F-stop blues

Tuesday November 23 2010

Lens Insight

For years, lens makers have fought hard to market lenses of wider and wider aperture. Wide apertures (e.g., f /1.4 instead of f /2) but a series of measurement published on cast some doubts on the real benefits, for digitally equipped photographers, of these progresses.

Loss of light at wider aperture

“We have been very surprised,” explained Frédéric Guichard, chief scientist at DxO Labs, “to find out that some of the gain from wider lens openings seems to be offset by the present state of sensor technology. Our measurements all point in the same direction: as you go further than f/ 4 – to f /2 and wider, the accrued quantity of light falls marginally onto the sensor. A stronger and stronger part of this additional light is blocked or lost. I am therefore inclined to question the real benefit of faster lenses.”

This loss seems to increase when the pixel size decreases, as shown on the figure below.

ΔEV at f/1.4 function of pixel pitch

Some sensors can have a loss of more than 1 Ev at f/1.2.

ΔEV at f/1.2 function of pixel pitch

Raising ISO to compensate light loss

A further assessment is still more troubling. It seems that many camera makers are raising the ISO sensitivity when the lens opening is at its widest, probably to cover up the lack of efficiency. “We have ascertained,” explained Guichard, “that on many cameras, the ISO is pushed up on the order of 1/3 to 1/2 stop when the lens opening comes close to its upper limit.

Sensor gain function of f/# (expressed in Ev)

“Accordingly, the photographer has no way to detect that he/she has not benefited from the increase of light transmitted. Of course this increase in ISO translates into other downfalls – mainly in accrued noise. But these remain unknown to the operator,

“We can suspect,” Guichard continued, “that sensors collect the incoming light all the more improperly, in that this light comes from a more oblique angle. Since faster lens have, by definition, a wider opening, they raise the proportion of oblique light, hence the proportion of lost energy which never lands on the pixels.”

DxO Mark measurements are based on an assessment of the effective T-stop for every camera-lens combination. T-stop, a measurement widely used in the industry, especially the motion picture industry, is a measurement of the effective quantity of light transmitted by the lens at a certain f-stop. Thus, T-stop takes into account every reflection or absorption due to the lens. It can be considered as an effective (versus theoretical) way of measuring the aperture.

A possible conclusion of DxO Labs’ measurements is that photographers should consider with caution the maximum f-numbers advertised for lenses. Indeed, depending on the performance of their camera body and sensor, they may not effectively benefit as they expect from such wide apertures.