[W.j.christy]

I have been bothered for some time about the depth of field of a lens for aps-c and have been doing some research on the subject. What I have come up with is of course, the longer the lens, greater the aperture, or closer to the subject you get the shallower the depth of field.

I get the larger aperture and closer to the subject bit, what I don't understand is why a longer focal length can create shallower depths of field more easily that wider. I honestly thought that is was the reverse due to the distance from the subject being shorter in order to maintain the same field of view (i know the angle will stay the same or it should for a given focal length) .From my readings the reason for this is how the light is bent for the shorter focal lengths that cause it to create fairly large depth of fields where as the light is straighter going into longer focal lengths and this somehow corresponds to shallower depths of field.

My question is, can someone explain this to me on perhaps a 9th grade physics level so i can understand what is actually causing the shallow depth of field in the longer lenses?

[GUB]

I have been bothered for some time about the depth of field of a lens for aps-c and have been doing some research on the subject. What I have come up with is of course, the longer the lens, greater the aperture, or closer to the subject you get the shallower the depth of field.

I get the larger aperture and closer to the subject bit, what I don't understand is why a longer focal length can create shallower depths of field more easily that wider. I honestly thought that is was the reverse due to the distance from the subject being shorter in order to maintain the same field of view (i know the angle will stay the same or it should for a given focal length) .From my readings the reason for this is how the light is bent for the shorter focal lengths that cause it to create fairly large depth of fields where as the light is straighter going into longer focal lengths and this somehow corresponds to shallower depths of field.

My question is, can someone explain this to me on perhaps a 9th grade physics level so i can understand what is actually causing the shallow depth of field in the longer lenses?

For any given sensor format - for any given subject magnification (The in focus subject is the same size in your viewfinder) - for any given fstop : then the Depth of Field is the same - that is for all lens.

[zestoi]

I have been bothered for some time about the depth of field of a lens for aps-c and have been doing some research on the subject. What I have come up with is of course, the longer the lens, greater the aperture, or closer to the subject you get the shallower the depth of field.

#2 (greater the aperture) and #3 (closer to subject) are correct.

but... when you shoot with a longer lens (from further away with the same subject size) it "sees" a narrower background, so there's less to fill the width of the background so it blur's out more. usually referred to as "compressing the background" as it appears closer to the subject.

which is why I usually prefer to shoot from further way with my 70-200 at 2.8 than my 24-70 at 2.8.

also consider the distance to subject, verses subject to background.

[GUB]

#2 (greater the aperture) and #3 (closer to subject) are correct.

but... when you shoot with a longer lens (from further away with the same subject size) it "sees" a narrower background, so there's less to fill the width of the background so it blur's out more. usually referred to as "compressing the background" as it appears closer to the subject.

which is why I usually prefer to shoot from further way with my 70-200 at 2.8 than my 24-70 at 2.8.

also consider the distance to subject, verses subject to background.

Ah I have been trying to get my head around that for a while. Now I have just found this link over on a thread in the K1 section . It shows nicely what you are saying.
How much blur? - A visual background blur calculator

[zestoi]

Ah I have been trying to get my head around that for a while. Now I have just found this link over on a thread in the K1 section . It shows nicely what you are saying.
How much blur? - A visual background blur calculator

ah... yep. this is the chap "A shorter focal length will have a wider field of view, and therefore it is likely that there will be more objects in the background at a closer distance."

if you have a zoom lens with a fixed maximum aperture try shooting with the subject the same size in your viewfinder from different distances away at the same aperture and it makes sense

for example the background looks so much more blurred out shooting at 200mm verses 70mm both at f2.8. it's not really... there's just less to stretch to the background width. so it is

so... which also seems to cause a lot of confusion. from the same distance on my cropped sensor k3 at f2.8 i can shoot a model at 135mm and at about 200mm on my full frame k1. so the k1 photo has a better blurred out background. even tho f2.8 is f2.8 no matter what the sensor size is. (not getting into ISO performance since that's another one, since the full frame sensor just collects more light (T-stops)).

[Alex645]

My question is, can someone explain this to me on perhaps a 9th grade physics level so i can understand what is actually causing the shallow depth of field in the longer lenses?

Excellent question. My best answer is that longer focal lengths create larger circles of confusion beyond the nodal point.

Have you ever made a pinhole camera? One that is a cube of equal height, width, and distance from pinhole to paper, creates a normal perspective. One that is less deep as it is wide and tall creates a wide angle effect as the image circle is smaller on the paper, thus a wider FOV. The worst or least sharpest pinhole camera emulate a telephoto lens as they are deeper than they are tall and wide.

The circle of confusion exiting the pinhole aperture expands in size and is larger and thus appears less sharp.

[zestoi]

Excellent question. My best answer is that longer focal lengths create larger circles of confusion beyond the nodal point.

circles of confusion are only part of the answer... shoot a 50mm at f1.4 and a 200mm at f2.8 with the same subject size and compare the backgrounds. not sure I've tested those two exact examples but you see the point...

[GUB]

Okay with the same camera using that link with the 3M x 2M that would put a apsc camera with a 50mm lens about 6 metres from the subject (27degree H FOV). If we used it at f5.6 and then we stepped back 6 metres and used 100mm @ f8 . According to the link a background about 9 metres behind the subject would have the same blur with both these combinations . But we have gained a stop of DoF control on the subject. Of course possibly also a simpler background from the perspective change. It is a useful amount of difference. How is my maths?

[zestoi]

Okay with the same camera using that link with the 3M x 2M that would put a apsc camera with a 50mm lens about 6 metres from the subject (27degree H FOV). If we used it at f5.6 and then we stepped back 6 metres and used 100mm @ f8 . According to the link a background about 9 metres behind the subject would have the same blur with both these combinations . But we have gained a stop of DoF control on the subject. Of course possibly also a simpler background from the perspective change. It is a useful amount of difference. How is my maths?

while I do actually have a degree in mathematics I'm not really sure as at this point way too sleepy plus university was a long time ago...

what I can say is that a simpler (narrower) background due to a perspective change makes a heck of a difference.

this thread has got me really interested though to do some actual tests since these days I have a decent set of lenses to test the theory with and models+locations.

edit: plus I was always bad at number theory so I'd better just take some photos

[HippyHippo]

This site explains it well, if it helps:

Understanding Depth of Field in Photography

As others have said, "Even though telephoto lenses appear to create a much shallower depth of field, this is mainly because they are often used to magnify the subject when one is unable to get closer. If the subject occupies the same fraction of the image (constant magnification) for both a telephoto and a wide angle lens, the total depth of field is virtually* constant with focal length! "

[GUB]

I actually did a set the other day with 28mm (@14M) , 50mm (25M) and 135 at 65M but threw them away because the background changed so much you couldn't make a comparison. Might have another go tomorrow.
Not sure about this telephoto background tending to be simpler though but I do agree that with a telephoto you can have a far higher degree over what your background is by just moving a metre or so.

[zestoi]

I actually did a set the other day with 28mm (@14M) , 50mm (25M) and 135 at 65MM but threw them away because the background changed so much you couldn't make a comparison.

that's entirely the point background compression plays a major part in how the final image looks.

[Penumbra]

You Tube

You Tube

you have a very nice, easy to understand (graphical) explanation.
This gentleman has a few other movies worth watching on the same photography subject.

[beholder3]

I have been bothered for some time about the depth of field of a lens for aps-c and have been doing some research on the subject. What I have come up with is of course, the longer the lens, greater the aperture, or closer to the subject you get the shallower the depth of field. ...

My question is, can someone explain this to me on perhaps a 9th grade physics level so i can understand what is actually causing the shallow depth of field in the longer lenses?

To everyone asking questions I always do suggest to use the following tool/website first and play around extensively:
Bokeh simulator & depth of field calculator
You will be surprised.

Things are not easy as there are so many parameters which can turn a correct assumption into a wrong one.

Example:
Just recently I was wondering if I should have taken a 200/2.8 lens or a 400m F5.6 lens to motocross shooting.
Using both lenses I wanted to frame the drivers exactly the same way: half filling the frame. The x2 focal length difference in this case was no issue, as the site allowed me to get closer or more far away at will.
Both lenses have the exact same background blur of far away background, due to having the same absolute aperture opening.
But the longer lens gives me exactly twice the DoF of the shorter one, so while getting the same background blur I get the whole motorcycle sharp, instead of blurry parts on the shorter lens.

[Lowell Goudge]

I have been bothered for some time about the depth of field of a lens for aps-c and have been doing some research on the subject. What I have come up with is of course, the longer the lens, greater the aperture, or closer to the subject you get the shallower the depth of field.

I get the larger aperture and closer to the subject bit, what I don't understand is why a longer focal length can create shallower depths of field more easily that wider. I honestly thought that is was the reverse due to the distance from the subject being shorter in order to maintain the same field of view (i know the angle will stay the same or it should for a given focal length) .From my readings the reason for this is how the light is bent for the shorter focal lengths that cause it to create fairly large depth of fields where as the light is straighter going into longer focal lengths and this somehow corresponds to shallower depths of field.

My question is, can someone explain this to me on perhaps a 9th grade physics level so i can understand what is actually causing the shallow depth of field in the longer lenses?

the answer can be a little math intensive, but it all comes down to what people call an acceptable degree of sharpness.

the old standard is 1/100 of an inch, when viewing an image on 8 x 10 enlargement. this same definition also applies to camera shake, and image blur for camera shake.

it is referred to as the circle of confusion, and when applied to a point source of light, it refers to the limit where it no longer appears to the unaided eye, viewing from a reasonable distance as a point .

without getting into too much math, think about the difference between a telephoto and a wide angle lens, when you take a photo of a subject, from the same place, using the same aperture with both lenses. the wide angle lens appears to have very high depth of field, only because it magnifies the image less. zoom into or crop to the field of view of the telephoto, and then look at the two images,

aside from perhaps loss of definition in the wide angle image due to lens and camera resolution, you should see the same subject with the same depth of field.

depth of field as it applies to image sharpness is all about magnification.

it does not care whether the magnification is between lens and sensor, or in the final enlargement of the image, although where this magnification takes place is of interest in terms of the bokeh (the subjective quality of the out of focus area) a simple way to evaluate this is to simply take a shot using a wide angle lens and zoom in on your monitor. ghe bigger the enlargement the less sharp things look

i hope this truly answers the question