Why are my Close-up Photos Underexposed with the EF 100mm f/2.0 USM?

Question

I'm using the EF 100mm f/2.0 USM lens on an EOS 5D Mark IV body. There seems to be a specific focusing distance (about 1.5 m) below which all photos come out at least one stop underexposed when relying on the camera's metering system.

The following two photos were both taken through the viewfinder in aperture priority mode using evaluative metering. The subject distance is about 1.5 m.

100 mm, f/2, 1/80 s, ISO 640

100 mm, f/2, 1/100 s, ISO 1600

As you can see, the camera's metering system chose a much darker exposure for the first photo, with the only difference being that the camera was slightly further away from the target in the second photo.

This happens in both AF and MF and seems to depend solely on the focusing distance, rather than the actual subject distance. When changing focus in MF mode, I can easily see the jump the metering system does when crossing the focusing distance at which the effect starts. I can provide more examples if needed.

This does not happen in live view. It seems to be specific to mounting the EF 100mm f/2.0 USM lens on the EOS 5D Mark IV body, as it does not happen with any of my other lenses on the EOS 5D Mark IV body and it also does not happen with the 100mm f/2.0 USM lens on my EOS 750D body. Both bodies and the lens in question run the most recent firmware at the time of writing this.

What is causing this effect? I will probably return the lens since this makes shooting with it very difficult and I didn't like it that much overall anyways, but it would be nice to know what is wrong so I can avoid this problem in the future.

I do not think that my question is a duplicate of this question. The jump in exposure is very sharp and happens at a specific focusing distance, rather than continuously changing with focusing distance as I would expect with light loss at high magnification factors. Furthermore, the magnification is not particularly high as the subject distance is about 1.5 m. I am fairly certain that the issue is with the metering. Indeed, when using manual mode to circumvent all metering or live view shooting to use a different metering system, everything works as expected.

Answer 1

The modern camera’s metering system measures exposing light thru-the-lens. We expect the results to be accurate, even when close focusing.

You should be aware that the f-numbers associated with a camera lens are based on imaging an object at infinity (symbol ∞). In other words, the math is performed when imaging a point source, like a star. When we image objects that are closer than ∞, the f-numbers become invalidated. The f-number error is super tiny when imaging objects 3 feet (1 meter) or further. When imaging object at about 1 meter, or closer, the error becomes significant.

This phenomenon is called “Bellows Factor” (BF). In the past, most camera makers stopped close focusing when the error approaches 1/3 f-stop. They did this because general purpose camera lenses are optimized for distance thus they are compromised when tasked to work close. Additionally the f-number error (BF) will likely result in an under-exposure unless a correction is applied. Modern cameras that read the exposure through the lens are generally immune as the metering system likely applies the needed correction.

On the other hand, a macro lens keeps the f-numbers valid when close focusing and they are optimized to image close-up., They are slightly challenged when tasked to image objects at a far distance.

Bellows Factor math: BF=(m+1)^2. M = magnification. If you close focus an achieve “unity” (life size or magnification 1), then m=1. At unity BF=(1+1)^2 (magnification + 1 squared). Solving for BF at unity: BF =( 1+1)^2 = 4 The BF is handled just like a filter factor; we multiply the exposure time by the BF to compute a revised shutter speed. If the factor is 4 the correction is 2 f-stops compensation (you open up the aperture since each f-stop is a 2X change). At unity the camera set to f/5.6 is actually functioning two f=stops more stopped down at f/11. Applying the BF to shutter speed: The shutter is set to 1/60 of a second, we multiply 1/60 by 4 = 1/15 (the revised shower shutter speed).

Why is your close-up images unexposed? Perhaps the camera’s logic does not fully compensate for BF. Perhaps the mundane background and it’s color / hue is fooling the metering logic. I suggest aperture priority thus allowing the camera logic to select the shutter speed. You can compensate in manual mode and open up the aperture or slow the shutter.