Sony a7s - Loss of saturation/contrast with small amount of ND

[jcarrig]

Hi all,

In searching haven't found any answers to this.

I have a Tiffen 4x5.65 WW ND kit that when used with my Sony a7s, even at .3 (1 stop), reduces saturation/contrast in recorded image.

Using the same ND on a Blackmagic Cinema Camera this does not occur. Am I looking at IR contamination? At such a low amount of ND and the fact that this also occurs under tungsten light also has me confused.

I have also tried this with a Tiffen Vari ND and the same loss of saturation and contrast occurs.

See attached photos, compensating for exposures by adjusting aperture by a stop, so there is a small change in depth of field:

 

[Ryan Patrick O'Hara]

What apertures are you on in each photo?

The simple fact you are using the aperture to compensate means the image with the wider aperture will probably be a little softer (if you are near wide open), adding to your problem. For example, a photography lens at a F/2 will probably be less sharp than the same lens at F/2.8... so already you have two variables at play in a test that is trying to sort out one.

Can you post two locked-off photographs and simply adjust exposure with your shutter, instead of aperture? That way we don't change more variables than necessary in the optical process.

Try to find an area with plenty of light. Take the photographs at a solid F-stop, like a F/4. or F/5.6... Use a typical and healthy ISO... Also use healthy and fast shutter speeds, something over 1/100 to help eliminate chances of vibration skewing results... and obviously compensate your exposure with shutter speed.

That should show us a better comparison to what the difference is via filter and sans-filter.

 

[Andrew Stone]

As usual, Ryan sums things up nicely. One thing that wasn't mentioned is the sensor size difference. The BMCC near half the size of a super 35 sensor, so this is affecting your depth of field greatly compounding the factors contributing to the discrepancy your are seeing.

Given what I am seeing in your images posted above, everything seems to be what you should expect when you factor in the wildly differing sensor sizes, opening the aperture when using ND which will lessen your depth of field more on the A7s more than the BMCC and of course Ryan mentioning the additional softening that occurs when you have the aperture wide open.

If you are using your A7s in APS-C mode you will get the resultant effect of a Super 35 sensor. If it is not enabled then you will be getting the result of a full-frame sensor which has even shallower depth of field relative to the BMCC.

 

[jcarrig]

Thanks guys.

I was aware of all the variables you brought up but thought I could get away with the aperture difference by just analysing the point of sharpest focus, but you're right, I was going from 2.8 to 4 and 2.8 is definitely softer.

I did it again changing only shutter speed and the results show me there is a very slight drop in contrast with Tiffen WW ND on, in both cameras.

a7s with ND


a7s without ND


bmcc with ND


bmcc without ND

 

[Samuel H]

* A variND is a couple of polarizers stlightly rotated, so they are always going to have an effect on color.
* It may be because of a huge aperture difference, but in the first example you are losing A LOT of sharpness with the variND. That would worry me even more than the color thing. This made me sell my faderND.