I am by far not an expert, but, hoping to do something nice for the community, I am submitting a report that would have helped me tremendously when choosing a lens for deep-sky astrophotography. I still think even a good doublet refractor is the way to go, since it is comparable in quality, however, I wanted something to use for general purpose daylight photography as well. I bought this lens because the non-IS version is very hard to find lately, and IS didn't hurt so I went for this one. With this out of the way, here is the very basic test I did. It is no myth that the non-IS version is sharper, however, this lens is still very good, and at f/4 sharpness was still very high, with only a noticeable contrast loss, especially on brighter stars. The star field is very flat, and chromatic aberration on this optic is very easily dealt with.
Test report of the Canon EF 300mm f/4 L IS USM for Astrophotography | from Simone Scarpa, Italy (15/12/2025)
Tested on a 20MP Canon 6D with LPF2 filter removed (H-Alpha Mod)
In-camera corrections for chromatic aberrations were turned on, as the lens profile is available for the Canon lens on the Canon body. This correction is also available in Lightroom and Camera Raw. It was left on as there is no point in not using corrections for better star rendition.
DxOMark reports say blue fringing should be very noticeable on this lens, and I can confirm it is prevalent and there is no purple or red fringing. This is good as blue is a color which is more pleasant on the eyes and star fields, and more easily correctable, since it appears mostly on blue and whiter stars.
Test #1 | f/4 vs f/5.6 on a 30-second exposure on the Pleiades for performance on bigger stars
- Applied NoiseXTerminator carefully checking star shapes were not altered nor were star aberrations removed. Sharpness was not altered with any Camera Raw tools.
- f/4 is considerably darker at ISO 3200 than f/5.6 is at ISO 6400. The lens’ actual transmission at f/4 is probably around f/4.5 as it seemed like a third of a stop was missing in the histogram which was slightly skewed to the left compared to f/5.6 shot.
Left: f/4 (no spikes) | Right: f/5.6
📷 f/4
📷 f/5.6
- Any sharpness difference is barely noticeable as the diffraction spikes mask any aberrations that might be going on with bright stars. However, small stars are a bit tighter in the f/5.6 exposure and micro-contrast is better, given that lower-magnitude stars appear more clearly.
- Also, since a modified camera was used, it’s very nice to see that the lens managed to bring all the color into the same focal plane nicely, without many aberrations other than those already listed. This has yet to be tested on areas of hydrogen and with more yellow stars, as the Pleiades area is mainly only composed of young stars.
Test #2 | f/4 vs f/5.6 on a single 30-second exposure of a random star field near Polaris
f/4 vs f/5.6
📷 f/4
📷 f/5.6
400% zoom
- Even for pixel-peepers there is almost no difference. Main difference may boil down to contrast on stacked and longer exposures. The white stars appear just a tad smaller in the f/5.6 image. Very small stars are already out-resolved by the sensor anyway.
- f/4.5 and f/5 were tested but not included, as any real difference was not noticeable on the 20MP sensor.
Test #3 | Step-down ring test on stacked data (4x10s at f/4.67 and 6x60s at f/4)
Left is f/4.67 using a 62mm step-down ring
📷 wide open vs step down ring (62mm)
- The data is not the same, and nebulosity appears more complex in the f/4 image because there is more data. Regardless, microcontrast is still better in the f/4.67 image stopped down using a 77-62mm step-down ring. The brighter stars are tighter, as previously noted already. The stars are less wonky.
- However, star halos due to contrast loss are minimized, and this is very noticeable in processed data.
Very quick process on the 43-minute stacked exposure at f/4
The processed image confirms the very good performance at f/4 with a small contrast loss. The stars after heavy stretching were starting to lose sharpness but basic stellar deconvolution brought them to astrograph level quality. In reality, you would do deconvolution before stretching the data, unlike I did for testing.
📷 Pleiades
Conclusions
The Canon EF 300mm f/4 L IS USM is a very good lens for astrophotography.
Unfortunately, I didn’t have time to stack a lot of data at different focal ratios to better check for aberrations on stacked data, but this is enough of an indicator for me to recommend this lens.
Corner performance at f/4 does not give perfect stars and center sharpness is a hair short of great, but this only marginally worse than any other 250-280mm f/4.5-f/4.8 quadruplet refractor. Especially considering that from f/4.67 upward the quality of this lens goes from good to extremely good. Chromatic aberration is similar to the one of a doublet per the design of this older lens (1997) but resides mostly in the blues and is easily taken care of with post-processing and camera corrections. The diffraction spikes are very ugly and I would not stop down this lens with the internal iris.
The focal length is also somewhat inaccurate, given the FOV results to be in the range of a 290mm optic. I checked this through Astrometry.net and then input resulting data from the uncropped image into a site with necessary formulae (“astronomy.tools useful CCD formulae”). This is indeed still a very nice focal length for imaging without autoguiding. This is also the reason why a 62mm ring brings it to around f/4.67 and not f/4.83 as you would think from 300mm.
