Deep Red filters and Exposure

[Turtle]

Hi,
Curious as to the exposure compensation you use with deep reds, such as the #29 or equiv?

I recently shot some using TTL metering on the MP with a #29 on and made sure I was being generous and instead got very under exposed negs. I have had this problem before (a long time ago...never followed up on it) and wonder if the camera is 'seeing' more light due to relatively higher meter sensitivity to red than on film?

My B&W filter says it has a filter factor of 8. I did not see what extra exposure the camera was giving (over without the filter) but I suspect at least 3 stops judging by the fact that I was getting two more indicated stops when I used a yellow filter. Judging by the negs I would have needed to have given a good 1-2 MORE stops (over the camera metered exposure) to have had any shadow detail worth speaking using.

Could it be that a prepoderance of blueish light (I was on top of a hill and much of the light in the area was perhaps reflected directly from the sky) means an additional compensation was required. I have not had anything like this with normal red filters and below. I've not used a deep red on the MP before so perhaps the metering is more sensitieve to red light than usual.

Any thoughts? I guess when down to such a small frequency range any changes in the colour of light could have a catastrophic impact on requierd exposure. I guess if the light was largely blue, the filter removed almost all - right?

[joachim]

Originally Posted by Turtle

Hi,
Curious as to the exposure compensation you use with deep reds, such as the #29 or equiv?

I recently shot some using TTL metering on the MP with a #29 on and made sure I was being generous and instead got very under exposed negs. I have had this problem before (a long time ago...never followed up on it) and wonder if the camera is 'seeing' more light due to relatively higher meter sensitivity to red than on film?

My B&W filter says it has a filter factor of 8. I did not see what extra exposure the camera was giving (over without the filter) but I suspect at least 3 stops judging by the fact that I was getting two more indicated stops when I used a yellow filter. Judging by the negs I would have needed to have given a good 1-2 MORE stops (over the camera metered exposure) to have had any shadow detail worth speaking using.

Could it be that a prepoderance of blueish light (I was on top of a hill and much of the light in the area was perhaps reflected directly from the sky) means an additional compensation was required. I have not had anything like this with normal red filters and below. I've not used a deep red on the MP before so perhaps the metering is more sensitieve to red light than usual.

Any thoughts? I guess when down to such a small frequency range any changes in the colour of light could have a catastrophic impact on requierd exposure. I guess if the light was largely blue, the filter removed almost all - right?

In my experience red filters (even 25 equivalent) do not work well with TTL metering. The spectral response of the metering cell and the film are typically to different. I get better results by either metering without the filter or using a handheld meter and in both cases applying the filter factor thereafter.

1-2 stops under exposure matches my experience from a #25 (Hoya) on various cameras.

[bottley1]

Forget the lightmeter. Open 3 stops

[Ronald M]

TTL metering is rarely perfect with any color filter, the darker the color the worse the problem.

Find the filter factor under the light you wish to use, say sun, by photographing a grey board lens focused on infinity. Add the filter and see how much more exposure is required to get the same density in the grey board by making more eposures. Vary the stop, not shutter.

By applying the increased exposure detirmined above, you will get perfect exposures.

You can get close and still meter thru the lens if you put the filter on and meter the grey board and see how much "compensation" is required AFTER you adjust the f stop for exposure increase to get proper exposure. Different colored subjects limit how well this can work.

[Turtle]

thing is the camera chose an exposure which represented approximately 3 stops more than without the filter, which would be about right for the 8x factor. I have to admit the factor stated on the filter seemed very low compared to the 16X factor that is normal for a deep red. I reckon one more stop might have made negs that looked vaguely reasonable. Two additional stops was what I needed! I am going to do some serious bracketing next time I am out and see what happens. I have no probs (not even slight) with an orange or orange/red, so was a bit surprised. I reckon the meter/film sensitivity differential could have been half the problem and the bluish light the other half. Maybe!!! Also, this was undoubtedly compounded by the fall off with the 21mm CV I used, meaing that altho the centers were underexposed, the edges were screwed! Frames using the orange filter were fine.

[larmarv916]

Most "Red" filers start with a minimum of increasing the exposure fo 4 stops. Most deep reds are going to require and extra 8 stops!! If your using a "Pan" films that is not unexpected.
I have seen some reds with a published filer rating of X32 If you using something like a ADOX film or ACROS.....it could get really crazy. I hope that helps.......Laurance

[richard_l]

If you are using a handheld meter, and the filter factor is N, divide the film speed by N, and set the meter accordingly. For example, if the film speed is 400 and the filter factor is 8, set the meter as nearly as possible to a film speed of 50 (400/8). This will increase the rmeter's readout by 3 stops.

Or you could leave the meter alone and simply adjust exposure compensation to give three more stops.

TTL meters obviously do not need to be adjusted for the filter factor, but their accuracy may vary when attempting to read highly monochromatic light which has passed through a strong filter.

Richard

[bottley1]

Originally Posted by larmarv916

Most "Red" filers start with a minimum of increasing the exposure fo 4 stops. Most deep reds are going to require and extra 8 stops!! If your using a "Pan" films that is not unexpected.
I have seen some reds with a published filer rating of X32 If you using something like a ADOX film or ACROS.....it could get really crazy. I hope that helps.......Laurance

surely you mean 4x and 8x? Deep red filters mostly have an exposure factor of 8x i.e. 3 stops. Light meter readings using TTL with filter attached are not accurate due to the spectral sensitivity of the light reading cell. Meter without filter and open 3 stops.