Lens Focus and Vignette Test

682 1024 Tim Breaseale

I’ve been shooting digital since the year 2000.  I was still shooting film until about 2004, which was medium format.  Most of my work is comprised of environmental portraits.  I would use Sailwind lens shades with the filters to achieve vignetting on my images.  Between the years 2004-2006, I bought my Canon 24-70mm f/2.8L IS and Canon 70-200mm f/2.8L lenses.  I was shooting all digital by then and was capturing images on a  Canon 1D Mrk IIn and a Canon 10D.  I never paid attention to vignetting, because I was so used to adding it in camera.  I still used the Sailwind filters on the 35mm camera bodies, even though I could add it in post.  In 2007, I bought a Canon 40D and stopped using filters.  Now fast forward to 2013.  I sold off most of my gear.  I only have a couple Canon lenses that I never sold.

I have been without a camera system for about 6 months.  For this time span, I have been using cameras from one of my clients, which is a Leica store (using the Leica S2).  I have been a dedicated Canon shooter for the past 17 years.  I decided that it was time to get a new Canon camera.  I purchased a Canon 6D at the first of February.  I also bought the new Canon 40mm f/2.8 STM lens to go along with it.  I have not taken that lens off the camera for about two months and love it.  Over the past 6 months, I still use the Leica S2, because of it’s medium format size sensor.  For the past couple of jobs that I’ve worked, I decided to use the 6D because I didn’t need the large resolution of the Leica S2.  I used my old Canon 24-70mm lens for the jobs.  I noticed that my images had way too much of a vignette and were a little soft.  I decided to do a little test with my camera to see if there were issues with front or back focusing and to also see how much vignetting was occurring on the lens.  So, here is my very non-scientific testing results and conclusion.  I used this focusing chart that I found online, a shoe box, 2 leaves, a Color Checker Passport, and my front porch.  Another note, I placed the Color Checker Passport in the frame to use as a white balance reference and forgot to use it.  So, my images are not color corrected.  Click any image to enlarge.

Here are some types of vignetting problems.  The information is from Wikipedia.  I think I have the optical vignetting problems.

“Mechanical Vignetting Mechanical vignetting occurs when light beams emanating from object points located off-axis are partially blocked by external objects such as thick or stacked filters, secondary lenses, and improper lens hoods. This has the effect of changing the entrance pupil shape as a function of angle. The darkening can be gradual or abrupt, depending on the lens aperture. The smaller the aperture, the more abrupt the vignetting as a function of angle. Complete blackening is possible with mechanical vignetting (when the corner of the image is essentially imaging the inside of the lens hood or filter holder).

Optical Vignetting  This type of vignetting is caused by the physical dimensions of a multiple element lens. Rear elements are shaded by elements in front of them, which reduces the effective lens opening for off-axis incident light. The result is a gradual decrease in light intensity towards the image periphery. Optical vignetting is sensitive to the lens aperture and can often be cured by a reduction in aperture of 2–3 stops. (An increase in the F-number.)

Natural Vignetting  Unlike the previous types, natural vignetting (also known as natural illumination falloff) is not due to the blocking of light rays. The falloff is approximated by the cos4 or “cosine fourth” law of illumination falloff. Here, the light falloff is proportional to the fourth power of the cosine of the angle at which the light impinges on the film or sensor array. Wideangle rangefinder designs and the lens designs used in compact cameras are particularly prone to natural vignetting. Telephoto lenses, retrofocus wideangle lenses used on SLR cameras, and telecentric designs in general are less troubled by natural vignetting. A gradual grey filter or postprocessing techniques may be used to compensate for natural vignetting, as it cannot be cured by stopping down the lens. Some modern lenses are specifically designed so that the light strikes the image parallel or nearly so, eliminating or greatly reducing vignetting. Almost all lenses designed for the Four Thirds systemare of this type, as telecentricity is a stated design goal.

Pixel Vignetting Pixel vignetting only affects digital cameras and is caused by angle-dependence of the digital sensors. Light incident on the sensor at a right angle produces a stronger signal than light hitting it at an oblique angle. Most digital cameras use built-in image processing to compensate for optical vignetting and pixel vignetting when converting raw sensor data to standard image formats such as JPEG or TIFF. The use of offset microlenses over the image sensor can also reduce the effect of pixel vignetting.”

The image at the beginning was shot with the 24-70mm f/2.8L lens shot @ f/2.8, and I overexposed by 2 f/stops.  My first thought when using the camera’s metering system (Evaluative metering) and checking the histogram is that it was underexposing the scene by 1 to 2 f/stops.  So, for the rest of the images in this post, they are 2 stops overexposed.  The image below shows a side-by-side of the lens set @ f/2.8 on the left and set @ f/5.6 on the right.  The image on the right still has the vignette @ f/5.6, but is gone by f/11.

Now to check the lens set at 70mm.  The image below is showing the lens at 70mm @ f/2.8.  You can see that the vignetting is just slightly there.  Now I am wondering about the rest of my lenses and how much vignetting they have at wide open apertures.

My new Canon 40mm f/2.8 STM lens is very sharp.  I love using this lens and camera combination as a walk around system.  Below you can see the side-by-side images of the aperture setting @ f/2.8 and @ f/5.6 with the 40mm lens.  There seems to be a little vignetting with this lens at wide open apertures.  The vignetting transition is very smooth and is barely noticeable.  By aperture setting f/8, the vignetting has disappeared.  Now, since I am a Lightroom user, I wanted to see how much Lightroom’s lens correction presets really work.  On LR 4, I didn’t use the presets, because it would soften the overall image too much for my taste.  On LR 5, the presets work much better, but there is still a little softening in the corners.  I can live with the little amount of softening LR 5 presents produce.

40mm2856

Below is a comparison using LR 5 with my 24-70mm lens set at 24mm @ f/2.8.  So much better, not 100% perfect, but very usable.

Below is a comparison using LR 5 with my 24-70mm lens set at 70mm @ f/2.8.  The presets work pretty well.  See the difference in the corners.

Now, for my 40mm STM lens (below image), how well will the presets work?  Pretty darn good.

I got thinking, if my 24-70mm lens set at 70mm has some vignetting, by theory my 70-200mm lens set at 70mm should have vignetting too.  So, I checked out my Canon 70-200 f/2.8L IS lens set to 70mm.  Below is what I found.

Canon70200

This animated Gif has 5 images, f/2.8, f/3.5, f/5.6, f/8, and f/11.  You can see the vignetting is gone at f/11.  How well does LR 5 work for this lens?  Below you can see the before and after using LR 5 preset.  It works pretty well.

Conclusion

Canon 24-70mm f/2.8L.  This lens has a pretty heavy vignette, which takes up at least 2/3rd’s of the frame.  I would say at least a 2 f/stop change from corners to center frame when set at 24mm @ f/2.8.  A little less vignetting when the lens is set at 70mm @ 2.8.  The vignette does start to fade and is gone by f/11 at all focal lengths.

Canon 70-200mm f/2.8L IS.  This lens has a medium vignette, which takes up a little more than 1/4th of the frame.  I would say at least a 1 f/stop change from corners to center frame when set at 24mm @ f/2.8.  A little less vignetting when the lens starts to zoom towards the 200mm mark.  The vignette is gone by f/11 at all focal lengths.

Canon 40mm f/2.8 STM.  This lens has a slight vignette.  The vignette is not as noticeable and has a very smooth transition from corner frame to center.  I would say maybe 1/3rd f/stop of change (or slightly more) of a transition.  The vignetting was gone by f8.

The focusing on all three lenses were right on center.  No front or back focusing issues.  I also checked out different focusing points from the camera and noticed that not all focusing points would lock focus.  The center point was the quickest to focus while the outer points were slower, but did lock on.  With the focusing points to the right and left of the center point, I could not get them to lock on the very center of the chart (the black bar), but it would lock focus on the 25% grey part of the chart (that did not make sense to me).

I guess I really need to update my lenses.  The two lenses in question, the 24-70mm (released in 2002) and 70-200mm (released in 2001),  were designed for film cameras.  Canon has new versions of the 24-70mm and 70-200mm lenses that are optimized for digital capture.  I think they are overpriced.  That’s one reason I didn’t sell them, because of the cost to replace them.  There are some third party lens manufacturers that have really revamped their quality and have been getting really great reviews.  I might have to consider some of these lenses in the future.  Until I replace the lenses, LR will have to fix the vignetting problem.

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