How to get maximum depth of field for a given framing

[Joan]

Depth of field (DOF) depends on lens focal distance, lens aperture and distance. If I want to take a picture of a given object, for a given framing and lens aperture (f), the shorter the focal distance of the lens, the biggest the DOF, and the biggest the distance to the object the biggest also the DOF.

However, focal distance goes against distance to object (for a given framing). If I use a short focal lens (this is good for DOF) I will have to reduce the distance to the object (and this is bad for DOF) to keep the desired framing. My question is, to have the maximum DOF, what is better: to use long focal lenses (and increase distance to object) or to use short focal ones (and reduce distance to object) ?

Thanks for your help

[arith]

I think DOF is based on magnification and aperture. If you fill the frame at say 25mm then the DOF will be the same as filling the frame with the same object at say 250mm, it is just the perspective that changes.

Wikipedia

[Graystar]

According to a demonstration found on the following page...
http://www.luminous-landscape.com/tutorials/dof2.shtml

The answer is...neither.

From the LL page above, the blur of the background is more prominent with a long lens than with a short lens, even though the object's blurred appearance is about the same.

This is generally why I slap my head and shake it when I see people advising beginners that to get a more blurred background, use a longer focal length. It doesn't work because the beginner will zoom in and then step back to restore the framing, thereby increasing the distance to the subject. And since they're not going from a 17mm to 400mm, like the test above, the different isn't noticeable. So they come back wondering what they did wrong. They did nothing wrong...it was the advice that was wrong. If you need more background blur for a given framing, then you need a lens with a wider aperture.

[Richard K]

Are there specific lenses that give you better background blur than others

[Inkanyezi]

Often people are confused about depth of field and its relation to bokeh, background blur. They are not the same.

DOF, the region in which the circle of confusion is not unacceptably larger than a sharp "point" where the maximum sharpness lies, is exactly the same at all focal lengths, at the same reproduction scale. Hence DOF will not change when you change focal length and adjust your distance to the subject accordingly. DOF is intimately related to the spatial angle of the lens aperture seen from the subject, the entrance pupil. It remains the same at the same reproduction scale and relative aperture, regardless of focal length at large distances.

Background blur is different. It does indeed depend on focal length, distance and the physical dimension of the entrance pupil. With a longer focal length, your background appears enlarged, compared to a shorter focal length, and with more enlargement comes more blur when it is not within the depth of field. The entrance pupil governs the circle of confusion, and your choice of focal length how much this circle of confusion will be enlarged. Therefore, a large focal length with a large physical aperture will produce more background blur at large distance, i.e. when the distance from the main subject to the background is large compared to the distance from the lens to the subject. A wide angle lens never has a very large entrance pupil. For example a 35/1.4 has an entrance pupil of 25 mm, while a 400/5.6 has an entrance pupil almost three times wider. Hence a long lens can give more blur in the background at long distances. You could also regard it as with adjusted distance to the main subject in focus, the background is more enlarged, so that you see a much smaller portion of it in the image.

[Graystar]

Are there specific lenses that give you better background blur than others

The quality of the blur (or bokeh) depends on the quality of the lens. Aberrations, along with the shape of the aperture, all have an effect on bokeh. If you have the MTF chart for the lens then the tracks of the meridonial and sagital lines will give you an indication of the lens's bokeh. The following page from LL described how to read an MTF chart and mentions bokeh.

http://www.luminous-landscape.com/tu...ding-mtf.shtml

[DanK]

Richard,

It is a confusing topic at first.

First, background blur is different from DOF. There is an excellent tutorial on this site, https://www.cambridgeincolour.com/tu...h-of-field.htm, that shows that as long as you frame the same--that is, vary the distance to the subject with lenses of different focal lengths so that the subject fills the same proportion of the frame--focal length has very little effect on DOF. The common assertion is that wider gives you more DOF, but that is true because when people switch to a wider focal length, they generally do NOT walk forward to keep the framing the same.

Background blur is something different, having to do with how rapidly things go out of focus as the subject is moved outside of the DOF area. As long as you "zoom with your feet" to get the same framing with different focal lengths, you will get more background blur with longer focal lengths. This is shown clearly in the set of images in the URL that Graystar posted. It is also shown clearly in the images at this page, which is perhaps the best explanation of DOF and background blur that I have found on the web.

As for the quality of background blur--now you are getting into the realm of "bokeh." Ask three photographers about bokeh, and you will get four opinions. But in a nutshell, it refers to the quality of the out-of-focus area. For example, for many photos, you might want a very smooth, "creamy" bokeh, like the one in the photo I'll post below. This is a combination of many things, including the amount of background blur, the aperture blades, and other aspects of lens design. For example, folks generally expect that more aperture blades will tend to produce smoother bokeh than fewer, and curved blades may be helpful. If you have a couple of lenses you are comparing for purchase, you can google them to get people's arguments about their bokeh. If you are trying to decide among lenses you already own, just take a bunch of shots and compare.

Dan


smooth bokeh. 50D, Canon 100mm L macro lens, several images stacked with Zerene for greater DOF.

[Mike Buckley]

Welcome to CiC, Joan! You might want to edit your profile to ensure that your location is displayed along with your other information on the left side of each post. That can be very helpful in certain situations for people who are attempting to provide helpful responses.

My question is, to have the maximum DOF, what is better: to use long focal lenses (and increase distance to object) or to use short focal ones (and reduce distance to object) ?

When considering the practicalities of making a photograph, as opposed to the physics of using a lens, I think that might not be the best question to ask. Instead, I think the better question is to ask which distance to the subject will provide the ideal perspective and thus the image that you envision before releasing the shutter. Once you determine the answer to that, you can then determine which focal length will be necessary to achieve your goals.

As an extreme example, you might want to fill the frame with a religious symbol at the top of a church, synagog or mosque while standing on the ground. You also might want to emphasize the symbol's height and you might want to isolate it against the background of a plain blue sky. To do that, you might have to get relatively close to it and beneath it. Otherwise, getting further away from it may allow parts of its building, other buildings or trees to intrude into the background. Positioning yourself farther away also might not adequately emphasize the subject's height. Once you have determined the ideal place to stand, you can then determine the focal length to use that will fill the frame.

My two points are that when considering "framing," the perspective is an issue that also has to be taken into account and that, once you have done that, only then can you determine the ideal focal length.

[Manfred M]

Are there specific lenses that give you better background blur than others

Fast (large aperture) lenses that are shot at or nearly wide open will give you the shallowest depth of field, which will throw the backgrounds nicely out of focus.

The closer you are to your subject the greater your ability to throw the background out of focus. This is why people prefer shooting with longer focal length lenses when they are trying for shallow depth of field shots.

You can enhance this by using depth of field and rather than focusing on the subject, pulling the focal point in front of your subject a bit, which throws the background more out of focus. This is fairly easy to do, but you have to override your autofocus controls and set your focal distance manually. I find some of the older lenses with DoF markings make this easier to do.

To address your question on "better" background blur. Nikon has a 105mm and 135mm DC lens; where the DC stands for "Defocus Control". These allow the photographer to improve the look (smoothness) of the bokeh or out of focus highlights by softening the out of focus areas. This is my primary portrait lens; the in-focus areas are extremely sharp and when using the appropriate setting, the bokeh is silky smooth. You have to use this lens either manually or in aperture priority mode as one has to dial in the DC setting manually, based on the aperture that will be used.

Note: This comment applies only to the Nikon lenses, the Sigma DC identifier refers to their crop-frame sensor specific lenses and has nothing to do with defocus control.

[Joan]

I'm absolutely surprised by the quality and the knowledge level of the replies to my first email in Cambridgeincolour. I asked the same thing in a Canon forum here in Spain and I got no real answer to my question.

Doing some home made tests I arrived to the same conclusion that has been pointed out by arith, Dank, Inkanyezi and Graystar, that is, that DOF does NOT change noticeably when trading lens focal length and distance to subject in order to keep subject framing. I took some pictures of a focus test chart keeping approx. the same framing (measured in the middle of the picture) and the same f, and changing the focal length of the lens as well as the distance to subject.

The results with a 200mm lens can be found here and the one with a 24mm lens can be found here.

As you can check there is no big difference of DOF in the cases I tested.

What it is true, as Mike Buckley and GrumpyDiver say, it is that the perspective of the subject is quite different when using a 24mm or a 200mm lens, for instance.

[Mike Buckley]

What it is true, as Mike Buckley and GrumpyDiver say, it is that the perspective of the subject is quite different when using a 24mm or a 200mm lens, for instance.

That's only true if you are referring to the perspective distortion or the lack of it offered by a particular focal length.

The perspective that I was referring to is not affected by the focal length (other than whatever distortion occurs.) Ignore the use of a lens to understand the perspective that I am referring to. Place a glass on a tabletop near its edge that is closest to you. Position yourself about one meter from the glass. View the glass from a kneeling position that positions your eyes well beneath the tabletop. View the glass from a position that places your eyes at the same height as the table top. Last, stand fully erect and view the glass with your eyes positioned well above the table top. The purpose of that exercise is to demonstrate that the perspective changes because of the changing relationship of the camera to the subject; it has nothing to do with the focal length.

[Joan]

I agree to what you say, but what is undeniable it is that keeping lens f and subject framing, shortening the focal length of the lens makes the subject more distorted, while the opposite makes the subject more "flat". Probably I have expressed myself wrongly. Sorry. I have difficulties with my photography skills and also with my English ;-)

[Inkanyezi]

No te preocupes, todos tenemos dificultades del itioma de distintos grados.

Distortion might not be the best expression for the way an image appears when we use a short focal length. The relative distortion depends on the viewing distance for the image and the resultant angle of view, compared to the angle of view of the lens when the picture was taken. Hence the most natural perspective is often perceived when the image was taken with a normal lens or a short tele lens, in the region between 35 and 50 mm for an APS sensor or 50 to 85 mm for full frame. Also longer lenses "distort", but in a different way, compressing distance, so that medium distance objects may seem pasted upon those farther away.

So when looking for an "undistorted" perspective, maybe lenses in that region are more suitable than either very short or very long focal length. We often find a portrait more pleasing when taken from a distance of between 1½ and 2½ metres than if taken at shorter or longer distance; in fact it is not the focal length that distorts, but the difference of viewing angle when looking at the picture, compared to looking at the scene directly. When those angles coincide, perspective is normal, and we experience no distortion. When you take a portrait from a very short distance, say about 1', with a wide angle lens, the short distance makes the nose appear much larger than if the portrait is taken from farther away. In fact, the wide angle shot gets its natural perspective when viewed from very close-up, although that is not the way we usually look at pictures. It is the same perspective as when you look at the person from that same close distance (with one eye).

[William W]

Depth of field (DOF) depends on lens focal distance, lens aperture and distance.

More correctly Depth of Field depends on:

• Focal length of the Lens used
• Aperture of the Lens used
• Subject Distance (Distance to the Plane of Sharp Focus)
• Camera / Film Format used

Various replies have mentioned that provided the FRAMING and the APERTURE remains the same, the DoF will not change. You have ratified this with your own test.

This is the Axiom of Depth of Field and will hold true for all practical purposes and for most Subject Distances we encounter in General Photography.

The Axiom will not hold true as we approach CLOSE UP work and is not true for MACRO work.

The Axiom becomes mostly irrelevant as we approach the HYPERFOCAL DISTANCE.

***

My question is, to have the maximum DOF, what is better: to use long focal lenses (and increase distance to object) or to use short focal ones (and reduce distance to object) ?

As a result of the replies and your own testing, you now know the answer to this question is “neither”.

However, your original supposition was lacking in that you did not consider all the aspects of DoF and you asked a question based only on three of those aspects.

Therefore - to correctly answer the title question "How to get maximum depth of field for a given framing"

If you wish to attain a Maximum DoF for any given FRAMING of any particular Subject, then you should use the SMALLEST Format Camera which is available.

WW

[Mike Buckley]

I have difficulties with my photography skills and also with my English ;-)

If your photography skills are as good as your English, I really look forward to seeing your images.

[William W]

I agree to what you say, but what is undeniable it is that keeping lens f and subject framing, shortening the focal length of the lens makes the subject more distorted, while the opposite makes the subject more "flat". Probably I have expressed myself wrongly. Sorry. I have difficulties with my photography skills and also with my English ;-)

The correct technical term for the effect of using a Wide Angle Lens close to the Subject (for example making a Portrait looking down on a small child, where the head appears very large and the feet very small) is – FORESHORTENING or FORESHORTENED.

The correct technical term for making a similar shot of the child's face and looking front on to the child, and using a very long telephoto lens is - COMPRESSION or COMPRESSED (aka 'flat').

PERSPECTIVE is also a technical term: and is often bandied around recklessly which eventually causes confusion we need to use the word accurately (as in this thread).

As already pointed out, PERSPECTIVE is dependent upon the Camera’s Viewpoint and Distance relative to the Subject: nothing more and nothing less.

Not necessarily directed as a comment to responses here: but the use of the casual phase ‘perspective distortion’ adds even more confusion.


WW

[DanK]

Joan,

shortening the focal length of the lens makes the subject more distorted, while the opposite makes the subject more "flat".

As a few have said, "distorted" is not the best word, but I think you have the idea correct. Think of two things in your image, one close to you and one far away. A longer lens will compress the perspective, making the two seem close together. A short lens will do the opposite. That is why a very wide angle shot of a face looks distorted: the nose looks big and bulbous. The neutral point, where neither of these happens, is a lens of about 30mm on a crop sensor camera or 50mm on a full frame camera.

Another place you can easily see this effect is watching a sports event, such as a soccer game. In many shots, people will seem close together (front to back) because the photographer is using a very long lens.

This variation in perspective is why many people like to use lenses of roughly 90mm on a 35mm camera (shorter on a crop sensor) for portraits.

However, this is completely independent of depth of field and background blur.

Dan

[Joan]

I agree that distortion is not the correct word fow what I wanted to mean when using "normal" lenses. Perspective is the correct term to use.

For the sake of curiosity: this is also valid for fisheyes ? There is no distortion there neither ?

[Inkanyezi]

For the sake of curiosity: this is also valid for fisheyes ? There is no distortion there neither ?

Now we enter the realm of philosophy. It can be viewed in different ways, but it is a matter of what cartographers relate to as projection. If you project the fisheye image onto the inside of a semi-sphere, there should be no distortion.

[Joan]

If you wish to attain a Maximum DoF for any given FRAMING of any particular Subject, then you should use the SMALLEST Format Camera which is available.

You mean the highest possible crop factor ? A crop factor greater than 1 results on a reduction of the field of view of the camera equivalent to what would happen if there was an increase of the focal length of the lenses, but we have seen that this does not significantly affect the DOF (at constant framing and f) ....