Camera Nerd
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Okay, you know how increasing focal length increases depth of field. Well anyway, I was wondering, is it the full frame equivalent or the actual focal length that matters.
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DOF questoin
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baloo_buc
Forum Pro
It the other way around. Increasing focal length makes DOF shallower at the same focus distance. You use the real focal lengths and the circle of confusion appropriate for your sensor size and print size.
For you Panasonic camera (four thirds) the circle of confusion for 8x12” (20x30 cm) print is 0.0144 mm.
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Victor
Bucuresti, Romania
For you Panasonic camera (four thirds) the circle of confusion for 8x12” (20x30 cm) print is 0.0144 mm.
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Victor
Bucuresti, Romania
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Solution
Confused of Malvern
Senior Member
Increasing the focal length doesn't increase the depth of field - it reduces it! At any given aperture and focus distance, there will always be more DOF with a shorter focal length than with a longer lens
Did you mean increasing the 'focus distance' rather than focal length? That wouldn't be entirely true either. For a fixed focal length and aperture, it is true that at close range there is more DOF when focussed at 10ft rather than 3ft, but at medium/long range once you start focussing at a distance beyond he hyper focal point then the DOF reduces.
There are various apps/depth of field guides you can download that will demonstrate the actual DOF for any given focal length, aperture and focus distance which you might find useful (although I don't personally use one becuase I'm no good at visualising how far 9.4ft is in real life anyway
). Always input the actual focal length of your lens rather than the full frame equivalent but the results are relative to the sensor size, so you have to input that as well.
Mark S Abeln
Forum Pro
Actual focal length is what matters.
Depth of field is approximately equal to:
2 x (subject distance)^2 x (f/stop) x (circle of confusion) / (focal length)^2
And all of the measures have to be in the same units, such as millimeters.
I do not. Increase FL and keep what else fixed? Aperture, f-stop, distance to the subject framing? You cannot keep them all fixed.
You are probably thinking about background blur, not DOF.
Well, as a few people pointed out, increasing focal length *decreases* depth of field. (Assuming we're talking about DOF and not background blur -- DOF is, effectively, the distance between the closest and furthest-away thing that will be acceptably sharp. Anything outside the DOF is blurry.)
So I think the question you might be asking is -- does DOF depend on the physical focal length of the lens or -- well, let's call it the relative FL of the lens? (i.e. an 18mm on an APS-C is roughly equivalent to a 27mm on FF.)
If all my assumptions are correct...
A DOF calculator like this one can answer this question.
Physical:
According to the calculator, if we take a 35mm lens at f/4, mount it to an APS-C Sony a6000, and focus at 2 feet, the DOF is 0.15 feet. On a full frame A9 it's 0.23 feet.
Relative:
If we take an 18mm lens at f/4 on the Sony 6000, subject distance 2 feet, DOF is 0.6 feet. If we mount an equivalent (27mm) lens on an A9, all other settings the same, DOF is 0.39 feet.
So either way, the answer, I guess, is "both".
Let's hope I'm doing this right and not making a complete fool of myself.
Aaron
Brian Kimball
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Honestly this is one of those times I wish DPReview was more like reddit, and more like itself in late 2012:
bobn2
Forum Pro
Everything that determines depth of field actually happens in front of the camera, ho the rays of light make their way from various parts of the subject to the aperture of the lens. So in fact focal length as of itself has nothing to do with depth of field, what matters is subject distance, aperture size, angle of view and the proportionate blur, relative to the whole picture, that you will accept as 'sharp'. It's unusual to do depth of field calculations using just these, but it can be done. More commonly it's done from the point of view of what's going on inside the camera, which actually results in finding out the 'depth of focus' which is then translated to depth of field. Doing it this way, angle of view gets replaced by focal length and frame size, aperture size by f-number, and 'proportionate blur' by the 'circle of confusion'.
So, in short, it's the angle of view that matters, so in turn it is the 'equivalent focal length'. But as many have pointed out, the idea that DOF increases with FL is sort of wrong in the first place. DOF increases with subject distance for the same aperture diameter since all the angles narrow out, and you'll usually use longer focal lengths for longer subject distances to narrow the angle of view, but that in turn reduces the absolute amount of acceptable blur.
bobn2
Forum Pro
I don't think it would be helpful or charitable to downvote a beginner who comes on these forums asking for help and makes a mistake. That maybe one reason why the thumbs down was removed.Honestly this is one of those times I wish DPReview was more like reddit, and more like itself in late 2012:
--
Is it always wrong
for one to have the hots for
Comrade Kim Yo Jong?
Michael Fryd
Forum Pro
The shooting factors that affect Depth of Field are angle of view, subject distance, and aperture diameter.
Given the same subject distance, you will get the same Depth of Field from:
- a 2X crop body, 24° angle of view (50mm lens), and a 25 mm aperture diameter (f/2)
- a full frame body, 24° angle of view (100mm lens), and a 25 mm aperture diameter (f/4)
Assuming the same shutter speed and recent models of camera, both of the above will produce images with similar noise levels.
On a car forum this question would sound something like: okay, you know how with a bigger tank you can travel shorter distances. Is it the tank capacity in liters or gallons that matters?
What problem are you actually trying to solve?
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Yes! Knowledge.
C
Christof21
Guest
This is impossible to answer this question, you can write the dof equations either with focal length or equivalent focal length.
I personally prefer to consider that it depends on equivalent focal length as Bobn2 explained because it is directly related to angle of view, which is a direct factor of dof.
photolando
Veteran Member
No. Neither does decreasing focal length. At least if you fill the the subject exactly with each lens. Meaning backing up or getting closer to match.
"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! This would of course require you to either get much closer with a wide angle lens or much farther with a telephoto lens". taken from Cambridgeincolour.com
Some of the disagreement in this thread may be a product of how folks use language.
For example, if we imagine a scenario in which a photographer is using a camera with a 70-200mm zoom to make a portrait. Standing in the same spot with their subject a few steps distant, the photographer may choose a combination of focal length and f-stop of 70mm, f/2.8, and make a photo. The photographer may then zoom in to a 200mm focal length keeping the f/2.8 f-stop and make a second photo. Let's compare:
Nikon D610 w/ Tamron 70-200mm f/2.8 Di VC USD at 70mm, f/2.8
Nikon D610 w/ Tamron 70-200mm f/2.8 Di VC USD at 200mm, f/2.8
As we compare the above two photos, let's keep in mind the difference between bokeh and depth of field. Bokeh is the quality of the out-of-focus specular highlights. Is there a visible "onion ring" effect within the broken balls - the defocused glints of sunlight reflecting off the Christmas ornaments - or are the bokeh balls evenly illuminated across their disks? Depth of field is the distance in front and to the rear of the subject within which focus is sharp enough that a physical element within that range is perceived as being well-focused.
Looking at the second photo, the photographer may say, "The size (diameter) of the bokeh balls relative to the hula dancer toy grew larger at 200mm, f/2.8. I kept aperture constant at f/2.8 in the two photos. All that changed was an increase of focal length from 70mm to 200mm. Therefore, increasing focal length makes depth of field more shallow."
I want to focus on the use of the term, aperture, in the above. It's commonly used as a reference to f-stop. In a casual setting, as long as there is an implicit agreement to use the term to describe the f-stop used to make a photo, that's all well and good. However, the more formal meaning of the term, aperture, is an opening. In photography, aperture describes the virtual entrance pupil of a lens. It's the diameter of the virtual opening through which light passes while en route to the sensor.
One way of evaluating depth of field is to compare the size of the bokeh balls relative to the size of the subject. If the bokeh balls appear larger in size with respect to the subject, we perceive that as a photo having a shallower depth of field. If the size of the bokeh balls is smaller with respect to the subject, we perceive that as a deeper or greater depth of field.
In the above photos, the bokeh balls are smaller with respect to the hula dancer toy in the 70mm photo than they are with respect to the toy in the 200mm photo. Also, the degree to which the folding tables are blurred is greater in the 200mm photo than the amount of blur they display in the 70mm photo. Visually, we interpret these differences in appearance as being the product of a difference in depth of field. Depth of field in the 200mm photo is shallower than depth of field in the 70mm photo.
Our hypothetical photographer attributes this difference in appearance to the change in focal length. After all, the camera hasn't moved. The subject hasn't moved. Aperture is f/2.8 in both photos. Only focal length changed. Therefore, that must be the central factor affecting depth of field. Unfortunately, that understanding is incorrect and it's the casual use of the term, aperture, that produces the misunderstanding.
The f-stop of a lens describes a focal ratio. This is the ratio of lens focal length to the virtual entrance pupil diameter. For example, a 70mm, f/2.8 lens has a virtual entrance pupil diameter of (70/2.8=25) 25mm. A 200mm, f/2.8 lens has a virtual entrance pupil diameter of (200/2.8=71.4) about 71mm.
It is this increase in aperture diameter that produces the perception of a shallower depth of field in the 200mm photo. To test this, let's take a look at this photo:
Nikon D610 w/ Tamron 70-200mm f/2.8 Di VC USD at 200mm, f/8.0
This photo was made with the same camera and lens as the first two photos. The camera & lens are mounted to a tripod in a fixed position. The tray tables, hula dancer toy, and ornaments are all in the same physical location with respect to each other in the three photos. While the f-stop of the lens in this third photo is different (f/8.0), the virtual entrance pupil diameter (200/8=25) of 25mm is the same as was used to make the first photo. The same aperture diameter at the same distance from the subject produces a perception that the two photos - despite the very different angles of view - have similar or same depths of field.
Because depth of field is a perceived quality of a photo, we need to allow room for folks to have different perceptions of depth of field in photos presenting subjects at very different image scales. That acknowledged if we asked a random group of folks to compare the depths of field in these three photos. most folks would describe the 2nd photo as having a very different, much shallower depth of field than the other two. Most people would also describe the first and third photos as having the same or similar depths of field.
The primary factor producing these perceptions is not focal length. It's aperture diameter - the size of the lens entrance pupil - that produces the change in perceived depth of field.
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Bill Ferris Photography
Flagstaff, AZ
SmilerGrogan
Senior Member
Neither matters very much. Depth of field is determined by the distance between you, your subject, and the background. You can get very shallow depth of field with an iPhone's 4.5mm f/1.8 lens and very deep depth of field with an 800mm f/5.6.
You use shorter or longer lenses depending on how small or large you want your subject to appear in the frame. You control depth of field by moving your subject closer or further from the background and by moving your position.
Some people say you can control DOF using your aperture, but that's poor, lazy technique because you're letting a decision about depth of field override more important aspects of the picture—namely sharpness and exposure.
You use shorter or longer lenses depending on how small or large you want your subject to appear in the frame. You control depth of field by moving your subject closer or further from the background and by moving your position.
Some people say you can control DOF using your aperture, but that's poor, lazy technique because you're letting a decision about depth of field override more important aspects of the picture—namely sharpness and exposure.
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bobn2
Forum Pro
...plus a few other things.
bobn2
Forum Pro
Great post.Some of the disagreement in this thread may be a product of how folks use language.
For example, if we imagine a scenario in which a photographer is using a camera with a 70-200mm zoom to make a portrait. Standing in the same spot with their subject a few steps distant, the photographer may choose a combination of focal length and f-stop of 70mm, f/2.8, and make a photo. The photographer may then zoom in to a 200mm focal length keeping the f/2.8 f-stop and make a second photo. Let's compare:
Nikon D610 w/ Tamron 70-200mm f/2.8 Di VC USD at 70mm, f/2.8
Nikon D610 w/ Tamron 70-200mm f/2.8 Di VC USD at 200mm, f/2.8
As we compare the above two photos, let's keep in mind the difference between bokeh and depth of field. Bokeh is the quality of the out-of-focus specular highlights. Is there a visible "onion ring" effect within the broken balls - the defocused glints of sunlight reflecting off the Christmas ornaments - or are the bokeh balls evenly illuminated across their disks? Depth of field is the distance in front and to the rear of the subject within which focus is sharp enough that a physical element within that range is perceived as being well-focused.
Looking at the second photo, the photographer may say, "The size (diameter) of the bokeh balls relative to the hula dancer toy grew larger at 200mm, f/2.8. I kept aperture constant at f/2.8 in the two photos. All that changed was an increase of focal length from 70mm to 200mm. Therefore, increasing focal length makes depth of field more shallow."
I want to focus on the use of the term, aperture, in the above. It's commonly used as a reference to f-stop. In a casual setting, as long as there is an implicit agreement to use the term to describe the f-stop used to make a photo, that's all well and good. However, the more formal meaning of the term, aperture, is an opening. In photography, aperture describes the virtual entrance pupil of a lens. It's the diameter of the virtual opening through which light passes while en route to the sensor.
One way of evaluating depth of field is to compare the size of the bokeh balls relative to the size of the subject. If the bokeh balls appear larger in size with respect to the subject, we perceive that as a photo having a shallower depth of field. If the size of the bokeh balls is smaller with respect to the subject, we perceive that as a deeper or greater depth of field.
In the above photos, the bokeh balls are smaller with respect to the hula dancer toy in the 70mm photo than they are with respect to the toy in the 200mm photo. Also, the degree to which the folding tables are blurred is greater in the 200mm photo than the amount of blur they display in the 70mm photo. Visually, we interpret these differences in appearance as being the product of a difference in depth of field. Depth of field in the 200mm photo is shallower than depth of field in the 70mm photo.
Our hypothetical photographer attributes this difference in appearance to the change in focal length. After all, the camera hasn't moved. The subject hasn't moved. Aperture is f/2.8 in both photos. Only focal length changed. Therefore, that must be the central factor affecting depth of field. Unfortunately, that understanding is incorrect and it's the casual use of the term, aperture, that produces the misunderstanding.
The f-stop of a lens describes a focal ratio. This is the ratio of lens focal length to the virtual entrance pupil diameter. For example, a 70mm, f/2.8 lens has a virtual entrance pupil diameter of (70/2.8=25) 25mm. A 200mm, f/2.8 lens has a virtual entrance pupil diameter of (200/2.8=71.4) about 71mm.
It is this increase in aperture diameter that produces the perception of a shallower depth of field in the 200mm photo. To test this, let's take a look at this photo:
Nikon D610 w/ Tamron 70-200mm f/2.8 Di VC USD at 200mm, f/8.0
This photo was made with the same camera and lens as the first two photos. The camera & lens are mounted to a tripod in a fixed position. The tray tables, hula dancer toy, and ornaments are all in the same physical location with respect to each other in the three photos. While the f-stop of the lens in this third photo is different (f/8.0), the virtual entrance pupil diameter (200/8=25) of 25mm is the same as was used to make the first photo. The same aperture diameter at the same distance from the subject produces a perception that the two photos - despite the very different angles of view - have similar or same depths of field.
Because depth of field is a perceived quality of a photo, we need to allow room for folks to have different perceptions of depth of field in photos presenting subjects at very different image scales. That acknowledged if we asked a random group of folks to compare the depths of field in these three photos. most folks would describe the 2nd photo as having a very different, much shallower depth of field than the other two. Most people would also describe the first and third photos as having the same or similar depths of field.
The primary factor producing these perceptions is not focal length. It's aperture diameter - the size of the lens entrance pupil - that produces the change in perceived depth of field.
--
Is it always wrong
for one to have the hots for
Comrade Kim Yo Jong?
C
Christof21
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