Crop factor - misleading

Reflex · Mar 23, 2014

Wondered if anyone else can verify my recent experience...

I bought my daughter an old lens for her APS-C camera. It was a relatively cheap Tokina 19-35mm designed for full frame use. 19mm is still a useful focal length for APSC sensors so I thought it would be worth a try. Set to 19mm however the view that the lens actually gives is more like a 29mm lens even if by 35mm the view is closer to its claimed focal length. Is this lack of wideness a result of the extreme distortion that comes with an extreme wide-angle lens meaning that the crop-factor calculation does not work with this kind of lens? Have others seen the same kind of effect?

Thanks for any of your thoughts.

astroNikon · Mar 23, 2014

Yes, that is field of view.
a Full Frame is basically 1.0x the focal length (thus a 19mm is 19)
a Nikon dSLR crop is basically 1.5x the focal length (a 19mm is 28)
and a Canon dSLR is generally 1.6x the focal length

here is a visual reference
http://www.thephotoforum.com/forum/...rence-between-full-frame-cropped-sensors.html

if you search "sensor size" in this forum you will get alot about this issue
http://www.thephotoforum.com/forum/...354984-sensor-size-focal-length-question.html

basically,
the lens projects an image. That image is static in SIZE
the sensor behind it varies dependent upon camera
and the sensor only captures the area that it covers, the full-size image thus "cropped"
That is what "causes" the "magnification" I state above of 1.5x etc

thus the reason when you buy small cameras they have small focal lengths which seems normal. as when you get smaller sensors you get more magnification ... at least that is my understanding.

astroNikon · Mar 23, 2014

And Welcome to TPF

Braineack · Mar 23, 2014

19mm is wide on a FF. It's not on a crop sensor.

Overread · Mar 23, 2014

Focal length is measured when the lens is focused on infinity and is universal no matter what the sensor/film size behind it is.

A 19mm lens when used on a 35mm gives you the angle of view that 19mm on fullframe will show. Because of the popularity of 35mm film its often the base-line to what other films and sensors are set to - ergo why 35mm sensors in digital are called "full frame".

Now when you put that 19mm lens on a camera with a different sized sensor you get a different angle of view; the crop sensor digital cameras (commonly 1.6 crop for Canon and 1.5 Crop for Nikon) the angle of view changes. What you get is similar to what you'd see if you used a longer focal length lens on a fullframe.

So a 19mm on a 1.5 crop camera (eg Canon) would be similar to the angle of view that a 30mm lens would give on a fullfame body.

Now there is also an added element here which is focus change due to focusing point. Many lenses often shift their focal length as the focusing point is moved closer and closer (esp in some zooms). As a result focusing nearer to the closest point of possible focus the lens might appear to have a different angle of view than the one it "should" based on its focal length. Typically this isn't too much of a problem as a small lean back or forwards will correct it.

480sparky · Mar 23, 2014

Not misleading....... merely misunderstood.

Reflex · Mar 23, 2014

Thanks so much for all the replies and the welcome. Really appreciated.

Well; this is my mistake here and I apologise for the confusion. The question was not meant as it was answered by all those kind and informed replies. What I am asking is this - with this lens set to 19mm it is not giving the field of view that one would expect from a true 19mm lens on an APS-C sensor camera. Instead it is giving the field of view one should see with a lens of about 29mm on an APS-C sensor. I hope that has eliminated the confusion in the earlier post.

[So if I translate to a 35mm film equivalent full frame sensor for completeness (and considering that this is on a Canon camera) that would equate to seeing the field of veiw equivalent to a focal length of about 30mm but actually seeing the equivalent of 46mm.]

At its longer end (the 35mm focal length) this lens is giving a field of view that is roughly right or perhaps a mm or two out like say 36mm or so.

So the question is whether anyone else has noticed a full-frame ultra wide-angle seemingly delivering the wrong focal length on an APS-C cropped sensor in this way? Having an apprently longer focal length than it shoud have (and for completeness I mean longer than it should have comparing like-with-like not comparing cropped with full frame)?

I have wondered if this is caused by the inevitable geometric distortion in any ultra wide-angle lens.

I hope that is now clearer.

Thanks again all for the replies so far.

Designer · Mar 23, 2014

As it has been explained here many times (see other thread from a couple of days ago) it is not "wrong".

No, it is not caused by "inevitable geometric distortion".

No, we are not confused.

Overread · Mar 23, 2014

Reflex read the latter part of my last post. I did wonder from your first post if you meant that it was appearing longer than it should. Try the lens at infinity and see if it appears closer to what you expect it should be.

Robin Usagani · Mar 23, 2014

That lens will mount to a full frame so they don't put the 35 mm equivalent because it IS a 35 mm lens.

Robin Usagani · Mar 23, 2014

My point is.. If you look at canon EF-S lenses and look at the description.. they will put the 35mm equivalent.

Reflex · Mar 23, 2014

Overread said:
Reflex read the latter part of my last post. I did wonder from your first post if you meant that it was appearing longer than it should. Try the lens at infinity and see if it appears closer to what you expect it should be.

Thanks for pointing that out. The lens was focussed at about 100 metres. I will try again focussing on something closer to infinity.

Reflex · Mar 23, 2014

Robin Usagani said:
My point is.. If you look at canon EF-S lenses and look at the description.. they will put the 35mm equivalent.

Robin

Hi. Thanks for your input. I am not a Canon user - I have more experience of Nikon myself. So niether I (nor in fact my daughter) have any EF-S lenses but I think what you are saying is that on such lenses they do not say their actaul focal length (like Nikon do) but the equivalent focal length after taking the crop into account.

Did I understand correctly?

Thanks for pointing that out.

Overread · Mar 23, 2014

Focal length is not dependant upon the sensor/film size. As such most cameras only state the actual focal length on the lens itself. 19mm is 19mm no matter if the lens is for fullframe, crop sensor, medium format or point and shoot.

The point and shoot market sometimes states things in equivalents, but normally states that pretty clearly (and normally they prefer to use the "zoom factor" as opposed to focal length -10*optical zoom on its own means nothing but that's enough for casual users and marketing).

Reflex · Mar 23, 2014

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.