Confused comparing DX vs FX exposure

Discussion in 'Nikon DX DSLR' started by keko, Aug 5, 2008.

  1. keko

    keko

    143
    Jul 20, 2007
    Barcelona
    I'm sure there's an error somewhere in my line of reasoning... so I need help sorting out my logic...

    Due to the difference in sensor sizes, I assume an FX setup will have to be stopped down 1.5x more than the DX to achieve the same DOF, and thus the resulting shutter speed should also be 1.5x slower, to achieve the same exposure. Doesn't that make the FX 1/2 a stop slower for all practical purposes?

    When comparing noise shouldn't we compare 1.5x lower values of the DX to the FX, instead of comparing equal settings?
     
  2. cotdt

    cotdt

    Jul 14, 2007
    Bay Area, USA
    there's nothing wrong with your reasoning (FX has about 1.1 stops shallower DoF). if you want to maximize DoF then DX and FX is equal. The advantage to FX are just the great lenses for it and the option of a shallower DoF.
     
  3. No, your exposure is still the same. You've reduced the aperture by a half stop and increased the exposure time to compensate. No change in exposure.

    DoF on an image is affected by more than just aperture and sensor size though they do affect it.

    Ronnie
     
  4. Exposure is affected by the amount of time the light is allowed access to the sensor.

    If conditions call for f/4, 1/500th at ISO200, then that is the correct exposure. Matters not if its film or digital.

    DOF is not exposure, its composition. It is affected by distance to subject, aperture and focal length used.
     
  5. LAW2

    LAW2

    358
    Oct 24, 2006
    JAX
    I think Gretchen has us on the right road.

    So would the DOF be different on a FX camera versus a DX camera?

    Seems to me the answer would be no. For example with a 50MM lens on a DX camera you’re not zoomed in to 75MM but only seeing what that lens can show on a smaller sensor. Its still a 50MM but cropped in.
     
  6. scooptdoo

    scooptdoo Guest

    i agree
     
  7. keko

    keko

    143
    Jul 20, 2007
    Barcelona
    I just reread my original post and it doesn't sound clear enough. Let me rephrase with a simpler question...

    Let's assume I'm shooting DX @50mm FL and FX @ 75mm, at the same aperture would I get the same DOF? The way I understood it is tha tthere would be close to 1 stop difference in DOF, such thtat the FX would have to be stopped down further to achieve the same result and thus to get the same picture at the same aperture and shutter speed I'd have to use a higher ISO.
     
  8. cotdt

    cotdt

    Jul 14, 2007
    Bay Area, USA
    yes, that's correct.
     
  9. keko

    keko

    143
    Jul 20, 2007
    Barcelona
    Thanks cotdt,

    so that leads me to he conclusion that (not considering lens limitations), I will need to increase the ISO of the FX by about 1 stop to get the same shot. So the 2-3 stop advantage of FX is actually a 1-2 stop.

    Thanks for clarifying...
     
  10. Same aperture and same shutter speed, same ISO.
     
  11. keko

    keko

    143
    Jul 20, 2007
    Barcelona
    Hi Phil,

    do you agree that for example the following settings would yield the same picture in terms of composition, exposure and DOF?

    DX FL: 50mm Aperture: f5,6 Shutter: 1/60 ISO 200
    FX FL: 75mm Aperture: f8 Shutter: 1/30 ISO 200 or
    FX FL: 75mm APerture: f8 Shutter: 1/60 ISO 400
     
  12. cotdt

    cotdt

    Jul 14, 2007
    Bay Area, USA
    FX has 2.3x the area of DX, so doing the math here, on FX you actually want f/8.5 against f/5.6 on DX. But other than that, yeah you're right.
     
  13. The size of the sensor has no bearing on exposure. So that's just nonsense.

    Ronnie
     
  14. These will give you the same exposure and composition. Depth of field is affected by more than just the aperture used it's also affected by the "enlargement" of the final image so we can't answer that part.

    Ronnie
     
  15. cotdt

    cotdt

    Jul 14, 2007
    Bay Area, USA
    You must have not read the thread. The OP was refering to the same DoF and composition. In which case, for FX you would want a lens that is 1.5x longer and stop down 1.1 stops more as compared to DX.
     
  16. I have read the thread. There are more variables in depth of field than just the aperture.

    Ronnie
     
  17. cotdt

    cotdt

    Jul 14, 2007
    Bay Area, USA
    Right. Aperture and magnification ratio are the 2 factors, and nothing else. And a larger sensor means a larger magnification ratio for the same composition, thus shallower DoF for the same aperture.
     
  18. Ok, I see that. I was only trying to point out that the final image size will also affect depth of field so what you are saying is correct for a given final image size. By that I mean print or whatever the final output is.

    Sorry for any confusion.

    Ronnie
     
  19. No.

    If you are shooting at shutter speed X and aperture Y, to get equivalent exposures you need to use the same ISO Z.

    ISO, aperture and SS are a triangle. If you keep 2 parts the same, the 3rd must remain, or you change the exposure.
     
  20. pforsell

    pforsell

    Jan 15, 2008

    Hi Luis,
    I agree with this.

    What is more important to me, is that to get the same image (same DOF, same FOV) that I shoot with

    FX: 50 mm aperture f/1.2 shutter 1/125 ISO 200 or
    FX: 28 mm aperture f/1.4 shutter 1/125 ISO 200

    I'd need

    DX: 33 mm aperture f/0.9 shutter 1/250 ISO 200 or
    DX: 19 mm aperture f/1.0 shutter 1/250 ISO 200