Go Back   Steve's Digicams Forums > Digital SLR and Interchangeable Lens Cameras > Canon Lenses

Thread Tools Search this Thread
Old May 8, 2008, 7:32 AM   #1
Join Date: Mar 2008
Posts: 31

Hi all,
I have read a lot about the magnification factor of the APS-C sensor. It appears that there is no magnification, the Field of View is just cropped…

The only thing that I can see that is really affected by this "Magnification Factor" is:
  • the "Circle of Confusion" is decreased per pixel by 1.6x[/*]
  • to get the 35mm FOV for a lens 1.6x more distance is required from the subject
In turn, both of these affect Depth of Field and possibly perspective by 1.6x.

In saying this I am still confused…sometimes I think I have figured it out, then at other times I am baffled again…

I would like to take portraits.

I read in a book the other day that a 90 - 105mm lens is good for taking portraits, as the perspective is nice to people's faces.

If there is a magnification factor, this would this mean that a 55 - 65mm lens will provide the same perspective as the 90 – 105 would on a 35mm format?

In other words, would a 55-65mm lens do the trick for me?

Confused...I am!

Thanks in advance for your replies.

alfabeta2 is offline   Reply With Quote
Sponsored Links
Old May 8, 2008, 8:25 AM   #2
Join Date: Jun 2003
Location: Savannah, GA (USA)
Posts: 22,378

The perspective you see is a result of your distance from the subject versus the focal length being used. Of course, if you want the same framing with a longer focal length lens, you'd have to move further away with a model using an APS-C size sensor, which is going to change your perspective. Here's a short article about perspective:


If there is a magnification factor, this would this mean that a 55 - 65mm lens will provide the same perspective as the 90 – 105 would on a 35mm format?
Right, because you'd be shooting from the same distance for the same framing using the shorter lens on a dSLR.

Depth of Field is another matter though (as you noticed you're going to have greater depth of field using the camera with the APS-C sensor for the same framing and aperture. which can be undesirable if you want to help your subject stand out from distracting backgrounds by using a wider apertuire for a shallower depth of field). So, for a shallower depth of field, the model with the larger sensor has the advantage.

Here's a very good program (Barnack) that can show you things like field of view, depth of field, and more for a given film or sensor size, even taking things like focus distance into consideration for field of view calculations (although I can't seem to connect to the site right this minute):


Here's an online DOF calculator:


What you don't see in charts is that a longer focal length lens tends to give you the illusion of a shallower depth of field, because of the perspective you have shooting from further away (even though depth of field at most focus distances you'd use with portraits is going to be roughly the same for the same subject framing and aperture setting, regardless of focal length).

That's because the out of focus background will appear to be more compressed if you shoot from further way, helping your subject to stand out more from the blurrier background. So, I would personally lean towards a longer focal length lens for portraits on a model with an APS-C size sensor, if you have room to use one (for example, I'd try to use an 85mm versus a 50mm for head and shoulders type work if I had room, even on a camera with an APS-C size sensor)
JimC is offline   Reply With Quote
Old May 8, 2008, 8:44 AM   #3
Super Moderator
peripatetic's Avatar
Join Date: Nov 2004
Posts: 3,599

Haha, if only it were that simple.

English (or any natural language) is not very good at precisely expressing mathematical concepts.

There is no inherent "magnification factor" nor in fact is there a "crop" there is simply a sensor of a certain dimension; one that is 1.6 times smaller than the 36mmx24mm of a frame of 35mm film.

But using fuzzy English expressions one might regard that as a cropped version of a 35mm frame.

And if you crop and enlarge to the same print size one might (in fuzzy terms) describe it as a "magnification factor". So the magnification factor is larger on a crop sensor.

The increased "magnification" leads to a increased CoC, which in turn reduces the DOF.

HOWEVER that assumes we are keeping the focal length of the lens constant. In practice however you usually don't keep the focal length constant, because as photographers we are more interested in field-of-view than focal length. So when we want a wide angle lens on a 35mm camera we might use a focal length of 28mm, to get an equivalent field-of-view on a camera with a smaller sensor we might use 28/1.6=18mm.

Now as you know DOF depends on lots of variables: focal length, aperture, distance from subject to camera, print size, viewing distance, etc.

So although the increased magnification/crop leads to a smaller DOF, there is an off-setting change in focal length which increases DOF. The net effect of these two changes is usually to result in INCREASED DOF with a smaller or "crop" camera, but the equations can throw up some funny numbers depending on aperture and distance to subject.

But even then it's not that simple because the sensor resolution and lens resolution both come into play in the determination of CoC, much depends on nyquist frequencies, use of Anti-aliasing filters, the structure of micro-lenses and gaps between sensor wells. So it's all very complicated, and you can really drill down into as much detail as you want.

The net result however is that you can distill it down into a very fuzzy and inaccurate rule-of-thumb - crop cameras have bigger DOF than full-frame: you will be experiencing increased DOF in your day-to-day photography when you compare say the "crop" 450D and 40D cameras to the 5D or 1Ds cameras.
peripatetic is offline   Reply With Quote

Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

All times are GMT -5. The time now is 9:34 PM.