How wide is wide?

What constitutes a “Wide” lens depends on whether you have a full-frame camera or a crop camera:

So on a full-frame camera a 24mm lens might be”wide” , while on a crop camera like a Digital Rebel or a D90 you would need a 15mm lens to get the same width.

You may want to keep this table in mind the next time you decide whether a lens is a wide-angle lens or not.


Crop sensors and depth of field

So a crop camera has wider depth of field?

Depends how you look at it. The way I look at it: “only because you will use it differently. If you use it to take the same picture, you’ll get the same depth of field”. But of course for that “same picture” you’ll need a different lens.

OK, this is complicated. So let me just show you by example.

Picture one: full frame 1Ds MkIII camera, 50mm lens at f/1.4.


(You can click to see the large version).

Picture two: crop camera, a Canon 7D, with a 35mm lens also set to f/1.4. That 35mm lens on a crop camera is about the same as a 50mm lens on a full frame camera; i.e. enables me to be at the same point and yet get roughly the same picture. Now I see this:


As you see, this gives you roughly the same depth of field. If there is any difference, it is minor. SO:

A 35mm lens on a crop camera gives you roughly the same picture as a 50mm lens on a full frame camera, and roughly the same depth of field.

A few notes, by the way:

  1. These results are consistent; not just “one randomly picked picture”.
  2. The DOF (depth of field, i.e. how much is sharp) is extremely narrow. Great care is needed when shooting at f/1.4!.
  3. I was aiming a single focus spot at the dot (near the 10) in all pictures (On the 7D, I was using the extra small focus spot).
  4. I have noticed that the Canon cameras (or is it the lenses?) focus too closely (they “front-focus”) when open at f/1.4. By f/2.8, this effect is either gone or too small to see clearly. In the last picture, I added an adjustment of +15.  This does not change the depth of field; it is just to make both pictures about the same.

Point 2 explains a few things to me, by the way. That’s part of the Canon-effect, more on which in the next few days; it’s why I have to adjust extra when wide open.

Crop factors explained

Cameras used to have a negative-sized negative (duh). That is what we call “standard size”. (It is 24×36 mm, if you are interested).

But many digital cameras, especially cheaper models, have a slightly smaller sensor than that. We call this a “crop factor” sensor. These can be 1.5 times smaller than a negative (some Nikon models), or 1.6 times smaller (some Canon models), and so on.

So let’s work though this to see how it works.

First, using, say, a 24 mm wide angle lens, take a photo on a “full frame camera” (a film camera, or an expensive DSLR like a Canon 5D) and you get picture 1:


Now put the very same lens on “crop factor camera” and take the very same picture from the very same vantage point. Same lens. Smaller sensor. So because of this smaller sensor, only the central part of the lens is actually used, and you now get picture two:


The lens is the same, but the sensor is smaller, so a smaller part of the lens is actually used.

Finally, if I now print or display that “crop” picture at the same size as picture one (after all, it probably has the same number of pixels), I get picture three:


And that looks just like a picture I would take with a 36 mm lens on a full-frame camera! See how the objects are larger?

That is why we say that a crop camera “appears to make your lenses longer”. So a 50mm lens on a full-frame camera, when used on a Canon Digital Rebel (crop factor 1.6) works like an 80mm lens on a full-frame camera. It has, as we say, “an effective length of 50×1.6 = 80mm”. There are other subtleties, but basically, that’s all.

This is excellent news if you like long lenses (your 200mm lens is now, free of charge, a 300mm lens). It is perhaps not quite such great news if you like very wide angle lenses: your 35mm lens works more like a 50mm lens (35mm x 1.6) would work on a full-frame camera. So to get what we used to get on a 16mm lens, you would need a 10mm lens.