An eclectic selection of technology with some cookery
Working out what factors of a lens / camera body govern the depth of field of an image.
Often know as a “F Number” in photograthry. Also the term “T number” is sometimes used in high end cinimatic lenses. To my understanding F numbers are calculated from the lenses design where “T” numbers are measured by sending light though a built lens. The latter being more accurite. For the purposes of the calcilation they will be used interchangeably.
The focal length of the lens, normall stated in “mm”. Commonly a 50mm lense is used with a 35mm film stills or “full frame” camera.
f not to be confised with the “F number”.
How far is the lense from the subject.
I’ll adopt the d/1500 calculation to simplify this.
| Sensor Size | CoC |
| Type 1 | 11μm |
| 4/3” | 15μm |
| APS-C | 18μm |
| 35mm | 29μm |
DOC = (2 ( u 2) N c)/ ( f 2)
This suggests f focal length and Distance to subject have the largest effect as they are square factors
Doing some calculations on a spreadsheet agrees with the above statment, sensor size also has a noticable effect but not as much as you might think. Also having F Numbers on lenses and reffering to f for focal lenght is very confusing.
If you want a shallow depth of field, it depends how far you are from your subject and there for focal length possibly more than having a large sensor / “fast glass”.
In my experience a 50mm F1.8 on APS-C is more than adequate enabling you to dial in just the right amount of background blur. This is quite a telephoto view, but also offers the “flattened look” common in portrait lenses without having to get too far away. I often find no need to be at F1.8 because you’re only aiming to draw the eye away from the background just enough not make it distractingly blurry.
If you do want to show the whole scene by having a large depth of field then “Stopping Down” to say F16 will easily do this. It’s just a matter of having enough light to do this without having a high ISO (More noise / less Dynamic range) or going to a shutter speed that will likely give motion blur. This is where having a short focal length i.e. 16mm on APS-C will and a smaller sensor say type 1 will give you a big advantage. i.e. a smart phone.