DOF Notes:1 - Head shots using a 50mm lens.

[RichardTaylor]

DOF - 1

50mm Head shots.

In response to posts asking "why are my portraits blurry when using my 50mm lens".

Part of the problem is that at short shooting distances the Depth of Field (DOF) or zone of sharpness of a 50mm lens can be very small.

DOF is controlled by.
(1) Lens Focal length
(2) Lens aperture
(3) Subject distance.

It is also controlled by how big and what distance you are viewing the final image, however I won't be looking at that.

When using a 50mm lens on a consumer DSLR and filling the frame (shooting distance 4 feet (1.2m) You have the following DOF.

@F2 ~1.5" or ~ 35mm to @F8 ~5.6" or ~140mm
That really doesn't give you a lot of room to play with.
So if you are shooting at F2 to have nice blurry backgrounds, and focussing just on the eyes you may find the eyes are the only things in focus and if you are shooting partially side on only on eye will be sharp.
If your subject or yourself move subject didn't move and the focussing isn't perfect then you have problems.

Here are some examples of what you can expect.
The bears head is roughly the same size as mine.
Subject distance ~ 4 feet - Canon 50MM F1.8 lens on a Canon 350d (Rebel XT - A "consumer" DSLR). I was shooting hand held.

The background is about 25-30 feet away (8-10m). All pics were given the same sharpening when PPing the original raw file.
Larger (1280pixels) pics are on my flickr stream.

(1) @ F2 - notice that it really isn't sharp, but the background is nice & blurry. That may be the lens or my technique for the bear that is.


(2) @ F2.8. Starting to get there. Notice the nose and eye are starting to sharpen up and the background is still blurry.


(3) The F4 pic can be seen here on Flickr.
20101103-IMG_0567 @ F4 | Flickr - Photo Sharing!

(4) @ F5.6 Here the whole head looks pretty sharp but the background is starting to become intrusive.
There is around where the consumer "standard zooms", when fully extended, start ie 18-55mm F3.5-F5.6.


(5) @ F8 The whole head is sharp (notice the back of the chair) but the background is intrusive.


For me the bottom line is, unless shooting profiles, shoot around F5.6 (if you want the whole head sharp) and make sure you have a "clean" background.

You may find this link handy It is a depth of field calculator.
Online Depth of Field Calculator

More to come.
Thanks for looking and feel free to ask questions.
Richard

Update.
The second part of this series is here. (Blurring the backgrounds)
DOF Notes:2 - Blurring backgrounds with shots of multiple people

[troutwhisperer]

Richard, thank you for the post, very informative and educational. I had an understanding of DOF but never looked at it with this kind detail, considering the f-stop progression.

There is still one (probably more) aspect of DOF that I don't completely understand, and that is the relationship of the distance between the subject and the background. There are times when I make a photo that I expect the background and the subject to be separated with blur but when I view that photo the background is more in focus that I had planned. Do you have any thoughts on the relation ship of the distance of the camera to subject and subject to background and the impact on DOF?

[sk66]

Originally Posted by troutwhisperer

Richard, thank you for the post, very informative and educational. I had an understanding of DOF but never looked at it with this kind detail, considering the f-stop progression.

There is still one (probably more) aspect of DOF that I don't completely understand, and that is the relationship of the distance between the subject and the background. There are times when I make a photo that I expect the background and the subject to be separated with blur but when I view that photo the background is more in focus that I had planned. Do you have any thoughts on the relation ship of the distance of the camera to subject and subject to background and the impact on DOF?

As the focal distance gets smaller (working closer to the subject) the DOF will become smaller. Distance of subject from background doesn't affect DOF, just whether the background falls within the DOF.

[RichardTaylor]

@ troutwhisperer

Thanks for the question.

Sk66 is correct

I've learn't a few things.
One of the variables is the distance between the subject and the background.
See this
http://en.wikipedia.org/wiki/Circle_of_confusion

and this

Depth of field and diffraction.

That's more math than I can handle.

This page will give you an idea of "how much blur", and how you can control the appearance it by using different focal length lenses with having the same subject size in the image Bokeh and Background Blur - Bob Atkins Photography

This page, on the same site, may help also.
Bokeh and Background Blur Calculator- Bob Atkins Photography

Note also the DOF (for a given image size) only varies slightly for changes in focal length. (The DOF calculator, in my first post, shows this also).

So to blur the background even more, if you can live with the change of perspective and field of view. Shoot with a longer lens, keeping the subject size the same.

Have a look at this page.
You will need to scroll down to the background blur section.
http://toothwalker.org/optics/dof.html

I tried this with some more teddy bear pics, however I won't re invent the wheel.

[troutwhisperer]

Sk66 and Richard, thanks for the clarification. I will have to noodle over this for a little while to make sure I understand. I certainly will follow up if I still do not understand. Thanks again for your input.

[mactheweb]

Really appreciate the research and time spent with the example photos.

For portraits, I prefer the slightly out of focus look at f2.8 - f4. People seldom want to be able to see each pore or blemish. If the eyes are in focus a slight blurring of the rest of the face is usually preferable. Pretty much what portrait software or Photoshop plugins do is blur the skin.

Of course if I were doing portraits of bears I'd want to see each hair.

One more point. The DOF of APS sensors is generally about one stop greater than for full frame ones. ie. f2.8 for APS ~ f4.0 for FF.

[RichardTaylor]

When at the same aperture and distance from the subject the DOF is the same for both full frame and APS-C.

However if you do move the camera so the image size is the same then yes you will get more DOF with the crop camera as you are increasing the shooting distance.

[troutwhisperer]

Richard, reviewed the information in the links you sent and I think it is becoming more clear. Thanks for your help.

[Celesta]

The distance from the subject does matter a lot. The further the camera is away, the larger are the circles of confusion. But it only matters for variable focal length (zoom) lens. A fixed length lens dictates the distance from the subject relative to how large you want the subject to appear on the photograph.

With the variable focal length, you can experiment with the distance from the subject. Say, you can stay 3 feet away and get a headshot with a relatively sharp background on 24 mm. Going back 10 feet, you can zoom to 200 mm to increase the subject's size on the photograph to be the same size as on 24 mm, but the background will appear nicely blurry. I like the recent article that shows examples:
How to use your zoom lens as a compositional aid
The model stays the same size, but the depth of field on 16-24-50 and 200 looks drastically different.

[RichardTaylor]

Thanks Celesta for the info and the link..
I wasn't aware of that posting.

Some more on that here.

DOF Notes:2 - Blurring backgrounds with shots of multiple people