Technical T Question

[jdlaing]

Advertisement (gone after registration)

On lenses.

 

The 23mm, for example, is actually a 35mm fov on the T. The lens also fits the Leica SL.

 

Why is it not labeled 35mm if that is the only fov it will produce?

[earleygallery]

Because it's not a 35mm lens, it's a 23mm lens. 

[Advertisement]

The Leica Q3 is available for pre-order at B&H Photo Video and Adorama

[jdlaing]

But you can't get a 23mm field of view.

[julian m]

On lenses.

 

The 23mm, for example, is actually a 35mm fov on the T. The lens also fits the Leica SL.

 

Why is it not labeled 35mm if that is the only fov it will produce?

 

For whatever reason it's the way the whole camera industry decided to label lenses when crop sensors became popular. They label it based on the field of view for a full frame lens used on that sensor and consumers are just expected to know that it's different depending on the type of camera. I suspect it's partially because so many full frame lenses already existed, so you could compare your fancy new lenses to what you'd have with the existing full frame lenses adapted for your camera. It's likely also because it's still a measure of a physical property of the optics of the lens, though it's now harder to understand.

 

The labeling does make it clear, for example, that putting a 35mm Leica M lens on the Leica T (via an adapter) will produce the exact same field of view as putting the upcoming 35mm T lens on a Leica T (roughly 50mm equivalent).

 

It makes it less clear, however, what field of view equivalent you'll get when you put that 35mm T lens on the SL!

 

I agree it makes even less sense now that you can put crop-sensor lenses on full frame cameras and still have them automatically crop. Sony has this issue as well.

Edited December 22, 2015 by julian m

[earleygallery]

Yes, you are getting a 23mm field of view, on an APS-C sensor. 

[jdlaing]

On an APS-C sensor I believe it is a 35mm fov.

[jdlaing]

Advertisement (gone after registration)

For whatever reason it's the way the whole camera industry decided to label lenses when crop sensors became popular. They label it based on the field of view for a full frame lens used on that sensor and consumers are just expected to know that it's different depending on the type of camera. I suspect it's partially because so many full frame lenses already existed, so you could compare your fancy new lenses to what you'd have with the existing full frame lenses adapted for your camera.

 

The labeling does make it clear, for example, that putting a 35mm Leica M lens on the Leica T (via an adapter) will produce the exact same field of view as putting the upcoming 35mm T lens on a Leica T (roughly 50mm equivalent).

 

It makes it less clear, however, what field of view equivalent you'll get when you put that 35mm T lens on the SL!

 

I agree it makes even less sense now that you can put crop-sensor lenses on full frame cameras and still have them automatically crop. Sony has this issue as well.

Thanks. I fully understand that. I've been doing this a while and it just seems senseless to do it in this day and age with crop lenses on a crop sensor. No matter how you slice it or dice it the 23mm T lens is a 35mm fov whether on the SL or T. Same goes for the zooms.

[julian m]

Yeah, I think the issue is that focal length is a measure of physical optics of the lens itself. That number used to be all you really needed to understand the field of view. But technically field of view and focal length are two separate calculations—they just happen to be easily related if you know sensor size.

 

What we actually care about is field of view, so it would perhaps be better to begin labeling lenses that only produce one particular field of view with that number. The 23mm T lens, for example, could indicate that it produces 35mm equivalent (even when used on the SL).

 

But then imagine, for example, a future camera with a sensor even *smaller* than the T sensor, but with an adapter to use T lenses. That camera would no longer produce 35mm equivalent field of view, it would become even narrower!

 

Perhaps this labeling could just happen digitally, so the physical lens wouldn't presume to know all future camera bodies it would be used on. The SL and the T could show the field of view (instead of just the physical optics focal length) when a lens is attached or as you zoom. This would make it clearer when you attach a T lens to an SL exactly what you're going to get.

 

I'm imagining this future world should clearly differentiate field of view and focal length—just say that it's 54º field of view across instead of equating it to some particular focal length reference.

Edited December 22, 2015 by julian m

[jdlaing]

Yeah, I think the issue is that focal length is a measure of physical optics of the lens itself. That number used to be all you really needed to understand the field of view. But technically field of view and focal length are two separate calculations—they just happen to be easily related if you know sensor size.

 

What we actually care about is field of view, so it would perhaps be better to begin labeling lenses that only produce one particular field of view with that number. The 23mm T lens, for example, could indicate that it produces 35mm equivalent (even when used on the SL).

 

But then imagine, for example, a future camera with a sensor even *smaller* than the T sensor, but with an adapter to use T lenses. That camera would no longer produce 35mm equivalent field of view, it would become even narrower!

 

Perhaps this labeling could just happen digitally, so the physical lens wouldn't presume to know all future camera bodies it would be used on. The SL and the T could show the field of view (instead of just the physical optics focal length) when a lens is attached or as you zoom. This would make it clearer when you attach a T lens to an SL exactly what you're going to get.

 

I'm imagining this future world should clearly differentiate field of view and focal length—just say that it's 54º field of view across instead of equating it to some particular focal length reference.

Agreed. 

[Michael Geschlecht]

Hello Everybody,

 

I think the reason many manufacturers have decided to publish the field of view relative to the field of a lens of the focal length equivalent necessary to produce the equivalent angle of coverage on a 24mm X 36mm image plane is demonstrated in this very Thread:

The ratios of the different angular image planes's coverages to different image sensors sizes relative to the angular image plane coverage of a 24mm X 36mm image plane varies somewhat manufacturer to manufacturer.

 

By using a reference that has been a standard for an extended period of time it makes it easier for the user to visualize the angle of coverage of the different image plane.

 

Best Regards,

 

Michael

Edited December 23, 2015 by Michael Geschlecht

[grillo]

It's what everyone does: even small sensor compact cameras without interchangeable lenses label their lenses with the real focal length, not the equivalent field of view in 35mm full frame.

[earleygallery]

On an APS-C sensor I believe it is a 35mm fov.

 

Wrong! It's 23mm. That doesn't change. But, if you were to compare it to a FF camera the EQUIVALENT would be a 35mm lens (approx). 

[julian m]

The 'mm' measurement is not a measurement of field of view. It's a measurement of focal length. And there's a short calculation involved to get from focal length to field of view. That calculation involves the sensor size.

 

But everyone uses focal length as a short hand with the sensor size part of the calculation assumed.

 

So when jdlaing says it's "35mm fov" as above, he's implying the assumptions to make that calculation. He did explicitly say "fov" there, not focal length.

 

If we were to be pedantic about it, we'd say that "the 23mm T lens on an APS-C-sized sensor produces the equivalent field of view of a 35mm focal length lens on a full frame sensor".

[jdlaing]

The 'mm' measurement is not a measurement of field of view. It's a measurement of focal length. And there's a short calculation involved to get from focal length to field of view. That calculation involves the sensor size.

 

But everyone uses focal length as a short hand with the sensor size part of the calculation assumed.

 

So when jdlaing says it's "35mm fov" as above, he's implying the assumptions to make that calculation. He did explicitly say "fov" there, not focal length.

 

If we were to be pedantic about it, we'd say that "the 23mm T lens on an APS-C-sized sensor produces the equivalent field of view of a 35mm focal length lens on a full frame sensor".

That's why I explicitly did NOT use the words focal length.

[earleygallery]

 

 

The 'mm' measurement is not a measurement of field of view. It's a measurement of focal length. And there's a short calculation involved to get from focal length to field of view. That calculation involves the sensor size.

 

But everyone uses focal length as a short hand with the sensor size part of the calculation assumed.

 

So when jdlaing says it's "35mm fov" as above, he's implying the assumptions to make that calculation. He did explicitly say "fov" there, not focal length.

 

If we were to be pedantic about it, we'd say that "the 23mm T lens on an APS-C-sized sensor produces the equivalent field of view of a 35mm focal length lens on a full frame sensor".

 

Yes, just as I said. 

Edited December 23, 2015 by earleygallery

[LexS]

On lenses.

 

The 23mm, for example, is actually a 35mm fov on the T. The lens also fits the Leica SL.

 

Why is it not labeled 35mm if that is the only fov it will produce?

It is not about focal length only: it is about depth of field also. This one has the dof of a 23mm lens. And the fov of a 23 mm lens.

[mjh]

But you can't get a 23mm field of view.

There is no such thing as a ‘23mm field of view’. The field of view depends on both the focal length and the image/sensor size, but also on the focus. Associating a focal length with a specific field of view is fallacious.

[mjh]

This one has the dof of a 23mm lens.

There is no such thing a the depth of field of a 23mm lens either. You cannot reduce all the parameters determining depth of field to just the focal length. (Having said that one could argue that comparing the depths of field you get with the same focal length and different sensor sizes is neither here nor there anyway – it would be difficult to imagine a situation where a photographer wonders whether to use a certain focal length with either a larger or smaller sensor.)

Edited December 24, 2015 by mjh

[jdlaing]

There is no such thing as a ‘23mm field of view’. The field of view depends on both the focal length and the image/sensor size, but also on the focus. Associating a focal length with a specific field of view is fallacious.

http://paulbourke.net/miscellaneous/lens/

[Advertisement]

Hallo,

Look here: Leica L-Mount

[bencoyote]

For whatever reason it's the way the whole camera industry decided to label lenses when crop sensors became popular. They label it based on the field of view for a full frame lens used on that sensor and consumers are just expected to know that it's different depending on the type of camera. I suspect it's partially because so many full frame lenses already existed, so you could compare your fancy new lenses to what you'd have with the existing full frame lenses adapted for your camera. It's likely also because it's still a measure of a physical property of the optics of the lens, though it's now harder to understand.

 

The labeling does make it clear, for example, that putting a 35mm Leica M lens on the Leica T (via an adapter) will produce the exact same field of view as putting the upcoming 35mm T lens on a Leica T (roughly 50mm equivalent).

 

It makes it less clear, however, what field of view equivalent you'll get when you put that 35mm T lens on the SL!

 

I agree it makes even less sense now that you can put crop-sensor lenses on full frame cameras and still have them automatically crop. Sony has this issue as well.

 

I would argue that what the camera industry did actually makes sense while the way that we photographers think about focal length and field of view is actually the problem.

 

There aren't actually full frame sensors (or film) and everything else. Especially back in the film days, there were a huge number of formats. 35mm was just one of many film sizes. We even have that today if we take into account all the sensor sizes found in compact cameras and smart phones as well the interchangeable lens cameras including all the smaller format cameras like m43, Nikon 1, as well as APS-C. The three most important parameters for a lens are: focal distance, aperture, and imaging circle. We tend to assume the latter based upon the lens mount so they don't tend to put that in the marketing materials.

 

The problem comes when we photographers associate focal distance and the implicit imaging circle into a familiar field of view. For cameras, especially cameras like the T, we shouldn't talk about a 23mm lens and crop factor. We should talk about the FOV of the lens when it is mounted to our camera. Then things would make a lot more sense.

A 23mm T lens has a field of view 64 degrees. It doesn't matter what camera you put it on.

A 35mm M lens also has a field of view of 64 degrees from one corner of its field of view to the other. However, since you are only using 2/3 of the illuminated circle the sensor only sees 47 degrees. Note that 2/3 is the reciprocal of 3/2 which is 1.5 and is the stated crop factor for an APS-C camera.

 

What we as photographers care about is not really the actual focal distance of a lens but rather the field of view of a lens. That being the case it would be better if we thought about:

This zoom lens has a field of view of between 75 degrees and 28 degrees rather than thinking 18-56mm

 

As an aside one amusing thing that I thought about is with the announcement of the SL Leica said that they are going to make L mount adapters for R, S, and PL lenses. The R adapter will be fairly straight forward. Probably similar to the M-Adapter-T (or are they calling them M-Adapter-L now)  but with mechanism to stop down the lens before shooting. The interesting thing will be if someone mounts a S lens on a T with the yet to be released S-Adapter-L. In that case does our camera suddenly need a new 2nd crop factor in relation to the S's medium format as opposed to in relation to the M or a full frame camera? Probably not. It is better to just think in terms of field of view as opposed to in terms of imaging circle and focal distance. (I wonder if the T will have enough battery current to drive the focus motors in S lenses or if the camera will just say: "Unsupported Lens".) The Vario-Elmar-S 30-90mm will behave just like any other 30-90mm lens on the T (but with 0 vignetting and amazing corner to corner sharpness ;-) ) The key is knowing the field of view for various focal distances and mapping that your impression the feeling of that FOV. Then you can forget the crop factor and how FD relates to FOV on other format cameras.