Crop factor and Short Focal Lengths

[Bebulamar]

What I am saying is that the angle is not directly proportional to the crop factor. The linear dimension is. For example when you look thru the viewfinder and a length of say 10ft. fill your frame. If you change the FOV so that only 5ft. fill your frame the FOV in angle isn't half but is greater than half.

 

[john.margetts]

The joys of geometry! The crop factor will be working on the tangent of the angle of view, not the angle itself (might be sine or cosine - it's 45 years since I learnt my geometry).

 

[Bebulamar]

Well. Exactly. That's my point.

Because when we're talking about focal length we're really discussing FOV there is a discrepancy. If a 50mm lens doesn't actually behave like a 100mm lens of 4/3, then approximation is inadequate.

It does. The 100mm on FF has the same FOV as the 50mm on 4/3. Just that the 50mm on FF has more than twice the FOV in angle as the 100mm on FF.

 

[unpopular]

Well. Exactly. That's my point.

Because when we're talking about focal length we're really discussing FOV there is a discrepancy. If a 50mm lens doesn't actually behave like a 100mm lens of 4/3, then approximation is inadequate.

It does. The 100mm on FF has the same FOV as the 50mm on 4/3. Just that the 50mm on FF has more than twice the FOV in angle as the 100mm on FF.

50mm is a bad example. The FOV factor starts to correlate with the crop factor at this point, not precisely, but fairly close.

Lets look at 20mm instead. Using the Arctan FOV formula for 4/3 an ideal 20mm lens will have a an equivalent FOV of a 33.32mm lens on FF sensor, which depending on who you're talking to is pretty significantly different from 40mm.

 

[unpopular]

The joys of geometry! The crop factor will be working on the tangent of the angle of view, not the angle itself (might be sine or cosine - it's 45 years since I learnt my geometry).

Well that sure makes things less complicated, doesn't it?

 

[Bebulamar]

Well. Exactly. That's my point.

Because when we're talking about focal length we're really discussing FOV there is a discrepancy. If a 50mm lens doesn't actually behave like a 100mm lens of 4/3, then approximation is inadequate.

It does. The 100mm on FF has the same FOV as the 50mm on 4/3. Just that the 50mm on FF has more than twice the FOV in angle as the 100mm on FF.

50mm is a bad example. The FOV factor starts to correlate with the crop factor at this point, not precisely, but fairly close.

Lets look at 20mm instead. Using the Arctan FOV formula for 4/3 an ideal 20mm lens will have a an equivalent FOV of a 33.32mm lens on FF sensor, which depending on who you're talking to is pretty significantly different from 40mm.

The 20mm lens on 4/3 has the FOV of 56.82 Degrees and the 40mm lens on FF has the FOV of 56.81 Degrees. You must make an error somewhere in the calculation.

 

[unpopular]

@Bebulamar - you're missing the point. What I am saying is that a 20mm 4/3 lens will have a similar FOV of a 33mm FF lens, while the crop factor of 2.0 predicts it will be more similar to a 40mm lens.

I am saying that for shorter focal lengths, the crop factor is not accurate when estimating FOV.

 

[Bebulamar]

@Bebulamar - you're missing the point. What I am saying is that a 20mm 4/3 lens will have a similar FOV of a 33mm FF lens, while the crop factor of 2.0 predicts it will be more similar to a 40mm lens.

I am saying that for shorter focal lengths, the crop factor is not accurate when estimating FOV.

That what I said 33mm on FF the FOV is 66.49 Degree not the same as 20mm on 4/3 which is 56.82 Degrees which is the same as 40mm on FF with the FOV of 56.81 Degrees.

 

[unpopular]

@Bebulamar - you're missing the point. What I am saying is that a 20mm 4/3 lens will have a similar FOV of a 33mm FF lens, while the crop factor of 2.0 predicts it will be more similar to a 40mm lens.

I am saying that for shorter focal lengths, the crop factor is not accurate when estimating FOV.

That what I said 33mm on FF the FOV is 66.49 Degree not the same as 20mm on 4/3 which is 56.82 Degrees which is the same as 40mm on FF with the FOV of 56.81 Degrees.

What formula are you using to calculate FOV?

 

[Bebulamar]

Your formula.
FOV=2*Atan( Diagonal of Sensor / (2* Focal Length))

 

[table1349]

 

[unpopular]

@Bebulamar yep. you're right. I'm too tired to try and figure what I did wrong. But obviously I did something wrong.

 

[Bebulamar]

So the crop factor is correct. The crop factor can not be applied to the FOV in angle but only to the FOV in linear dimension.