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Sunday, 07 February 2010

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I'll be honest, I thought the title said 'Brooks Jensen's Lens Tasting' and I was immensly curious to see what new and scientific method we were going to be introduced to today.

My excitement was dashed when I put my reading specs on :-(

interesting site!

[came across it through a search for john szarkowski -whose the photographer's eye i am currently reading- yielding a post in your original blog]

Dear folks,

I would add one modest caution to the use of this chart.

It's an excellent tool for measuring lens performance and focus accuracy near the center of the field. It's extremely difficult to align a target like this parallel to the film/sensor plane. It has to be nearly perfect, or you'll see falloff in image quality at the corners that you will confuse with poor lens performance. Your depth of field is extremely small in these circumstances; a matter of millimeters!

On axis, though, it should work very well. And, by locking down the focus and then making a series of exposures moving your camera backwards and forwards in half centimeter increments, you'll easily be able to see if your focusing system is off (for that particular lens and distance, anyway) and how much resolution you're sacrificing because of it.

Absent such fussing around, understand that what you will be measuring here is your total system resolution including focus errors, not the true performance of the lens or the sensor.


~ pax \ Ctein
[ Please excuse any word-salad. MacSpeech in training! ]
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In the great spirit of "I should go out more."

Oh the tasty Lens Culinaris aka the lentil bean, lens being Latin for lentil. Or perhaps you are French since lentil beans and camera lenses are spelled the same in French. According to Google translate, lenses for cameras in French are "lentilles de caméras" and lentil bean soup in French is "soupe aux lentilles"

I'd be afraid of what I might find out about the two lenses I currently use!

Two bits of advice -- first, one way to get your camera's sensor aligned fairly well with the target: hang a mirror in place of the target and make sure the dead center of your framing is pointed at the dead center of your lens in the reflection. (Then make sure your target is exactly where the mirror was.)

Also, Live View is very handy for these tests, especially if you want to measure your lens's acuity and not your camera's autofocus accuracy. Zoom in tight in Live View to manually focus on your target until it's as perfect as you can get it. (That is, zoom in the LCD, not your zoom lens.)

The first of the following two paragraphs is outright incorrect. (Text copied form the link given.)

QUOTE "The trick with this chart — and I assume other charts are used similarly — is to photograph it from a precisely measured distance. This precise distance times the focal length of any lens allows direct comparison between lenses of different focal lengths that are meaningful. Even zoom lenses can be tested with comparable results.

The important target pattern in the chart is a series of lines that are pre-sized to be photographed at this specific distance — btw, exactly 26 times the actual focal length of the lens, not the digital 35mm equivalent BS which we see so often these days. Here is what it looks like." UNQUOTE

It is not "This precise distance times the focal length" but rather:
The distance from camera to test chart should be the focal length of the lens multiplied by a factor which is specific for each brand or type of chart. In this case the number is 26.

Of course it does not matter which factor you use as long as you use the same factor for all lenses you want to compare.

The only reason to use the factor given by the chart manufacturer is that you can then (more or less accurately) read off the resolution of the lens in lpm by looking at a negative. The values interpreted by one person may be different from the interpretation by another person, so the same person should interpret the test negatives for all lenses compared.

There are other statements in the text with which I disagree, but that has more to do with opinion than facts.

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