Wildlife and Depth of Field

[WayneF]

"It is not about wide angle. At typical landscape distances (whatever that means), a general rule of thumb is to focus about 1/3 into the scene. But at close distances, its more like 1/2 way. This varies with distance of course, but it won't be far wrong."

Isn't that what that chart is showing?

I thought the diagrams were excellent, and shows the point very well. There are the three factors, and of course, they can combine and interact.

The one little thing I didn't like was the middle one with subject distance, seeming to show the subject at the back of the distant zone. The problem of course is that infinity is only a couple of feet beyond 20 feet (does not show the rear zone, but it was linear up to that point). And rotating focus rings on lens barrels are marked that way too, but somehow it seems to miss something right at that one spot.

But overall, very good charts.

 

[kluisi]

Now that makes a lot of sense, if we think of changing distance or focal length as magnification, the topic of DOF comes into focus (no pun intended!!)

I don't believe you. I think you intended that pun very much.

 

[STM]

Focal length does not affect DOF

Good tutorial here:-

Understanding Depth of Field in Photography

Actually it does, and to a great extent. Imagine taking a photo of a woman. You start with a 28mm lens @ f/5.6 and set the distance to fill the frame. Now, put on a 180mm lens, also at f/5.6 and step back until she fills the frame the same amount. You will see a MARKED difference in depth of field. Although it seems to be left off pretty much most lenses nowadays, compare the depth of field scales on two lenses of widely varying focal lengths for the same aperture. You will see big difference.

 

[WayneF]

Actually, if you step back to fill the frame the same, what you are comparing is perspective. Portraits ought to always stand back at least maybe six or seven feet.

Here is one: Stephen Eastwood|Beauty and Fashion Photographer | Tutorials

If you adjust distance to fill the frame the same with the subjects head, sure, the background might still show differences, because it was the head, not the background, that was adjusted to be same magnification size.

There are a lot of DOF demonstrations on the web, and I'd guess most are correct (except Cambridge is full of it sometimes. )

Everyone knows about the three factors. This has been known for at least 100 years.

 

[Revet]

So maybe to make everyone happy, it might be better to state that DOF is largely influenced by two factors, 1) Aperture, and 2) Magnification. Magnification then is affected by focal length, distance, and sensor size.

 

[Eyelight]

Warning: This post uses numbers and math and I don't always 'splain things the best. It could have an adverse effect on life in general. Turn back now or proceed at your own risk.

Been playing with the numbers some more, just to see, because quite frankly, this is one of those things that is difficult to visualize without some kind of picture aid. So using @STM 's idea from above, I created the chart below that compares Total DOF as well as the front portion and rear portion using f/5.6 and 28mm focal length at a focus distance of 1 meter and then calculating a focus distance to achieve the same magnification at a focal length of 180mm. Plotted interim focal length/distance combinations to add to the visual aid.

Based on a DX sensor size, the total DOF at a focal length of 28mm is 286.16mm and the DOF at a focal length of 180mm (at roughly the same magnification) is 280.67mm for a difference of 5.49mm.

Being fair to @STM if a shorter focal length were stated like 20mm, the difference in DOF jumps to roughly 45mm and at a focal length of 10mm the difference jumps to roughly 245mm. So, even at the same magnification, moving toward ultra-wide angle increases the DOF. Also, the difference in DOFs will increase if the starting focus distance is increased; if the subject size is larger, the DOF's will have a greater difference.

Caution: I'm kind of lazy when it comes to triple checking numbers and I am relying on equations from the webernet, so would advise anyone to whip out their DOF calculator or table and compare the results to the table below the chart. Better yet, use a camera and zoom to make a real life comparison.

DOF in Front means the depth of field that extends toward the camera from the focus point.

DOF in Rear means the depth of field that extends away from the camera behind the focus point.



Here are the calculated values that generated the chart lines.

Focal Length	28	47	66	85	104	123	142	161	180
Depth of Field	286.16	282.51	281.53	281.13	280.93	280.82	280.75	280.70	280.67
DOF in front	123.01	129.45	132.39	134.07	135.16	135.93	136.49	136.93	137.27
DOF in Rear	163.15	153.06	149.14	147.06	145.77	144.89	144.26	143.77	143.40

 

[wornish]

Thanks Eyelight for this
Is this where I am supposed to say "told you"
but ... as with all things at the extremes yes DOF does change with focal length but in most normal use cases it makes no difference as long as the subject fills the same percentage of the frame !

You can disagree, but the math speaks for itself, its physics.

 

[Silven]

As I was taking photos for this months challenge, Birds, and I got a little confused on Depth of Field. I have read and understand the 3 things that affect DOF (aperture, distance, focal length) but as I was putting this into action photographing the birds, I found out I didn't understand it as much as I thought.

Here is what happened, I set my camera up with tripod, speed light, and remote trigger. I started at a zoom of about 200 to stay back from the birds at my feeder. I got some great photos but I wanted to increase the DOF. I couldn't use a smaller aperture since I already was using a high ISO and max flash (already increased to +3), so i decided to put my depth of field knowledge to use. I said to myself, if I use a smaller focal length (ie. 70 mm) I will increase my DOF. But in order to get the same field of view, I would have to move in closer (less DOF!!!!). Oh No!!!

I could do some experiments but I am lazy and I don't have a DOF preview on my Nikon D3100 (and I don't know the distances I used to plug into a DOF calculator). In general, which is going to give me more DOF with the same field of view, 1) further back with zoom, or 2) closer with a wider angle (assuming same f/stop, etc).

Closer with wide angle.

 

[Revet]

Thanks for doing the math for me Eyelight. When I originally posted this, I wanted to do exactly what you did with a DOF chart but I had no clue how to make sure the subject remained the same %age in the field of view without setting up the camera outside and using a tape measure.

 

[Revet]

Here is from Depth of Field in Wikipedia:

[h=2]Factors affecting depth of field[edit][/h]

A 35 mm lens set to f/11. The depth-of-field scale (top) indicates that a subject which is anywhere between 1 and 2 meters in front of the camera will be rendered acceptably sharp. If the aperture were set to f/22 instead, everything from just over 0.7 meters almost to infinity would appear to be in focus.​

Out-of-focus highlights have the shape of the lens aperture.​

Several other factors, such as subject matter, movement, camera-to-subject distance, lens focal length, selected lens f-number, format size, and circle of confusion criteria also influence when a given defocus becomes noticeable. The combination of focal length, subject distance, and format size defines magnification at the film / sensor plane.
DOF is determined by subject magnification at the film / sensor plane and the selected lens aperture or f-number. For a given f-number, increasing the magnification, either by moving closer to the subject or using a lens of greater focal length, decreases the DOF; decreasing magnification increases DOF. For a given subject magnification, increasing the f-number (decreasing the aperture diameter) increases the DOF; decreasing f-number decreases DOF.

I think it clearly states that DOF is affected by two things; 1) f-stop, and 2) Magnification (which is affected by sensor size, lens focal length, and distance to subject). Thus to the photographer who wants to blur a background, if you zoom in on a subject or get closer (without changing f stop in either case) you will decrease your depth of field (and blurring your background) because of magnification.