How to determine exposure when shooting macro under studio light

How can you determine the correct exposure when you use a macro lens under a

studio light setting? Of course, this is for a film camera. Using a hand held

meter can't compensate for the lens extension light loss, and TTL metering does

not work with studio lights. I know on the Contax 645, you can actually use the

camera to trigger the flash (but not exposed any film) and the camera then gives

you the proper exposure setting, but this feature is not available on typical

35mm cameras.

Divide the bellows extension squared by the focal length of the lens squared to get an exposure factor. Bellows extension is the distance from the ground glass to the rear element of the lens. For instance you are using a 12 inch lens with an extension of 15 inches. Divide 225 by 144 and you get approx 1.5. Increase your exposure by 1/2 stop.

Camera type would help as would specifc lens.

 

35mm with behind the lens meter will compensate for small subjects.

 

For cameras without meters and a symetrical lens, a film size subject requires 2 stops increase. Double film size is 1 stop.

 

Define film size as 4x5 inch subject with a 4x5 view camera. 24x36m subject with a 35mm camera. Double film size are 8x10 and 3x6 for the two types.

 

Telephoto and wide angle design lenses hare slightly diffrent. I said telephoto, not long focus. A true telephoto design.

The easiest way is to measure bellows distance between the lens and the film plane and compare that distance with the focal length of the lens.

 

You don't have to divide by anything or use the inverse square law etc. or square any numbers. There's very little computational math needed.

 

Using the same unit of measurement ie: inches or cm or mm for both the bellows distance and lens focal length just compare those numbers as if they were f-stops.

 

For example, say you are using a 110mm lens. At an infinity focus distance, ie: no bellows factor, the distance from the film plane to the nodal point of the lens will be 110mm. But where's the nodal point of the lens?

 

You could find the nodal point of this 110mm lens just by focusing on infinity and placing a small tape mark on the lens at 110mm from the film plane.

 

Now, suppose in a close-up situation you measure from the film plane to that tape mark on the lens and the distance is 160mm. Compare 110mm (focal length) to 160mm (bellows distance).

 

Drop the zero from both measurements and the comparison is 11 to 16. Consider those numbers to be f-stops and the difference between f-11 and f-16 is one stop. Your bellows factor is 1 stop so increase the exposure by 1 stop and you will have corrected for that bellows factor.

 

Maths is HARD. #8^)

It looks like Brooks' method is the most painless one. Thanks.

Actually, probably the easiest method is to place a ruler as the subject at the same distance as the actual subject will be and work out the magnification allowing for the reduced field of view of the viewfinder (e.g. 95%), since you know the frame width will be 36mm on 35mm film - or simply read off the magnification from the lens barrel if it is marked, as many macro lenses are. The bellows factor is then simply 1+M where M is magnification: multiply the aperture by the bellows factor to get the effective aperture.

 

Strictly speaking, you should also adjust for the pupil magnification ratio P, which is the apparent diameter of the aperture viewed from the back of the lens divided by the apparent diameter of the aperture viewed from the front of the lens: this can be a variable with macro lenses according to magnification. The corrected factor for aperture is 1 + M/P, which can make quite a difference at higher magnifications. Incidentally, the corrected aperture should be considered when working out the impact of diffraction on softening the image.

Mark,

 

It's still infinitely easier to take two simple measurements along the side of the camera and lens.

 

Sometimes you can't place a ruler in mid air, against a flower for example, to get a measurement of subject size.

 

Taking into consideration the reduced field of view of the viewfinder is another step that's unecessary, as is multiplying the aperture by bellows factor to get the effective aperture.

 

A pupil magnification ratio? Why?

 

Just measure the bellows extension with a simple ruler and compare it to the focal length of the lens. It doesn't get any easier than that.

When working with 35mm lenses, as mooted by OP, the method I suggest becomes almost essential absent TTL flash control, TTL metering or extensive exposure bracketing. Here are the reasons why:

 

1) The pupil magnification ratio varies significantly between infinity and close focus with many macro lenses. For example, on my Tamron 90 it is only about 0.6 at close focus, yet close to 1 at infinity. Ignoring it at life size would produce an aperture factor of 2 at life size, compared with the adjusted factor of 2 2/3, so e.g. at an indicated f/11, the true effective aperture is f/29 not f/22, so you would get significant underexposure (as well as slightly more depth of field and diffraction blurring).

 

2) Many 35mm macro lenses pose difficulties with measuring barrel extension. Even some with unit focus (i.e. all elements are moved together, so focus merely acts as a variable extension tube) may have strongly recessed front elements, making measurement quite tricky. Others have floating elements (sometimes more than one group, moving differently), which result in significant changes in focal length at close focus, so even if you can measure any accompanying physical lens extension it is a poor guide to the actual magnification being achieved. The Canon 100mm f/2.8 USM macro is among several lenses that are entirely internally focussing, so there is no extension at all to measure.

 

3) Whilst modern micro Nikkors do actually report a true effecctive aperture, obviating the need for complex calculations by the user, I believe many (most?) other manufacturer's lenses do not: they mostly assume that the camera will meter the exposure, whether ambient or flash.

 

As to methodology: frame the subject using manual focus. Now without adjusting focus, dismount it from your tripod if necessary and aim the camera at the ruler until you get a sharp image, necessarily at the same focus distance and magnification to get the measurement of magnification.

 

An alternative might be to estimate the compensation factor using TTL metering under ambient light if the camera offers that: the technique is to meter an evenly lit subject (e.g. a white wall) with the lens set to infinity focus: then frame the subject as desired and focus, remove the camera from the tripod and without adjusting focus, use the same evenly lit subject to take another meter reading. The difference in readings provides the exposure correction factor to use.