Discussion in 'Beginner Questions' started by jeanie_cottrell|1, Oct 28, 2012.
How is depth of field affected by different lenses?
Depth of field is affected by focal length, aperture, distance to subject, and film or sensor format. It is not affected by the details of lens design (how many elements in what configuration, etc.). So if you have, for example, four different lenses that can shoot at 50mm f/2.8, and you try to shoot exactly the same picture with all of them on the same camera, depth of field will be identical for all of them.
However, the quality of out-of-focus areas may be quite different between lenses. One lens may render very smooth, creamy backgrounds, while another may produce a busier, more nervous look. Also, bright points in out-of-focus regions may be different shapes according to the design of the lens' aperture iris. Any lens shot wide open makes circular bokeh, but stopped down, an iris with five straight blades will produce pentagons, while one with six blades will produce hexagons. Some lenses have curved blades that more closely approximate a circle when stopped down.
Also, here's a good tutorial on depth of field:
It is a bit too simplistic to say focal length affects dof, subject distance is far more important. Yes if you use a shorter focal length from the same place then the same aperture will yield deeper dof, but as always that isn't the whole story.
For example if you use a 50mm at f2.8 and a 200mm at f2.8 but the subject remains the same size in the viewfinder, a common scenario for a portrait for example, (ie you moved back for the longer focal length) then the dof is the same. This is counter intuitive and seems to go against the "telephoto compression" meme, but it is true, your perspective is different but your dof is the same.
It is funny, people say the $5,000+ 200mm f2 gives incredibly narrow dof, well the 50 f1.8 at under $100 actually gives a narrower dof for the same subject size.
It's hardly "simplistic"; it's just an accurate high-level summary without all the complex details. Focal length does affect DOF; anyone who says otherwise doesn't understand the equation. Subject distance also affects DOF. I said that too. Your point, Scott, is simply that the two balance each other such that if the subject is the same size in the frame (that is, if with a longer focal length you increase subject distance enough to compensate) DOF will be more or less the same. This is true, but it's a special case, not in any sense a correction of anything I wrote. It's also somewhat misleading in a different way, because while DOF will be the same, neither the subject nor the background will look the same because you're shooting from two different positions with two different focal lengths.
The original question was simply, "How is depth of field affected by different lenses." The simplest correct answer is just to spell out the different factors involved so that the OP understands that lenses of the same focal length, when shot at the same aperture, focused to the same distance and shot on the same camera (or the same film/sensor format, actually), will give exactly the same DOF regardless of differences in their design. The Cambridge in Colour tutorial fills in all the details about how the different factors relate to each other and even provides a online calculator (which, you will note, asks for focal length, aperture, and subject distance, not subject magnification, which is merely a side effect, not one of the principal variables).
I didn't write that to cause offense or to correct, I actually agreed with you "if you use a shorter focal length from the same place then the same aperture will yield deeper dof, but as always that isn't the whole story".
I merely intended to head off the common misconception that people seem to extrapolate from that information, and assume without thinking, that wide angles always give more dof and longer lenses always give less dof for the same aperture, like I pointed out, there is a very common situation where people do think about this incorrectly. They assume if they take a portrait with a longer lens at the same aperture it will give them less dof. This is a very common real world situation, hardly a special case.
Rather than a technical exposé, for which there is plenty of room and useful links, I was adding a real world practical effect that is often overlooked and misunderstood.
With regards what factors affect dof, in truth, focal length and subject distance are not needed to work it out. DOF is determined by subject magnification and aperture, though coc plays a part too (and that bit covers your sensor size). The reason online dof calculators ask for focal length and subject distance is so they can work out the subject magnification for you as few people know how to do this themselves.
It's the beginners forum guys. Let's keep it as simple as necessary, but no simpler.
For now, it's simple enough to say that different focal lengths do affect the DoF. A quick test with some of the online "DoF Calculator" links will help you get an idea of it. If you get through that and want a more in depth discussion of why and how you can work around that, and the rest of the above ^ , ask at that point.
I'd ask Jeanie to expand on her question and also perhaps put it in some practical context of why she wants to know.
It is a beginners' forum and it is understood that some OPs on this forum might not have the precise words to phrase an exact question: or perhaps it was meant as a general question and the OP is indeed lapping up and understanding the broad range, including the 'in depth' answers.
So, I'll wait for a response from the OP - and also, welcome to Photonet.
[aside - “meme” and “exposé” (completed with the aigu) - nice words!]
I think there is a pretty simple answer for the OP, if you skip some of the fine points:
1. If you stand in the same location and switch lenses, a shorter focal length will give you more depth of field.
2. If you switch lenses but then change position so that the subject takes up the same portion of the frame, the change in focal length will not have much impact on DOF.
This summary omits some details, but it is a reasonable set of guidelines.
A very good explanation of this, along with photos that clearly show it, can be found at http://toothwalker.org/optics/dof.html
I like this reference http://www.trenholm.org/hmmerk/#TIAOOF.
Depth of field varies with the length of the lens and with the subject distance but there is no difference if you maintain the same image size .......... is that crisper?
Thank all for your input. It was what I was looking for and more. I'm new to the photography world and I have many
questions, and you all answered one of my bigger ones. So once again thank you very much. And sorry I have not
responded sooner. I live in the Jersey Shore, and finally I have a little free time, so thank you for your patience. I'm a new
member to photo.net and so for I'm very impressed with fellow users knowledge of everything dealing with photography
and their generosity with sharing that with the other users.
Jesnie, just like William, I am glad we could point you in the right direction.
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