Fx and Dx lens conversions

[WayneF]

What I find amazing is back in my film days, there was no such thing as a 'conversion factor' to compare lenses between 135, 120 and 4x5 formats. I never heard of a number to multiply (or divide by) when changing from 35mm to 6x4.5 format, or to 6x7 format, or to 4x5 format. I never had to 'convert' the 80mm lens of my Mamiya 645 to 'the equivelant of __mm on my 35mm camera". Nor convert the 150mm on my 4x5 to 'the equivelant of __mm on my RB67 camera".

That is a bewildering statement, because it certainly was well known back then. I for sure knew it (very many years ago), and I seriously doubt my knowledge was special. Everyone I discussed things with knew it too, it was just facts of life, how thing were.

The lenses were not interchangeable between film sizes then, but changing film size cameras (35 mm to medium to 4x5, etc) certainly required this specific knowledge to buy lenses for the new format (assuming we had previous experience to know something, instead of just asking at the camera store). It had to be known what lens to buy for the purposes, and then, known when to use which lens. You apparently knew this too? Perhaps you just did not recognize it for what it was?

35 mm focal length knowledge did not work directly on the larger film without this conversion, but the conversion told us things. We didn't know this as "crop factor", but we did know it as the various normal lens focal lengths, and we converted from that. We knew the normal lens was approximately the film diagonal, and we knew different film had different diagonals, and we also knew what focal length multiples of that diagonal did.

The case where we did have to interchange lenses was in the darkroom enlarger. The columns were just so tall, so 50mm for 35 mm negatives, and 75 or 80 mm for medium 120 film. Same as the cameras required as a "normal" lens. This was no surprise to anyone. We understood it.

I understand why the manufacturers created the 'conversion' factor. It was a marketing tool to help sell fledgling digital SLRs to an uneducated populace. The populace that had grown up with one format: 35mm. Everyone and their uncle owned a 35mm lens. Most carried a 50mm 'standard' lens. Many purchased 28mm wide-angles and 135mm telephotos. So that was 'the gold standard' back then.

It is of course the sensor diagonal that requires this, not any whim of the manufacturers. Uneducated might have significance though, except I would word it "unfamiliar". Since so many of us spent years (and decades) with the 35 mm film system, we well knew what the lenses did. Then all the different new digital sensors sizes make the "crop factor" be extremely handy info to know for example what would be 3x telephoto on this new-fangled little sensor.

For the newbies never experiencing 35 mm, crop factor won't have any meaning, but I for one am glad for the help. It has significant meaning to me.

 

[480sparky]

..... what's so hard to understand about cropping images?.............. .

And this is EXACTLY the problem. YOU have been through the transition from film to digital. You already understand it. Those entering photography today have NEVER BEEN DOWN THIS ROAD before.

...............because it certainly was well known back then...................

Yes. We knew about it. But again: My Point: We understood WHY. That 'why' is not in any of the advertising or marketing of the manufacturers. Newbies just get told, "Multiply the FL by 1.5". They have to clue what that means. Hence, the confusion.

...............We understood it....................

But what are manufacturers doing today? Zip. Nil. Nada. Goose Egg. Just the 'conversion factor'.

................For the newbies never experiencing 35 mm, crop factor won't have any meaning, ..................

I swear I already stated this.

 

[WayneF]

Yes. We knew about it. But again: My Point: We understood WHY. That 'why' is not in any of the advertising or marketing of the manufacturers. Newbies just get told, "Multiply the FL by 1.5". They have to clue what that means. Hence, the confusion.

Crop factor is probably not at the top of the list of things that newbies don't understand yet. Shutter speeds and f/stops come to mind. Camera automation might be good for the camera, but knowing more about how things work doesn't seem much help on a cell phone. I guess the exception is that one could still control external things, if one cared to.

But you're right that things are not explained well today, if at all. The one that bugs me is that the Nikon models that can do 1/320 second sync state a warning casually. In the rear spec chart, it says: Flash sync speed: flash range drops at speeds between 1/250 and 1/320 s.

I cannot comprehend that as stated. I can easily believe that 1/320 shutter can truncate a full power level flash duration (which IMO is all this can mean), but all of the lower power levels should have no bad effect. That is my own experience, but that's not what it says. I think a couple more words could help it considerably. But it seems the style now to not actually say what it means.

And yes, there probably is as much incorrect confusion about crop factor as about anything. That does not make it be a bad thing. It is very useful information if there is a need to know. It would seem to need full frame experience to be useful, but that does not mean it should be omitted, as there are many that can use it. If contemplating buying some new little camera with a X to Y mm zoom lens and a Z Crop Factor, now those many with 35 mm experience know exactly what it will do (field of view wise), without ever picking it up. IMO, that's the purpose, and it works well.

 

[Woodyg3]

If you couldn't use FX lenses on a DX camera, I guess I wouldn't see the point of a conversion factor. Since you can, it's important for people to know what to expect. I don't know a better way than the 1.5x crop factor explanation, even though it does open a huge can of worms.

 

[480sparky]

If you couldn't use FX lenses on a DX camera, I guess I wouldn't see the point of a conversion factor. Since you can, it's important for people to know what to expect. I don't know a better way than the 1.5x crop factor explanation, even though it does open a huge can of worms.

On a DX camera, it matters not whether the lens is FX or DX. A 35mm FX lens and a 35mm DX lens will render the same FOV.

 

[Woodyg3]

On a DX camera, it matters not whether the lens is FX or DX. A 35mm FX lens and a 35mm DX lens will render the same FOV.

Yes, yes, yes. LOL. The point I was making was that if FX lenses didn't work on a DX camera, there would be no need to explain the difference in field of view between using a 35mm FX lens on a DX camera instead of an FX.

Then again, since even cell phone manufacturers use the "full frame equivalent" focal lengths, I guess I'm probably wrong.

If lenses were marketed as field of view instead of focal length, maybe the world would be a simpler place.

 

[WayneF]

Right. The way this term Equivalent Focal Length is used (e.g., the DX 1.5 crop factor, and 1.5x longer Equivalent focal length) is that this Equivalence only applies to the FX sensor, and NOT at all to the DX camera.

The lens is what it is, it does the same thing on DX or FX. The focal length is always what is printed on the lens body.

The DX camera sees the field of view as defined by the size of the sensor. A smaller field of view if a cropped smaller sensor. However, DX Crop 1.5 means that the FX sensor is 1.5x larger, and the DX sensor is 2/3 the size of FX (1/1.5 = 2/3). So the field of view of FX (with this same lens) is 1.5x larger than as seen on DX (only because the DX sensor is cropped smaller). The meaning of Equivalence is that the FX sensor with a lens 1.5x longer would then see the same smaller field of view as DX with the original lens. This "Equivalent Focal Length lens" is mounted on the FX body (for the comparison). Not when mounted on the DX body. This real focal length on the DX sensor has the same field of view as the Equivalent (1.5x longer) focal length on the FX body.

If we are using the new fangled DX sensor (compared to decades of 35 mm film use), we are not at that moment much interested in what FX might do. However, the big exception is that if our experience knows exactly what focal length X does on full frame (from years of 35 mm film), then we already know what X/1.5 mm focal length will do on DX. My own notion is that this experience is the ONLY reason to imagine comparing them (and it would seem very valuable then). But if we are already standing there holding the DX camera with this lens on it, then Equivalent Focal Length (on FX) is not of much interest to us.

The lens does only what it does and can do, on either body (it is only the sensor sizes that vary the field of view). What we seem to need is more clear words to describe the effect of the smaller sensor. To describe the actual smaller view instead wording it so much in terms of what another sensor might do. Crop seems such a word, but 1.5 is in terms of the full frame sensor. And Equivalent is in terms of the full frame sensor. We have boxed ourselves in. It would seem more clear with DX described as a 2/3 size sensor instead of a 1.5x crop which describes larger FX instead.

But the crop factor thing does seem to come clear when we actually try to visualize the smaller DX sensor cropping the field of view of the lens to be smaller... cropped. And when that understanding becomes clear, there would seem to be little problem with it. But we do have to understand to understand.

 

[hark]

And this is why I wish the 'crop factor conversion' idea should disappear from the face of the earth. Erase it. Delete it. Send it to the Trash Bin. Eradicate it. As if it never existed....

....I understand why the manufacturers created the 'conversion' factor. It was a marketing tool to help sell fledgling digital SLRs to an uneducated populace.

Unless DX cameras and lenses are marketed differently or redesigned completely with different specs, this is how many people will continue to explain it. And quite frankly, they are in their own right to do so. You are preaching to the wrong people. Go after the manufacturers to get them to change their designs.

​

If you couldn't use FX lenses on a DX camera, I guess I wouldn't see the point of a conversion factor. Since you can, it's important for people to know what to expect. I don't know a better way than the 1.5x crop factor explanation, even though it does open a huge can of worms.

On a DX camera, it matters not whether the lens is FX or DX. A 35mm FX lens and a 35mm DX lens will render the same FOV.

I already addressed the difference in FOV earlier. And Woody's comment makes sense to me. If the mounts for DX bodies and DX lenses were different than mounts for FX, then the 1.5 crop factor would be a moot point. I'm surprised you didn't understand his idea.

 

[Woodyg3]

I feel sorry for new photographers reading this thread.

It's not as complicated as we are making it seem. Honest!!!

 

[WayneF]

If the mounts for DX bodies and DX lenses were different than mounts for FX, then the 1.5 crop factor would be a moot point. I'm surprised you didn't understand his idea.

Wow, you would really want to intentionally destroy this quite nice compatibility simply because newbies don't understand crop factor yet?

Heck, medium film might have become popular if they could have mounted 35 mm lenses. (worded to stir the pot )

 

[480sparky]

............. I'm surprised you didn't understand his idea.

Please stop assuming I don't.







I guess at this stage, I'm done here. I've led the horses to water.

 

[hark]

Wow, you would really want to intentionally destroy this quite nice compatibility simply because newbies don't understand crop factor yet?

Heck, medium film might have become popular if they could have mounted 35 mm lenses. (worded to stir the pot )

Changing the DX mount would simply end the confusion of the crop factor concept. Keeping the mounts the same is like having stepping stones to move from DX to FX so I doubt the manufacturers would do that.

Lol at the idea of medium film format with 35mm lenses. :surprise: 35mm lenses wouldn't completely cover the area of 120 film although in theory, it would have been great if they did, Mr. Pot Stirrer. :encouragement:

 

[Woodyg3]

Wow, you would really want to intentionally destroy this quite nice compatibility simply because newbies don't understand crop factor yet?

Heck, medium film might have become popular if they could have mounted 35 mm lenses. (worded to stir the pot )

LOL. That reminds me of many years ago seeing some of the crazy ways fully manual lenses could be mounted on various cameras with all sorts of adaptors.

 

[WayneF]

The medium conversion was just meant as a joke. There would be several unsolvable problems, the wider coverage of the larger format, and the mount, and the distance from lens mount to film could not be shortened about an inch so the lens could reach the film. And it would leave most medium cameras without a shutter too.

The DX DSLR is another story. I think it's great all the FX lenses work great on DX, there's obviously much use made of that. And the FX bodies can even accommodate the DX lenses too (at the cost of pixels). Some newbies may not bother to learn much of anything, but those that want to know can know anything.

 

[Horoscope Fish]

hai guyz!!

Golly... What's going on in this threaOHMYGAWD....






*backs out slooooowly*