Why Field of View difference between Canon and Sony?

[freshtracks]

Hi,

I've undertaken an exercise in camera lens calibration in order to identify how each lens should be adjusted to achieve a 50mm field of view. I have two cameras: a Canon 350D with an ultrasonic 28-200 lens, and a Sony DSC-F828 with a 28-200 lens which is housed as part of the camera body.

According to my calibration, the Canon lens should be adjusted to the inverted 'L' mark just after the 28 on the lens (does that inverted L, printed onto the camera lens, have a technical name?). However, if I do the same with the Sony lens, it must be adjusted to the '50' on the lens. I would be grateful if someone could explain why. My presumption is that this is so, because the lens is incorporated into the Sony body and so has been calibrated specifically for that camera body, where-as the Canon lens is interchangeable and so the 50mm FoV will change for each camera body? But there's quite a big difference between adjusting the lens to 31mm or thereabouts on one lens, and 50mm on a different lens...

The other question is, if accounting for distortion I need to adjust any digital SLR lens to '35' focal length to achieve a fixed 50mm equivelant, would I adjust the lens to 35mm FL on both the Canon and the Sony, or does the Sony have a different focal length to achieve the equivelant of a 50mm FC on a 35mm camera?

Many thanks for any explanation.

[Sue Allen]

If I've read this correctly [apologies if I've misunderstood!], you want to take pictures at 50mm - or at least with that field of view. The Sony 28-200 means just that - the range is 28-200. Because of the 'crop factor' of Canon DSLRs of that type being 1.6x, the lens on your 350D would need to be set to around 31mm to give the same results as your Sony at 50mm. The FOV on your Canon lens equates to roughly 45- 310mm in comparison.

[KeithT]

I think the differences come with the different crop/lens factors, eg, Sony is 1.5 and Canon 1.6, so there would be a slight difference in the distance scales of both lenses. 50mm is generally the standard focal length for a full-frame digital camera, or 35mm film camera, which covers a sensor/film area of 36x24mm; whereas crop sensors are 24x16mm. Because of this variation in ratio sizes I would doubt that you could match a true 50mm field of view exactly, though I guess you could get fairly close.

At a crop of 1.5 you would need to set the lens at 33-1/3mm to come close to a 50mm ratio and with 1.6 you would need to set the lens to 31.25mm. I am sure of this, but someone may have different thoughts on it.

[Sue Allen]

The Sony 828 is a bridge camera not a DSLR -so setting the lens at 50mm should give you 50mm surely?

[chris-p]

Well, yes it should give you 50mm but it rarely will... The thing with these is that they're such different formats and focal lengths are ALWAYS estimates.

We all say the Canon has a 1.6x crop or multiplier (or whatever) but thats not really true. Horizontally the crop factor of the 350D is 1.58x and vertically it's 1.62x so it's averaged at 1.6x. This doesn't sound like a lot but it makes a difference.
Couple this with the 3.92x - 3.94x crop of that particular bridge camera and it's lens needs to be set at about 12.7mm to get a 135 format 50mm field of view.

These cameras also have slightly different aspect ratios, which means that even if you set the exact focal lengths (in reality, a feat thats impossible on both cameras) to gain the exact 46.8 degree diagonal field of view you get with on 135 at 50mm, they will still look different.

[greenwing]

The main reason that the 50mm mark on the Sony corresponds (roughly) to 50mm on a 35mm camera is that, not to put too fine a point on it, the markings are lies. The lens is more like 7-50mm, what is marked as 50mm is really about 12.5mm focal length. It's marked with the '35mm equivalent' focal lengths.

Chris