Contest: Depth of Field & Focal Length
Several times over this past week I’ve been asked the same question about depth of field and how it relates to focal length? I find most photographers have a hard time understanding the relationship between these two. One of the people who asked me this question was a mentor student of mine, a really sharp engineer type. I had a bit of trouble explaining myself so we set up a small demonstration in his living room to prove my point. The problem was I was proven wrong…. or so I thought. So here’s the question:
You’re shooting at 18mm at f/4, then you decide to shoot the same subject with the same magnification (meaning the subject fills the frame in the same way) at 200mm at f/4. What happens to your depth of field?
Here’s what I want you to do. I would like you to leave your answers or your thoughts in the comments below. Don’t Google the answer – that’s cheating. Work it out in your head with what you know about photography or do an experiment of your own! Tell us why or how you came up with your answer. I will then pick a winner from the right answers on Friday and award you with a prize. I haven’t found a prize yet but I think it’ll be nice. If you need clarification of the question let me know and I’ll try to clarify it below. What are you waiting for? Get with it!
UPDATE: I promised you a great prize for the winner – and I always keep my promises. Think Tank Photo has graciously given us an Urban Disguise 40 as the prize for the winner! That is a $175 value and a great bag!
The winner receives this Urban-Disguise 40!
I *think* it becomes deeper because you need to move further away to fill the frame.
DOF will get smaller. No need to google that 🙂
Now, I could be wrong…and generally, DOF does get smaller with a longer lens, but the trick here is to “fill the frame the same way”. So…I think…the DOF actually gets deeper when you switch to 200 because you need to move back so far to match the framing.
Morning Matt. The depth of field remains almost constant with changing focal length. If you change your physical position such that the subject occupies the same area of the frame (in other words, move your feet to compensate for the difference in the angle of view of the two lenses), the depth of field will be approximately identical.
Just to throw a spanner in the works, depth-of-field is perceived and actually doesn’t exist – there’s only a plane of focus, perceived depth-of-field will alter depending on the size of the image. An image printed the size of a postage stamp will have a greater perceived depth-of-field than the same image printed on the side of a building. I know that wasn’t the answer you were looking for but it’s a scientifically accurate one 🙂
Probably.
You are right. So lets keep everything a constant. Same print size for each focal length. You can choose the print size. 😛
I remember a physics class at school where the teacher asked all the kids in my class to go home and measure rain-fall for a week. It took us a while to figure out why we all returned with such different results. It was because the teacher purposely hadn’t specified the size of the aperture for our rain-collecting devices. It was a lesson in the need for consistency when conducting experiments. So there are lots of variables when debating depth-of-field including size of image, visual acuity of the viewer, distance of their eyes from the print, print resolution. I know all of these can be consistent for your experiment but it’s worth knowing that depth-of-field isn’t a hard and fast measurement depending only on focal lengths, subject to camera distance and apertures. But this probably isn’t helping. I’ll get my coat 🙂
Don’t let the door hit you on the way out!
From what I’ve noticed, it *appears* to become more shallow.
Since you’re saying you’ve been proven wrong, I’m starting to wonder… As I recollect from reading and seeing on dps and then experimenting on my own, the depth of field get shallower with a longer lens.
What I’m wondering, is the subject to background distance here, because if that is too shallow, even the longer lens would keep it focus despite compression.
I’ll go with the point that it’s shallower with the longer lens (200mm) unless you’re delving into the hyper-focal distance.
If I recall correctly, while the angle of view and lens-to-subject distance changes perspective, the depth of field remains the same for any given f-stop if subject is at the same magnification….ps – I made sure not to cheat or read other’s comments 😉
I’m with Ian Mylam and Bob Abela here. DoF does not change. I’ve seen the experiment somewhere, and will not try to better Ian’s description.
However, the apparent ‘blur’ in the background will be larger with the telephoto, as it will simply enlarge the equally blurred items more.
So…Thinking I knew the answer I started to post here all confident and such but then I second guessed myself and got out my camera to do a test. Turns out, in my late night rudimentary test, the DOF stays pretty constant. At 24mm (my widest lens at the moment) my DOF was about 2 and a half inches. Second shot at 200mm, again about 2 and a half inches. Now, at the wide end moving away from focus happens very gradually and is much more difficult to determine where the focus/out of focus line is. But at 200mm moving from focus to out of focus is very abrupt and much easier to determine where the focus begins and ends.
Hmmmm… you learn something new everyday.
Assuming focus accuracy, if you fill the frame with a brick wall and nothing else I doubt you will notice any difference 🙂
And assuming you’re shooting with the sensor plain parallel to the plain of the brick wall 🙂
OK folks – Good news. Think Tank Photo has provided me with an Urban Disguise 40 as the prize for the winner! That is a $175 value!
You would end up with a DEEP dof with 200mm @f4, but I believe it would need to be a prime lens. Using my 25-200mm telephoto drastically decreases the ‘in-frame’ scene i would get. This is from trying my hand at deep DOF landscapes and not being at all impressed with the resulting images… If you increase the aperture it narrows the focal length.
The DOF of 200mm become larger & the subject & the background are more visible compare to 18mm.
For eg (just a rough calculation):
18mm f4 = DOF is 1cm
200mm f4 = DOF is 6cm
Without looking it up/trying it I’m going to take a guess that the DOF would remain constant if the subject was the same size in the frame. This is because the get the same FOV from a telephoto you would have to be standing along way away and DOF is effected as much by subject to camera ratio as it is by aperture. I would however imagine the bokeh to be a lot different (if there was any at that range) due to the different lens constructions but this isn’t in the question!
Whatever the books say I did the test and the DOF was shallower with the 200 than the 24mm….
I would say there is another variable in the equation that has not been defined as far as I can see. The size of the subject will make an impact on what you observe. The smaller your subject (and the closer you are to it at the wide angle), the more difference you will see in DOF between the focal lengths, with DOF narrowing as the focal length increases. As the size of the subject increases, you I think you will approach a point where DOF is the same. That’s my hunch, anyway.
Depth of field (DOF) is a function of focal length and aperture, with subject to camera distance another factor. When you consider that for any focal length the ratio of DOF in front of and behind the in-focus subject will always be 1/3 in front of subject and 2/3 behind the subject. To achieve the same subject size in your frame, you’ll have to increase your distance from the subject. The longer lens will “flatten” perspective narrowing your depth of field to only a sliver of sharpness in the plane where you focus your lens, magnifying the point where in focus and out of focus converge in an almost magical juxtaposition of ultra sharp and ultra soft.
We all love dramatic depth of field!!!
I feel so stupid all of a sudden. LOL Congratulations to the winner!
I belive that the DOF will be the same. Since the distance to subject will change.
Depth of field will be perceived as shallower – further away from the subject means the subject covers more of the background. (i think…:)
it is perceived as being a lot more shallow.
The object in picture get closer so think you would lose depth and 3D quality your shot!
Nothing. The depth of field would be the same if the magnification is the same. Depth of field is dependent of magnification not focal length.
Focus distance would increase, so DOF would decrease.
I’d say that it gets shallower as a longer focal length isolates your subject( reduces the amount of bg) taken as well. Well, thats to my personal experience anyways and definitely less distortion and distraction captured! ( oops out of topic LOL )
But then again , you did mention it is of same magnification value, hence you’d be stepping back quite a bit with a 200mm at f/4.. so i would say same if not thicker as focus distance plays a part
I think I read about this somewhere – it’s the same, or close enough to not be able to tell. The DOF with a telephoto is only more shallow if you take the photo from the same position as the wide angle photo. If you back up to get the same field of view, the DOF should appear the same. I think there’s more to it, but to anyone just viewing the photos, it shouldn’t make a big difference.
The DOF depends on focal length, distance and lens aperture.
less focal length (18mm) = more DOF
more focal length (200mm) = less DOF
more aperture (F4) = less DOF
less aperture (F16) = more DOF
less distance (1m) = less DOF
more distance (15m) = more DOF
If you change the lens from 18mm to 200mm you have less DOF.
If you want the same magnification you have to increase your distance so you have more DOF.
Finally the DOF are more or less the same.
Daniele
In order to fill the frame with the 200mm lens with the subject that is equally filled with an 18mm, you would have to back up and increase the distance between you and the subject.
Depth of Field increases as the distance between the subject and the focal plane increases. I don’t know the exact amount without using a calculator or tool to figure it out, but the general answer to your question, at the same f-stop, the DoF would increase as you would have to back up / increase the distance between the camera and subject.
-Scott
18mm tends to be a wide angle lens with a shallow depth of field widening your subject. 200mm is much more narrow lens and it’s going to lengthen and gives a deeper depth of field lengthening your subject. However; this is all dependent on light and shadow which either enhances or negates the effect in a printed photo.
Q: You’re shooting at 18mm at f/4, then you decide to shoot the same subject with the same magnification (meaning the subject fills the frame in the same way) at 200mm at f/4. What happens to your depth of field?
Answer: The DOF will be reduced. Going from 18mm to 200mm will give you:
(1) more bokeh (which is to say it will (a) seperate the subject from the background and (b) reduce background detail))
(2) more compression, which (I think) is more flattering for people as their faces aren’t distorted with the 18mm lens (the fisheye look).
Cheers,
Mike
PS – If someone gives you their point & shoot, taking a snapshot of people together is nicer (my opinion) using the tele end of the lens. Instant camera upgrade, of sorts!
From my understanding DOF should stay the same. A 50 mm at f/2 at 15 ft. should give the same as a 200 mm at f/2 at 60 ft.
Or to line up better with your question make that 18mm at 5 ft vs 200 at 55.55 ft. both at f/4.
“What happens to your depth of field?” Nothing. Same object size, same aperture, same DOF. It’s a rule.
DOF–Stays the same
I can see the issue. I too had a problem trying to explain the DOF issue to a friend who was trying to come to terms with it.
The general perception is that longer lenses tend to five a shallower depth of field and is therefore useful for isolating subjects from the background e.g. sports events. And that wide angle lenses are best for deep depth of field such as when used in landscapes where everything from front to back is in focus. This isn’t a bad perception, but when, as you describe the subject is the same size in the viewfinder for both lenses, the depth of view is to all intense and purposes the same.
Of course this means the 200mm lens will necessitate the photographer being further away from the subject than when shooting with the 18mm lens.
The wider angle lens will probably give you just a smidge more depth of field, we’re talking mm here. However, the difference is probably imperceptible when viewed by the naked eye, but measure the distance by photographing something like the scale on a ruler and you just might see the difference.
Dave
Indeterminate – missing information about distances of each shot.
The DOF will be shallower with a 200mm at 4.0 than with 18mm at 4.0 with the subject remaining at an EQUAL distance.
However, the DOF is shallower as distance to subject is decreased, so in this scenario in which you are moving closer to subject in order to fill frame while shooting with 18mm, the DOF is decreasing as you approach the subject.
Bottom line: without knowing the distances of each subject with each lens, we can’t determine. It is possible at close distances that the 200 will have a greater DOF. The DOF will approach infinity as distance increases – faster for the 15 than the 200 (and even faster the larger the sensor). In practice the DOF will typically be greater with the 18mm.
If keeping subject the same size in frame, the composition effect to be discussed is perspective more than DOF. The 200 will appear to compress the background, thus the desirability of opening up the aperture to blur the background and limit the DOF for subject portraiture. Conversely, wide-angle, stopped-down is preferred for the expansive feel of a landscape shot.
The DOF should say the same. If the subject fills the frame in the same way then the the DOF should not change since you have to move so far back to get the same magnification.
The depth of field will get shallower as the focal length gets longer. Since the subject fills the frame the same way, the distance to the subject is larger with the longer focal length.
Based on my experience, the DOF should get deeper. Shooting portraits is so much easier when using a longer lens, even at wider apertures to keep everything in focus. Did I mention how much I LOVE Think Tank gear?!?!?!
A winner has been announced. Find out by clicking HERE who won.