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DPChallenge Forums >> Hardware and Software >> FX/DX Perspective Equivalence
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 10/29/2009 04:08:27 PM · #1
I've got a Nikon DX camera and I've been wondering something about how the DX sensor relates to the FX sensor and how the same lens would behave with the different sensors.

I understand that since the DX is smaller, a lens' image will get cropped (1.5x). This means that a 35mm on a DX has a roughly equivalent FOV as a 50mm on an FX. What I'm curious about is perspective, though...

A dolly zoom treats the focused FOV of a shot (roughly) the same, but foreground and background elements in the picture drastically change in size and perspective. //en.wikipedia.org/wiki/Dolly_zoom

So, I assume a similar thing is happening between lenses with FX/DX "equivalent" focal lengths.

Is this a problem? Should DXers buy "equivalent" lenses and deal with the change in perspective, or should they buy non-equivalent lenses and deal with the crop factor? Or does it even matter?
 10/29/2009 05:01:42 PM · #2
Buy "equivalent" lenses. What determines perspective is the angle of view, and the APS-C (DX in Nikon-speak) format crops, thereby reducing the angle of view as well. In other words, the lens acts *just like* a longer focal length lens on APS-C, including having the perspective behavior that you'd expect for the longer focal length.
 10/29/2009 05:31:37 PM · #3
Hmm. I'm trying to wrap my head around all the variables going on... I don't have a full-frame camera. I wonder if someone would be willing to do me a favor in the name of science.

Take a crop-frame camera with an "equivalent lens" (e.g., a Nikon DX with a 35mm DX lens) and a full-frame camera with an "equivalent lens" (e.g., a Nikon FX with a 50mm lens), and then take a shot of the same scene from the same distance (preferably with things in the fore and background). I'd imagine using f/22 (or something low like that) would render fore and background elements in focus better so the perspective test could be run fairly. Would that be too much to ask? For science?

And, kirbic, is what you're saying true for "DX" lenses too? Would a hypothetical 50mm DX lens on a DX camera be the same as a normal 50mm lens on a DX camera?
 10/29/2009 05:38:59 PM · #4
Originally posted by dustingooding:
Hmm. I'm trying to wrap my head around all the variables going on... I don't have a full-frame camera. I wonder if someone would be willing to do me a favor in the name of science.

Take a crop-frame camera with an "equivalent lens" (e.g., a Nikon DX with a 35mm DX lens) and a full-frame camera with an "equivalent lens" (e.g., a Nikon FX with a 50mm lens), and then take a shot of the same scene from the same distance (preferably with things in the fore and background). I'd imagine using f/22 (or something low like that) would render fore and background elements in focus better so the perspective test could be run fairly. Would that be too much to ask? For science?

And, kirbic, is what you're saying true for "DX" lenses too? Would a hypothetical 50mm DX lens on a DX camera be the same as a normal 50mm lens on a DX camera?


I think he is saying that a 50mm lens on a dx camera will behave exactly like a 75mm lens would on a ff camera
 10/31/2009 05:32:24 AM · #5
I've got FF lenses and use them on my APS-C (SF) D90 and D80, to future proof my glass investment. If I ever get a D700 or D3(s) I can use those lenses there as well. Crop factor in Nikon is 1.5x. Image quality has been really good and I have the choice of buying older glass 2nd Hand at half the price of equivalent new, even new SF only glass. From my profile you will see I'm using Sigma DG not DC and a 15 year old Nikkor 35mm-70mm behaving as a 52.5mm-105mm for my daily lens. (But have recently bought better f/2.8 Sigma DG glass as well.)
 10/31/2009 05:39:41 AM · #6
I am currently looking at a 2nd Hand MF (Manual Focus) Micro-NIKKOR 105mm f2.8 MF MACRO Lens for under USD $330-00 in a local South African Camera Shop as an experimentation lens which weighs less than my Sigma 180mm Macro HSM f/3.5. Of course I'd like the new AFS Micro-Nikkor 105 mm f/2.8 ED-IF VR G but at USD $900-00??? And it is said to be a bit of a pain with VR on a Tripod...
 10/31/2009 09:59:44 AM · #7
Originally posted by Magnum_za:
....And it is said to be a bit of a pain with VR on a Tripod...


VR should always be turned off if the camera is on a tripod or firmly supported.
 10/31/2009 10:17:53 AM · #8
The perspective question has pretty well been answered as to how lenses work with the two different sensor sizes. No one mentioned that the equivalent lenses I/E 35mm f2 DX would normally weight a lot less than a 35mm f2 for a FF camera. The lens is covering a smaller sensor, so the diameters of all the parts can be smaller and lighter.

The MF Micro 105 f2.8 Ai is an outstanding lens. I have one and use it as often as I can. It not only shoots beautiful macro, but handles shots to infinity with exceptional sharpness and contrast. The camera that I use does not meter with the Ai lenses, but that don't seem to be a problem because I can look at the shot on the LCD. For macro on the move, I usually take a monopod. It allows me to move with much less restriction than a tripod, and focusing becomes a matter of getting close then moving the camera in or out a little to get the fine focus on the chosen spot of the subject.
10/31/2009 02:24:02 PM · #9
This issue of "perspective" is kind of tricky because the word itself means different things to different people. But the effect that's being discussed here is the relationship that different objects in the frame have to each other, nothing more than that. It's really very simple. Hold your arm out and make a fist with your thumb sticking up. Close one eye. See what the thumb is occluding. Now move the thumb in closer to your face; see it's occluding more now? Switch eyes. See the change?

That's the changing relationship of objects to each other, and of "camera" (the eye) to objects.

Here's the basic rule: set up your tripod in a particular spot and, with your wide angle lens, frame up a shot and capture it. Now start switching focal lengths, without moving the tripod or its head at all, taking a new exposure with each focal length.

Now, in photoshop, open all these images, and crop the wider ones down to cover the same angle of view as the longest one.

You can paste every one of these images in a stack over the uncropped shot, and you won't be able to see a difference between them, as far as "perspective" goes. The most-cropped ones will look less sharp, of course, but the coverage will be identical.

If, on the other hand, you were to move progressively further from the subject with each longer lens, so the framed coverage was the same in each shot, THEN you would see a change in "perspective", because the relationship of the objects to each other within the frame has changed with the changing camera position.

R.

Message edited by author 2009-10-31 14:27:30.
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