Confused about Sigma 10-20mm

jbushee · Oct 21, 2008

The blurb for this "designed exclusively for digital" lens http://www.sigmaphoto.com/lenses/lenses_all_details.asp?id=3301&navigator=6 offer "super wide-angle photography and it is a very powerful tool for indoor shooting and landscape photography with APS-C size image sensors of digital SLR cameras".

Those sensors are around 25.1 × 16.7 mm, I think.

It further states that "Vignetting will occur if the lens is used with digital cameras with image sensors larger than APS-C size or 35mm SLR cameras"

My K10D uses a 23.5 X 15.7 sensor, so I thought I should be able to get nearly the full field of view (a full 10-20mm equivalent), yet when I questioned Sigma, they said I'd be subject to the 1.5 crop factor, making it a 15-30mm equivalent.

If it will vignett on a 35mm, and is made for an APS-C sensor, aren't they mistaken when they told me the crop factor applies?

If not, I'm really missing something.

Steph · Oct 21, 2008

They are talking about field of view. The field of view of a 10-20mm mounted on your K10D will be equivalent to the field of view of a 15-30mm lens mounted on a 35mm (or full frame sensor) camera. The focal length is a property of the lens, which does NOT depends on the camera it is mounted on.

tsaraleksi · Oct 21, 2008

A 'full-frame' sensor is 24X36mm. Your K10D has a sensor that is about half the size, hence the 1.5x crop.

jbushee · Oct 21, 2008

tsaraleksi said:
A 'full-frame' sensor is 24X36mm. Your K10D has a sensor that is about half the size, hence the 1.5x crop.

Right, but this lens wasn't designed for a full frame sensor, it was designed for an APS sensor-

"Vignetting will occur if the lens is used with digital cameras with image sensors larger than APS-C size or 35mm SLR cameras"

Why does 35mm come into play at all?

icassell · Oct 21, 2008

jbushee said:
Right, but this lens wasn't designed for a full frame sensor, it was designed for an APS sensor-

"Vignetting will occur if the lens is used with digital cameras with image sensors larger than APS-C size or 35mm SLR cameras"

Why does 35mm come into play at all?

Because the lens will fit on a Canon EF 35mm camera (with vignetting). BTW, I love that lens ... I use it on my 30D.

tsaraleksi · Oct 21, 2008

jbushee said:
Right, but this lens wasn't designed for a full frame sensor, it was designed for an APS sensor-

"Vignetting will occur if the lens is used with digital cameras with image sensors larger than APS-C size or 35mm SLR cameras"

Why does 35mm come into play at all?

All lenses, no matter what format of film or sensor they are being used on, use the same numbering scheme. Therefore, your 10-20mm lens is still a 10-20, even though it was designed for a smaller sensor.

Steph · Oct 22, 2008

jbushee said:
Right, but this lens wasn't designed for a full frame sensor, it was designed for an APS sensor-

The focal length is a physical property of the lens and has nothing to do with the camera it is mounted on.

jbushee said:
"Vignetting will occur if the lens is used with digital cameras with image sensors larger than APS-C size or 35mm SLR cameras"

The lens is designed for APS-c sized sensors. Its image circle is just large enough to cover these sensors. A piece of 35mm film (or full frame) sensor, is larger and therefore not fully covered by the image circle (hence the vignetting).

jbushee said:
Why does 35mm come into play at all?

Because people who learnt photography on 35mm photography are used to the field of view on these cameras, for any given focal length. That's why some people give the equivalent focal length. If you never used a 35mm camera, don't worry about the crop factor. Mount the lens on the camera and what you see through the viewfinder is what you get.

Mitch1640 · Oct 22, 2008

what i dont understand is why dont they just use the equivalent focal length on digital only lenses.

jbushee · Oct 24, 2008

Steph said:
The lens is designed for APS-c sized sensors. Its image circle is just large enough to cover these sensors. A piece of 35mm film (or full frame) sensor, is larger and therefore not fully covered by the image circle (hence the vignetting).

That would indicate to me then, that as my sensor is virtually the same size as the APS-C, I should enjoy the full field of view the lens can offer, and there would be no "crop factor", resulting in a 10-20mm (35mm equivalent) apparent focal length.

Dao · Oct 24, 2008

Please understand the CROP FACTOR is really mean nothing

if you are not used to the Full frame DSLR or 35mm SLR camera.

10-20mm lens is a 10-20mm lens. It's focal length can "zoom" within the range of 10mm to 20mm.

So when you mount the lens on your camera and set it on 20mm, you will see how wide you can see (angle of view). And that is your reference point. So if you want to take a picture with that angle of view, you will know which lens at which focal length can do that.

Now, let say a person that use 35mm SLR all his/her life. And know if he/she want to capture a image that cover certain area, he/she will know what lens/focal length to use. Now, when the person moved to DSLR with a smaller sensor. Everything changed.

For example. Person A took a picture with his Canon 35mm film camera of a house (exterior) using the focal length of 29mm. Few years later, he moved to digital and has a DSLR that has a crop factor of 1.6x.

He now want to take a picture from the same spot with his new DSLR camera (same zoom lens) of the same house. He found that he cannot see the whole house with the same 29mm he used when he took the image with his old 35mm film camera. Now, since he remember the crop factor, so he divide 29 by 1.6 and that is about 18. So he change the focal length of the lens from 29mm to 18mm and boom ... he can see the whole house just like the old days.

My point is, if you are not used to the full frame nor the 35mm film SLR camera, don't worry about the crop factor. It really doesn't matter. Since your reference point is what you have now. But of course, if later on you move to Full Frame or Film based camera, you may need to worry about it.

Village Idiot · Oct 24, 2008

Mitch1640 said:
what i dont understand is why dont they just use the equivalent focal length on digital only lenses.

Because not all digital SLR's have the same crop factor. There's FF sensors, APS-H 1.3x crop factor, APS-C 1.5x crop, APS-C 1.6x crop, and 4/3 2.0x crop for most standard DLSR cameras.

They're all digital and all have different sized sensors.

The lens on a APS-C Nikon would have a 15-30mm field of view. 16-32mm FOV on a APS-C Canon, and 20-40mm FOV on a 4/3 Olympus.

It's still a 10-20mm lens (as DSLR cameras are based on 35mm SLR cameras and film lenses can generally be used with digital bodies, so keeping the standard focal length rating is some what necessary.) but it gives you a different field of view depending on what type of sensor it's used with.