Mark's Essay ... for dummies ...

[jeremypayne]

I read the essay and the follow-up discussion here with great interest.

Here's how I thought it worked in the "good old days" ... please correct my romance with science.

Assume I have a film SLR with two lenses ... a 50mm prime and a 70-200 zoom.  Let's assume for argument's sake that the 70-200 steals one stop of light at f/5.6 compared to the 50.

I setup the camera and shoot both lenses at f5.6 and 1/60 with the same film.  I would have expected the shot with the 70-200 to be 1 stop underexposed relative to the 50 due to the difference in transmission.  If I had used the TTL meter on the camera, I would expect to get a different "correct" exposure for each camera based on the difference transmission characteristics of the two lenses.  If the meter read "correct" with the 50 at 5.6 1/60, I would have expected the meter to read correct at 4 1/60 with the 70-200.

After reading the article, I assume that I should expect my Nikon D700 to work very differently ... I would now expect that the meter would tell me that both scenes were correct at 5.6 1/60 ... except that instead of shooting at ISO 200 as I intended, I would be shooting at ISO 400 with the zoom ... is that correct?

[Joe Behar]

I think that pretty well sums it up Jeremy, with the addition of one thing.

What Marc is saying is that this change from ISO 200 to 400 happens "covertly" In other words, the camera does not tell you its doing it and you think you're still exposing at ISO 200. I also think Marc says this happens only at wide apertures, so if you shoot at f/5.6 or f/8 you have nothing to worry about.

Remain calm. Use the designated exits only and leave the area of the fire

[jenbenn]

I read the essay and the follow-up discussion here with great interest.

Here's how I thought it worked in the "good old days" ... please correct my romance with science.

Assume I have a film SLR with two lenses ... a 50mm prime and a 70-200 zoom.  Let's assume for argument's sake that the 70-200 steals one stop of light at f/5.6 compared to the 50.

I setup the camera and shoot both lenses at f5.6 and 1/60 with the same film.  I would have expected the shot with the 70-200 to be 1 stop underexposed relative to the 50 due to the difference in transmission.  If I had used the TTL meter on the camera, I would expect to get a different "correct" exposure for each camera based on the difference transmission characteristics of the two lenses.  If the meter read "correct" with the 50 at 5.6 1/60, I would have expected the meter to read correct at 4 1/60 with the 70-200.

After reading the article, I assume that I should expect my Nikon D700 to work very differently ... I would now expect that the meter would tell me that both scenes were correct at 5.6 1/60 ... except that instead of shooting at ISO 200 as I intended, I would be shooting at ISO 400 with the zoom ... is that correct?

Not quite correct. According to MArks essay and data, light is lost on the sensoronly at ultra large apertures. Which means that the camera only increases the iso without the photographer's knowledge at apterures larger than f/2. YOur 70-200 zoom would consequently not be be affected. You can only observe the phenomenon when shooting at f/2 or larger. And than its only half a stop at the most.

[welder]

I assume the camera then must need to be communicating with the lens for this to happen. So if you are shooting with an old lens where you need to manually set the aperture, the camera has no idea what the f-stop is and presumably maintains the ISO as set?

Not quite correct. According to MArks essay and data, light is lost on the sensoronly at ultra large apertures. Which means that the camera only increases the iso without the photographer's knowledge at apterures larger than f/2. YOur 70-200 zoom would consequently not be be affected. You can only observe the phenomenon when shooting at f/2 or larger. And than its only half a stop at the most.

[Alan Goldhammer]

I assume the camera then must need to be communicating with the lens for this to happen. So if you are shooting with an old lens where you need to manually set the aperture, the camera has no idea what the f-stop is and presumably maintains the ISO as set?

I can only tell you that the Nikon D300 knows what the f-stop is.  You have to program the camera to accept the lens by giving it the maximum aperture.  Without this data, there would be no way to meter the exposure.  Whether the ISO is adjusted is uncertain.  In any event none of my three older prime lenses is faster than f 2.0.

Alan

[fdisilvestro]

In the nikon D300 as well as other nikon bodies with the AI ring, ther is a "mechanical communication" between AI lenses and the camera. If you configure the camera with the focal lenght and maximum aperture, then you will be able to use matrix metering and the camera will know the aperture.

If you don't enter the lens data in the camera you can still meter, but not in matrix mode, only center weighted or spot. The camera will not know the aperture value, but it will know how many f-stops from max aperture is.

This of course with AI or AI-S lenses. Older non AI lenses will work only stopped down (provided you can mount them without damaging the AI ring on the camera)

[welder]

Hmm, well I've got several old lenses I use on Canon and Pentax cameras and they work just fine with no "programming" required. I'm not clear on why the camera needs to know a max aperture value for the lens in order to meter a manual exposure. Stop-down metering may not be convenient, but it still does work.

I can only tell you that the Nikon D300 knows what the f-stop is.  You have to program the camera to accept the lens by giving it the maximum aperture.  Without this data, there would be no way to meter the exposure.  Whether the ISO is adjusted is uncertain.  In any event none of my three older prime lenses is faster than f 2.0.

[Alan Goldhammer]

Older non AI lenses will work only stopped down (provided you can mount them without damaging the AI ring on the camera)

The three I mentioned are all circa 1975 Nikkors that have been modified so that they don't damage the camera.  See John White's website.  Since I mainly shoot in aperture priority it's no big deal that the lens does not auto index the f stop.  As the famed Ron Popeil commercial states so well, "set it at f8 and forget it!"