Correct DoF control thinking?

The way I'm seeing DoF control now:

To get the same image (i.e. same field of view at the plane of focus), the only way to change (want to increase) the DoF is with aperture.

If I move back I have to change the focal length to get the same image at the plane of focus and the DoF is back where it started. Correct?

If I move back and don't change the focal length, I have to crop it and enlarge it and end up with the same DoF. Correct?

If I use a smaller sensor (and move back), I have to enlarge it and end up with the same DoF. Correct?

Terry

DOF is a function of focal length, aperture, and subject distance. Alter any one and the DOF will automatically be affected.

As for moving back and cropping (either by post processing or with a smaller sensor camera), no the DOF will be vastly different. Imagine shooting on the sidelines of a football game with a 50mm while the person next to you has a 500mm. At that distance, your subject will be in focus as will the other sideline, bench, and fans so your cropped images will seem to have an extremely deep DOF. Meanwhile, the fellow with the 500mm doesn't crop any and has an extremely shallow DOF image - even when you both used the same aperture.

Now if you walked onto the field with your 50mm to get the same uncropped subject size as the person did with the 500mm at the same aperture, your actual DOF might be similar but it will still look deeper because the compression effect of the longer telephoto accentuates the OOF foreground and background.

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Originally Posted by Loupey

As for moving back and cropping (either by post processing or with a smaller sensor camera), no the DOF will be vastly different. Imagine shooting on the sidelines of a football game with a 50mm while the person next to you has a 500mm. At that distance, your subject will be in focus as will the other sideline, bench, and fans so your cropped images will seem to have an extremely deep DOF. Meanwhile, the fellow with the 500mm doesn't crop any and has an extremely shallow DOF image - even when you both used the same aperture.

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But when you enlarge the cropped 50mm image (a lot!) to get the same size print, won't the visible cut off points in image sharpness (DoF) be right back to where it was with the 500mm?

Terry

Here's a quick example (I have lots of different examples :p ).

Both shot from the same location one with 50mm and one with 300mm. Both at f/11.

See the cropped image of the 50mm shot and note the DOF verses the uncropped image of the 300mm with the same FOV.

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Originally Posted by Loupey

Here's a quick example (I have lots of different examples :p ).

Both shot from the same location one with 50mm and one with 300mm. Both at f/11.

See the cropped image of the 50mm shot and note the DOF verses the uncropped image of the 300mm with the same FOV.

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Just to let you know I haven’t given up on this (probably just pig headed :) ), but I haven't yet found a good location to show what I mean. Your two photos don't address the issue to me. The 50mm isn't blown up to the same size and is so blurry I can't tell where the DoF is anyway. The fence in the 300 shot is more blurry, but that's perspective, not DoF. DoF is that point (or plane) where the image goes from perceived as in focus to perceived as OOF. I still think the two points will be in the same place if the 50 is blown up so that the FoV at the plane of focus is the same for both (assuming the focus for both was on the cat tail). – Terry

Here is that same 50mm shot but just the cropped part.

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Originally Posted by OldClicker

The fence in the 300 shot is more blurry, but that's perspective, not DoF. DoF is that point (or plane) where the image goes from perceived as in focus to perceived as OOF. I still think the two points will be in the same place if the 50 is blown up so that the FoV at the plane of focus is the same for both (assuming the focus for both was on the cat tail). – Terry

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Yes, the 300mm shot will have the shallower DOF when shooting from the same distance and the same aperture. The perspective did not change (see how the grid of the fence overlays the background reeds in exactly the same spot in both images) because I did not change the shooting position.

Once you have taken the photo, the DOF of that photo will not change when you crop or enlarge the image regardless of any cropping or editing.

DOF will change due to focal length and F stop. But if you have the same image (same size in view finder) on two different lens the positions of the camera will be different but the DOF will be the same if each lens has the same F stop setting.

My understanding is that DoF is the plane at which it goes from in focus to OOF when a certain size print (traditionally) is viewed a certain distance, correct? It is not something that is inherant in the system, it's totally dependent on how it is viewed. If you move closer to that print the DoF is less, just as if you enlarged it. Is this wrong? - Terry

I think I see where you are going.

when you are closer or farther from a print, the perceived depth of field does change. Same reason why you have more perceived depth of field with a crop sensor - the image didn't change, just the viewing distance. (I'm pretty sure the only way to measure viewing distance is in terms of magnification)

Right?

however, that's dependent upon the native f/stop and magnification of the image too. It's not the image itself that changes, just your perception.

An out of focus image can only come into focus so much; loupey's fence will never be in focus at any distance because the information isn't there. And of course if you are close enough, the whole image will be out of focus and you will be looking only at individual pixels. This is the same reasoning behind focal length not being related to depth of field, but rather distance to camera (measured in magnification) being the other factor in DoF control.

To achieve identical depth of field, the magnification needs to be relative. The same object, viewed the same size in print, will have similar depth of field from any lens assuming the same magnification and f/stop. Proximity to the camera will change greatly, but so long as magnification and f/stop are the same, the differences between lenses are not great enough to effect the depth of field in a perceptible way. Likewise, the depth of field of things perceived with the human eye are dependent upon magnification.

Where I think Loupey is going; depth of field falls off sharply with longer focal lengths, which is why we can use them to isolate subjects - but that's again related to magnification (of the background) and the myth of "telephoto compression."

yes?

I think I understand Terry, but when you start talking about distance from a print, you're talking about visual acuity.

So let me explain what I've learned about visual acuity, and see if it helps. MY particular reason for learning this was try to determine a formula for minimum acceptable resolution for a print, given a particular viewing distance:

The generally accepted definition of normal (20/20) visual acuity is the ability to resolve a spatial pattern whose features are separated by one minute of arc, or 1/60th of a degree. This number comes directly from the retina. The lens of the eye projects one degree of the scene across 288 microns of the retina

In the area of the retina where photoreceptors are most tightly packed (the fovea), a linear 288 microns contain about 120 photoreceptors. So if more than 120 alternating black and white lines (60 cycles) are projected onto this area, someone with normal visual acuity will see a solid gray mass.

This is exactly the phenomenon that printers take advantage of, allowing us to perceive continuous tone from separate dots of ink.

But if I'm reading you right, it also speaks to what you're talking about- how the sharpness of an image is dependent upon how it is viewed. I'm not sure exactly how that works into your solution, except to say that if you stood farther away from the subject, the only way to compensate for the visual acuity would be to magnify the view so that those 60 cycles are spread across the same area of the scene. Does that make sense?

Or am I just way off base again?:p

- Joe U.

Is Circle of Confusion the missing term here? See this article in Wikipedia.

http://en.wikipedia.org/wiki/Circle_of_confusion

It's all explained with formulae and what it means for different camera sensors. Gave me a headache before getting to the end of the second page.

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Originally Posted by Franglais

Is Circle of Confusion the missing term here? See this article in Wikipedia.

http://en.wikipedia.org/wiki/Circle_of_confusion

It's all explained with formulae and what it means for different camera sensors. Gave me a headache before getting to the end of the second page.

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Note that CofC is as, "...perceived by the human eye..." and assumes a print size and viewing distance. DoF calculators start with the CofC and seldom explain that the CofC has already made this assumption. – Terry

This has a good tutorial on DOF, Link: http://www.cambridgeincolour.com/tut...h-of-field.htm