Tuesday, September 14, 2010

TTL vs. Manual Flash: A False Dilemma



Among flash users, there has been a rift between manual flash proponents and TTL flash advocates, with some proponents of one declaring their method superior to the other.

MANUAL VS. TTL: WHAT'S THE DIFFERENCE

Manual flash means the user sets the power output himself in order to get the desired amount of flash recorded in the picture (i.e., flash exposure). The flash exposure is affected by the available power of the flash (its guide number), the aperture, the ISO, and the distance of the flash to the subject.

With TTL flash, the camera and flash set the output automatically in order to achieve a more or less _constant_ flash exposure (with almost any combination of guide number, aperture, ISO, or flash-to-subject distance), analogous to your P, A, or S modes trying to achieve a middle grey, which the user then adjusts to his or her liking using exposure compensation. With flash, the equivalent control is flash exposure compensation (FEC).

THE CONTROVERSY AND THE MYTH

Some manual flash users scoff at TTL. One of the nicknames for TTL in manual flash circles is "idiot mode." I think that reputation among some manual flash users is ironically based on ignorance about how TTL is controlled. It is true that TTL may be left to operate automatically, and no doubt the majority of TTL users do exactly that (assuming we include point-and-shoot cameras). Perhaps it is from that phenomenon that TTL gained a reputation for being completely automatic like the green mode on some cameras. However, I would wager that any person that calls TTL "idiot mode" does not really know how to control TTL. (Specifically, I doubt any such person would pass Lesson 11 of the TTL Flash Tutorial.)

Serious TTL users advocate adjusting TTL through flash exposure compensation, in addition to minding other lighting fundamentals such as balancing flash and ambient.

TTL CONTROL

A myth about TTL is that it doesn't offer as much "control" as manual flash. With respect to range there is an element of truth to that. With a good manual flash, you can choose any setting from full power to 1/128 power, a range of 7 stops (though some units have a range less than that). Pretty amazing control huh?

Well here's the thing: on my TTL flash, I can go anywhere from -3 FEC to +3 FEC, in 1/3 stop increments. A range of 6 stops. Not bad for an idiot mode. If I use cumulative FEC adjustments (meaning I make an adjustment on the camera's FEC button and the flash's FEC button), the range expands to -6 FEC to +4 FEC. A range of 10 stops. Moreover, a flash on TTL can go below 1/128 power, without any neutral density gels or workarounds.

The partial-stop increments are not to be ignored. Some flash units with manual flash can be adjusted in 1/3 stop increments, such as the SB-800 (indeed, the manual YN-560 flash can be adjusted in 1/8 stop increments). However, not all manual flash are capable of partial-stop increments between full power and minimum power. For those manual flash units, users adjust the distance between the flash and subject, alter the aperture or ISO, or use a neutral density gel to adjust the flash output in smaller increments.

THE CATCH

The catch with TTL is that the 0 FEC point is determined by the camera and flash, and as TTL becomes more advanced, the way that flash exposure determined becomes more complicated to understand. On the other hand, it's very unlikely that the camera and flash can be 6 stops away from the 'right'amount of flash.

The other complaint against TTL is consistency. With TTL, it is theoretically possible for the flash exposure to vary from shot to shot when the composition or zoom varies. If the ambient light, aperture, ISO, subject-to-flash distance, and the guide number of the flash do not change, then manual flash users point out that the flash exposure should also be consistent. In practice though, I haven't seen a huge variation shot-to-shot under conditions when flash exposure is supposed to be constant. In addition, in some cameras, there are tools such as FV lock (flash value lock -- analogous to auto exposure lock) that can lock the flash exposure for consistency.



IS TTL FLASH SUPERIOR TO MANUAL FLASH?

Short answer: in my opinion, no. Just as no single exposure mode (P, A, S, M) is superior in every situation, neither do I believe that TTL or manual will be superior to the other in all circumstances.

TTL is quicker in fast-changing shooting conditions (e.g. weddings) when ambient light varies constantly and subject to flash distance can vary greatly (especially if bouncing the flash). In such conditions, it is difficult to get the keep adjusting flash output with manual flash with each small change to maintain the result we want.

On the other hand, in setup shots where the flash is on a stand and won't be moving, and the subject will stay in the same place for the shots, using manual would guarantee control and consistency from shot to shot. There are also situations where we are pushing the flash to the limits of its power and we know in advance that we want to use full power -- it would make sense to switch to manual flash. Finally, there are rare occasions when TTL will simply fail for reasons that are not immediately apparent. In such cases, it would be great to know that you can always switch to manual.

Unless you take only one type of photo, I believe it makes sense to learn and be comfortable with both TTL and manual flash. I must confess I often use TTL (because most of my shots are candid) but I can switch to manual when needed. The shot at the beginning of this article was taken on an occasion when I was switching from one to the other, and sometimes combining both. Link to album.  I doubt that you can tell which shot was shot with which method. :)

WHICH TO LEARN FIRST

You can learn either one first, but I believe you can get more-or-less ok results faster with TTL flash. I analogize again to the P, A, and S modes. Many beginners, instead of jumping to manual exposure mode, start out on those semi-automatic modes which frees them to think of composition and other aspects of the photo. However, there are photogs who do believe you learn faster with manual flash. It's up to you!

If you shoot with manual flash only or TTL flash only, I suggest you give the other flash method a shot. You can only gain from it.