Thinking of getting into FX but have a few reservations

Started Jun 18, 2014 | Discussions thread
michaeladawson Forum Pro • Posts: 11,456
Re: Turns out 2 of my 3 lenses

stromaroma wrote:

michaeladawson wrote:

It doesn't matter whether you leave the camera in FX mode and crop down to DX size in post or put the camera into DX mode when taking the shot. You will get the same DOF as DX.

The reason full frame gives you less DOF is because you are using a longer lens to give you the same field of view. Two photographers stand on the same spot to take a photo. Both shoot at f/1.8. The DX shooter uses a 35mm lens. The FX shooter has to use a 50mm lens to get the same scene area captured. The 50mm lens gives you less depth of field at the same distance as a 35mm lens.

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Mike Dawson

Does that mean that if I put for example a 50mm FX lens on my DX camera, I am going to get a different image than if I put a 50mm DX lens on my DX camera?

The very first thing you have to understand is that 50mm = 50mm = 50mm.  In terms of focal length there is no difference between FX, DX, Nikon V series, medium format, large format, Micro 4/3, etc.  The only differentiation between an FX lens and a DX lens is the image circle that the lens projects onto the sensor.  A DX lens projects a smaller cone of light onto the sensor.  It is more expensive to project a larger cone of light onto the sensor (FX) and minimize distortions, vignetting, etc.

I was under the impression that all lens focal lengths are standardized for 35mm film "sensor" size.That's why a 300mm on a V1 gives way more reach than a 300mm on the D610. And that's why it won't matter whether you put a DX or FX 50mm lens on your DX D7000 camera; you have the same field of view in either case.

That is part of the flaw in your understanding of the topic.  Sorry if I sound blunt.  There is NO standardization of focal lengths to 35mm sensor size.  As I explained above, a given focal length is just that...  a focal length.  Some manufacturers may give a 35mm full frame equivalent to their lens for a given sensor size but that is only to help the novices understand the field of view of the lens they are buying for their given format.

Yes.  Putting a DX or FX 50mm on your D7000 will give you the same field of view.  The only difference between a DX and FX 50mm is the diameter of the cone of light that they project onto the sensor.

Therefore, I should be able to put my DX 35mm f/1.8 on the FX D610 camera and get a wider field of view than if I put it on my DX D7000 camera, albeit with some vignetting.

Yes.  Except that there is vignetting.  So now you have to crop.  So you really didn't get that entire 35mm, did you.  You really got something more like 50mm.  So to get the exact same scene with your 35mm DX lens on a full frame camera you would have had to move further away.  And when you move further away the DOF for a given f-stop increases.

Therefore, I should get the exact same field of view on my D610 if I put either the DX 35mm or FX 35mm lens on it.

Yes.  But you don't have to crop the 35mm FX lens.

This I am pretty sure of. What I am not so sure about is if the DX lens now changes max f stop to say f/2.8, because now it has a wider field of view for its focal length.

No.  F-stop stays the same.

I am not sure how f stop is calculated, so this may be why I'd get the same depth of field as if I was using it on DX crop mode, because it is in fact becoming a slower lens?

As already explained in this post, and in other posts by me and others, you don't get the same DOF.  You have to stop believing that.  A given focal length lens on an FX body, ONLY when you use the entire sensor frame, gives about 1 extra stop of DOF over the same lens on a DX body.

This is all predicated on the supposition that you are trying to achieve a given field of view.

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Mike Dawson

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