Thinking of getting into FX but have a few reservations

Started Jun 18, 2014 | Discussions thread
OP stromaroma Senior Member • Posts: 1,158
Re: Turns out 2 of my 3 lenses

michaeladawson wrote:

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.

Yes, I agree.

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.

Yes, I'm with you.

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.

Yes I agree. (except that I wouldn't have to crop down all the way to a DX frame to get rid of the vignetting, but that is a side-issue). But here is where I am having trouble:

And when you move further away the DOF for a given f-stop increases.

I agree with this statement, except that the photo has already been taken -- I'm not moving away so the DOF isn't changing. If I take my vignetted uncropped image from the DX lens on the D610, and then crop that after the fact in PP to make it equivalent to a DX frame (in effect, "moving closer" virtually), the DOF is not going to change, it's physically impossible. If the DOF in the image of my flowers is 4 cm front to back, then no amount of cropping after the photo has been taken is going to ever change this -- it's going to be 4 cm, forever. As Wikipedia states,

"depth of field (DOF) is the distance between the nearest and farthest objects in a scene that appear acceptably sharp in an image".

That is set when I take the photo. And similarly, so is the bokeh quality. So therefore the OOF regions are going to look just as nice cropped as if it was not cropped (it's the same image).

Therefore, if I'm using the DX 35mm f/1.8 lens on the FX camera, I'm getting the benefits of the better bokeh rendering of FX, but by using a DX lens and cropping in PP. It will be as if I'm using an f/1.4 lens on my DX camera.

But you say this isn't the case... (and I'll take your word on that!), so the only other solution to this is that the effective f-stop of the DX lens changes when used on an FX body, in order so that the DOF also changes.

I am pretty sure that this is the case and the source of the disagreement, because f-stop is defined as "the ratio of the lens's focal length to the diameter of the entrance pupil" in Wikipedia. What we are doing is effectively increasing the size of the entrance pupil when we put it on FX, by using light from around the sides of the projected image that is otherwise wasted on a DX sensor. That's why we get the vignetting. The f/1.8 35mm DX becomes an f/2.8 35mm when used on an FX sensor. So when you crop down to DX you are negating any benefits, the "pupil" decreases back down in size.

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 stromaroma's gear list:stromaroma's gear list
Nikon D300 Nikon D7000 Nikon 1 V1 Nikon AF-S DX Nikkor 35mm F1.8G +1 more
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