Friday, July 26, 2013

Picking lenses based on Focal lengths and Sensor sizes

Good Friday from Prospect
1.5 Crop Sensor
Today I want to talk about lens focal lengths and the apparent differences when placed on cameras with different size film/sensor planes.
This refers to the surface area not resolution. This post is as much about image production and ultimate image feel as a discussion of hardware.

It took me a bit to get my head around this, when I moved to digital from film, since everyone obsesses about crop as the most important variable when choosing a lens.
This is where it threw me.
When I select a lens, the first factor is it's ability to compress or expand a subject. For instance I shoot portraits using a compressed, telephoto, focal length, not an expanded one. The flatter the subject can be the more attractive the outcome (there is of course a sweet spot but that's more subjective than technical). When using, what I call expanded focal lengths, wide angle, you distort your subject by pulling them forward from the background, which by the way also effects a face making it look deeper than actual dimensions.
The other factors like wide open apertures, fast glass for low light, bokeh, IS, filter sizes etc are the next deciding factors but not what we're discussing here today.
My division between the two is this. above 50mm = compressed, below 50mm = expanded. Focal length numbers from the lenses, period. 50mm is the zero point that acts like both, depending on the distance from your subject.
Full Frame Sensor
Focal Length Explained
First, You need to forget about the "crop factor." The focal length of a lens and its accompanying perspective characteristics don't change just because a lens may be mounted on a crop sensor camera. The crop area has absolutely nothing to do with the lenses characteristics in producing an image. A 100mm lens on a 35mm 1.5 crop, full frame or a medium format has the same result. Great compression and wonderful Bokeh at open apertures. 
The only time it should matter is when you are composing your subject and how close/far you are standing from that subject and how much stuff you want in the frame.
Yes it is true that in a small interior space you want to get as much as you can in the frame. But using an equivalent "crop factored" lens does not produce the same result.

Here's an example of the effect of different focal lengths and there equivalents. 
Shooting your subject up close with an ultra wide (like a 14 or 12mm) pushes the background farther away giving you that super depth effect you want with an ultra wide. They also allow more content in the frame. A 14mm is suggested to be a 21mm on a crop sensor so you would need to use a 9.5mm (approx.) on a 1.5 crop sensor camera to achieve the same frame using a 14mm on a full frame sensor camera.. Problem is in the real comparison. An image produced with a 14mm is vastly different than a 9.5mm. Perspective, although deep, is usually very distorted with a 9.5 (almost fisheye) than a good 14mm ultra wide. Two different animals entirely. Lens distortion does not change because you cropped in your photo, just the apparent visual effect because you don't see as much content in the composition. Same can be said in comparing a 35mm vs a 50mm. People can look very weird shot with a 35mm up close compared to a 50mm which is what our eye sees (subjective arguments not with standing).
One notable difference between full frame and crop sensor is the difference in distance between you and your subject, to achieve the same composition. You need to step back with a crop sensor camera to achieve the same framing as a full frame.
That does change a number of settings and ultimately the outcome of your image, like real depth of field. Those differences are for a discussion between full frame and crop cameras. Later.

Tech Note
Focal length refers to the distance from the main lens element to the film/sensor plane, period. Whether you are cropping it down or not, the lenses characteristics are still the same. And that plane distance calculation remains the same whether it's a full frame 35mm or 1.5 cropped sensor (or even medium format). 
This brings me to another factoid, how your subject will look in relation to what the human eye sees and what views more realistically. For instance the human eye actually sees the equivalent to a 22mm, not a 14mm cropped in, when discussing total input including the periphery but 50mm is what we see in the middle, not 33mm cropped in. 

f 2.8 crop sensor, fuji 35mm f1.4
(original size 3264 x 4896-XPro-1)
click here for high-res
Point 1 This point is the most important in relation to your subject matter. It relates to depth of field as much as perspective.: Focal length affects perceptual perspective. As the focal length and magnification of a lens increases, the image appears more compressed, resulting in less visual distinction and separation between the foreground, middle ground, and background. This is also the most important factor when attempting good BOKEH. Good bokeh allows you to create images with content awareness and focus with few distractions (what compositional focus is about).
f 2.8 full frame sensor
canon 16-35 II f2.8 at 35mm
(original size 2912 x 4368 - 5D mk1)
click here for high-res
Point 2 This is what everyone seems to be concentrating on. Crop factor: Ignore your crop factor when choosing your lens. The beauty in focal lengths in not the crop for composition, it's the subject itself and the perceptual perspective.
f 8 crop sensor , fuji 35mm f1.4
(original size 3264 x 4896-XPro-1)
click here for high-res
Point 3 The misdirection of crop factor effects on focal length is merely that, mis-direction. A subjects distance to film/sensor plane is what makes an image either sharp, or in focus. The lens has a better capacity to produce sharp images when the plane is closer to the subject, not unlike your own eyes. It's easier to see things in detail up close than far away. So in that point all 85mm lenses on any camera should produce the same detail and fidelity whether you crop it down or not, taken from the same distance to subject.
f 8 full frame sensor
canon 16-35 II f2.8 at 35mm
(original size 2912 x 4368 - 5D mk1)
click here for high-res
The samples on the previous post show that explicitly although I'm going to show it clearer here.
Those images where shot using only two focal lengths at the same distance from the subject. The only differences are the amount of excess image the frames carry due to the area available on the different sensors.

Point 4 Lastly It's always about the glass. Good glass on a sub camera produces better images than a crap lens on a good camera. So take your time in picking what's right for you. What are you shooting and what are your desired results are the two most important factors in choosing lenses.

Hope this helped.
Enjoy Derek

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