The most significant difference between the newer TS-E 50mm f/2.8 L Macro and the older TS-E 45mm f/2.8 is the way the relationship between the axis of shift and the axis of tilt can be altered on the fly without partially disassembling the lens.
In this respect, it is the same difference between the TS-E 24mm f/3.5 L II (2009) and the original TS-E 24mm f/3.5 L (1991). From my answer to What is the difference between Canon TS-E 24mm f/3.5L II and I?
The most significant difference is the ability of the TS-E 24mm f/3.5 L II to do TS rotation. This means that the relationship between the tilt axis and the shift axis can be altered from between 90º and 0º and any point in between. This can be accomplished on the fly during normal use of the lens. The original TS-E 24mm f/3.5 L can only be set at either 90º (standard orientation when the lens comes out of the factory) or 0º between the tilt and shift axes, and requires that the lens be partially disassembled to change the angle. This feature vastly expands the possibilities of how tilt and shift may both be used in combination to affect the perspective of an image. If you are interested in taking photos that only require either tilt or shift, but not both, then this feature is not much of a factor. If, on the other hand, you are interesting in discovering the full creative possibilities of a perspective control lens, then it is revolutionary.
The review of the TS-E 24mm f/3.5 L II at The-Digital-Picture includes many comparisons to the original TS-E 24mm f/3.5 L it replaced in Canon's line up.
As the linked review above relates, the "II" version is also superior optically, especially in terms of sharpness and lack of chromatic aberration. If you look at the sample images the review provides, you can see that the differences are more than trivial - they are rather significant.
There are similar optical differences between the TS-E 50mm f/2.8 L Macro and the older TS-E 45mm f/2.8.
Just as Bryan's review of the TS-E 24mm f/3.5 L II included some comparison to the older TS-E 24mm f/3.5 L, near the end of his recent review of the TS-E 50mm f/2.8 L Macro he includes comparisons to the older TS-E 45mm f/2.8.
The Canon TS-E 50mm vs. 45mm image quality comparison shows the L lens being noticeably sharper and the difference would appear even stronger if magnified to the EOS 5Ds R's resolution. The difference dissipates as the aperture is narrowed, but not until f/8 does the difference become difficult to discern... Also, the L lens is better-resistant to flare despite its element/group count increasing slightly from 10/9 to 12/9.
Some other things to note:
- The TS-E 45mm f/2.8 is not considered an 'L' series lens. Although well made, the TS-E 45mm f/2.8 was released in 1991 at the same time as the original TS-E 24mm f/3.5 L. For whatever reasons, the 24mm was considered an 'L' lens and the 45mm was not.
- Optically, all of Canon's TS-E series of lenses are great. But the current lineup of TS-E lenses released beginning in 2009 take that to a higher level than the earlier ones released in 1991.¹
- You can compare the flare performance of both lenses here.
- You can compare the optical quality of both lenses here.
For both comparisons at The-Digital-Picture you can change the aperture, the camera used for the test, and even add 1.4X or 2X extenders into the mix using the drop down menus. If you change the aperture of one lens, don't forget to the change the aperture of the other lens for an equivalent comparison.
There are plenty of examples that were made using the TS-E 45mm f/2.8 at Flickr. There aren't that many yet that were made using the TS-E 50mm f/2.8 L Macro. As always, the final quality of an image has a lot more to do with the ability of the photographer than the image quality of the lens used.
Do you know how much resolution the 45mm can resolve?
Measuring resolution of a tilt-shift lens is like aiming at a moving target. Every time the lens is tilted or shifted a different part of the image circle cast by the lens is being projected onto the camera's film or sensor. Most resolution tests and (theoretical) MTF charts released by manufacturers are based on the lens being centered with no movements applied.
There's a small version of the MTF chart released by Canon for the TS-E 45mm f/2.8 at this listing by allphotolenses.com. Unfortunately, it does not seem Canon has released any MTF charts for the TS-E 50mm f/2.8 L Macro. It must be kept in mind that MTF charts released by Canon are based on the theoretical design performance of the lens, rather than measurements of actual copies of a lens.
ePhotoZine has reviews of both lenses, but their resolution chart changed between the time the TS-E 45mm f/2.8 was reviewed in 2014 and the TS-E 50mm f/2.8 L Macro was reviewed in 2017.
As the "verdict" at the end of the TS-E 50mm f/2.8 L Macro says, it takes a lot of skill to use a tilt/shift lens well (emphasis added):
Canon TS-E 50mm f/2.8L Macro Verdict
It has bulk, it has the features. It has a high price, it has the features. In other words, another of those situations where the feature set and quality are not in question but the price will limit the market to those who really need this as a working tool. It would be very nice indeed to own, but it does need to pay its way, as well as needing a high degree of photographic skill to get the best out of its potential.
Given the need and the cash, the photographer will certainly not be disappointed in the results from the Canon TS-E 50mm f/2.8L Macro lens.
¹ The TS-E 24mm f/3.5 L, TS-E 45mm f/2.8, and TS-E 90mm f/2.8 were all released together in 1991. The TS-E 17mm f/4 L and TS-E 24mm f/3.5 L II were released together in 2009. The TS-E 50mm f/2.8 L Macro, TS-E 90mm f/2.8 L Macro, and TS-E 135mm f/4 L Macro were released together in 2017.