28mm on reversal ring

[Soeren]

Maybe I am only confusing myself but..
I am doing some macro using my 28 mm on a reversal ring thus magnifying about 2.5X. Metering is a PITA because of false light through the wievfinder so Im in trouble each time I preparing the exposure. Lets keep the PK13 I'm mounting it on at times out of the question for now
I find The meter is fooled to underexpose even when the wievfinder is blacked out (feature on F90X). I would like to use a handheld meter instead.
So Whats the "focallenght" of a reversed lens.
Does "(effective aperture) = (set aperture)* (1 + [mag ratio])" still apply
I do think so myself but when calculating magnification I need the focallenght and extension. So is a reversed 28mm still 28mm to the camera/film/meter ?
If the focallenght is not 28mm when reversed what about the F-stops ?
Thanks Søren

[Soeren]

Is this possible ? something you don't know

[David A. Goldfarb]

I think the reason there's some hesitation, is that retrofocus lenses behave a little strangely, and you have to start thinking about entrance and exit pupils, if you calculate it in that way.

My suggestion: take focal length out of the equation. I think this should work independent of the issue of the design of the lens, and it's the formula I use. It works for me, but then again, I don't think I've ever had to reverse a retrofocus lens, but a quick test should tell you if it's right.

Change in exposure=(1+magnification)^2

where a change in exposure of 2x will be 1 stop, 4x is 2 stops, 8x is 3 stops, etc.

An attraction of this method, is that it works with any format. I have a magnification/focal length table taped to the back of each of my LF cameras and in my photo notebook for other cameras, so I can quickly estimate the magnification by comparing the width of the scene to the width of the film frame and convert it to an exposure factor in f:stops, or I can measure it more precisely by putting a ruler in the frame and seeing how much of it I can see in the finder or on the groundglass.

[John Koehrer]

Why not use a focusing cloth while metering to cut out light to VF?

[Soeren]

Thanks for your reply David
I just tried comparing my sekonic and the internal meter.
The internal wont spotmeter below 1/3 sec so at lightbulb that didn't work.
The CV on a greycard goes.
1/3-1/4 @2.8; 1/2,5@4.0; 1/1,6@5,6; 1sec@8.0; 1,3sec@11; 1,6sec@16 and 2sec@22.
The sekonic said 1/15@2,8 + 6bars meaning 1/2stop less than f/2,8
This leads me to a preliminary conclusion that the camera meter is somehow fooled. Or does things work different in makro ?
I will test this in better light if the sun is out tomorow.
Hmm yes the thing definitely behave strangely but when I gt it right the pics are worth the work. Ever seen a pepergrain at 3X
Søren

[Soeren]

Why not use a focusing cloth while metering to cut out light to VF?

Might try that
Søren

[David A. Goldfarb]

I think what you may be seeing with the internal meter is that it's becoming non-linear at low light levels. This is why stopped down metering often gives different results from wide open metering, and wide open metering is often more accurate. Make some test exposures with slide film

[Soeren]

nonlinear meter. I didn't know that. Ill try the handheld and the "true F-stop" formula.
Regards Søren