Dx vs Fx and lens focal length (need help)

[laurence85]

Hi,,,

I know there are many image sersor format eg. DX format , FX format which i believe is 35mm or there are larger image sensor. Here, i just want to concentrate on Nikon's DX and FX format. I was trying to find out how the focal length differs by using different types of cameras with a lens. I will make it simple. For example, Nikkor 50mm AF f1.4 lens, if i use that lens on DX format camera, the focal length 50 mm become 75 mm. (or is equalvalent to) Is that correct? so if i use 105 mm lens (which is not DX lens) to a DX format camera (eg. D90, D300, etc ) what is the focal length i can get will be? How to calculate those ratio? or can i have that in a table comparing all the different focal length for DX and FX? thanks.

--
help me to improve my photography skills. thanks.

 

[ShadowDiver]

simple - for DX-Nikons just multiplicate with 1.5 (for Canon it is 1.6) - no matter if the lens is DX-lens of FX-lens. That makes a 50 to a 75 and a 105 becomes a 157,5

On FX-bodys the FX-lenses be just what they are (50 = 50, 105 = 105). The DX-lenses can only be used in DX-mode (with less resolution/MP). There are ways of using DX-lenses on FX-bodys in normal FX-mode - but there are limitations in range uasable etc ... see other threads here...

Chris

 

[drmatiz]

It has to do with the size of the sensor. In nikon's case the sensor in the D300, D90 and so on is 24x18 mm (and called 'DX') in stead of the old film format (36x24 mm) which is used in the D700 and called FX. The conversion rate therefore is 1,5. So if you have a 50 mm lens it will be a (50 x 1,5) 75 m lens on a DX body. A 200 mm lens will be a (200 x 1,5) 300 mm on DX and a 24 mm lens will be a (24 x 1,5) 36 mm lens on DX.

So it's 1,5 for all lenses on a DX body.

What lens manufacturers do for Nikons and canons is use the 35 (film) format. When you buy the 18-200 DX lens effectively it will be equal to the (non-existant) 27-300 lens on a FX body. Since we photographers are very used to the film format and the 'old' film format focal lengths we use something called the '35 mm equivalent'. This basically means that we calculate the focal length to find out the 'old' known focal length.

Eh, this gets a little complicated. Most important to know: most lens manufacturers I know give the focal length of their DSLR lenses in the 35 mm eq. and we have to calculate what that lens will be on a DX camera. So the 'travel'-lens 18-200 will behave like a 27-300 on a film or DX camera.

I hope this makes it a little clearer, it's quite hard to explain when english isn't your native language.

--
you don't need eyes to see, you need vision

 

[David Rosser]

First the focal length marked on all Nikon interchangable lenses is the true focal length of that lens. (Being technical focal length is the distance between the rear nodal point of the lens and the film/sensor plane with the lens focused on infinity.)
Second by simple geometry with the help of this grotty diagram



you will see that the angle of view of the lens depends on the sensor size, the same focal length of lens has a wider angle of view on a large FX sensor than a small DX sensor.

Third a lot of us old stuck in the mud photographers used 35mm film and built up a feeling for what focal length of lens to use to get a particular angle of view. When we suddenly started using digital cameras with smaller sensors we found that our experience led us astray and where in the past we would use a 50mm lens this gave too narrow an angle of view and we had to use a 35mm lens instead to give the same angle of view. Again simple geometry came to our rescue and it was realised that with DX we had to use a lens of 2/3rd the focal length we would have used on 35mm film to get the same angle of view.

Fourthly all lenses designed for 35mm/FX will work on DX but lenses designed specifically for DX may not work properly on FX because the lens designer may have designed it to only fully illuminate a DX frame.
--
Dave
http://www.rosser.pwp.blueyonder.co.uk
http://www.pbase.com/dgrosser

 

[psandham]

http://www.dpreview.com/learn/?/Glossary/Optical/

a good primer, especially the secions on "focal length" and "focal length multiplier" for the questions you posed.

--
pjs
RIP Kodachrome 1935-2009

 

[pocketfulladoubles]

50mm is 50mm is 50mm - I don't care what image circle you choose. All you are doing is changing the image circle size and squeezing more pixels into the DX sensor size. It gives the "effect" of having a longer lens, and people call this a multiuplying 1.5x "equivalent". The equivalent relates to what one would expect on an FX sensor in terms of what is shown in that cropped image circle, but the focal length itself is still the same. It's the same reason that if you take cameras with much large image circles than FX, say medium or large format cameras, their "normal" (ie 50mm on FX) lenses are 80mm, 150mm, 300mm, etc. We could call this a dividing factor "eqivalent", but the focal length is still what it is.

What does change, and this could be important, is the effect it has on depth of field, and that is directly related to focal length. Say you want to take a picture of something and at a given distance 58mm on DX and 85mm on FX are your selectied lenses to yield the same composition. If you take both shots wide open, the subject will be more isolated from the background with the 85 than with the 58, since the 85 at the same f# has less DOF.

 

[ltdan84]

What I found to be easiest is just for got the equivalent focal length, crop factor, whatever you want to call it.

Just start thinking in DSLR terms if you use a small sensor (or DX) format camera (which is even easier if you haven't shot a lot of film like me).

10mm is super wide, 18mm is wide, 35mm is not, 55mm is slightly long, 105mm is getting into telephoto, etc.

Saves the effort of all this multiplying, and makes more sense too since 50mm can never be anything besides 50mm.

 

[shaocaholica]

DX lens on DX camera: 1.5x FOV/crop factor (compared to the original 35mm spec)
FX lens on DX camera: 1.5x FOV/crop factor (same as above)

FX lens on FX camera: No FOV/crop factor (This is the standard 35mm scenario)
DX lens on FX camera: No FOV/crop factor but you'll most likely get vignetting

 

[LedFox]

the 18-200 DX lens will be equal to a 27-300 lens on a FX body

FX lens on FX camera: No FOV/crop factor
FX lens on DX camera: 1.5x FOV/crop factor
DX lens on FX camera: No FOV/crop factor but you'll get vignetting
DX lens on DX camera: 1.5x FOV/crop factor

I'd like a clarification on this:

FX 50mm on FX body = 50mm (35mm equivalent)
FX 50mm on DX body = 75mm (35mm equivalent)
DX 35mm on FX body = 52.5mm (35mm equivalent) or 35mm (35mm equivalent) ?
DX 35mm on DX body = 52.5mm (35mm equivalent)

There's tons of info on the web about FX lenses on DX bodies.

Or what you get with DX lenses on FX bodies. (vignetting, decreased megapixels)

But none outright tells you the '35mm equivalency" itself when you use a DX lens on an FX body.

Try a search: and you'll get the most minute explanations about the intricacies of focal lengths, but it's always about the former scenario. If they mention the latter, you get the usual "vignettes, less megapixels, ramblings about sensor sizes etc.."

This is the first thread I've seen it mentioned explicitly.

And even here, the two guys who mentions it (bless them), contradicts each other...

Or perhaps shaocaholica has it right: DX 35mm on FX body = the field-of-view of a 35mm fullframe lens.

And drmatiz did not actually mean "putting an 18-200 DX lens on a D700" but is rather giving the usual "35mm equivalency" explanation.

So to clear it once and for all:

DX 35mm on FX body = 52.5mm or 35mm ?

Thanks to anyone who can help.

 

[dradam]

the 18-200 DX lens will be equal to a 27-300 lens on a FX body

FX lens on FX camera: No FOV/crop factor
FX lens on DX camera: 1.5x FOV/crop factor
DX lens on FX camera: No FOV/crop factor but you'll get vignetting
DX lens on DX camera: 1.5x FOV/crop factor

I'd like a clarification on this:

FX 50mm on FX body = 50mm (35mm equivalent)
FX 50mm on DX body = 75mm (35mm equivalent)
DX 35mm on FX body = 52.5mm (35mm equivalent) or 35mm (35mm equivalent) ?
DX 35mm on DX body = 52.5mm (35mm equivalent)

There's tons of info on the web about FX lenses on DX bodies.

Or what you get with DX lenses on FX bodies. (vignetting, decreased megapixels)

But none outright tells you the '35mm equivalency" itself when you use a DX lens on an FX body.

Try a search: and you'll get the most minute explanations about the intricacies of focal lengths, but it's always about the former scenario. If they mention the latter, you get the usual "vignettes, less megapixels, ramblings about sensor sizes etc.."

This is the first thread I've seen it mentioned explicitly.

And even here, the two guys who mentions it (bless them), contradicts each other...

Or perhaps shaocaholica has it right: DX 35mm on FX body = the field-of-view of a 35mm fullframe lens.

And drmatiz did not actually mean "putting an 18-200 DX lens on a D700" but is rather giving the usual "35mm equivalency" explanation.

So to clear it once and for all:

DX 35mm on FX body = 52.5mm or 35mm ?

Focal length is focal lenght and is a physical feature of the lens that does not change between DX and FX lenses. The conversion factor is dependent ONLY on the body.

DX body => apply 1.5x conversion factor to ALL lenses to achieve 35mm equivalent
FX boxy => already 35mm equivalent so no conversion factor is needed on ANY lens

A twist:

In DX crop mode on FX cameras you WOULD apply the conversion factor as you will doing in camera (cropping and expanding the center of the image) what DX cameras do physically.

As a direct answer to your question:

DX 35mm lens on FX body (in FX mode) = 35mm lens (35mm equivalent), but will include vignetting or a image that doesn't cover the whole sensor

DX 35mm lens on FX body (in DX mode) = 52.5mm lens (35mm equivalent), and will not have the peephole appearance or the vignetting

 

[LedFox]

DX body => apply 1.5x conversion factor to ALL lenses to achieve 35mm equivalent
FX boxy => already 35mm equivalent so no conversion factor is needed on ANY lens

A twist:

In DX crop mode on FX cameras you WOULD apply the conversion factor as you will doing in camera (cropping and expanding the center of the image) what DX cameras do physically.

As a direct answer to your question:

DX 35mm lens on FX body (in FX mode) = 35mm lens (35mm equivalent), but will include vignetting or a image that doesn't cover the whole sensor

DX 35mm lens on FX body (in DX mode) = 52.5mm lens (35mm equivalent), and will not have the peephole appearance or the vignetting

Excellent, dradam.

So shaocaholica was partially right.

I wanted to get an idea of the field/angle-of-view, and I can get that the easiest using 35mm equivalency.

I've been careful to ask for the equivalency because one of the things always repeated everywhere on the web is the actual focal length never changing.

Plenty of repetitions of those. (amongst the vignetting etc..)

But this may be the first time the equivalency has been explicitly written & then confirmed by someone who sounds like they know the subject.

Your 'twist' is interesting and makes sense of the stuff that I found confusing.

(there's been some attempt at explaining your twist but because they always assume that only half of the twist was possible, I was getting some contradicting signals)

All this may sound like it's obvious and shouldn't have to be said on the web, but just google it:

Every info has been repeated ad nauseam:

• focal length never changes
• simply 1.5x
• vignetting
• only 6 megapixels!
• it's easy, just print a 4x6 and cut 1.5x!!!
• compression effects is due to distance not zoom lens!!! dam u!!1!
• f2.8 is not affected!!!1one1!!!

;-)

And yet, nothing about the twist.

It's like everyone just conveniently stops short of putting it all together.

(a few made it close but because of false assumptions, just end up contradicting each other)

Every conventional, obvious, easy explanation has been repeated except this.

So thanks, dradam.

It's my first question to the web, and I had to ask because for once, Google came up empty.

Cheer!

.

 

[I Beam]

The thread you have chosen is not a good one is not a good one in some ways as there is a lot of sloppy terminology and there are plenty of statements that are plain wrong.

'radam' has put thing right.

You mention 'ramblings about sensor sizes' but sensor sizes is the key to understanding the difference between FX and DX. Once this 'clicks' in your mind then it all becomes easy to understand.

An FX sensor is 36 x 24mm and a DX sensor is 24 x 16mm. The FX sensor is 1.5 times bigger in each direction than the Dx sensor and this is where the x1.5 crop factor comes from.

In the diagram below the DX sensor is represented by the blue rectangle and the FX sensor by the red rectangle. For 'DX Image Circle' read 'DX lens Image Circle' and for 'FX Image Circle' read 'FX Lens Image Circle'.



The last thing to understand is that if you had an image of a house say in the middle of diagram made by a 35mm DX lens and a 35mm FX lens, the image would be exactly the same size regardless of the lens and regardless of the sensor size.

Regards

Paul

 

[I Beam]

The last thing to understand is that if you had an image of a house say in the middle of diagram made by a 35mm DX lens and a 35mm FX lens, the image would be exactly the same size regardless of the lens and regardless of the sensor size.

Here is another diagram to illustrate my last point.



Regards

Paul

 

[LedFox]

You mention 'ramblings about sensor sizes' but sensor sizes is the key to understanding the difference between FX and DX

My bad. I should've explained.

I understand the stuff about sensor sizes completely.

I understood it for years, hence it got tiresome going through reams of repeated information on Google when people got long-winded about sensor sizes etc..

It's bad enough going through repeats about actual focal lengths not changing - at least those lines didn't take long to read.

But it's fine. I go through it because that's part of the searching process.

The problem was: these are not the info I need.

I already knew this stuff.

That's why I was surprised that everything was a complete re-hash and that I'd actually have to ask the question myself.

And because I knew the exact answers that could be re-hashed back at me, I had to phrase my question delicately & clearly.

And it worked.

First reply -- and dradam came up with exactly what I was looking for.

Which was field-of-view gauged through 35mm equivalency.

But not for FX lens on DX body -- that's been re-hashed to death.

But rather 35mm equivalency for DX lens on FX body.

That's what I was looking for.

And not more of these:

• focal length never changes
• just 1.5x it
• vignetting
• less megapixels
• print a 4x6 and cut 1.5x
• f2.8 is not affected
• sensor sizes

Hell, I even got dradam's reference to peepholes when describing the uncropped vignetting.

if you had an image of a house say in the middle of diagram made by a 35mm DX lens and a 35mm FX lens, the image would be exactly the same size regardless of the lens and regardless of the sensor size

Not exactly.

That only works when you include cropping.

Without cropping, the larger sensor affects the house size.

And the resultant image would be a smaller house.

This change-in-relationship is one of the big selling points for going FX. (among others: low noise-high iso, better dynamic range etc..)

.

 

[I Beam]

if you had an image of a house say in the middle of diagram made by a 35mm DX lens and a 35mm FX lens, the image would be exactly the same size regardless of the lens and regardless of the sensor size

Not exactly.

That only works when you include cropping.

Without cropping, the larger sensor affects the house size.

And the resultant image would be a smaller house.

This change-in-relationship is one of the big selling points for going FX. (among others: low noise-high iso, better dynamic range etc..)

Perhaps I should have been clearer - on the sensors the house image will be the same size. So if the images from the two sensors were printed then the DX image would be 1.5 times bigger.

Rgards

Paul

 

[LedFox]

Perhaps I should have been clearer - on the sensors the house image will be the same size. So if the images from the two sensors were printed then the DX image would be 1.5 times bigger.

Ahh, fair enough. I see what you mean.

Turns out, we're talking about different things.

I figured it was just a phrasing thing that could be made clearer.

.

 

[MichaelGGSantaFe]

Re: Dx vs Fx and lens focal length (need help) In reply to shaocaholica, Jul 24, 2010

the 18-200 DX lens will be equal to a 27-300 lens on a FX body

I'd like a clarification on this:

[...] And even here, the two guys who mentions it (bless them), contradicts each other...""""""""""

LedFox, I don't think the two people contradicted each other...

I think drmatiz meant that if you put a 18-200 DX lens on a DX body, it is LIKE PUTTING a 27-300 lens on a FX body. He didn't mean that if you put a 18-200 DX lens on an FX it become 27-300

Michael

 

[toomanycanons]

Holy Resurrected Thread, Batman! How does this happen? This is the COOLPIX forum now, not Dx, not Fx, not Dx vs Fx.