jon thornton

According to John Shaw, mirror vibration lasts between 1/8 sec and 1/15 sec. John argues that shutter speeds within this range should be avoided if maximum sharpness is required. John also notes that mirror vibration is only really an issue when lenses with a focal length of 100mm or greater are used.

Mirror vibration has no effect on long exposures. If for example you use a shutter speed of 1 sec, 14/15 of the exposure will be completely unaffected by mirror vibration.

Mirror vibration has no effect on short exposures for fairly obvious reasons.

Thanks for the links David! I have learnt a great deal from participating in this thread.

Although I now agree that the D70 histogram is plotted from luminance data, I am still a bit confused by the highlight display. As I mentioned before, when taking IR shots, I regularly clipped the red channel, but the highlight display did not flash. Do you think that it is possible that the highlight display might take luminance data as input too?

Happy shooting!

Jon

I agree with David. The D70 plots a luminance histogram. The luminance histogram is green channel weighted to reflect human brightness perception according to the following formula (or something very close to it):

Luminance = (Red x 0.30) + (Green x 0.59) + (Blue x 0.11).

Open Photoshop, change the histogram from RGB to luminance and compare the Photoshop graph to the one on the back of your camera. It will be identical.

Don't trust the clipping display. When I first started shooting IR and before I learnt to tak a custom white-balance reading from a white sheet of paper, I regularly clipped the red channel, but the highlight display did not flash.

Bracket tricky shots!

Hi Lex

I am participating in this thread because I want to know whether the Nikon D70 uses green channel data, luminance data or RGB data as the input to its histogram and clipping display functions. In short, I am here because I want to better understand what my camera is telling me.

Jon

Hi David

I wasn't able to find your posts at the other sites you mentioned, so I can only comment on what you have posted here.

The D70 highlight screen can be misleading. As I mentioned in my first post, I sometimes use a Hoya R72 filter to take infrared shots with my D70. The first time I used the filter, the shot looked dark on the LCD and the histogram was bunched up on the left of the graph. So, I increased my exposure half a stop at a time until the highlight screen indicated that I had clipped some highlights. My in-camera test indicated that 2.5 stops of postive exposure compensation was required to correctly expose IR shots. The story was entirely different on my PC. According to Photoshop, the only exposure that did not contain clipped highlights was the first shot taken with zero exposure compensation.

My IR exposure problem might indicate that the D70 produces unreliable histograms and highlight information when the white-balance is set to a value that varies hugely from reality or when the subject matter is predominantly blue or red.

My best advice is to bracket tricky exposures and to keep notes on the circumstances which result in misleading highlight and histogram indications.

Jon

Hmm, I thought I knew the answer to your question, but now I'm not so sure...

Apparently, the luminance histogram is weighted to reflect human brightness perception according to the following formula:

Luminance = (Red x 0.30) + (Green x 0.59) + (Blue x 0.11).

If these figures are correct, it would seem that humans are better at judging differences in the brightness of greens than reds or blues. I am not sure however, that this information is of practical use when plotted on the camera. I for one am much more concerned about avoiding clipped highlights than seeing a weighted average that may or may not reflect human brightness perception.

I don't think either of us have quite got to the bottom of this issue yet.

The resemblance of your Photoshop RGB histogram to the D70 histogram is coincidental.

The easiest way to prove this is to fit a Hoya R72 infrared filter to your D70, take a couple of shots and make the histogram comparison again. If you do this you will find that the D70 completely ignores the red channel when plotting its histogram.

If you were to perform a similar test with a dark blue filter, I suspect you would find that the D70 histogram ignores the blue channel too. If you don't have a blue filter, some blue cellophane would suffice.