Crop factor - misleading

[Braineack]

how are you measuring the AOV? I'd expect around 90° on a FF at around 19mm, but I'd only expect the 42° on a crop at the 35mm focal length...sure you didn't zoom in between your measurements? That's suggests a really large crop to go from 90° to 42° on the same focal length.

 

[Reflex]

Braineack

I use a sort of large protractor that comes off a big woodworking machine. I am sure that the zoom was in the right position - one thing I double checked when I saw the result. I need to try again at infinity focus and there is certainly some measurement error but I am sure the result is closer to what I measured than to what it should be.

 

[unpopular]

The crop factor certainly is misunderstood, if not entirely misleading. While I cannot put it into exact words, but I've always enjoyed shooting with a 50mm lens FF, when I switch to APS-C, I am finding that the 50mm has more in common with 50mm on FF than 80mm-ish and 35mm-ish on APS-C has less in common with a 50mm lens of FF.

I still continue to shoot primarily with 50mm lenses, and rarely touch my 35mm lenses. I've found that I tend to shoot the same with similar lenses regardless of what sensor size I'm using.

 

[astroNikon]

For the sake of absolute clarity then... The angle of view I am seeing in the horizontal plane is forty something degrees, around 42 degrees. It is not the 60 degrees I would hope for.

For reference; on a full-frame camera the angle of view in this plane for this focal length would be around ninety degrees. The crop factor accounts for the reduction from 90 to 60 degrees but not the further reduction to 42 degrees.

This is with the lens focussed on around 100 metres though as mentioned above.

I hope that is clearer.

I'm certainly getting more confused.

But I use the same lenses on FF as on my crop, a Nikon d600 and d7000. The exact same lenses.
They are FF lenses, which essentially provides a Full Size image vs a crop lens.

to get a "19mm" lens on a crop sensor you actually need a 12mm lens

If that 12mm lens is put on a FF camera you would get a 12mm lens.

 

[480sparky]

Braineack

I use a sort of large protractor that comes off a big woodworking machine. I am sure that the zoom was in the right position - one thing I double checked when I saw the result. I need to try again at infinity focus and there is certainly some measurement error but I am sure the result is closer to what I measured than to what it should be.

So where is the 'hinge' of the protractor placed, relative to the camera & lens?

 

[Designer]

For the sake of absolute clarity then... The angle of view I am seeing in the horizontal plane is forty something degrees, around 42 degrees. It is not the 60 degrees I would hope for.

For reference; on a full-frame camera the angle of view in this plane for this focal length would be around ninety degrees. The crop factor accounts for the reduction from 90 to 60 degrees but not the further reduction to 42 degrees.

This is with the lens focussed on around 100 metres though as mentioned above.

I hope that is clearer.

No.

Please read the thread from a couple of days ago.

(Note: This part needs to be deleted, but I want everyone to see what it was that should be deleted. )The angle of a lens does not change merely by placing it on a different camera.

http://www.thephotoforum.com/forum/...354984-sensor-size-focal-length-question.html

(edit) So I'm all wet. Forget most of my post. What more is to be said after Gavjenks and 480sparky post?

 

[Gavjenks]

Oh my god @everyone in this thread, seriously? He's obviously saying it is not as wide as advertised... not that he doesn't understand crop factors. As in, the lens is actually a 25mm lens straight up, when the barrel numbers say it's supposed to be 18, or whatever. Which, Mr. OP, in answer to your question, that happens ALL THE TIME, and is not weird at all, companies constantly lie or fudge their millimeters, or at least mislead.

As suggested by somebody i think, do note that lenses will change their angle of view sometimes based on how near or far you are focusing (this is a phenomenon referred to as "focus breathing," and most good lens reviews will cover how much focus breathing a given lens has, because it is a factor of critical importance to videographers. Lenses that perfectly maintain ACTUAL focal length for all focal distances at a given written number on the barrel are called "parfocal" lenses. Generally a good hint that a lens might be parfocal is a constant max aperture, but not a guarantee). So the lens might be accurate at, say, infinity, or some specific distance of focus, but not any other. Which is one way that sketchy manufacturers can get away with advertising one number and having it not actually be that wide most of the time, without legally getting in trouble.

Other times, though, companies just flat out lie or round to the nearest marketable number even if that actual FL isn't achieved at any focal distance, blatantly in disregard of ethics. This is especially prevalent at the telephoto end, where 278mm will often just be called 300mm and they call it a day. I wouldn't be surprised at the wide end either, though, even if somewhat less common.

The angle of a lens does not change merely by placing it on a different camera.

By the way, yes it absolutely does. In fact, not only does it change based on sensor length, but angle of view isn't even defined without reference to a specific film or sensor size. If I just tell you "300mm" without reference to any specific sensor or film, it is completely impossible to give me any corresponding angle, because you don't have all the parameters you need.

A 300mm lens on a crop sensor is about 5 degrees of visual angle, and a 300mm lens on a large format camera is 29+ degrees of visual angle...

 

[Gavjenks]

So where is the 'hinge' of the protractor placed, relative to the camera & lens?

There is probably no way this would account for a 20 degree difference no matter whether he put it an inch too far forward or not, since he is almost certainly using objects in a room several feet away as the reference endpoints.

 

[480sparky]

So where is the 'hinge' of the protractor placed, relative to the camera & lens?

There is probably no way this would account for a 20 degree difference no matter whether he put it an inch too far forward or not, since he is almost certainly using objects in a room several feet away as the reference endpoints.

Between the front of the lens, to the actual sensor plane, I'm sure it can. If he's using the protractor itself in the VF.

 

[Gavjenks]

So where is the 'hinge' of the protractor placed, relative to the camera & lens?

There is probably no way this would account for a 20 degree difference no matter whether he put it an inch too far forward or not, since he is almost certainly using objects in a room several feet away as the reference endpoints.

Between the front of the lens, to the actual sensor plane, I'm sure it can. If he's using the protractor itself in the VF.

K, actual numbers:



Blue here chosen as the absolute worst case scenario for a point he could have picked (for a normal medium-large 4 inch long lens). Any other possible point (like the flange mount) would be less dramatic of a difference. Image very roughly to scale-ish, but I based the math on trigonometry, not the image.

Distance being 9 feet, about as far as furniture and stuff next to the opposite wall would be if standing in a normal room with room for the photographer and a tripod.

(If using objects near the horizon outdoors, by contrast, the difference would be a tiny fraction of 1 degree. So again, this is a conservative assumption)





Also worth noting: 42 * 1.6 = 67.2, not 60, so this is almost certainly not an issue of him/her being confused about crop factors.

 

[buddha1200]

Good read.lots of info here.

Sent from my SM-N900T using Tapatalk

 

[Reflex]

Gavjenks

Thank you for understanding what I was asking about. I must be a complete idiot for not getting people to understand. Never mind. Really appreciate your input. here What appears to be the error between the marked value of the lens and what it seems to measure appears to be enormous. I thought, given the size of that error, someone might have had the same experience and it seems from what you say that is the case.

Apologies to everyone for not replying sooner by-the-way but I have been away.

In answer to 480sparky I have to admit that measuring the angle is not so precise that the point from where I am measuring makes much difference. I aim for somewhere on the lens. I could be a few degrees out but like Gavjenks diagram suggests the pivot position does not seem that important compared with the other errors. However I am sure that my measurements show the lens is not what it says (on the cropped sensor at least).

 

[Gavjenks]

Actually now that I think about it more, I'm not even sure where the correct point is. Might be the sensor, might be the optical/mathematical points in or just outside of the lens, etc. But yeah, shouldn't explain 18 degrees difference.

 

[sk66]

So where is the 'hinge' of the protractor placed, relative to the camera & lens?

There is probably no way this would account for a 20 degree difference no matter whether he put it an inch too far forward or not, since he is almost certainly using objects in a room several feet away as the reference endpoints.

Not so sure, some lenses "breathe" quite heavily... I.e. Nikon's 18-200 is ~ 130MM when set to 200mm and focused at short distances.

BUT, focus breathing works in the opposite direction. The lens is shorter/wider than one would think. The issue here is that it's longer/narrower than expected. I have no idea how to account for that except maybe it's a "comparison issue" and the "other lens" has the issue.

 

[table1349]

The angle of a lens does not change merely by placing it on a different camera.

By the way, yes it absolutely does. In fact, not only does it change based on sensor length, but angle of view isn't even defined without reference to a specific film or sensor size. If I just tell you "300mm" without reference to any specific sensor or film, it is completely impossible to give me any corresponding angle, because you don't have all the parameters you need.

A 300mm lens on a crop sensor is about 5 degrees of visual angle, and a 300mm lens on a large format camera is 29+ degrees of visual angle...

NO the angle of a lens DOES NOT change by putting it on cameras of varying sensor size. The physics of a lens DOES NOT CHANGE. The angle that various sizes of sensors can record changes with their size, but the angle that the lens projects does not change. A 300mm lens had the same FOV no matter what camera you put it on. The FOV that can be recorded is determined by the sensor size.