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DX verses FX

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Dscarinci1
5 Oct 2011 - 2:21 PM

I thought I completely understood the crop factor and the difference between DX and FX lenses until I tested my lenses for image size and compared them with each other. I went from 11mm to 105mm using five different lenses and then compared the image to the image on a different lens with the same millimeter. Surprisingly, each image is in perfect step regardless of whether it is a DX or FX lens. 70mm on my NIKON 70-200 f2.8 VR2 (a known FX lens) is exactly the same image as 70mm on my Nikon 17-105 kit lens (a known DX lens). The 16mm setting on my Tokina 11-16 (which I thought was an FX lens) is exactly 1mm less than the 17mm image on my Nikon 17-105 kit lens (a known DX lens). So, either my Nikon D7000 is automatically adjusting for DX/FX lenses on the image it records or I am wrong about which lenses are DX and which are FX because they all work in step on my D7000 as if they are ALL DX lenses. Here is what I own:

Nikon 7000 (DX camera body)
Tokina 11-16 (FX lens, I thought)
Nikon 17-105 kit lens (DX lens)
Nikon 35mm f1.8 (DX lens)
Nikon 50mm f1.8 (DX lens) (the new lens)
Nikon 70-200mm f.2.8 VR2 (FX lens, I thought) New

I need a better understanding of the difference between DX & FX lenses and whether it matters on camera if nikon camera bodies adjust for the lens they are using anyway? In addition, is there a list of Nikon lenses sorted by DX & FX? I have an older 60mm f2.8 Micro Nikkor lens which does not indicate whether it is FX or DX. The newer 60mm F2.8 Micro Nikkor lens clearly indicates on it that it is a DX lens. Is it possible that the older one is FX and the newer one is DX?

Please help.

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5 Oct 2011 - 2:21 PM

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oldblokeh
oldblokeh  3767 forum posts United Kingdom
5 Oct 2011 - 2:31 PM

The D7000 has an APS-C sensor, i.e. DX rather than FX. The only difference between a DX lens and and FX lens is how it will perform on a camera with an FX-sized sensor. I believe that most if not all Nikon FX cameras automatically produce a cropped image when used with a DX lens, as otherwise there would be sufficient vignetting to render a large portion of the image unusable.

Does this help?

Dscarinci1
5 Oct 2011 - 2:39 PM

When I tested my different lenses and compared them, there was no visible difference in the size of the image regardless of whether I used a DX or FX lens. A 70mm FX looks exactly the same size as a 70mm DX. Is this correct?

If this is correct then there is no problem using FX lenses on DX camera bodies but using DX lenses on FX camera bodies will have issues around the edge of the image because of the more narrow crop on the DX lens when used on the wider FX sensor of the FX camera body.

Do I have it right?
Donald

oldblokeh
oldblokeh  3767 forum posts United Kingdom
5 Oct 2011 - 2:48 PM

Basically, yes. 70mm physical focal length is the same whether it's DX, FX, or 120-roll film. The image size on the sensor of an object shot from the same distance will be the same whatever the combination of DX and FX lenses and bodies. However, that image size will be a smaller proportion of the sensor size on an FX body. The FX, being 35mm frame size is kind of treated as the standard. any sensor smaller than that is said to have a crop factor. You can multiply the actual focal length of the lens by the crop factor to get an 'effective focal length'. It isn't any such thing. The so-called effective focal length is the focal length on an FX body that would give you the same image size proportion on the DX or whatever body.

User_Removed
5 Oct 2011 - 2:54 PM

A focal length of 70mm is a focal length of 70mm quite irrespective of whether it is used on a DX or an FX camera.

The focal length of any lens is the distance at which it will resolve light from a distant (theoretically infinite but that doesn't exist) into a single point.

The crop factor of a DX camera exists solely because the sensor has smaller dimensions and, therefore, "catches" a smaller portion of the image projected by a lens of any given focal length.

Where you do get variation is that if you enlarge the portion of a DX image (taken with the same lens from the same position) and the (smaller) portion of the same scene taken with an FX camera, to the same size, then the perspective of the image will be different. That is why they say that a 70mm lens used on a DX camera will give an image "equivalent" to that taken with a 105mm lens on an FX camera.

One of those days, once all us oldies who remember 35mm photography have died out, they will probably start using "angle of view" instead of focal length to describe camera lenses.

.

Last Modified By User_Removed at 5 Oct 2011 - 2:59 PM
Dscarinci1
5 Oct 2011 - 3:09 PM

I think I get it. So when I use an FX lens on my DX body the image sensor on the DX body will not capture as much vignetting or distortion from the edges of an FX lens (particularly a super wide angle FX lens) because there are smaller pixels on a smaller sensor and the larger pixels on the larger FX sensor would make any vignetting or distortion from the FX lens appear greater. However, the image I see and print using an FX lens at 70mm is the same size image I see using a DX lens at 70mm regardless of the camera body.

The reverse is that a DX lens used on an FX body will always have edge distortion because the DX lenses are not designed (and priced) to fill a full frame FX sensor with larger pixels. Defects at the edges of the DX lens are there for all to see until corrected in post processing.

Do I get it or am I wrong?

Thank you so much for this dialog. It is the most helpful discussion I have ever had on this topic. There is too much talk of "crop factor" and not enough clarity about what it means when you compare images. Very confusing.

mikehit
mikehit  46154 forum posts United Kingdom9 Constructive Critique Points
5 Oct 2011 - 4:29 PM

The DX sensor is 2/3 the linear size as the FX sensor. This means the lens does nto need to cast as big an image circle to cover the sensor so they can make a 70mm lens smaller for a DX sensor.

What you are referring to in this post is linear magnification and that is dependent only on the focal length of the lens. The physical size of the lens is irrelevant regarding linear magnification.


NOTE: the numbers in the following explanation are pulled out of the air - I am sure they are not right but the principle remains.


Imagine you have a camera body without a sensor in it.
Attach your 70mm focal length DX lens to it and focus on an object. The lens will cast an image onto the back surface of the camera body: let us imagine that the circle is 25mm diameter and the object is projected as a 5mm line.
Take off the DX lens and atach a 70mm focal length FX lens. The object will still be projected as a 5mm line (because the focal length has not changed) but it will be within an image circle of 36mm diameter (a larger circle than cast by the DX lens).

Now insert a DX sensor and repeat the tests:
DX lens casts a circle that (just) covers the whole of the DX sensor. The object is a 5mm line
FX lens casts a circel that covers the entire DX sensor plus some of the surrounding camera back. The object is a 5mm line

Replace the DX sensor with a FX sensor
DX lens casts a circle that does not cover FX sensor - the light does not reach the corners, the corners of the picture will be dark and you see this as vignetting. The object is a 5mm line
FX lens casts a circle that (just) covers the whole of the FX sensor. The object is a 5mm line

You will see that pixel size is not relevant.

Last Modified By mikehit at 5 Oct 2011 - 4:30 PM
mikesavage
mikesavage  12233 forum posts England2 Constructive Critique Points
5 Oct 2011 - 6:41 PM

Just remember that the focal length of a lens is a physical property of the lens, and it doesn't change when you switch the lens from one camera body to another.

redken60
redken60  5 United Kingdom
5 Oct 2011 - 7:24 PM

Wink Don't get too confused by 'crop factors'.Modern DX lenses are designed for the the new,smaller sensor Nikon DX cameras.
It is only when you get to the wide angle lenses on FX that it becomes a problem.Any lens around the 17-18mm range on an FX camera will start to show signs of vignetting @ full aperature and the more you stop it down,the more it will show (the darker the corners become). All lenses designed to work on the old film cameras will operate on both DX AND FX cameras.Select your lenses for your FX cameras very carefully! As a general rule 'full frame' lenses will suffer very little or no vignetting
I 'm glad I kept a lot of my old Manual focus and early auto focus lenses,as they are of course still very workable with a lot of the new Nikon Cameras

thewilliam
5 Oct 2011 - 9:18 PM

There isn't much choice of wide angle primes for DX format.

I get frustrated when I use a manual focus "wide" on the D200 that I use for walkabout. The 20mm on DX acts like a 30mm on FX.

DX kit is so much smaller and lighter than FX, which is why I use it for walkabout.

Last Modified By thewilliam at 5 Oct 2011 - 9:21 PM
Dscarinci1
5 Oct 2011 - 10:08 PM

I just double checked my Tokina 11-16mm. It is a DX lens. This lens performs so amazingly well that I was convinced it was an FX lens. This is the only non-nikon lens I own and I bought it after I read all of the great reviews. Those reviews understate the performance of this lens. It is sharp with much less distortion at 11mm than my Nikon 17--105 kit lens at up to 20mm. The Tokina has imperceptible distortion at 14, 15 & 16 mm. A great lens.

LenShepherd
LenShepherd e2 Member 62434 forum postsLenShepherd vcard United Kingdom
6 Oct 2011 - 3:56 AM

What you are probably overlooking is, whilst 70mm is 70mm on any format at infinity focus many modern lenses change the angle of view away from what you might expect when you focus close.,
Depending on lens construction and how close you focus 70mm on one lens is not always exactly the same as 70mm on another lens if you focus closer than a subject about 2 feet high by 3 feet wide.

thewilliam
6 Oct 2011 - 2:07 PM

You're right Len. We have a couple of 100mm (ish) macro lenses: the Nikon 105 VR has internal focussing and so shortens to 52.5mm focal length at 1:1, whereas the Zeiss has only a minimal change of focal length because the floating elements are there for correcting aberration.

LenShepherd
LenShepherd e2 Member 62434 forum postsLenShepherd vcard United Kingdom
16 Oct 2011 - 8:39 PM


Quote: the Nikon 105 VR has internal focusing and so shortens to 52.5mm focal length at 1:1,

Using the standard formula for focal length other than at infinity I make it 78.5mm equivalent - with a maximum equivalent aperture of f2.1.

thewilliam
17 Oct 2011 - 11:34 AM

Len, I fitted the 105 VR Micro to a body, set the aperture wide open to f2.8 and then manually focussed closer. At half life-size, the aperture display said f3.5 and at 1:1, it was f4.8.

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