Lens focal length

[Jim Benton]

I was idly looking at the details of the Sigma 10-20 on Amazon when I saw something that didn't make a lot of sense.

A couple of people had written reviews in which they mentioned that the 10mm setting was 'equivalent to 15mm' on 35mm.

Firstly, I'd always assumed that when a lens for a Canon or Nikon DSLR was sold the focal length specification was given for 24x36mm negative/sensor size.

Secondly, surely is the FL spec was given for a 1.6 crop sensor the angle would be wider for full frame rather than narrower?

Can someone explain what's going on here?

 

[adamwilliamking]

The writing on the lens is the exact mm on a full frame camera.
Determine the crop of your camera and times both numbers by it.

10 x 1.6 = 16 + 22 x 1.6 = 35.2

Full frame: 10-22
1.6 Crop sensor: 16-35.2

 

[Big Mike]

You are reading too much into this.

When a lens lists it's focal length...that's what it is (within their tolerances anyway). This has nothing to do with the 'crop factor' or the size of the recording medium. The Sigma 10-20mm has a focal length range of 10mm to 20mm.

Firstly, I'd always assumed that when a lens for a Canon or Nikon DSLR was sold the focal length specification was given for 24x36mm negative/sensor size.

Again, the focal length is a lens spec and doesn't really have anything to do with the sensor size.

Now, where the confusion come in, is that most DSLR cameras have smaller sensors and thus we have the 'Crop Factor'. People (mostly camera reviewers) needed some way to compare the Field of View that these new cameras were giving us....and the standard they choose to use was the prevailing standard at the time...35mm film cameras. Also, since most of the lenses were designed for 35mm film cameras, the comparison makes sense.

So when they say that 10mm is 'equivalent to 15mm' on 35mm, they are referring to the FOV on a 35mm film SLR camera (or full frame digital body).

You only need to consider the 'crop factor' if you have a preconceived notion of what FOV you would get from a given focal length...on a 35mm film camera. For example, if you were a film shooter and used a 50mm lens....you might find that the same lens on a crop body camera gives you too narrow of a view. But if you have not been a film shooter...then just ignore the crop factor and know that shorter focal lengths gives you wider FOV and what you see through the viewfinder is what you get.

 

[bigtwinky]

You are correct in that the focal range listed is for a full frame camera, ie a sensor size of 24x36

So when its listed as a 10-22mm, thats on a full frame. You have to multiply each variable by 1.6 (1.5 for Nikon) to get the equivalent focal range on a crop sensor camera

Here is a decent article, with pictures, that explains the difference fairly well

Crop Sensor (APS-C) Cameras and Lens Confusion

 

[Jim Benton]

That's exactly how I understand it.

So it seems people are just being sloppy with their use of English when they refer to a '35mm' equivalent when what they mean is the focal length you'd need on a full frame to get the effective field of view of the lens in question on a 1.6 crop sensor.

 

[adamwilliamking]

Yeah most people learn backwards these days anyways, so they don't sit there and go hmm I need a 24 mm focal length for this shot. Instead they pop on their lenses and zoom in and out until they compose the frame. I assume in a film (more patient) world it may be very different.

 

[Jim Benton]

Firstly, I'd always assumed that when a lens for a Canon or Nikon DSLR was sold the focal length specification was given for 24x36mm negative/sensor size.

Again, the focal length is a lens spec and doesn't really have anything to do with the sensor size.

Yes, I appreciate that. I just couldn't see why they were talking about a 35mm equivalent.

I'm guilty of joining the 'sloppy' English user myself by talking about focal length when I mean field of view.

However I have seen POS cameras with tiny sensors claiming a zoom range of 35-105mm and I'm sure this is not what they actually have as that would be a telephoto zoom on such a camera.

 

[Big Mike]

However I have seen POS cameras with tiny sensors claiming a zoom range of 35-105mm and I'm sure this is not what they actually have as that would be a telephoto zoom on such a camera.

Those cameras use the same standard for comparison. So while the actual focal length might be 2mm to 20mm....they give the "equivalent" value of 20-200mm...not really telling you that it's equivalent to that focal length on a 35mm film camera...which by now, is something that many of these people have never used.