Lens and Field Of View

[Lijo]

Consider two lenses:-

1. CANON EF 50mm F1.8 II and
2. CANON EF-S 18-55 IS II

In the first one the mm value is constant. Does it mean that the lens can have only one focal length? If it is so, is there a way to change the Field Of View with this lens?

Lens Review: http://www.the-digital-picture.com/R...ns-Review.aspx

[Manfred M]

The first one is a prime lens, which means the focal length is fixed and by definition the FoV (which is dependent on focal length) is also fixed, i.e. it is a 50mm lens. If you are looking at a crop frame (APS-C) sensor camera, it is not the first lens I would recommend. 50mm is the "normal" lens for a full-frame sensor camera; and the equivlent for a crop frame would be around 30mm.

The second lens you list is a short zoom and the focal length varies from 18mm - 55mm. It is a common kit lens sold by a number of manufacturers for crop-frame cameras and makes a good first lens. Obviously the FoV will vary as well.

[tclune]

Consider two lenses:-

1. CANON EF 50mm F1.8 II and
2. CANON EF-S 18-55 IS II

In the first one the mm value is constant. Does it mean that the lens can have only one focal length? If it is so, is there a way to change the Field Of View with this lens?

Manfred's answer is correct. However, it is both possible and common to change the field of view of a prime lens by cropping the image in post.

[Manfred M]

Manfred's answer is correct. However, it is both possible and common to change the field of view of a prime lens by cropping the image in post.

Just to add, the FoV can be narrowed in post, but not widened.

[rpcrowe]

Additionally, I might mention that zoom lenses only deliver the advertised focal lengths when focused at infinity. When focused at distances shorter that infinity the focal lengths are not what the lens specifications indicate. That is just the nature of zoom lenses.

We occasionally get postings by indignant lens owners that their zoom lens does not deliver the focal length as advertised. This is no great problem, just something that a lens owner should be aware of so that the owner doesn't think that the lens is malfunctioning...

OTOH, since a prime lens doesn't change focal lengths; the focal length is the same whether focused at the minimum focal distance or infinity. This is also just the nature of fixed focal length lenses and, IMO, is no great advantage...

[William W]

The ANGLE of View remains the same for a Prime Lens: in this sense the FIELD of View does not change for a Prime Lens.

However, sometimes when we speak of the FIELD of View, we are referring to (or state) the LINEAR measurements of the SCENE at the PLANE of SHARP FOCUS.

For example, in a previous answer to you I wrote something like “with a 50mm lens a on a 135Format Camera the FoV at 15meters will be about 10m x 7m”

This means that what we expect to be in sharp focus, when we focus the lens at 15m - is a rectangle, about that size: 10m x 7m.

Obviously the linear measurements of that rectangle get smaller, as we focus closer, but the Angle of View of the Prime Lens, remains the same.

WW

[NorthernFocus]

Need I remind this esteemed group that ALL lenses are capable of changing the FOV. It's just that with some one must employ one's feet to achieve the desired effect

[jcuknz]

As I understand it and contrary to Richard's and William's statements the focal length of a lens changes as it is focused, otherwise how would the aperture be effectively smaller when expressed as an f/value. Something has changed in the formulae and it wasn't the aperture. The focal length of a lens is only the stated value when focused at infinity ... sorry to be a nit-picker

The FOV also changes with all lenses as they are focused... perhaps not much but there is a change. I gather this is one of the problems when one manually focus stack. Fortunately I have an editor which will re-size as a percentage of the original should I want to engage in this [ PSP ]. A feature I frequently use as it is easier to remember a percentage applying to both dimensions than a whole lot of numbers of pixels

[William W]

Picking Nits -

As I understand it and contrary to Richard's and William's statements the focal length of a lens changes as it is focused, otherwise how would the aperture be effectively smaller when expressed as an f/value. Something has changed in the formulae and it wasn't the aperture. The focal length of a lens is only the stated value when focused at infinity ... sorry to be a nit-picker

The “Stated” Focal Length of a Lens is indeed a statement applicable to when the lens is focussed at infinity.

But it would be rarely correct, anyway.

It will only be an indicative: certainly NOT always correct to 1mm, as the labelling would have a mathematician interpret. So a 50mm lens might be in fact 50.53mm or 49.47mm Lens – Perhaps even a 51mm lens!

We will find this error to be more erratic for Zoom Lenses.

***

The FOV also changes with all lenses as they are focused... perhaps not much but there is a change. I gather this is one of the problems when one manually focus stack. Fortunately I have an editor which will re-size as a percentage of the original should I want to engage in this [ PSP ]. A feature I frequently use as it is easier to remember a percentage applying to both dimensions than a whole lot of numbers of pixels

This is a phenomenon (now) referred to as ‘Focus Breathing’: a result of one of the compromises of lens design.

High Quality Cine Lenses usually do not have this anomaly – but they do cost a bit more than a Canon L Series.

***

For the application of this thread and most DSLR Photography - I think neither, though correct (I agree, jc) is overly relevant.

Though noted and I agree, a good point about the relevance of Focus Breathing re Focus Stacking, to present to the OP as Macro work is an high priority for him/her: – but how much Focus Breathing might there be in a typical macro stack is another question and I don’t know the answer to that question, off the top of my head.

WW

[DDK]

Need I remind this esteemed group that ALL lenses are capable of changing the FOV. It's just that with some one must employ one's feet to achieve the desired effect

That's so old-school that you have to respect it

[jcuknz]

That's so old-school that you have to respect it

I don't respect it but curse it and glad we now have zoom lenses for stills the way I had for movie work from way back