Depth of Field techniques Rebel XT

[peter_uzzi]

I am an amateur with little understanding of manual depth of field control. I

would really like to understand how to achieve a shallow depth of field for some

portraiture using my Rebel XT + 72mm 28-135 IS USM. I typically can achieve this

by accident, but would really like more control. Can anyone suggest any

automatic features or techniques I might understand? Thank you!

[tdigi]

Depth of field is a controlled by how wide open your lens is ( lower F stop number )

How far you are from the subject and how far the subject is from the background.

[danield]

Hi Peter

<p>

You can obtain shallower depth of field by:

<ul>

<li>using the widest aperture of the lens

<li>using a longer focal length

<li>getting as close to the subject as you can

</ul>

<p>

You can use the "portrait mode" on your camera to some extent, but I'd advise against it. Instead, I would use AV mode, increase aperture as much as possible, and follow the suggestions above. But be careful with flash, flash in AV mode works quite differently than what you may be used to.

[photo_dark]

Daniel wasn't clear on something you may not understand.... Using the 'widest aperture' or 'increasing aperture as much as possible means using the SMALLEST number. IE, f5.6 will give you a 'shallower' depth of field than F16. Using F16 would bring everything into focus. Great for landscape, not that great for portraits.

 

More expensive zoom lenses and primes usually allow for a much larger aperture. IE the $2000 70-200 F2.8 allows in twice as much light as the $700 70-200L F4. The larger aperture of f2.8 also allows for 'less' depth of field, or a more blurred background.

[ty_mixon]

If the bkground is further away, then it's more out of focus, correct?

 

And how does the ratio of camera distance to subject vs bkgrnd distance tosubject, affect DoF?

[tdigi]

http://www.dofmaster.com/dofjs.html

[peter_uzzi]

Thanks, guys. Will setting a smaller aperture in AV mode require me to use manual exposure control? This is where I get nervous, since I typically rely on automatic settings. Will AV or any other setting allow me to manually select my F-Stop while allowing the camera to do everything else for me? '

 

What does "use a longer focal length" mean and in what order do I do these things?

 

Thanks so much for everyone's help!

[JDMvW]

In focal length, longer is bigger number, so a 135mm is longer than 90mm. The apparent depth of field (area in which the human eye accepts the image as being more or less in focus) is less with longer focal lengths, as it is with larger aperture (f/2.8 as opposed to f/11). Thus by shooting "wide open" with say a 80mm focal length, you will have very shallow depth of field. You may need to manually focus to get the part you want in focus, if in doubt, choose the eyes as your sharpest focus--the human viewer will normally perceive a portrait with the eyes sharp as being focused, not so much the nose.

[davebell]

Peter, set the camera to Av mode, select your desired focal length and then set your aperture to the minimum value your lens is capable of. Try get close to the subject if the composition permits. Monitor the shutter speed the camera "selects" and make sure it is at least at fast as the "inverse" of the focal length, e.g. if you have a focal length of 50mm make sure your shutter speed is greater than 1/50. If it isn't, then increase your ISO setting by as little as possible, until the shutter speed is adequate. Strictly speaking, you shutter speed should be 1.6 times the "inverse" of your focal length due to the smaller sensor on you camera. But in my experience, blessed with steady hands, I can hand-hold at slower speeds with good results. You need the fast enough shutter speed to avoid blurry pictures. Good luck.

[peter_uzzi]

I did some experimenting and I think I finally understand this. Thank you all for your help!

[jeff_conrad]

For the same subject framing, DoF is approximately the same for all focal

lengths, so using a longer focal length won't help reduce DoF. The best

technique for shallow DoF is to use the largest possible lens opening.

Getting as close to the subject as possible can also help, but the subject

distance will often be dictated by aesthetic considerations.

<p>

Although you didn't expressly say so, it sounds as though you might like to

blur the background as well. The best way to do this is to keep the

subject as far away from the background as possible. In some cases, a

longer focal length might help, but often not as much as you might think.

Although the background blur is greater with a longer focal length, the

background magnification is also greater, and if the background contains

something really obnoxious such as a sign or a piece of paper with printed

text, that object is usually just as recognizable with a long lens as it is

with a short one. There is one big advantage to the longer lens, however:

the angle of view is narrower, so a slight change in position can often

move a distracting background element out of the picture. The best way to

control a background is to carefully choose it so that it's far enough away

and without distracting elements.

<p>

The subject perspective is determined by the camera-to-subject distance.

Although there aren't any hard-and-fast rules, settings of about 50 mm

to 85 mm might be good starting points using a 1.6-crop-factor camera

for portraits of a single person. It's usually better to choose the camera

position that gives the perspective you like than to play around with

distance and focal length to achieve effects that you can better achieve by

other means. Avoid sacrificing composition for technical considerations

whenever possible.

[sarah_fox]

Jeff, I don't disagree with you from an object field perspective. Using an object field approach (which examines how large the blur patterns appear to be out in front of the camera), you're absolutely right about the DoF being the same for all focal lengths at a given aperture number. However, from a circle of confusion perspective (i.e. viewed at the focal plane), a longer lens does result in larger blur patterns corresponding to objects beyond the point of focus. Hypothetically, if you focus on an object 5 ft away with a 100mm lens at f/4, the blur pattern from a point of light 10 ft away will appear about the size of a quarter (25mm, give or take). If you focus on the same object with a 200mm lens at f/4 at twice the distance (10 ft), the blur pattern at 15 ft (the farther point) will be the same quarter size. However, when projected to the focal plane, the blurred background object is not twice the distance (where it would produce the same size projection onto the focal plane), but rather 1.5x the distance. Thus it appears larger on the focal plane. Similarly, its blur pattern, which is the same size by object field methods, also appears larger.

 

So if one is trying to blur out something recognizeable, such as text, it makes little difference what focal length of lens is used. However, if one is trying to generate a field of large, soft, fuzzy, diffuse spots, the longer lens is the better choice.

 

Interestingly, if one is trying to fuzz out the foreground, a wider angle lens works better by the same rationale, assuming the same framing of the main subject.

[jeff_conrad]

Sarah, it seems to me that's what I said in my post ... sometimes the

larger blur spot may have a beneficial effect, but sometimes it may

not—it's a subjective call that may even vary from image to image.

And the same result obtains whether one looks at it on the image or object

side.

<p>

Although at first glance it may seem surprising, the blurring of a

foreground object <em>is</em> greater with a short-focus lens because the

distance from the camera is so small.

<p>

This all is covered in some detail in the Wikipedia article on

<a href="http://en.wikipedia.org/wiki/Depth_of_field">Depth of field</a>.

Paul van Walree's article on <a

href="http://www.vanwalree.com/optics/dof.html">Depth of field</a> offers a

similar discussion as well as some excellent illustrative photographs.

<p>

Again, though, playing with focal lengths to control background blur is at

best a secondary technique that makes sense only when specifically getting

large background blur spots is more important the perspective of the

subject and other compositional considerations. Don't get me wrong—a

portrait with a 400 mm <i>f</i>/2.8 can sometimes be very interesting,

especially because optics in that class usually yield a very pleasing

bokeh. But it's important to consider the big picture as well as the

background blur.

[William Michael]

> Can anyone suggest any automatic features or techniques I might understand? Thank you! <

 

Yes I can, but it steps outside the limits of your question.

 

Notwithstanding all the advice and detailed technical input, perhaps one very easy method of getting the results you want, and to learn more and to be able to experiment better, is to buy an EF50mmF1.8MkII.

 

The point is, the zoom lens you are using at the moment most likely has a maximum attainable aperture of F5.6 from about 50mm through to 135mm.

 

As Jeff mentioned, on a RebelXT a lens with a FL of 50mm to 85mm is a good place to start to get `nice` perspective for portraits.

 

Without going into the detailed maths of it: if you take a Full Length, subject at 12 ft using your zoom at 50mm and with aperture at F5.6 you will have a depth of field of about 4 ft, quite a lot really.

 

If you took the same shot with the 50mm prime, and set the aperture to F2, the DoF would only be about 1ft 4inches, just enough for the thickness of an average person. So even if the pretty flowers were not yards away in the background, but only a few feet behind, they would be rendered `more blurry`, for want of a better way of describing the effect

 

(I am not wanting to get into detailed bokeh arguments, but rather making a practical suggestion for easier shallow DoF portrait work.)

 

The availability of the much wider aperture becomes very effective should you want to isolate one person in a crowd, for example.

 

That could be more difficult with you zoom lens if you were limited to the F5.6 range.

 

At apertures of F2, you can more easily, at close quarters isolate just the eyes, in a tight shot, for example.

 

I do not necessarily suggest the F1.8 version of this lens, I just mentioned that lens because it will do a good job, and comparatively, it is very inexpensive.

 

I have the EF50F1.4 and it is a very nice portrait lens on my 20D and allows very shallow DoF work.

 

WW

[sarah_fox]

Jeff wrote: "Sarah, it seems to me that's what I said in my post" Ahem! Er... OK, I can see that now. ;-) Then we agree! :-)