stevei wrote:
But why would anybody do this? The two photos are different compositions, so any comparison between them is meaningless.
I agree - you'd either change the lens so that you frame the same image, or you'd move which would change the perspective. If the final image is different, then you're really not comparing things in the same situation. I certainly wouldn't use my various cameras in that manner.
I’ll explain what I call the physics by use of an example. Let’s say we take a picture of a point with a 4x5inch camera and crop out a 6x7cm, 6x4.5cm, and a 35mm frame sizes. (If you like, consider all these formats with the same focal length lens, at the same aperture, and the same distance to the point). According to the applets that take into account format size, the DOF changes for each crop. Does this make sense? Of course not! It is the same point on the same piece of film. As such, it can’t be magnified any more for each format and still look like a point and not a disk. Thus, we can conclude that COC is not a function of format. This leaves DOF as a property of the lens and focusing distance(review the above equations)....Show more →
Now take each piece of cut film and make an 8x10 print of each and view each print from 10 inches. The DOF will be different in each print because of the CoC. The original equations for determining DOF were from contact prints from 8x10 and 11x14 sheet film. Contact print a 35 mm negative and the entire image will look sharp. Enlarge the negative to 8x10 and DOF becomes apparent. View a B&W contact sheet of 35 mm negatives and try to judge sharpness when holding the sheet at 10 inches. It is almost impossible to pick out sharp frames under those conditions. The small size of the image and distance viewed from are beyond the limits of what the eye can resolve and the entire negative look sharp. Move the contact print closer and the eye can resolve detail and the eye will discern in focus and out of focus content.
Edited by Jeff Donald on Aug 13, 2005 at 03:41 PM GMT
stevei wrote:
But why would anybody do this? The two photos are different compositions, so any comparison between them is meaningless.
They would do this to compare DOF on two different sizes of films. So what if they are two different compositions? I don't recall the original poster asking for any comparison on different compositions.
If someone asks you what the temperature of one liter of boiling water is compared to the temperature of two liters of boiling water, does it matter if the same pot is used for both tests.
Imagemaster wrote:
If someone asks you what the temperature of one liter of boiling water is compared to the temperature of two liters of boiling water, does it matter if the same pot is used for both tests.
We don't need an analogy, the matter being discussed is simple enough to understand as it is, your analogy just makes the subject harder to understand by not directly mirroring the subject being discussed.
The only reason someone might ask about DoF varying with sensor size is to understand the characteristics of different cameras with different sized sensors. For the same shot, with the same composition, you can achieve different DoF with a different sensor size. That's all there is to it.
Imagemaster wrote
Take a Mamiya 645 camera and take a photo. Now, without changing the lens, aperture, or lens-to-subject distance, put on a 35mm roll-film holder and take a second photo. Now take your loupe and see if you can find greater or less DOF on the 645 film. Surprise, it is the same regardless of the change in film size.
The DOF was determined by the focal length of the lens, the aperture, and the lens-to-subject distance. I could put a piece of lettuce on the film plane and it would not change the DOF just ahead of that plane.
One last try and then I'm going to give up (sorta like evolution vs creation )
NO-ONE is saying that the piece of film cut from the 645 image is IN ANY WAY different from the 35mm film in your example above. If you think that's what they're saying, you have not been reading the posts.
[ BTW "Depth of Field" has nothing to do with the areas above or below film plane - there is another term Depth of Focus, but that is not what is under discussion here - Depth of field has to do with the plane of focus on the subject side of the lens]
Your example, however, allows me to ask you How YOU evaluate the DoF on those pieces of film? (Imagine you took a shot of a ruler on the ground in your yard ) You could look at the film- either one will do since they are identical - on a light box on the other side of the room (not likely that's how you'd do it) or on a light box a foot away from your eye, or with a 3x loupe or a 6x or 10x loupe. Let's say you find something that is *just* slightly less sharp than the object originally focused on - for example the mark for 36 1/4" on the ruler shot , where you focused on 36", is ever-so-slightly fuzzier with the 10x loupe and the 36 1/2" mark is even fuzzier. You will not see this difference in apparent sharpness if you don't use a loupe or if you use the 3x loupe. Perhaps with the 6x you could see it. This is because of the limitations of your eye so IT MATTERS WHICH LOUPE YOU CHOOSE when you try to decide which of the marks on the ruler appears as sharp as the sharpest one. This determines how much the image is magnified - just like we've been saying it matters how big you print and how far away you view it.
It is not until you *look* at the image somehow, that the concept of DoF has any meaning at all. You cannot define DoF without saying how you are going to *look* at the image....
cineski wrote: Does anyone one this thread actually remember how to actually take a picture?
The problem seems to be people who can't remember making prints
The argument is pretty academic though. Nobody in their right mind uses the same lens at the same focal length and distance for the same subject on both a 35mm and 1.6 crop body.
But some people like to be able to control all aspects of their shooting, so understanding DoF and its control are not academic, unless you accept what you get after having selected the other parameters...... We sort of assumed the OP wanted to understand the issue of DoF, but hey - perhaps they were just trolling
1. Print photo
2. Observe DOF in photo (take notes if it will help you learn)
3. Take scissors and crop printed photo
4. Observe DOF in photo (compare notes)
Did DOF change? NO!
What changed? FIELD of VIEW changed.
If your cropped off half your model's face in step 3, you are SOL with your crop frame sensor. You must either change lens or move farther back.
Crop frame sensors are waste of glass. FF sensors are better.
1. Print photo
2. Observe DOF in photo (take notes if it will help you learn)
3. Take scissors and crop printed photo
4. Observe DOF in photo (compare notes)
Did DOF change? NO!
What changed? FIELD of VIEW changed.
If your cropped off half your model's face in step 3, you are SOL with your crop frame sensor. You must either change lens or move farther back.
Crop frame sensors are waste of glass. FF sensors are better.
You have posted this repeatedly and many respondents have countered your inaccurate comparisons and illustrations. Nothing can be learned from your experiments. Did you read the link I've posted twice already to Van Walree's site? If not please take the time to do so and respond in an academic fashion that will contribute to the discussion. Your continued posting of the same meaningless thoughts and personal opinions are pointless.
Edited by Jeff Donald on Aug 13, 2005 at 07:26 PM GMT
AJSJones wrote:
But some people like to be able to control all aspects of their shooting, so understanding DoF and its control are not academic, unless you accept what you get after having selected the other parameters...... We sort of assumed the OP wanted to understand the issue of DoF, but hey - perhaps they were just trolling
If you have, say, a 1.6 crop camera then you need to understand how DOF works with your lenses on that body, but you can't control the size of your sensor and there's no point worrying about comparative DOF with two completely differently framed shots.
You do however need to know that the larger you make a print the narrower DOF appears when viewed from a fixed distance, and that cropping an image and printing the cropped section out has a similar effect to making a large print.
But he's from the land of Nikon where they don't get a choice. Any CF you like providing it's 1.5
BTW: I note that "where fools think crop factor affect DOF" is NOT what the poster said, these are your own words and are not implied at all by his post.
