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Just looking at Fuji X mount for this post but there are others.

 

Why oh why do manufacturers still designate the focal length of lenses as if they were being produced for a FF (35mm) sensor camera when in fact they are made for use on crop sensor models and will probably never be used with an FF model. (Not unless there is an adaptor and the tog wishes to obtain a certain effect.)

 

It would be so much simpler when looking at which lens to use with a crop frame sensor body if it was marked with the crop frame focal length without having to carry out complicated maths to find the true focal length for a lens which is actually made for that camera.

 

Just in case I am not making myself clear.  Fuji XF 18 - 55mm lens (as marked) is actually 27 - 84mm (according to Fuji) on the X-T1, X-E2 etc. Would it not have been simpler to have marked it with 27-84?

 

Or are we all now brainwashed into the "standard" designation and it would be foolish to change at this late stage?

 

Allan

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Perhaps it has become a marketing strategy at this point. Most people entering photography these days have probably never used 35mm cameras -- i.e. they actually think they are buying an 18-55mm lens with a wider angle of view.

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Or are we all now brainwashed into the "standard" designation and it would be foolish to change at this late stage?

 

 

Yes.

 

Don't forget there are some lenses which are used regularly on both formats. How are you going to designate those?

 

And even if that were not the case I still don't agree with a change. An 18mm lens has an 18mm focal length. The field of view is a function of the sensor, not the lens.

 

Alan

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It's arbitrary and born out of familiarity.

 

By way of comparison, a focal length of 50mm is considered wide angle on a medium format camera yet surprisingly, you don't see "35mm equivalent" focal lengths quoted for, say, a Hassy.

 

It's also worth saying that as a result of this arbitrariness, this "35 equivalent" thing is responsible for all of the wibble you see about DOF with a given aperture.

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Not sure that there is a problem here Alan. My understanding is that the actual focal length is 18 - 55mm, which is equivalent to whatever on a FF camera depending upon the size of the sensor. Would it not be more confusing if they marked the lens with the equivalent focal length?

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My 18-55 fuji lens says 18-55 on it and my 35mm says 35mm on it so i am not sure what you are getting at...

 

What would you call your 35mm lens if mounted on an APS-C sensor? I.e. - you took it off your full frame body and put it on an APS body?

Focal length is focal length, plain and simple, it does not change (well, unless your zooming it ;-) )

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Ah yes, having read your post again Allan, you seem to have things back to front.

 

Every lens I've ever seen on any camera - including small sensor compacts - is marked with actual focal length. It's only the marketing bumph that then gives the 35mm equivalent (which as I said above is arbitrary but familiar).

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You need a fixed point to set the standard. Is a 55mm lens not a 55 mm lens when mounted on a tele converter? Sensor sizes vary, even with the same manufacturer, across its range.

 

dov

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Thanks for the interesting replies.

 

As a long standing (my legs are starting to ache) photographer I do have knowledge of focal length, field of view, image circle, etc, etc, from when I first started proper about 40 years ago.

 

What I was trying to say was that a lens specifically made for a camera with a "crop sensor", even if that lens has a manufactured focal length of 18-55, is produced smaller in diameter, thus giving a reduced image circle to suit the smaller sensor and therefore would not be suitable for use on a FF (for sake of argument 35mm sensor) sensor which requires a larger image circle. Then because of the necessarily reduced image diameter giving a consequent reduced field of view the perceived focal length is 27-84mm. Which, in my opinion, is then the true focal length of that lens and should be marked as such.

 

@Alan - The field of view is a function of the sensor, not the lens.

If the lens is manufactured with a reduced image circle then it would not be able to achieve the F of V for use on a FF sensor.

 

@Russell -By way of comparison, a focal length of 50mm is considered wide angle on a medium format camera yet surprisingly, you don't see "35mm equivalent" focal lengths quoted for, say, a Hassy.

Interesting point raised by Russell.

 

@Bryan -Not sure that there is a problem here Alan.

No problem Bryan just interested in my perceived paradox in lens designation for purpose made lenses for use on smaller sensor cameras.

 

@Julie - My 18-55 fuji lens says 18-55 on it and my 35mm says 35mm on it so i am not sure what you are getting at...

The XF lenses were manufactured specifically for APS-C sensors and, by Fuji own admission, have an actual focal length on those cameras of 1.5x the marked designation.

 

@dov -You need a fixed point to set the standard. Is a 55mm lens not a 55 mm lens when mounted on a tele converter? Sensor sizes vary, even with the same manufacturer, across its range.

I don't have an answer for dov.

 

Very interesting discussion though. Keep the replies coming.

 

Allan

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...

What I was trying to say was that a lens specifically made for a camera with a "crop sensor", even if that lens has a manufactured focal length of 18-55, is produced smaller in diameter, thus giving a reduced image circle to suit the smaller sensor and therefore would not be suitable for use on a FF (for sake of argument 35mm sensor) sensor which requires a larger image circle. Then because of the necessarily reduced image diameter giving a consequent reduced field of view the perceived focal length is 27-84mm. Which, in my opinion, is then the true focal length of that lens and should be marked as such

...

But the physics of it are that the focal length, as measured on an optical bench, is 18-55mm. The image circle doesn't enter into it.

 

Turn it around. Put a 24-70mm "full frame" lens or that 50mm Medium Format Hassy on an APS-C sensor. What are their focal lengths now?

 

Another way to think about it is to use the 24-70mm on a full frame but crop the image to the field of view of an APS-C sensor. It now looks like a 36-105mm field of view but the focal length hasn't changed. You've just (arbitrarily) cropped your image to look like it was shot with a longer focal length.

 

Remember. This is all arbitrary (sorry for repeating myself). The "35mm equivalent" descriptor arises because of familiarity with that format. If no-one ever used the 35mm or full frame format and instead used nothing but 120 format film until APS-C came along, we'd be applying a different crop factor.

 

And the only reason we have full frame now is so that all of our lenses designed for our old 35mm fillum cameras work on a 1-to-1 basis.

 

These are all accidents of history.

Edited by Russell Watkins

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The focal length of a lens refers to how the optic focuses light and is totally independent of what it might be attached to.

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You need to know the actual focal length of a lens for various reasons, but not all that often if you're a beginner. It would make more sense to mark lenses with their angle of view, say 30°-75° or whatever, but since that has never been done in the past it's too much of a departure.

 

Makers of cameras with very small sensors do sometimes put both the actual, and the 35mm format equivalent, either on the lens or in the instructions. That's because focal lengths like 5.7mm or 8.8mm are really not very easily translated.

 

David

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It would make more sense to mark lenses with their angle of view, say 30°-75° or whatever, but since that has never been done in the past it's too much of a departure.

But that also depends on the sensor size (and aspect ratio selected). I agree angle of view is easier to understand (as a simple concept when taking pictures) than focal length. If camera bodies had to be marked with a crop factor (versus a full frame 24 x 36mm format) and lenses had angle of view when used on a full frame sensor marked on them, then with some simple maths, you'd always know your angle of view (unless you changed the aspect ratio). But then who carries a protractor around? Also the angle of view quoted is corner to corner, (rather than horizontal or vertical), making it slightly less useful. Digital zoom factors add another layer of complexity, not to mention the distortion corrections now being applied in some cameras which reduce the angle of view quite noticeably.

 

I'm happy with lenses showing their true focal length (i.e. based on the physics of light focusing and ignoring the sensor size). At least then it's easy to build up a set of lenses and know you've got the range covered. If I want to see my "angle of view" I look through the my 100% electronic viewfinder with distortion correction already applied...  :)

Edited by M.Chapman
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I'm happy with lenses showing their true focal length (i.e. based on the physics of light focusing and ignoring the sensor size). At least then it's easy to build up a set of lenses and know you've got the range covered. If I want to see my "angle of view" I look through the my 100% electronic viewfinder with distortion correction already applied...  :)

 

Absolutely! Can you imagine the confusion if quoted focal length was anything other than . . . well . . . focal length?

 

When I use the converter and put my 135mm Nikkor on my daugther's nice new Nikon 1 V3, it's still a 135mm lens. I know however that its coverage is different to when it's on my fulll-frame, as it is to when it's on my old D200 . . . but it is and always will be a 135mm lens. I certainly don't need or want any further description engraved on it by the lens manufacturer to "tell" me what I already know . . .

 

dd

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My understanding of it is that focal length is focal length regardless of what sensor is being used - so it makes perfect sense to call an 18-55 an 18-55; that's what it is!

 

Take a nifty fifty for example...

 

On the 5D --> 50mm

On the 1D mkIII --> 63mm

On the 7D --> 81mm

Mounted on a Nikon 1.5x --> 75mm 

 

and so on...

 

What makes more sense - to label that lens with its actual focal length and inform your customers of how to factor in sensor size, or to manufacture various editions of the lens with arbitrary focal markings? 

 

You mentioned lenses designed solely for crop (i.e. <35mm sensors). As highlighted by the 1D mkIII above, the sensor crop sizes aren't universal and can change. What if fuji designed a super small compact with XF mount but with a smaller sensor? Would they have to re-tool their factory to produce the exact same lenses but with different numbers etched on?!

 

As for the habit of refer to focal length by 35mm - that's just a habit. It's a bit like people referring to electric car efficiency by referring to mpg; that's what we've come to know and understand. If I mentioned 200mm focal length people know exactly what I mean without me having to explain which sensor the camera had or if a teleconverter was used, etc. 

Edited by RichardM
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