I recently looked into how playing the sun field affects an outfielder's defensive performance. I was inspired by Craig Wright's discovery that Babe Ruth regularly switched between left and right field throughout his career to avoid playing the sun field, as I wanted to know what kind of effect this knowledge would have on his defensive value.
As far as I can tell, there isn't much of an effect. You can stop reading here if you care whether reading material is interesting, but I'll detail my methodology below so that those interested can know what I mean when I say I didn't find an effect.
First, I compiled as best I could a list of sun fields for all open air stadiums in the Retrosheet era (1950-2012 for my current database). Sun fields were estimated based on diagrams and images from Seamheads ballpark database, Ballparks.com, and AndrewClem.com. I was able to corroborate or correct some parks by looking around the web for written mentions of sun fields or photographs showing shadows during a game.
This is actually trickier than it sounds--you can get a decent idea of where the sun should set from maps or diagrams that include stadium orientation, but the sun's position also depends on the stadium's latitude and changes based on the time of day and time of year (which also means you can get conflicting results from photographs depending on when they were taken--see the shadows pointing to CF vs the shadows pointing to RF in Busch Stadium). Still, I did the best I could to identify a primary sun field, and while I doubt I came up with a perfect list, it should be good enough to detect an effect if there is one.
Once I had a list of sun fields for each stadium, I looked at putouts per ball in play for corner outfielders in each stadium. I then divided these into day and night games, so that I had average number of putouts per ball in play for left and right fielders in day and night games for each stadium. Using these figures, I checked the difference between PO/BIP between day and night games. Parks with roofs (retractable or not) and parks with CF sun fields were ignored.
From there, I checked how much the average PO/BIP went up or down for fielders playing the sun field. If playing the sun field makes impairs the fielder, then they should see a drop in performance from night to day games. However, it is also possible that playing the outfield is generally easier or harder in day games, so I also checked the change in putout rate for the opposite corner outfield position to use as a control group. Rather than compare the sun field's day game performance to its night game performance, I compared the change from night to day for the sun field to the change for the non-sun-field.
For example, in 2012 Busch Stadium, left fielders recorded putouts on 6.13% of balls in play during night games, and 5.94% during day games. Right fielders were 6.62% for night games and 7.49% for day games. That means that left fielders dropped their PO/BIP by .0019 in day games, while right fielders raised their PO/BIP by .0087. I have right field as Busch's primary sun field, so the sun field was associated with a gain of .0106 putouts per ball in play over the control group in day games.
Doing this for every season in every stadium included in the study, I got an average* of 0.0002 gain in PO/BIP for the sun field over the non-sun-field, which is practically zero and very slightly in the wrong direction to indicate an effect.
*the average was a weighted mean, with the weight given to each stadium-season being the harmonic mean of day BIP and night BIP. For example, 2012 Busch stadium had 2872 night BIP and 1549 day BIP, which is a harmonic mean of 2012.5.
Since I was concerned that poor data on which field was the sun field may have masked any potential effect, or that only some parks might have a bad sun field, I checked to see if individual stadiums displayed any effect. If that were the case, it should still show up in the overall data as a diminished but still visible effect, but it was worth checking. Individual stadiums did vary from zero effect, but not any more than they would by random chance. When splitting stadium-seasons into even and odd numbered years, there was no correlation between the observed effect for a stadium in even years versus the same stadium in odd years.
Finally, I checked the same thing for individual fielders, to see if there was any evidence that particular fielders had notable trouble with the sun field that would show up in PO/BIP. The result was the same as the test for individual parks--fielders varied from zero effect but no more than expected by chance, and the even-odd season correlation for fielders was 0.
This does not necessarily mean that the sun does not affect fielders--I assume that when the ball is actually in the sun, it adds a great deal of difficulty. It is likely that this does not happen often enough to significantly alter a fielder's defensive numbers, though. At the very least, it appears that finding an effect would require much more precise data. For example, you could probably find something by using the sun's position at the time of the play and the trajectory of the batted ball to identify specific plays that are likely affected. Even if this data were available, however, it would be impossible to use it to evaluate Ruth specifically, and the overall effect I saw indicates that there is likely no need to adjust his defense valuation down simply because he rarely played the sun field.