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Author Topic: How (and why) does sensor size change DOF?  (Read 28322 times)

Knut Skywalker

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How (and why) does sensor size change DOF?
« on: July 15, 2013, 01:59:12 AM »
Hey guys,
I've read it multiple times now and i really dont understand it. Why does the sensor size affect the DOF and why do FF cameras have a smaller DOF? Seems like a pretty basic question but i really couldn't wrap my head around this concept.

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How (and why) does sensor size change DOF?
« on: July 15, 2013, 01:59:12 AM »

ecka

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Re: How (and why) does sensor size change DOF?
« Reply #1 on: July 15, 2013, 02:33:45 AM »
DoF ~ focal length * aperture * subject distance
If you want to shoot the same picture using the same lens with both FF and crop sensor cameras, you need to be closer to the subject with FF camera to get the same framing and that's the only difference.
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privatebydesign

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Re: How (and why) does sensor size change DOF?
« Reply #2 on: July 15, 2013, 02:39:11 AM »
DoF ~ focal length * aperture * subject distance
If you want to shoot the same picture using the same lens with both FF and crop sensor cameras, you need to be closer to the subject with FF camera to get the same framing and that's the only difference.


That is only part of the story, and there are many ways to tell it........

For a very good explanation look here. http://en.wikipedia.org/wiki/Depth_of_field

Specifically:-
Quote
Relationship of DOF to format size

The comparative DOFs of two different format sizes depend on the conditions of the comparison. The DOF for the smaller format can be either more than or less than that for the larger format. In the discussion that follows, it is assumed that the final images from both formats are the same size, are viewed from the same distance, and are judged with the same circle of confusion criterion. (Derivations of the effects of format size are given under Derivation of the DOF formulas.)

“Same picture” for both formats
When the “same picture” is taken in two different format sizes from the same distance at the same f-number with lenses that give the same angle of view, and the final images (e.g., in prints, or on a projection screen or electronic display) are the same size, DOF is, to a first approximation, inversely proportional to format size (Stroebel 1976, 139). Though commonly used when comparing formats, the approximation is valid only when the subject distance is large in comparison with the focal length of the larger format and small in comparison with the hyperfocal distance of the smaller format.

Moreover, the larger the format size, the longer a lens will need to be to capture the same framing as a smaller format. In motion pictures, for example, a frame with a 12 degree horizontal field of view will require a 50 mm lens on 16 mm film, a 100 mm lens on 35 mm film, and a 250 mm lens on 65 mm film. Conversely, using the same focal length lens with each of these formats will yield a progressively wider image as the film format gets larger: a 50 mm lens has a horizontal field of view of 12 degrees on 16 mm film, 23.6 degrees on 35 mm film, and 55.6 degrees on 65 mm film. Therefore, because the larger formats require longer lenses than the smaller ones, they will accordingly have a smaller depth of field. Compensations in exposure, framing, or subject distance need to be made in order to make one format look like it was filmed in another format.

Same focal length for both formats
Many small-format digital SLR camera systems allow using many of the same lenses on both full-frame and “cropped format” cameras. If, for the same focal length setting, the subject distance is adjusted to provide the same field of view at the subject, at the same f-number and final-image size, the smaller format has greater DOF, as with the “same picture” comparison above. If pictures are taken from the same distance using the same f-number, same focal length, and the final images are the same size, the smaller format has less DOF. If pictures taken from the same subject distance using the same focal length, are given the same enlargement, both final images will have the same DOF. The pictures from the two formats will differ because of the different angles of view. If the larger format is cropped to the captured area of the smaller format, the final images will have the same angle of view, have been given the same enlargement, and have the same DOF.

Same DOF for both formats
In many cases, the DOF is fixed by the requirements of the desired image. For a given DOF and field of view, the required f-number is proportional to the format size. For example, if a 35 mm camera required f/11, a 4×5 camera would require f/45 to give the same DOF. For the same ISO speed, the exposure time on the 4×5 would be sixteen times as long; if the 35 camera required 1/250 second, the 4×5 camera would require 1/15 second. The longer exposure time with the larger camera might result in motion blur, especially with windy conditions, a moving subject, or an unsteady camera.

Adjusting the f-number to the camera format is equivalent to maintaining the same absolute aperture diameter; when set to the same absolute aperture diameters, both formats have the same DOF.

Also here, http://www.josephjamesphotography.com/equivalence/ for a very detailed insight into sensor sizes and their interaction with focal length, dof, aperture and iso. Yes, even iso has a crop factor!
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ecka

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Re: How (and why) does sensor size change DOF?
« Reply #3 on: July 15, 2013, 03:17:20 AM »
DoF ~ focal length * aperture * subject distance
If you want to shoot the same picture using the same lens with both FF and crop sensor cameras, you need to be closer to the subject with FF camera to get the same framing and that's the only difference.


That is only part of the story, and there are many ways to tell it........


Well, for me it is a very simple thing and I gave a simple answer to the question "Why?". You don't need to write a book to answer a simple question, because that's how people get confused.
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tortilla

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Re: How (and why) does sensor size change DOF?
« Reply #4 on: July 15, 2013, 04:08:27 AM »
Why does the sensor size affect the DOF and why do FF cameras have a smaller DOF?

It doesn't. But you use larger focal lengths on FF cameras, and the larger the focal length, the smaller is the DOF.

mik14

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Re: How (and why) does sensor size change DOF?
« Reply #5 on: July 15, 2013, 05:12:07 AM »
That is only part of the story, and there are many ways to tell it........
There's no "story". DOF is a characteristic of a lens and has nothing to do with sensor size. End of story.

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Re: How (and why) does sensor size change DOF?
« Reply #6 on: July 15, 2013, 05:19:17 AM »
There's no "story". DOF is a characteristic of a lens and has nothing to do with sensor size. End of story.

You couldn't be more wrong. Perhaps you're stuck in the circle of confusion...   ::)
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Re: How (and why) does sensor size change DOF?
« Reply #6 on: July 15, 2013, 05:19:17 AM »

rs

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Re: How (and why) does sensor size change DOF?
« Reply #7 on: July 15, 2013, 05:50:57 AM »
Another way of looking at it is to exaggerate and think of a very small sensor camera (iPhone) vs very large sensor camera (medium format). If both are equipped with a lens to give the same angle of view, then other than scale, they do pretty much the same thing.

Image a scene with a subject at about 2m and a background at about 10m away.

The phone sees that scene in front of it as being relatively massive - almost like how an insect would see it - so anything beyond about a metre is effectively infinity, meaning its pretty much all in focus. Put that same scene in front of the medium format camera, and everything is much smaller relatively speaking to the camera - so none of it gets close to infinity (more like a macro shot), so the depth of field is much narrower.

It's a little bit like taking a photo of a really scaled down version of that scene in macro mode with the iPhone.

Obviously the differences are much less when comparing FF and APS-C, but the differences are still there. 
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badgerpiper

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Re: How (and why) does sensor size change DOF?
« Reply #8 on: July 15, 2013, 05:51:33 AM »
There's no "story". DOF is a characteristic of a lens and has nothing to do with sensor size. End of story.

You couldn't be more wrong. Perhaps you're stuck in the circle of confusion...   ::)

If you take two pictures at the same position with the same lens on both a crop and a full frame, the distance between the nearest in focus object and the furthest away in focus object will be the same, so the depth of field does not change. Obviously the image captured will be different on each.

However, if you're talking about the depth of field for an equivalent picture, you'd have to move the full frame camera closer which would result in a reduced depth of field.

Grumbaki

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Re: How (and why) does sensor size change DOF?
« Reply #9 on: July 15, 2013, 06:08:43 AM »
Let's try to help

DoF ~ focal length * aperture * subject distance

in this "equation",

Focal lenght =
- on FF, just the number you see on the lens/EXIF
- on ASP-C/H, number of the EXIF * 1,6/1,3

thus just changing the sensor changes the focal lenght and by repercussion the DoF.

I can't find a simpler way to put it.

ecka

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Re: How (and why) does sensor size change DOF?
« Reply #10 on: July 15, 2013, 06:32:58 AM »
Let's try to help

DoF ~ focal length * aperture * subject distance

in this "equation",

Focal lenght =
- on FF, just the number you see on the lens/EXIF
- on ASP-C/H, number of the EXIF * 1,6/1,3

thus just changing the sensor changes the focal lenght and by repercussion the DoF.

I can't find a simpler way to put it.

Wrong. The only thing that changes is the field of view. Focal length stays the same.
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GaryJ

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Re: How (and why) does sensor size change DOF?
« Reply #11 on: July 15, 2013, 06:42:22 AM »
Love you guys...... ;D
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Re: How (and why) does sensor size change DOF?
« Reply #12 on: July 15, 2013, 06:50:20 AM »
I don't think it does.  Yes there are people who claim it is greatly affected, but I'm not sure they are right. 

The lens focuses the image onto the sensor.

With full frame, more of the area is captured, but depth of field is unchanged.

http://improvephotography.com//wp-content/uploads/2013/02/how-a-lens-works.jpg

So the corners are improved because they are excluded.  I've heard there is a half stop of depth of field that is changed... and that seems minor enough to be true... but that I'm not sure about.
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Re: How (and why) does sensor size change DOF?
« Reply #12 on: July 15, 2013, 06:50:20 AM »

tdrive

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Re: How (and why) does sensor size change DOF?
« Reply #13 on: July 15, 2013, 07:01:57 AM »
My understanding of it is that compact cameras have a larger Depth Of Field to a full frame because it is dictated by ACTUAL focal length, NOT EQUIVALENT. A 35 mm camera on a standard lens wide open is about 24mm. On a cropped camera it is about 18mm and on a compact somewhere about 6mm
http://dpanswers.com/content/tech_crop.php

privatebydesign

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Re: How (and why) does sensor size change DOF?
« Reply #14 on: July 15, 2013, 07:53:06 AM »
Oh dear,

Anybody that wants to argue the point really needs to read my two links, and get a background understanding on the CoC, circle of confusion.

The first thing you have to do when trying to understand this dof stuff is set a standard for your comparison. If we are going to assume a same sized reproduction, ie 8"x10" print or "fill screen" on the same monitor, and frame the image the same while shooting from the same place, you have to take everything into consideration.

Here is a comparison I did a while ago for another thread much like this one. It is a comparison based on the above criteria. The three images are identical in dof, noise, perspective, angle of view, etc etc.

If you read and understand the quote from Wikipedia I posted you will understand if you change your comparison criteria you will change the dof characteristics.
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Re: How (and why) does sensor size change DOF?
« Reply #14 on: July 15, 2013, 07:53:06 AM »