Quoted focal ranges

[bwelsher]

Quick question (and prob asked but no luck on search) -
when a lens is specifically designed for crop cameras (e.g Canon EF-S or Sigma DC) is the quoted focal range the actual range you will get or is it a 35mm focal range?
To give a real world example would a Canon EF-S 60mm lens give a 60mm focal range on a Canon 550D or would it be 60x1.6 = 96mm?

[OsmosisStudios]

focal length is focal length. The lenses are always marked and advertised as 35mm focal lengths, because they always have been. What happens is that when you put the lens on a crop-sensor camera (like the 550D) then it "crops" part of the image (you do not notice this) and gives you an apparent 35mm equivalent focal length (which is focal-lengthx1.6)

So, a 60mm lens is marked as a 60mm lens because it is a 60mm lens. EF-S or not, its a 60mm lens. But because it's mounted to a crop-sensor camera, it gives the same field of view as a 96mm lens.

Put another way, if you have a 50mm f/1.8 on a full-frame sensor and on a crop-sensor camera, the lens' focal length stays the same, but it *appears* to be a longer focal length (by a factor of x1.6) on the crop-sensor camera because of the sensor.

The trick here is that it the focal length is always marked as 35mm equiv, but your camera will alter how it looks. If you have a full-frame camera, the focal length you get is what is written on the lens. If you're using a crop-sensor camera, then you have to calculate the equivalent focal length. This is, as I've indicated, regardless of the EF-S (or other crop-sensor designation).

In short: if you have a crop-sensor camera (like a 550D) you ALWAYS have to figure out your 35mm equivalency.

[bwelsher]

Cheers mate, that explains it nicely.
It never occurred to me that the 18-50mm kit lens I have is actually a 29-80 and the 55-200mm is 88-320mm.

[OsmosisStudios]

Originally Posted by bwelsher

Cheers mate, that explains it nicely.
It never occurred to me that the 18-50mm kit lens I have is actually a 29-80 and the 55-200mm is 88-320mm.

In a sense. You'll get a lot of arguing on this point. The important part is to understand that you get the same EFFECTIVE focal length. What that means is that, for instance, a 100mm lens on a crop-sensor will give you the same field of view as a 160mm lens, but it's the same actual focal length (so you get the same depth of field and so on).

What that means is that taking the same photo with a 100mm on a crop sensor and a 160mm lens on a full-frame sensor should give you roughly the same scene (ie objects at the edge of one image should be pretty much at the same edge on the other), but the depth of field will be different and you'll get a different perspective rendition.

[dcclark]

Originally Posted by OsmosisStudios

What that means is that taking the same photo with a 100mm on a crop sensor and a 160mm lens on a full-frame sensor should give you roughly the same scene (ie objects at the edge of one image should be pretty much at the same edge on the other), but the depth of field will be different and you'll get a different perspective rendition.

This is really key. A 160mm lens on a full-frame camera, and a 100mm lens on a 1.6x crop body camera, really are not identical. The perspective especially will be slightly different, which is more obvious when you have very wide angles involved.

To see what Osmosis is saying about depth of field, try to get a shot with a shallow depth of field while using a point-and-shoot. They have crop factors of 5x or even 10x, and it really effects the depth of field a LOT.

[bwelsher]

Lol, the wonderful complexity of optics eh?
Thanks for all the explanations though fellas, I think I'm getting my head round it.