Why Don't We Use Field of View?

Why Don't We Use Field of View?

Focal length and maximum aperture are the most common measures for summarizing the principle characteristics of a lens, but are they the best to use? Given the range and proliferation of different camera types, why don't we see field of view used more often?

Most photographers are familiar with the meaning of focal length and maximum aperture, and their impact upon the photographs they take and make. Focal length determines how much we can see in an image, whether it is wide angle, normal, or zoom. And we’ve become familiar with a "normal" nifty-fifty, street photography at 35mm, going wide at 24mm and shorter, the flattering effects of a portrait lens at 85mm, and then specialist zooms beyond 100mm. But what does 75mm on the Pentax 645Z, 10.4mm on the Sony RX100 V, 105mm on a Nikon D850, 129mm on the Panasonic Lumix ZS50, or a 4.47mm Google Pixel 2 really mean?

How much we can see is more commonly known as the field of view. As the lens opening is circular, it projects a cone of light rays onto the camera sensor; the further away the lens (i.e., the focal length), the narrower the field of view. As most cameras have a rectangular sensor, the horizontal and vertical angles of the cone (measured in degrees) define this.

However, it is the combination of focal length and sensor size that determines field of view. So when we talk about focal length, camera manufacturers (and reviewers) have defaulted to using this value for a 35mm (full frame) system. This is incorrect for non-full frame cameras and so we have to use the crop factor (usually assuming the same aspect ratio) as a multiplier to give the effective focal length, showing what you would see if it was a full frame system. So for example, the Nikon D7500, a DX sensor at 23.5 x 15.6mm, has a 1.54x crop factor when compared to the full frame (FX) D850 with a 35.9 x 23.9mm sensor. A 35mm lens on the D7500 therefore has an equivalent field of view to a 54mm lens on the D750.

Isn't it time for manufacturers and reviewers to use a physically meaningful measure? For over 50 years 35mm has had a virtual monopoly in terms of film sales so sensor size and focal length were synonymous. Only professionals used medium and large format and, arguably, understood the impact of focal length on field of view. But that's definitely no longer the case. We have APS-C, Micro Four Thirds, and 1-inch all being very common consumer formats, and that's before you get in to the myriad range of sensor sizes in smartphones and action cameras. Indeed, it's noticeable that some manufacturers, such as drone maker DJI, report field of view first for their cameras, then 35mm equivalent.

Sure, carry on using focal length as many people still understand it, but the plethora of sensor sizes makes this a redundant measure for what photographers need to understand and is irrelevant for the general public. And for the record, that would be 33 degrees, 65 degrees, 19 degrees, 3 degrees, and 75 degrees respectively.

Image by fernandozhiminaicela via Pixabay.

Mike Smith's picture

Mike Smith is a professional wedding and portrait photographer and writer based in London, UK.

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That would be great for a lot of people. Even beginners would understand. For instance the cone of visual attention for humans is 54 degrees. We see more things with our peripheral vision, but we concentrate on that field of view.

I guess it's also easier to advertise "more = better".
More megapixels, more resolution, more hertz, more frames, more millimeters, ...

Field of view (angle of view) is probably very important in panoramas. I'm planning one for tomorrow morning and went looking for this spec for my lens. The information I found was lacking. For a Canon 70-200mm f/2.8L IS II, I got no info from Canon's site. I went to B&H and all they gave me was 34 deg-12 deg. I'm assuming those are for 70mm and 200mm respectively. What about at 100mm and 135mm?

Why not look up some 100mm and 135mm prime lenses and see what their angle of view is? For example, the Canon 135mm f2L is 18 degrees, the Sigma 135mm f1.8 ART is 18.2 degrees, the Canon TS-E 135mm f4L is 18 degrees, and the Canon 135mm f2.8 Soft Focus is 18 degrees.

It's probably not going to be 100% spot on, also considering that focus breathing exists since you're not working with a cine lens and the specs don't say whether the angle of view is based on minimum focus distance or infinity. But it'll get you in the ballpark at least.

I figure I can just assume a 12-deg angle of view (AoV) throughout the focal lengths and base each shot on that. Then, pan 10 degs for each shot. The actual view at any focal length will always be wider than 10 degs. That way, I'll always have the overlap I need. I'll just be taking more shots than necessary. But, I can live with that.

Just look it up. The fov is not vendor specific: https://www.nikonians.org/reviews/fov-tables

I find focal length useful because they represent categories of perspective, which FOV is only a part of. The visual characteristics for a given focal length that you become familiar with over time are what inform my lens choices, and that may or may not have anything to do with "how much can I fit in the frame?" Maybe the distinction is more subtle than I'm making it out to be, and maybe I'm just thinking from habit.

I totally do this...I see in terms of the focal lengths of the camera I am using.

Focal length is an attribute of the lens but FoV varies with the application of that lens. A 50mm lens has a focal length of 50mm no matter what size the film/sensor size is (yes, I know falloff on the edges limits the practical application of this but I'm hardly practical). Identifying a lens as 40 degrees only defines its function in a given application. What might I get if all I ask for is a 40 degree lens? The essential elements (focal length and sensor size) are absent from that spec. By the way, the specs on that file I need: 300 dpi.

Does not make sense using FOV. It varies a little with focus breathing, and I think it would be confusing when we talk about FF lenses that still work on M43, APS-C, APS-H...or better yet, when we use a 6x7 or 645 lenses on FF.
A 50mm is about normal on FF, but com a 6x7 it's wide, and it's a tele on a M43. The FOV woud change in these three situations, but it's still a 50mm. How would someone advertise that?

I have thought this for a long time - the field of view could be a much better way to compare the outcomes provided by lens/camera combinations. And, as photographers, outcomes is what matters most.

But I foresee two potential areas of confusion if FOV (ie an angle, measured in degrees) is used instead of focal length:

1. Higher levels of zoom require smaller FOV numbers, which would be counterintuitive to many beginners. A 3 degree field of view results in a huge magnification, whereas a 120 degree field of view means the opposite. It's the same confusion that many beginners have with f numbers, where the numbers get smaller as the lens allows more light through.

2. Conversions would be much more difficult when using FOV. What field of view does a full frame 64 degree FOV lens project, when the lens is attached to an APS-C or M43 sensor? Rather than being able to multiply by 1.5 or 2, like we do for focal length, we would instead need to use trigonometry to make the conversion.

So I think that focal length is likely a better stat than FOV for both beginners and experienced photographers. It avoids the counterintuitive numbering system caused by FOV figures, that often also confuses beginners when it comes to aperture. And using focal length is likely better for experienced photographers, who would suddenly be required to use trigonometry to convert FOV angles when previously they only needed to use straight multiplication.

I would have no objection to FOV being used more often, as a supplement to focal length figures, though.

Talking in terms of angle of view would work fine, despite "nattering nabobs of negativism."

So someone asks me, "what lens did you use?" If I say "I used my 55mm," the next thing they have to ask is what my format is. If I say, I used my 50 degree lens, it's up to that guy to figure out what that would be with whatever format he's using. That's just something people should know about their own equipment to call themselves "photographers."

Angle of view is actually the direct answer to the real question. The only convention we'd have to form is whether we're talking about the angle of view on the diagonal or the long dimension (nobody thinks about AOV of the short dimension).

But how do you know that, by asking the 'which lens' question, the real answer is FoV? They may have liked the shallow/deep DoF and wanted to know the focal length of the lens. In this case FoV is useless.

I completely support this idea. The most important factor in choosing a lens is fov; the focal length needed is a result of the fov plus my sensor size. Using this nomenclature might start down the path of never again debating so-called "equivalence".

This is the best idea since sliced bread. I'd love to always know the angle of view I'm getting with any lens on any sensor.

The problem is, we would wind up having to change the name of lenses whenever they are mounted on different bodies. This would be even more confusing than the current situation, IMO.

Nope, leave the nomenclature on the lens just as it is. That's an accurate description of the lens design.

Think of water bottles, for instance. For some purposes, a liter bottle is the "big" bottle; for people in that context, "get the big bottle" is a fine reference. For other purposes, a liter is small; for people in those contexts, "get the small bottle" is a fine reference. But the bottle should always be marked "one liter."

Agreed. I just wish that the angle of view were more commonly used, period. But at least it's usually listed in the specs of lens' sales pages, so I think that's the most we'll ever get.

I've been shooting for along time, and I used to shoot almost entirely primes back in the day, and although I may know a lens as a Xmm, it's really their field of view that makes me think of them.

I see lots of reviews for action cams and cell phones that use FoV! Only interchangable lens system-folk still use focal length.

this makes perfect sense...FoV when the lens is fixed (we cannot choose the application so let us know practical effect of the lens) and focal length for interchangeable (we have choices, let us know the actual focal length).

I really like using angle of view/field of view when talking about the behavior of a lens. It is way more "real world' useful in terms of understanding what's going to happen when you put a lens on a camera and point it at your subject than simply using the focal length. The problem, as some have pointed out, is that FoV depends on the imager size while focal length stands independent.

What I feel we should get away from is talking about crop factor when we talk about lenses on different cameras. It doesn't matter to me that a 24mm lens on my APS-C sensor behaves like a 35mm lens on a full frame because I haven't shot enough with full frame lenses to be able to visualize what that looks like.

I wrote a little article for my company about using FoV/AoV instead of crop factor. You can check it out here http://zsyst.com/2018/01/demystifying-crop-factor/

"What I feel we should get away from is talking about crop factor when we talk about lenses on different cameras. It doesn't matter to me that a 24mm lens on my APS-C sensor behaves like a 35mm lens on a full frame because I haven't shot enough with full frame lenses to be able to visualize what that looks like."

I don't understand this. How can you not visualize it when it's right in front of you? The answer is in your statement: you have a 24mm lens on an APS-C camera, which is pretty much the same field of view as you would get with a 35mm lens on a full frame camera. Knowing this fact when looking through the viewfinder, you don't have to "visualize it" in your mind, it's right there in front of your eye. Want to know what an 70-85mm-ish lens looks like on a full frame camera? Slap a 50mm lens on an APS-C camera. Bam, there you go. You can see it without "having" the full frame camera. That's the whole point, being able to compare what you would see from 2 different cameras/lenses.

You've got the same confusion in the article you linked when you say that you don't like crop factor because, "It doesn’t tell you anything concrete about the camera that is right in front of you", but precede that remark with "Because it is a relative term that is a comparison of one camera with one type of sensor to another camera with another type of sensor." Yes, that's the whole point, being able to compare 2 different camera/lens systems to have an understanding of what the field of view would look like across the two.

Here's a practical example of why it matters to some people:

Let's say you're shooting a wedding/event/concert/whatever, and you're carrying two cameras with prime lenses, but one is a full frame and one is APS-C. If you only went by the focal length of the lens, you might say, "I'm shooting at 50mm and 85mm", but if the 50mm is on the APS-C camera, you've basically got 2 cameras with the same lens (close enough)... and what's the point of that? The statement may technically be "true", but if your goal was to have 2 different field-of-view options then you pretty much failed. So yes, it's relevant.

By "visualize" I mean before I even put my eye up to the viewfinder. I am often in situations where being able to know ahead of time what lenses I need and where the camera should be placed is invaluable. I shoot almost exclusively APS-C or Super35, so I know without even picking up a camera what sort of look I am going to get from a 24mm lens. It is completely irrelevant in that moment that I would get the same look from a 35mm lens on a full frame camera.

My hole point is that crop factor is only useful in a comparative setting. It's great if you have multiple cameras with a variety of sensor sizes. But I see far too many people getting caught up in "what's the crop factor of this camera" when they have never and will never use a full frame camera, so knowing that the crop factor of the Sony a6500 is 1.5x, making their 50mm lens have the same angle of view as a 75mm lens would on a Sony a7Rii doesn't get them any closer to understanding what their camera is doing than they were before the question was asked.

If instead you were able to say that a 50mm lens on an a6500 has an angle of view of roughly 27 degrees they would know that on the a6500 the 50mm lens has a fairly tight field of view. In this scenario there is no need to bring up any piece of equipment other than what is right there in front of you.

I get what you're saying but you're fighting a nonexistent fight. Saying that crop factor is bad because it's only useful for comparison is like saying food is bad because it's only useful for eating. The whole point of crop factor is for comparison. That's what it is. Trying to use it for something else is using it for something for which it's not intended, and criticizing it for an unintended use is just missing the point.

Regarding your statement that it's pointless for somebody to want to know the crop factor of a camera/lens combo even if they never intend on using a different sensor size, I still disagree. I think a great example would be somebody who is drawn to really wide angle shots and wants to replicate those on a smaller sensor. Let's say they research photos they like and find that they were shot at 16mm-20mm on a full frame camera. Fields of view tend to change more noticeably at extreme ends of the focal spectrum, so the difference between 12mm and 22mm is way more pronounced than the difference between, say, 50mm and 60mm. But if a person buys a Sony a6500 and gets a 16mm-50mm kit lens because "16mm is ultra-wide", they might be disappointed that they're not achieving the look they're going for because they're essentially using a standard 24mm-70mm lens. In this case knowing the crop factor would be extremely useful because when they see that the image they liked was shot on full frame, they would know that the Sony 10-18mm lens would be a more appropriate choice for their a6500.

You could argue that the "actual" field of view number (120 degrees, 50 degrees, etc) would be a better metric but the fact remains that you still need a starting point to visualize what that means, and in order to visualize that you need an example. And if you have an example then you already have a sensor size/focal length combination, which you can use in the same manner (via comparison). So regardless which value you use, you'll still need some manner of comparison in order to use it for a practical purpose.

Seems to me that either way you have to ask a follow up question, so where’s the advantage of changing? Serious question!

I think from an education (vs product) perspective it's important to understand how field of view relates to the focal length / sensor size combo, which is why any time somebody asks me "what focal length did you use for that picture" I always include both in the answer (i.e. 24mm on full frame), because most of the time the question they're *really* trying to ask is what field of view were you using on your camera when you took the photo. I think something that's helpful for beginners to learn is "how to ask what you're really asking". The example I like to use is that people see pictures and constantly ask about the lens, but on its own that's a fairly arbitrary question as it relates to the picture itself. Are they wondering because of the sharpness of the picture? The focal length/field of view? The bokeh? And so on. Most of the time what they're really trying to ask is, "what aperture did you use?", "what field of view is this?", etc, they just don't know how to ask so they default to "what lens did you use?".

Getting back to the product side, as to whether manufactures should do a better job of pointing this out, I think that's a complicated question because as was mentioned, field of view requires both the focal length and the sensor size, so you still can't put that spec on a single item (i.e. the lens). What I do find really interesting is the discrepancy between how manufacturers describe their product lines across different sizes/models... look at most point & shoot cameras and they'll say 24mm-70mm (or whatever the 35mm equivalent range is for that model), not "8.8mm-25.7mm" (using the RX-100M3 as an example). Sure, that # is stamped on the lens but the advertised product description doesn't use it. Then you jump up to APS-C and they use *only* the focal length, then you jump to full frame and yes, they still only use the focal length but *this* focal length is the reference point for field of view on their point & shoots but not APS-C cameras. I think that ends up being the source of a lot of confusion.

This is a great point. I think lens manufacturers need to start advertising this with their lenses and it will become a standard.

FOV doesn't account for sensor ratios. Eg. APS-C is much wider than m43rds but is only marginally taller. Medium format is also usually not 3:2.

You could measure FOV diagonally but that's probably even more confusing for consumers.

Mike Smith is basically correct -- it's tradition. Those who own interchangeable-lens cameras quickly become familiar with the image each lens creates. No one has to tell someone owning a Nikon full-frame and a Rollei 2000 that the 80mm standard lens for the latter is going to cover a wider field of view than a 105mm Micro-Nikkor than suggested by the.30% difference in focal length.

The current confusion seems to be caused by lens interchangeability among cameras with different sensor sizes. The arguments among users about "focal length", effective f-stop, field of view, etc, are appalling. These folks generally know little or nothing about basic photography. No matter how carefully you try to explain things, they will not listen -- or think.

There is no way to define a lens's field of view with a meaningful single number, if the formats have different aspect ratios. Any attempt to do so will only further muddy the waters, which are already badly polluted. Anyone who doesn't understand the optical principles of photography using an interchangeable-lens camera should learn them. Maybe the light will go on.

My view is that this is one those technical deals you have to understand in photography, just like shutter speed, just like f stop, like T stop, like ISO. You can choose to try to understand or you can choose to ignore. Angle of view can not replace the focal length data, but it can be a welcome addition.
At least the number of people complaining about those confusing mm's instead of inches and/or meters in the focusing scale, has gone down over the years.
I still find it hilarious that even in most american websites they give measurements in a 50/50% ratio imperial/metric.
From the fstoppers article on the new Tamron lenses:
"...This lens weighs in at 1.89 pounds and measures 2.99 inches by 6.90 inches...."
So...That is 1 pound and how many ounces? 6 inches and how many quarters of an inch?
How does 17. 17/16 cm sound? Just as ridiculous.