MTF Charts – (Modular Transfer Function)

A few weeks ago I was doing a photoshoot outside for the first time in a while. I’ve almost exclusively been doing studio work for about the last year or so and I noticed some big differences when I went to go shoot outside again.

When I’m in the studio I’m usually trying to keep my camera between f/11 and f/22 to keep things nice and sharp. It shows in the final image especially with a 60D which doesn’t really have too many other scenarios where it can really shine. I’ve been getting so used to seeing images that are nothing but crisp front to back that it was kind of jarring shooting outside at f/4-f/5.6 again. Focus was noticeably less sharp on some but not all photos, making me a little uneasy but that’s of course to be expected given such a wide lens opening and the many other factors of outdoor shooting.

One of my big pet peeves with shooting outside is that I have to account for potential blurriness. In the studio you can keep your shutterspeed around 1/250th or even 1/125th of a second and still rarely encounter blurriness because the flash is what is freezing the action if there is any type of movement at all. Also, unless you bring a reflector, or some flash units with you, you’re at the mercy of mother nature. As I’ve been shooting in the studio I’ve really been enjoying all the control I have with lighting to create a specific look, feel or mood. Outside I find there are the two extremes of super flat, soft light from a cloudy day, or the ultra-contrasty-racoon-eye sunny days. There are a few in between situations but that’s generally what you’re dealing with.

More recently, I found that there’s also another factor that contributes to the sharpness, or rather the unsharpness of outdoor photos given the low f/numbers needed to create an exposure. Most big lens manufacturers like Canon, Nikon and Sigma have created a chart that represents what’s called the “Modular Transfer Function” of a lens. Essentially what the chart does is it maps out the relative sharpness/resolution and contrast of a lens on a scale for a lens that is opened at its widest f/stop and then stopped down to f/8. I was surprised to find that some lenses become almost 1/2 their total sharpness/resolution quality at their widest f/stop as opposed to f/8 which is significantly better.

The MTF Chart for the 24-105L IS from Canon's website.

The MTF Chart for the 24-105L IS from Canon’s website.

 

The black lines represent the lens wide open, the blue lines represent the lens stopped down. The thick lines represent contrast, and the thin lines represent sharpness/resolution. The Y axis is a 0-100% numbering system, meaning that for the wide angle of the 24-105L it is at 100% of it’s total potential sharpness/resolution and contrast. The X axis is from 0 (which here is the center of the lens) to the edge of the lens (20). Ideally, all the lines should stay grouped together in a straight line across the top of the chart. This is found more often with prime lenses at least on paper. There are many photographers and gurus on the web that will argue, given the right zooms and prime lenses, that both types of lenses are equally sharp. I think in today’s age, the difference between a zoom and a prime lens is negligible and even less so if you stop down to f/8 or higher on a zoom and on a prime lens. There are still many photographers that will only shoot their images using prime lenses.

 

Canon's EF 400mm f/4 DO IS USM lens MTF chart

Canon’s EF 400mm f/4 DO IS USM lens MTF chart

So before you think that you may have back-focused your lens while shooting and that’s why you’re getting soft images, check your lenses accompanying MTF chart on your lens manufacturer’s site. It could be that your lens is much more sharp at one f/stop down from where you’ve been shooting. For the future I will definitely be referring to the MTF charts before buying another lens.

If you need another explanation of the subject, there is a beautiful Luminous Landscape article that discusses MTF as well.

http://www.luminous-landscape.com/tutorials/understanding-series/understanding-mtf.shtml

-Alex

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