What does focal length mean when same lense different FOV

Sorry about the previous post

I am wondering what Focal length means when the same lens means a different Field of View (FOV) depending on the camera format used. For example a 50mm "standard lense" may mean 75 mm (35 mm equivalent) on some formats while it equals 50mm on other formats.

When photographs submit camera data, do they submit the actual lense used or the FOV equivalent in 35mm?

I assume it is the actual lense used. If so, isn't the lens length irrevelant info unless you know what the 35mm equivalent is?

Thank you.
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It's all about light.

Virtually always the focal length quoted for a lens is what it would be with a 35mm film ("full frame" in current terminology) camera. ie 70-200mm will behave as roughly 105-300mm on most DSLRs (except full frame ones and fourthirds models). So you have to adjust focal length depending on whether the body you are using is fourthirds, APS-C, etc. See DPReview glossary for better information on this and on sensor sizes.
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Sandy

spikedog123 said:

Sorry about the previous post

I am wondering what Focal length means when the same lens means a different Field of View (FOV) depending on the camera format used. For example a 50mm "standard lens" may mean 75 mm (35 mm equivalent) on some formats while it equals 50mm on other formats.

Click to expand...

Focal length doesn't mean much at all, EXCEPT in the context of which size sensor/film format it is used on. As you have noticed yourself, f-length and format size BOTH have their effects on Field of View, so both have to mentioned whenever the field of view is being specified.

For this reason, many people think it would be better if f-length-with-format was abandoned altogether, and we just specified the field of view, in degrees, probably. It wouldn't take long for people to get used to it.

spikedog123 said:

When photographs submit camera data, do they submit the actual lens used or the FOV equivalent in 35mm?

Click to expand...

Usually actual lens f-length, but sometimes the 35mm equivalent is stated as well.

spikedog123 said:

I assume it is the actual lens used. If so, isn't the lens length irrelevant info unless you know what the 35mm equivalent is?

Click to expand...

No, there's no need to know what the 35mm equivalents are unless you are actually running a 35mm FF camera in tandem and need to match FoVs, say. All you need do is make yourself familiar with the f-lengths that apply to you camera of choice, and don't bother with any other sizes you are NOT using...

.... after all, I don't suppose you are using a 5x4" camera alongside your dSLR, so I'm sure you don't feel any great compulsion to be aware that the 35mm lens of your APS is equivalent to 150mm on that Large Format size, do you?
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Regards,
Baz

Good points all.

I have been looking at the contest images and noting the camera data. It usually indicates lense, fstop, speed, etc. but NOT the 35mm equivalent. So I have to know for example that a Canon 5d is FULL format ( I think) and a Canon T1i is APS to make sense of the lense length.

Just wondering if any FOV standard nomenclature exists.

Thanks again

--
It's all about light.

spikedog123 said:

Just wondering if any FOV standard nomenclature exists.

Click to expand...

Hi,

Standard way of quoting FoV is in degrees, eg 45° and, to confuse the issue further, it's measured across the diagonal of the CCD or film. So (again) the format or aspect ratio comes into it.

Regards, David

The only thing you need to know is how focal length pertains to YOUR camera. Focal length is a physical property of the lens that has to do with it's magnification properties, not field of view - that is a property of the sensor. Focal length never changes from one format to the next, it is absolute. In the film days people who shot 35mm never spoke of medium format equivalent FOV. And people who shot compact cameras never spoke of 35mm FOV. The were only concerned about what the lens did on their camera.

spikedog123 said:

Good points all.

I have been looking at the contest images and noting the camera data. It usually indicates lense, fstop, speed, etc. but NOT the 35mm equivalent. So I have to know for example that a Canon 5d is FULL format ( I think) and a Canon T1i is APS to make sense of the lense length.

Click to expand...

Why? Does it make the picture any better or worse to know this?

spikedog123 said:

Just wondering if any FOV standard nomenclature exists.

Click to expand...

The best thing to do is to educate people to get over it (FOV equivalence). It's too abstract for people to think in terms of 33 degree field of view or whatever. The true focal length of a lens tells you a lot more about what you'll get than the FOV equivalent does.

iMac said:

Focal length is a physical property of the lens that has to do with it's magnification properties, not field of view - that is a property of the sensor. Focal length never changes from one format to the next, it is absolute.

Click to expand...

Exactly.

Imagine fixing a lens in a clamp on your kitchen table so that it casts an image of the garden on the back wall. How big things appear in the image (on the wall) depends on the focal length of the lens.

Then stick an A4 piece of paper on the wall in the middle of the image circle. This is your sensor and the field of view depends now much of the image is captured by that particular sensor. if you replace the A4 sheet of paper with an A5 sheet, you capture less of the image and the field of view is therefore narrower, but the lens and the image it casts have not changed.

The fact the you capture a smaller part of the image gives you the appearance of having zoomed in when the image is enlarged to give a print, hence the 'effective focal length' (a horrible term) is higher. You are just taking a smaller part from the centre of the image (A5 instead of A4) and blowing it up twice as much when you view it.

Best wishes
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Mike

Barrie Davis said:

For this reason, many people think it would be better if f-length-with-format was abandoned altogether, and we just specified the field of view, in degrees, probably. It wouldn't take long for people to get used to it.

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But wouldn't that leave user with the same problem? If an EF lens was marked as having a 20° FoV, would that still be 20° on an EFS body anymore than the 18-55mm lens really gives an effective 18-55 focal length range on the same bodies?

Mjollnir said:

But wouldn't that leave user with the same problem? If an EF lens was marked as having a 20° FoV, would that still be 20° on an EFS body anymore than the 18-55mm lens really gives an effective 18-55 focal length range on the same bodies?

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Oh yes. In those lenses that can be used on more than one format, it WOULD still need specifying by format size in use. There is no way 'round that .....

(##) see below)

.... but at least people would know it was Field of View that was under discussion, rather than what's a "real" focal length and what's an "equivalent" focal length on some other format than that which is in use at present.....

..... the degree ° symbol itself being a good indicator of that.

(##) It happens that Large Format lenses , the sort of lenses which can be used in front of any sized film/scan-back you care to place behind them, have their "circle of coverage" specifically defined in data sheets. This gives users the means to know....

• the largest format size those lenses may be used on with no camera movements, for argument's sake, a maximum size of 10x8" with the camera locked up on its zeros.....

• and/or the amount of movement off-axis that might be employed in a smaller format behind the same lens, say, 2" rise and 2" fall on the 5x4" format, all done within the circle of coverage and before that coverage misses the an edge of the image...

In short, you can't leave the format size out of FoV considerations.. it is far too intimately involved in them.
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Regards,
Baz

iMac said:

In the film days people who shot 35mm never spoke of medium format equivalent FOV.

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Medium format was never targeted at casual photographers, and the transition from medium format to 35mm (to the extent that there was one) was over long ago. So most 35mm photographers wouldn't have cared about medium format FOVs.

The transition from film cameras to digital cameras is more recent, and involves a lot more casual photographers, and sensor sizes. There is a greater need for a common yardstick, and since cameras never traditionally indicated FOV in neutral ("degrees") terms, "35mm-equivalent focal length" became that yardstick by default.

iMac therefore iAm said:

And people who shot compact cameras never spoke of 35mm FOV.

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Once 35mm film was out for a while, most (if not all) compact cameras used it. So there was no mapping required. With digital compacts, sensor sizes vary from one compact to another -- and virtually all are much smaller than those of DSLRs. Thus you would have a "multiple mapping" problem even if all 35mm cameras (compact, SLR, full-frame DSLR) were to vanish from the face of the Earth.

Mjollnir said:

But wouldn't that leave user with the same problem? If an EF lens was marked as having a 20° FoV, would that still be 20° on an EFS body anymore than the 18-55mm lens really gives an effective 18-55 focal length range on the same bodies?

Click to expand...

If a lens has a mount that allows use with multiple sensor sizes, then you are correct. That means that large parts of the Canon, Nikon, Pentax, etc. lens catalogs could not use physical FOV markings on the lens barrel.

But modern bodies are computerized, and modern bodies know where the lens focal length is set (how else would they come up with a number for the EXIF data?). The bodies also know their own sensor sizes (a constant for any given model).

Thus,

1. A suitably designed body can display a live Field Of View measurement anywhere that there is a suitable display (viewfinder, top-panel LCD, back-panel LCD).

2. A suitably designed body can record field of view information in the EXIF data -- along with the absolute focal length and the sensor size.

Thanks for your answers. The question came to me while I was viewing some contest pictures. They list the lense used (ex. 80mm) but not the format so unless you also know the camera and the format used, you can't actually know what lense length means with regards to the image displayed. (of course you can guess).

I am still stuck in 35mm mentality. It is kinda like converting English system to Metric. Rather I simply understood the original measurement rather than converting.
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It's all about light.

spikedog123 said:

Sorry about the previous post

I am wondering what Focal length means when the same lens means a different Field of View (FOV) depending on the camera format used. For example a 50mm "standard lense" may mean 75 mm (35 mm equivalent) on some formats while it equals 50mm on other formats.

Click to expand...

Focal length is simply a measurement inside the lens. FOV of the lens is determined by the sensor size it is used on.

spikedog123 said:

When photographs submit camera data, do they submit the actual lense used or the FOV equivalent in 35mm?

Click to expand...

Different people do different things. Some list the actual focal length, some list the 35 mm equivalent. Most digicam users list the 35 mm equivalent while most dslrs users list the actual focal length, not the 35 mm FOV.

spikedog123 said:

I assume it is the actual lense used. If so, isn't the lens length irrevelant info unless you know what the 35mm equivalent is?

Click to expand...

Don't assume anything. And yes the focal length is somewhat useless to know if you don't know the sensor size. But if you know Canon is 1.5x and Nikon is 1.6x and Olympus is 2x, the it's easy to figure out just by knowing the brand of camera. When the 35 mm equivalent is used, most people have the sense to mention it is the 35mm equivalent. But again, never assume anything.