Factors Affecting Depth of Field

spraynpray

@roombarobot: Of course it does. You are magnifying an image so the DoF gets shallower. Just like if you look at an image from further away, the DoF looks deeper.

Paperman

@roombarobot

Why are you refusing to accept what a DoF calculator shows you for same lens/same distance/same lens - different sensor size comparison ?

roombarobot

@roombarobot: Of course it does. You are magnifying an image so the DoF gets shallower. Just like if you look at an image from further away, the DoF looks deeper.

The DoF does not get shallower. The DoF of a picture is the DoF of a picture. It is already set once you take that picture. If you follow that logic, then when you zoom in on a picture on your computer the DoF changes. That's crazy.

roombarobot

@roombarobot

Why are you refusing to accept what a DoF calculator shows you for same lens/same distance/same lens - different sensor size comparison ?

@paperman, because I don't believe that this calculator is being interpreted in the correct manner in this case. I think that the calculations are assuming one is keeping the same field-of-view as well, which would mean that the photographer moves.

If you think that it is true, then please explain to me why the depth of field would change in the simplest case possible, the one I put forth above. I take a picture of a stationary object with my D800E in FX mode. I switch the camera to DX mode and take a second picture without moving or changing the lens or aperture. You are saying that the depth of field changes, I am saying that it won't. Why would it change?

Remember, the actual subject height will be the same, as will the distance to the subject. The lens and aperture will be the same too. The only thing that will change is the field of view, it will be smaller at DX than FX. This is exactly the same as if after I took the FX shot I cropped the image on my computer. That changes the field of view, but not the depth of field, which was fixed when I took the shot. You can't change the inherent depth of field of a shot by cropping it on your computer!

ThatNikonGuy clearly explains it here =
However my thought experiment using the same camera and switching from FX to DX is even simpler.

Paperman

Please read about the subject somewhere first - maybe it will help . This has turned into a discussion where you are defying facts/science/common knowledge. Punch in the numbers in any DoF calculator you think is right and let me know the result.

The 2 images have different DoFs because the object has to cover same area/same size in frame for the DoF to be same - that is the rule.

Regarding ThatNikonGuy video - he is wrong and one wise guy has expressed it in comments. As you said. don't believe everything you see on internet :-)

And if you only trust your eyes , look closely at the images above ... You will see clear cut lines in background on left where the magnified image on right is all blurry in those areas. When you magnify an image, what is a bit blurry becomes a lot blurrier as you are now looking at the enlarged blurriness ! ! Again, what may be acceptably sharp to your eyes may come out to be unacceptable in sharpness when you magnify it - just like the above example.

The best way to see it is by exaggerating the crop . Just crop it to 3x or 5x instead of 1.5x ( since you say sensor size makes no difference ) . What I'm trying to say should be more visible to your eye.

roombarobot

@paperman, I have read a lot about it, and I do understand science, I have a PhD in physics.

Here is an example of a good explanation of where I think many are misunderstanding the situation:

As sensor size increases, the depth of field will decrease for a given aperture (when filling the frame with a subject of the same size and distance). This is because larger sensors require one to get closer to their subject, or to use a longer focal length in order to fill the frame with that subject. This means that one has to use progressively smaller aperture sizes in order to maintain the same depth of field on larger sensors. The following calculator predicts the required aperture and focal length in order to achieve the same depth of field (while maintaining perspective).

http://www.cambridgeincolour.com/tutorials/digital-camera-sensor-size.htm (emphasis mine)

This clearly states that the reason the depth of field often decreases when the sensor size increases is because the photographer needs to move closer to the subject to get the same field of view. If you do not move closer to the subject, depth of field does not change, instead the field of view changes.

Here is another simple and clear example, showing the same result:
http://www.have-camera-will-travel.com/field_reports/full_frame_vs_crop_sensor_-.html
Full frame vs. crop sensor, only changing the sensor size. The depth of field stays the same, but the field of view changes.

Paperman

I'm tired - please write your theory to DoF calculator guys and to those that prepared them as tables during the 50 years at film times ( nothing has changed since then - maybe accepted CoC ) . Let's see if they will adjust their calculators/tables to give same DoF for same lens/same distance/same aperture but different sensor sizes ( MF,FF, APS-C, 1/2.5" etc ... )

As a scientist, you must have more faith in numbers :-)

heartyfisher

LOL .. sorry .. sigh .. discussions like this sometimes make me wonder if my belief in God is actually right 8-> Then it makes me wonder if the atheists are actually right. :-) Then I go grab my camera and go take more photos..

Correlli

@roombarobot: the reason why dof does change when cropping or when switching from FX to DX is that the magnification of the final image changes. You are of course right that the pixel information itself does not change. But you are not printing individual pixels.

Just think of printing a 30 x 40 cm print from your image and viewing this from say 50 cm distance. Your eyes are the limiting factor here. Details that are in the image but that your eyes cannot resolve in that print from that distance are not interesting as they do not give you information (you can't see them). This is what defines the CoC. Details that have the size of the CoC are still regarded as in-focus, details with a larger size are not.

If you now use the same image but cropped (or switching to DX) you need to magnify the image more to get to the same 30 x 40 cm print. This also means that details that were just in-focus on the first print get blown up more and are now slightly out of focus. Hence the dof has changes.

Same thing is true if you post an image on PAD with 640x480 (or so) or if you look at the D800 file at 100%. The PAD image might look much sharper than the 100% magnification.

The problem with digital photography is - as far as I see it - that many think the file they get is the final product. But in reality the final product is the image that you view. Be it a Facebook thumbnail or a poster print. And this determines what details you can see and how you judge the dof.

Hope this helps.

spraynpray

@roombarobot: I don't know if this will help or hinder but no offense meant of course: The CoC is when a point of light becomes a disc so surely you can see that the points that are close to becoming discs actually become discs when you zoom in to an image/walk closer to an object/use a longer lens? Using your eyes, try to see an inch at 100 yds then hold the ruler in your hand - you can easily see it - same thing.

SquamishPhoto

This thread is awful!!!!!!! :]

spraynpray

Well Mike, if you think you know the answers and can lay it out eloquently and convincingly, please join in.

Golf007sd

WestEndBoy

Despite the background, the image is beautiful.

I even notice the difference between my 85 and 135. Perhaps the difference boils down being able to isolate undesirable elements out of the background more easily.

If I am looking for this effect, do you think a 200 is sufficient, or would a 300 or even 400 be required to really achieve this look Pitchblack?

WestEndBoy

I think that there are some really good principles here that could be very useful. For example, under certain ideal situations, it might be possible to achieve similar looks at shorter focal lengths. For example, shooting my 135 at f/4.0 or f/5.6 with a distant enough background to enable the subject to remain in focus. It would seem the trick is finding a background that is not too cluttered, though that might be difficult, particularly if you don't have a wide choice of locations.

And if so, it would stand to reason that what the longer focal length is practically doing is increasing the opportunities to achieve looks similar to this.

Of course, you need enough distance to get far enough away from the subject as the focal length increases.

Thoughts?

tcole1983

+1 at pitch black. I with my 18-200 which would be f5.6 at 200mm and my friends 70-200 f2.8 it isn't the same as 300+. You can get decent results with some bokeh but as pitchblack says it doesn't look the same. I constantly say my 300 f4 has amazing subject isolation and bokeh. If I could control it more it would be the best portrait lens I own.

I don't have portraits, but I concur with what pitchblack said about the subject being in focus...here are my examples with the 300 f4.