and full frame sensors out for a minute. All else being equal, will the same lens result in different DOFs between a film camera and a digital camera with a full frame sensor?
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Ray
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« Reply #24 on: September 04, 2006, 09:36:28 AM »
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Lets leave small and full frame sensors out for a minute. All else being equal, will the same lens result in different DOFs between a film camera and a digital camera with a full frame sensor?
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Your question is confused. In the first sentence you write, 'let's leave small and full frame sensors out'. In the second sentence you bring 'full frame sensors' back in. Which is it? Out or in?
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Chris_T
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« Reply #25 on: September 05, 2006, 08:39:30 AM »
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Your question is confused. In the first sentence you write, 'let's leave small and full frame sensors out'. In the second sentence you bring 'full frame sensors' back in. Which is it? Out or in?
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I meant instead of comparing DOF differences between small and full frame sensors, will there be any DOF differences for a lens when used on a 35mm film camera and when used on a 35mm digital camera with full frame sensor? That's a mouthful, hope I got it right this time.
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Ray
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« Reply #26 on: September 05, 2006, 09:04:56 AM »
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I meant instead of comparing DOF differences between small and full frame sensors, will there be any DOF differences for a lens when used on a 35mm film camera and when used on a 35mm digital camera with full frame sensor? That's a mouthful, hope I got it right this time.
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Only in respect of differences in resolution at the plane of focus. It's theoretically possible that a fine grain B&W film could deliver more resolution than, say a 5D, but color film wouldn't stand a chance.

DoF calculators do not bring such matters into their calculations, but it seems clear to me that either a lens or film that is able to deliver greater resolution at the plane of focus than a lesser lens or lower grade film (or coarser sensor), will produce an effect of less DoF because the difference between what's in focus and what's out of focus will be greater.
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Jonathan Wienke
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« Reply #27 on: September 05, 2006, 12:23:59 PM »
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Lets leave small and full frame sensors out for a minute. All else being equal, will the same lens result in different DOFs between a film camera and a digital camera with a full frame sensor?

The correct answer is a definite "maybe". Because of it's higher resolving power, the digital sensor has a smaller effective circle of confusion, and therefore less DOF, but only if the lens used is as good as the sensor. If your lens is a cheap coke bottle that is outresolved by the film and digital sensor, DOF will be identical between digital and film. But if you have a good lens that outresolves the film but not the digital sensor, then the more sharply-focused areas found only in the digital image will raise the bar of what appears to be "in focus", and areas that appear "in focus" in the film image will suffer in comparison to other areas in the digital image that are even more sharply focused, and therefore will appear "out of focus".

What we perceive as "in focus" is based on comparison to other parts of the image, so any area that is very sharply focused will cause less sharply-focused areas to appear "out of focus". So the more resolution you extract from a given imaging area (like 24x36mm), the narrower your DOF will be--you're effectively decreasing your CoC size when you do so. As prints get larger, the difference will become more noticeable. Here's an example:



In the web-sized JPEG above, everything appears to be "in focus" because the entire image is equally sharp--resolution is limited by downsampling to 800x640 pixels, not the lens or sensor. But when looking at the full-resolution version of this image, you can see that the right edge of the image is slightly OOF compared to the center and left side. By reducing overall resolution when downsizing this image for web display, I increased the effective CoC and increased DoF. But when increasing resolution, (going back to the original sized image) I decrease the effective CoC and DoF decreases proportionately.
« Last Edit: September 05, 2006, 12:35:01 PM by Jonathan Wienke » Logged

Chris_T
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« Reply #28 on: September 06, 2006, 07:35:32 AM »
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The correct answer is a definite "maybe". Because of it's higher resolving power, the digital sensor has a smaller effective circle of confusion, and therefore less DOF, but only if the lens used is as good as the sensor. If your lens is a cheap coke bottle that is outresolved by the film and digital sensor, DOF will be identical between digital and film. But if you have a good lens that outresolves the film but not the digital sensor, then the more sharply-focused areas found only in the digital image will raise the bar of what appears to be "in focus", and areas that appear "in focus" in the film image will suffer in comparison to other areas in the digital image that are even more sharply focused, and therefore will appear "out of focus".
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Now it's my turn to be confused. When "less DOF" is used, I always take it to mean that the distance range within which everything is sharp is *shorter*. But in your explanation above, you seem to say that a digital camera will have a *longer* distant range within which everything is sharp than a film camera, everything else being equal.
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Jonathan Wienke
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« Reply #29 on: September 06, 2006, 10:40:15 AM »
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But in your explanation above, you seem to say that a digital camera will have a *longer* distant range within which everything is sharp than a film camera, everything else being equal.

Re-read what I wrote: Digital achieves higher resolution, smaller CoC, narrower DOF. The sharper the sharpest focus is, the narrower the distance range that will be that sharply focused in the image.
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Ray
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« Reply #30 on: September 06, 2006, 06:15:33 PM »
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In broad terms, if the lens and f/stop is the same and the distance to subject is the same then the DoF will be the same, whether film or sensor. However, if you wish to put a fine point on it, there will be subtle differences in DoF depending upon the resolving power of the film or sensor being used. For example, a 1Ds2 should produce a marginally shallower DoF than the 5D, but so marginally shallower that you probably wouldn't notice it.

Likewise, a T-Max ISO 100 B&W film, that can apparently resolve 100 lp/mm at 60% MTF would produce a shallower DoF (everything else being the same) than an average slide or color film, given a big enough print.
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George Barr
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« Reply #31 on: September 08, 2006, 08:09:07 AM »
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Depth of field is simple. The mathematical formula for final sharpness in the print has depth of field inversely proportional to magnification from sensor to print (you make bigger prints, you have less depth of field, you use smaller sensor to make same size print, you have less depth of field), BUT, depth of field is inversely proportional to the SQUARE of the focal length, thus changes in the focal length of the lens used (the actual focal length, not some fudged 35 mm. equivalrent) have a much larger effect on depth of field.

This applies no matter what sensor size you have, whether it's film or digital, contact print or enlargement, sharpness fanatic or prebyopic baby boomer who's lost his reading glasses.
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Ray
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« Reply #32 on: September 08, 2006, 10:33:32 AM »
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Depth of field is simple. [a href=\"index.php?act=findpost&pid=75858\"][{POST_SNAPBACK}][/a]

DoF is not simple. It's complicated by all sorts of factors which have an influence one way or another, including print size, lens quality, focal length, sensor quality, sensor size, objective factors and subjective factors.

There have been many protracted debates on this forum about these issues, some arguments addressing only the properties of a perfect lens using simplistic mathematical formulas and other arguments taking in a broader spectrum of variables that have both a direct and indirect influence on DoF.

Some folks will argue that sensor size has no bearing on DoF, for example, completely ignoring the practical reality that sensor size determines choice of focal length and therefore indirectly affects DoF. But because sensor or film format does not appear in the DoF formulas, such people will strenuously deny that sensor size has any bearing on DoF. It's a sort of photographic fundamentalism.
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Olivier_G
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« Reply #33 on: September 09, 2006, 01:13:19 PM »
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DoF is not simple. It's complicated by all sorts of factors which have an influence one way or another, including print size, lens quality, focal length, sensor quality, sensor size, objective factors and subjective factors.
Most of the debates I see about "DOF" could be sum up as "how do you define DOF?". Many people use different meanings... without even being aware of this...  

I like the traditional way (ie: DOF formulae and CoC=1/1730 or whatever of the diagonal format) because its mainstream since years and provides also a good comparison basis for Background blur, even between different formats.
In another forum, I had to define "useful DOF" because people wanted to take into account the system real limitations and even "absolute DOF" to please one who considered DOF to be a characteristic of the lenses only... Oh well...  

Olivier
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Ray
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« Reply #34 on: September 09, 2006, 07:16:55 PM »
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(ie: DOF formulae and CoC=1/1730 or whatever of the diagonal format)


Olivier,
I take it you mean CoC should be 1/1730 of the diagonal of the format. I suppose that's good enough for an 8x12" print, although some demanding people will claim the CoC should be equal to the pixel pitch of the sensor if maximum sharpness at the plane of focus is desired on the maximum size print the camera can produce, without interpolation, at 240 ppi.

Where I find these DoF calculators begin to break down is in the area of lens quality and sensor pixel pitch. For example, DoF Master treats the D30 the same as the 30D. The sensors are the same size. However, the D30 has 3mp and the 30D 8mp, yet DoF Master uses the same CoC of 0.019mm to get the same result at the same aperture and focal length of lens and same distance to subject.

I suppose this is fair enough if you make an assumption that the D30 with an average lens will deliver detail as sharp as the eye can see when viewing an 8x12" print from a distance of 10" or so, and that any sharper result the 30D might produce with a better lens on a larger print is irrelevant for this size of 8x12" print.

Perhaps someone who owns both a D30 and 30D could test this   .

I notice that DoF master does not emphasise that their figures apply only to a print size of 8x10" (or 12"). In fact I can't see it mentioned. But maybe I haven't looked hard enough.
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AJSJones
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« Reply #35 on: September 09, 2006, 10:12:24 PM »
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Perhaps someone who owns both a D30 and 30D could test this   .

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Ray, I started with a D30 and have a 20D now.  Without formally doing any comparisons , my recollection/experience tells me the following.  I would say that 8x12's from the D30 with a good (enough) lens - I had a good 28-135 - came close to that performance.  I also had some poor lenses that didn't - suggesting something about system resolution and that it didn't take much loss in lens resolution for the overall loss to be noticeable;  i.e. the D30 was at its limit at 8x12 at 240ppi original pixels, my (preferred) limit.  The 20D goes beyond 8x12 at 240.  

The DoF concept is definitely a moving target, given the variability in print size, degree of crop/enlargement that's so easy with digital.  With the temptation to use original pixels at the printer's limit making it so easy to either crop/enlarge or just print bigger, it makes sense to use the pixel (or close to it) as the CoC to guide capture apertures during exposure

Andy
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Ray
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« Reply #36 on: September 10, 2006, 12:16:20 AM »
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Andy,
As you describe it is pretty much as I imagined, but a thorough camparison of the D30 and 30D on an A4 size print would be interesting, especially if the D30 shot was taken with a zom at, say 50mm, and the 30D shot was taken with the Canon 50/1.4 prime.
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Olivier_G
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« Reply #37 on: September 10, 2006, 04:30:56 AM »
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Where I find these DoF calculators begin to break down is in the area of lens quality and sensor pixel pitch.
Ray, it's just that "DOF" is defined without those.
Historically, DOF has been defined by using the 'perfect lens' formulae and a 1/x ratio of diagonal format that matched both an "acceptable" quality for viewing print AND the resolution of film used at that time.
Resolution of lenses and films and then digital cameras has improved, but this hasn't changed DOF definition... and for good reasons: simplicity, relationship too Out Of Focus Blur and probably the most important one => photos are meant to be viewed, not to be scrutinized with a microscope at 100%...  
That's why we keep finding it in most litterature and in all online DOF calculators. This is also what is used by all manufacturers (Canon, Schneider, Leica, etc...).
Using a different 1/x ratio of the format depending on one's needs is still within this definition.

Taking into account the Lens resolution, Sensor resolution, Diffraction, more accurate formulae depending on lens design, non planeity of focus area, whatelse... is a different modelization.
Anyone can use and refine such modelization if they want.
The only thing I am asking is to use a different name than "DOF/Depth Of Field" which is assumed to be the traditional definition (and is really mainstream) in order to avoid misunderstandings. This is why I suggested "Useful DOF", etc... for those different concepts.

Olivier
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Olivier_G
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« Reply #38 on: September 10, 2006, 04:57:02 AM »
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If you are interested in more comparisons, have a look at those shots by Jean-Marie Sepulchre and Michel Denis-Huot (in french). They are quite interesting about 24x36 vs APS, although Jean-Marie and I didn't agree on the conclusions...  

If you want to go down the 'Lens&Sensor quality' road, don't forget:
- you won't have any formulae to calculate that "useful DOF" (change name here...)
- using an excellent lens and a bad one won't give you the same "useful DOF" even when using exactly the same settings
- of course, "useful DOF" will be different in the center and in the corners where lenses are less sharp
- there won't be any relationship between DOF and OOF blur
- Diffraction will have to be taken into account (and will you get more DOF with larger CoC or something else like DOF=0... you choose)
- it will be sensor+lens dependant... and will have to be updated with every change
- don't count me in!!!  

Olivier
PS: if you do so, please don't forget to use a different name than just "DOF/Depth Of Field" so that I can understand what you're talking about and leave you alone in that mess...  
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Ray
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« Reply #39 on: September 10, 2006, 08:00:33 AM »
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If you want to go down the 'Lens&Sensor quality' road, don't forget:
- you won't have any formulae to calculate that "useful DOF" (change name here...)
- using an excellent lens and a bad one won't give you the same "useful DOF" even when using exactly the same settings
- of course, "useful DOF" will be different in the center and in the corners where lenses are less sharp
- there won't be any relationship between DOF and OOF blur
- Diffraction will have to be taken into account (and will you get more DOF with larger CoC or something else like DOF=0... you choose)
- it will be sensor+lens dependant... and will have to be updated with every change
- don't count me in!!!   


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Olivier,
I assure you I have no intention of going down that road. The DoF issue for me is mainly a conceptual one of academic interest and a matter of semantics and meaningful definition. As a practical photographer who uses zoom lenses most of the time, outdoors and in the field, I have no way, nor the time, to make precise DoF measurements which take into consideration lens resolution, pixel density or print size. Even using a basic DoF calculator such as DoF Master would be too cumbersome for me. For example, if the nearest point which is reasonably sharp is 12.8 metres, how do I measure that distance? I'm not interested in carrying around a 30 metre measuring tape or wading acroos a river to make a precise measurement.

It is sufficient for me simply to be aware of the factors that can influence the perception of DoF on the final print. For example, with my D60 and 20D I was more reluctant to use f16 than I am with my 5D because the greater pixel density of the 20D revealed the softening effects of f16 more. With the 5D the softening effect is hardly noticeable. F16 is very usable with the 5D because the pixel density is less. At the same time, I need to use f16 with the 5D more than I do with the 20D because DoF is shallower with the 5D at the same f stop and FoV.

These are issues I need to be aware of to make reasonably accurate guesses in the field.
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