1.6x crop/magnification (?) while wearing 35mm lens

[jun_ea]

Does the 1.6x field of view crop commonly found in digital slrs

correct the perceived distortion caused by a 35mm lens? I want to buy

a Canon 20-35mm lens with the intent to use the 35mm setting on a 10D

for some portrait work. I know that it may not be ideal for portraits,

but I'm try to avoid buying an extra lens for portraits. I also like

the relative small size of it for a zoom lens. What do you think? Bad

idea?

[aardvarko]

No

[jcuknz]

If you have worked with a 50 or 55mm lens with film you can answer the question for yourself since that is very close to what you will get ... it is a personal and artistic choice. I would avoid doing serious portrait work with such a lens.

[awindsor]

My main portrait lens for my 20D is Canon's 50/1.8. It is only about $70 new, and even lighter and smaller than the zoom.

 

Most people prefer to use lenses slightly longer than normal for portraiture. The "classical" portrait lenses were 85mm, 100mm and 135mm.

 

The prime lens is faster than the zoom which is useful for the shallow depth of field common in portraiture.

[scott_eaton]

What Alistair said.

[leonard_evens]

There are two issues.

 

First, you have the multiply the actual focal length by 1.6 to get the focal length of a 35

mm full frame camera lens with the same angle of view. Or equivalently, you should

divide your favorite 35 mm focal length by 1.6 to get the DSLR focal length to use. People

differ about what 35 mm focal lengths they use for portraiture. It could vary from

something like 90 mm to 135 mm. If you chose 90 mm for 35 mm format, you should

use 90/1.6 = 55 mm for the DSLR. (That is the real focal length, not the 35 mm

equivalent, which is 90 mm.)

 

Second, if you use the choice of aperture to limit depth of field and throw the background

out of focus, you have to divide the f-number you would usually use with a 35 mm full

frame format camera by 1.6 to get the same depth of field with your SLR and the

equivalent angle of view lens as described above. So, if f/5.6 worked for you with a 90

mm lens on a 35 mm camera, you would have to use f-number 5.6/1.6 = 3.5 with the 55

mm lens. This can be a problem because you may not be able to find a lens with a large

enough relative aperture to limit depth of field with your chosen focal length. Of course,

it is always possibly to throw the background out of focus by editing your image in a

photoeditor if you know what you are doing. So, worrying about limiting dof may not be

so important.

[preston_merchant]

In terms of distortion, the crop factor means you are utilizing the center of the lens, not the edges, so there is less distortion. I have a 20 mm lens on a 20D (which translates to 32 mm in the other format). On a film camera, that lens has all the edge distortion you would expect. On the DSLR, there is none.

[godfrey]

Explained another way:

<br><br>

If you're talking about *perspective* distortion, remember that perspective is a function of

camera to subject distance. Focal length is irrelevant to perspective, only distance matters.

Traditional portraits are normally made with the camera to subject distance at about 7-9

feet to achieve the correct perspective.

<br><br>

Where focal length comes in to play is in determining subject magnification ... With a

35mm film camera, you normally want something on the order of a 70-100mm lens for

head&shoulders portraits so that you can fill the frame with your subject. You use a 50mm

or shorter lens to include a wider field of view *at the same distance* for an environmental

portrait.

<br><br>

With the 10D's smaller format (approximately 15x22mm instead of 24x36mm), you need a

shorter focal length lens to achieve the same subject magnification. The difference in focal

length required is approximated by the so-called "1.6x crop factor". So, given a portrait

taken at the same 7-9 foot distance, you would use a 50mm lens for the head&shoulders

crop, a 35mm or shorter lens for the environmental portrait.

<br><br>

With my 10D, I use a Canon 28mm f/1.8 for environmental portraits (often a bit more

coverage and a bit closer in) and a Canon 50mm f/

1.4 lens for head&shoulders to 1/2 length work. They're both excellent, with very similar

imaging

qualities, and fast enough to allow a great deal of control of focus zone at portrait

distances.

<br><br>

Examples: <br>

50mm lens: <a href="http://homepage.mac.com/ramarren/photo/PAW4/10.htm"

target=new>http://homepage.mac.com/ramarren/photo/PAW4/10.htm</a><br>

28mm lens: <a href="http://homepage.mac.com/ramarren/photo/PAW4/18.htm"

target=new2>http://homepage.mac.com/ramarren/photo/PAW4/18.htm</a><br>

<br>

A 20-35mm (or even better, a 17-40mm f/4L) will make an excellent lens for general

purpose use and environmental portraiture. The f/4 maximum aperture is a little less

flexible in being able to limit DoF for some kinds of shooting, but I find this focal length

range to be very good for a tremendous amount of work. I'd still want a fast 50mm for

portraits in addition, however ... one of my favorite walkaround kits is a 17-40/4L +

50/1.4 (for the Canon 10D) or 20-35/4+50/1.7 (for the Pentax *ist DS).

<br><br>

Godfrey

[ralph_jensen]

I'm with Alistair and Scott: the 50/1.8 will is sharper, faster, has a larger aperture, and

costs only $70. If you can buy both, that would be ideal.

[byronlawrence]

spending the very little money necessary to get the 50mm lens (as stated previously) is a really good idea for portrature. get the other lens if you still want it.

[jun_ea]

THANK YOU. THANK YOU. THANK YOU, everyone for all the insights and suggestions. You have helped me to resolve the issue with just about total clarity.