Methods of Testing Your Lenses

I recently wrote and article for PentaxForums.com. It was my most complex lens review to date, with a lot more technical information and visual examples than the ones I currently have here. I plan to get more into reviews like that one once I finish going through my lens collection with general opinion centric reviews.

In this article today I’ll go over a few methods I’ve attempted so that I can better understand the strengths and weaknesses of my lenses. Keep in mind that this is what works for me and I don’t claim to have the best methods. It’s always going to be a learning process to improve results I’m sure of that.

Technical tests:
Tests that measure properties of lenses are a great way to understand more about your tools. If you learn by taking measurements or making comparisons with your lenses, you can in turn understand more about settings and configurations that will produce results you want most out in the field.

Download an ISO 12233 lens testing chart or similar (eg. search Google for “lens test chart”). Print the chart out in the highest resolution your printer can manage. Keep in mind that this is the “poor man’s” lens testing chart as you can alternatively buy a real chart for a few hundred dollars. The differences are far as I can tell are that the tend to be physically larger than most printers can handle and also higher resolution. I think taking the simple and cheap route as I have done is sufficient.

Attach the chart printout to your wall and photograph the image at various aperture settings with a few lenses. Try to use lenses that have at least one similar property (focal length, aperture, etc). When photographing, use a tripod and attempt to have the resulting image from all lenses look the same focal length wise.

Devise a range of apertures you want to test at. Generally, I think testing the widest aperture, one around MTF (approximately f5.6 on most lenses), and one toward the smallest aperture (say f16 or f22) to get a good understanding of capabilities without spending too much time.

Make certain that for each lens, have the same camera settings (ISO, aperture, and shutter speed). Otherwise your tests won’t be a true comparison. Along with settings, test in a setting with consistent lighting that doesn’t change, so your results are even between testing subjects.

Analyzing technical results:
Now that you have charts, compare each sub-test (aperture, ISO, shutter speed).

Center:
Notice sharpness or lack of. The center of the frame should be the sharpest part of the image. The center is what you always want to achieve maximum focus on in your tests.

Corners:
Look at the edges of the image. Are lines the same amount of blurriness in all four corners? If not, you probably did not get your camera directly facing the chart. Try setting up your test again and getting your camera directly parallel to the chart. If the difference in corner sharpness isn’t much, just leave it to expected error and continue analyzing the images. How do the corners of multiple lenses stack up? You might be surprised how poor some lenses are at rendering details toward the edge of the frame.

Lines:
Example the lines in the photo. On the chart some lines are very close together. This is to help you see how much resolving power the lens has. If the lines close together quickly blur together, your lens is lacking in resolving power. If you see distinct lines, your lens is sharper. Look at the edges of lines again and see if you notice colored halos of say purple, red, green or similar. These are chromatic aberrations that are something you don’t want in the image. If you see them in the test chart, chances are you could see them in your real photos (say when you photograph a blue sky and see purple lines around tree branches in the picture). There are various types of aberrations that can distort the image, it’s best to do some research online to learn more.

Understanding and trying features:
It’s important to understand what your lens is capable of. Some lenses provide special features like high magnification, excellent background blur (bokeh), or mechanical abilities. Read up on what features your lens has and try each feature out. After understanding the feature you might even see additional uses.

Pentax FA 50mm f1.4 example feature analysis:
Standard “normal” prime lens: It’s a normal prime lens, but that’s only the case on 35mm full-frame cameras. Pentax doesn’t offer any in digital, so you can expect the lens to fit better in a portrait lens category (50mm x 1.52 crop factor = equivalent 76mm focal length).
Aperture ring: This a feature that isn’t available on most new lenses these days. This is a ring that allows the user to manually select the aperture setting completely separate from the camera body. Possible uses of this feature could be reversing ring macro photography, or using the lens on an older film camera that is completely manual without the ability to control aperture.
Build quality: While not exactly a feature, looking at overall build quality will help you know where you can take the lens and how dependable you can expect it to be. The FA 50mm isn’t weather resistant, has quite a bit of plastic, but does have a metal lens mount. It’s a pretty average lens build quality wise, so it’s probably a good idea to not use it in extreme weather or climates.
Filter size: The FA 50mm has a filter size of 49mm, which is one of the most widely used sizes. There are a multitude of products you can attach to the filter thread, so knowing the size can help you weed out stuff that won’t fit.
Minimum focus distance: The FA 50mm has a minimum distance of 18 inches. Knowing this value before you buy the lens helps you understand what types of subjects you will be able to photograph in a given location.
Aperture range: A smaller number means a bigger opening. Generally, a larger aperture (smaller value) will give you more power over depth of field and background blur. The FA 50mm has a pretty large aperture at f1.4, making it easy to blur the background.
Accessories such as lens hoods: A good lens hood can help reduce lens flairs and improve contrast. In the FA 50mm’s case, it doesn’t come with a hood.

Create an opinion of lenses by comparison:
By analysis of features and technical image qualities between lenses, you can improve your overall knowledge and skill. With more knowledge about your tools in general, you can better be able to select tools that fit your needs and tools that help you accomplish your ideas.

The more you know about the lens, the better.

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