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11-20-2009, 11:16 AM   #31
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QuoteOriginally posted by zx-m Quote
I am somewhat surprised that no one has chimed in with an immediate answer, .
Maybe you need to wait more than two hours?

11-21-2009, 02:07 PM   #32
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QuoteOriginally posted by Szten Quote
Yes, it's the sensor. More specific, it's the CoCs in the sensor. The problem is that it seems like you don't know what a circle of confusion is, and thus understanding the DOF is not possible.
Take a look at a DOF calculator. The calculated DOF will be different with cameras with different sensor sizes. Why? Because the maximum permissible circle of confusion will be different.
You should look after and understand the meaning of CoC first, and everything will be clear.
A few links:

Circle of confusion - Wikipedia, the free encyclopedia
Understanding Depth of Field in Photography
Circle of Confusion: Optical: Glossary: Learn: Digital Photography Review

Read them and you will understand what Coc is, and how it affects the DOF.
I have known for a very long time what a circle of confusion is.

What I have been trying to tell you is that DOF is RELATIVE to the sharpest item in the print. If you print a photo from a 6 MP camera at 30x40 then nothing is sharp, and the even less sharp areas are outside the area covered by the DOF. It all scales evenly.

Getting back to the DOF scale on the lens - I took two photos today at f22 with a 28mm lens. One was taken with the lens set directly to infinity, one was taken with the infinity mark right at the edge of the DOF scale. I can not tell which is which, and I highly doubt you will be able to either. You can view them full size here.

Once again the physical experiment destroys the theory.
11-21-2009, 03:59 PM   #33
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And you call this a relevant test? It has nothing to do with our argument - F/22, infinity - you have to be kidding.
But I realized one thing. I was wrong, the main problem is, that you define DOF like you did, and with that definition, you were right all the time. But DOF is not relative, not subjective etc, but it is an exact, calculable value. There's an exact definition of DOF, you cannot define DOF to be something absolute different. In fact you can, but that will leads to misunderstandings, like this.
If DOF means that to you, we can't say anything to prove you're wrong, because you aren't. It's just that we think about DOF another way - the formal way. The official way if you like. (The official definition of DOF can be read in the second link I gave you before)
11-21-2009, 05:32 PM   #34
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QuoteOriginally posted by Szten Quote
And you call this a relevant test? It has nothing to do with our argument - F/22, infinity - you have to be kidding.
The OP asked about the DOF scales on his lenses. I provided a simple test of DOF scale accuracy. I found mine to be reasonably accurate. If you wish you are more than welcome to try and prove me wrong - through experimentation, not on paper. Untill it is backed up with a real world test it is just a theory.

11-21-2009, 06:08 PM   #35
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QuoteOriginally posted by Steve Beswick Quote
Everybody really makes this much too complicated.

The DOF of any lens at a specific focal length and f stop will always be the same. The DOF scales will always be as accurate as they always were.

The lens does not know or for that matter care what sort of camera it is attached to. It will project the same image onto a digital sensor, film, ground glass, or a blank sheet of paper.

The only exception I know of is the focus screens on DSLR's. They use a few optical tricks that have the effect of making fast lenses look like they have the DOF of f2.8 or so.
Sorry Steve, but this is not true. DOF is not a physical property of the lens, per se. The variables are:
  • Focal Length
  • Aperture
  • Capture dimensions (film/sensor size)
  • Final viewing dimensions (print/image size) and viewing distance
Interestingly, the scales on a traditional lens are an approximation based on a standardized print at a standardized viewing distance. There are some huge threads on these forums regarding this issue as well as many comprehensive articles on the Web.

The general rule is that a given focal length will deliver somewhat greater DOF with an APS-C sensor than with 35mm film.


How much more (stops, front/back range, whatever) depends on final image size and viewing distance. This greater DOF is one reason why people are so interested in the f/1.2 glass and why there are so many complaints about lack of fast glass at about 35mm focal length.

Now in case, you don't believe me regarding image size and DOF...consider the fact that thumbnail images ALWAYS appear to have better focus than the same image at 100%. Consider too, viewing a photo mural from across the room as opposed to 6 inches.

One other consideration that should be emphasized is that the DOF in the viewfinder is not a good predictor either. Typically the stock Pentax focus screen will show somewhat greater DOF than what you will see in a print of reasonable size. The reasons are hard to explain, but it is enough to say that the viewfinder will show a DOF equivalent to about f/4 at narrowest.

Steve

(Taking cover...the **** is about to fly...)
11-21-2009, 09:02 PM   #36
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Caution - long quote from an old book

QuoteOriginally posted by stevebrot Quote
Sorry Steve, but this is not true. DOF is not a physical property of the lens, per se. ...<snip>...
(Taking cover...the **** is about to fly...)
I am quoting the following book in support of stevebrot, and against the other Steve.
The focal guide to lenses, 1977, Focal Press.
The book above has a whole chapter on depth of field, in which they give a lengthy discussion of depth of field, and how to calculate the depth of field for a given lens and format.

I will quote from page 68, first:
QuoteQuote:
... At a given focused distance, depth of field is the same when the effective aperture is the same. ... At a given f-number, depth of field is greater with shorter focal length lenses because the effective aperture is then smaller.
In other words, the depth of field is based on the physical diameter of the lens aperture. A 50mm lens has twice the depth of field of a 100mm lens using the same diameter aperture at the same distance from the subject. The effective aperture of the 50mm lens at f/4 is the same as the effective aperture of the 100mm lens at f/8.

Page 67:
QuoteQuote:
... taking the generally accepted figure of a circle of confusion of .25 mm diameter for a 25 cm viewing distance,...
QuoteQuote:
...the circle of confusion should be the same for all focal lengths and it must aim at producing a disc on the final image no greater than .25 mm for a viewing distance of 25 cm. Thus, the degree of enlargement must also be taken into consideration.
Page 69.
QuoteQuote:
A reasonable compromise that imposes fairly strict standards is in fact, a circle of confusion of .025mm for 35 mm work, .04 mm for 6x6 cm cameras of .12 mm for 5x4 cameras.
In other words, the circle of confusion on the final print is the criterion by which depth of field is measured.

End.
11-22-2009, 05:33 AM   #37
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QuoteOriginally posted by stevebrot Quote

The general rule is that a given focal length will deliver somewhat greater DOF with an APS-C sensor than with 35mm film.

Steve, surely this is only true when the image size of the subject in the frame is the same, ie the camera to subject distance is increased with the DSLR. It is the increase in camera to subject distance which is the main factor in increased DOF on the DSLR.

Where the distance to subject stays the same on both formats the actual DOF will be greater on the picture produced by the 35mm film camera. I maintain this is because the DSLR image has to be enlarged more, and thus the COC has to be increased more. The two images will have different fields of view of course. This is why i believe the answer to the OP, is that he will have to stop down more when using a lens originally designed for the 35mm format. Because the scale on the lens when built did not anticipate the exta enlargement required on the APS-C sensor.
11-22-2009, 05:57 AM   #38
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QuoteOriginally posted by KungPOW Quote
This was written by Clayton Jones. I think it is one of the better explinations for DOF effects on the web. And he uses a Pentax lens in his example.

"However, because the DOF calculations are different with APS sized sensors, we can no longer rely on these lens DOF scales. We will have less DOF than they show us. But while we get less DOF from each lens, the smaller sensor is actually giving us greater DOF at equivalent fields of view (FOV)."

Here is the link:

Hyperfocal Focusing With APS Sized Sensors

It was a good read.
A good comment and a good link. The point of all this is that for an equivalent field of view, we get more DOF with our APS-C sensor DSLRs, but if we use "film" lenses we must be aware that the DOF scales are 1 to 2 stops over optomistic.

11-22-2009, 03:10 PM   #39
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Well, now that seems to be settled....again. What I find hard to understand is that this fact would keep the OP from going digital. These facts may seem a disadvantage to him in theory (it's theory to him because he hasn't shot digital on APS-C or else why the question). Sure, there are differences in the digital world compared to film and even though there are disadvantages there are also some great advantages.
I can't think of a single time I've felt I lost a great picture because of this DOF issue on the 1.5 crop sensor. Admittedly it may be that I've never taken a great picture in the first place. I could maybe see the point if he took most of his pics with the A 15mm and there was no comparable APS-C equivalent lens or some such thing. There are times I wish I had a super wide angle and I could use one for sure but on the other hand the APS-C isn't hurting my 300mm.
When I now see a scene and think I need the 31mm I'm looking at the scene with the 1.5 crop, I don't do any conversions in my head anymore. My 31 is a 31, the 77 is a 77...... on my K10D or K20D! The DOF is the DOF and I don't even worry about what it would be on film when I'm shooting digital. I can't change it so why worry about it? We can argue these things till the cows come home but we can't change them. Might as well get out and take some pics!
11-22-2009, 04:33 PM   #40
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QuoteOriginally posted by zx-m Quote
How about shooting with a digital-specific lens--such as the DA 40/2.8? Would this also need the one-stop reduction for accuracy, or would its DOF markings be correct?
DOF markings are never anything more than rough approximations. Stop worrying about them and pay attention to the DOF preview in the viewfinder - or just shoot the picture and check for yourself to see if what you want is in focus. This isn't rocket science, and you're allowed to do a trial and error.
11-22-2009, 05:01 PM   #41
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QuoteOriginally posted by stevebrot Quote
Sorry Steve, but this is not true. DOF is not a physical property of the lens, per se. The variables are:
  • Focal Length
  • Aperture
I never disagreed with either of these points.

QuoteQuote:
The general rule is that a given focal length will deliver somewhat greater DOF with an APS-C sensor than with 35mm film.
Thats funny, considering that is the exact opposite of what roughly half of the posters in this thread think.

QuoteQuote:
How much more (stops, front/back range, whatever) depends on final image size and viewing distance. This greater DOF is one reason why people are so interested in the f/1.2 glass and why there are so many complaints about lack of fast glass at about 35mm focal length.

Now in case, you don't believe me regarding image size and DOF...consider the fact that thumbnail images ALWAYS appear to have better focus than the same image at 100%. Consider too, viewing a photo mural from across the room as opposed to 6 inches.
According to this, DOF scales would almost always be WAY off, so why put them on the lens in the first place?

QuoteQuote:
One other consideration that should be emphasized is that the DOF in the viewfinder is not a good predictor either. Typically the stock Pentax focus screen will show somewhat greater DOF than what you will see in a print of reasonable size. The reasons are hard to explain, but it is enough to say that the viewfinder will show a DOF equivalent to about f/4 at narrowest.
Not only do I agree with this, I wrote essentially the same thing in the very post you quoted. I said f2.8, but the point was the same.

QuoteQuote:
(Taking cover...the **** is about to fly...)
Was this really necessary? I was just starting to think "Finally! someone who wants to have a rational discussion!", and then I read this.
11-23-2009, 01:02 PM   #42
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QuoteOriginally posted by Steve Beswick Quote
According to this, DOF scales would almost always be WAY off, so why put them on the lens in the first place?
Good question. As I've said, they're never more than rough approximations, valid only for "typical" prints viewed at "typical" distances by a person with 'typical" visual acuity.
11-25-2009, 12:51 AM   #43
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QuoteOriginally posted by stevebrot Quote
... DOF is not a physical property of the lens, per se. The variables are:
  • ...
  • Capture dimensions (film/sensor size)
  • Final viewing dimensions (print/image size) and viewing distance
This is very interesting, and if correct, leads to something that a lot have highlighted here - that DOF is predicated on the size of print (or digital image if you wish) that is displayed.

I was a strong opponent of this initially, but after thinking it through, i realise that it does makes sense! As the others have pointed out, depth of field is used to describe the photo (e.g. "woah, this depth of field is this picture is amazingly small"). As such, it is affected by at least 3 variables:

(i) the negative/sensor size, for any given lens of a particular FL.
(ii) The f number of the lens, which affects the image that is "projected" on the sensor / negative
(iii) the final print, which is affected by how it is cropped, etc.

How I understood the principle was through an example: Take for example the picture below:



Now if the image is printed as it is, only the area near the "main course" would be sharp, while the rest of the picture would be blur. The depth of field of the image is relatively shallow.

Say you crop the image, such that only the area near the words "main menu" is shown in the print. The whole print, which consists of that 2 -3 rows of words, would be sharp (assuming that the image taken from our wonderful lens and sensor/negative does not suffer from loss of resolution in the enlargement - I'm illustrating using an ideal case). The print can then be said to have very large depth of field.

---------------------------------------

The same concept applies when the sensor size is reduced - for the equivalent depth of field on film negative, the depth of field on a digital sensor will be bigger, relatively speaking.

Say for on a film negative, the area in focus is 5 x 5 mm, on a 36 X 24 mm negative. The area in focus is approximately 2.9% of the image. Compared to the same lens mounted on an APS C sensor. The area of focus is the same, i.e. 5 X 5 mm because this depends on the optical properties of the lens, which is unchanged. The APS C sensor is only 25 x 17 mm. Therefore, the area of focus on an APS C sensor is 5.9%. The depth of field of the final print has increased. Do note that we're taking about 2 images here, 1 from the film negative and 1 from the digital sensor, which are different.
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