Do you need the "best" lens

rsn wrote:

Valdai21 wrote:

What matters to me is good enough

The "good enough" concept is well known in another hobby I participate in, its kind of a "devil in the details" and you can get bogged down in technical details and over prep. There is also another concept in the hobby which I have adapted to photography. Does an image have a "Wow!" factor. If a photo is a Wow! photo, it can get away with many sins.

The ability of a lens to collect light is an interesting topic for me, an f1 in a 1.5 crop sensor is really an f1.5 lens. Many understand one has to apply the crop factor to lets say a 100mm lens - which becomes a 150 lens - but don't realize the crop factor also applies to the f number as well.

And interestingly lens from the same and different manufactures, all with lets say a f1.4 build do not collect the same amount of light. The movie industry ran into this problem when lenses that were in theory the same f1.4 but each lens collected more or less light than other lenses used that were an f1.4. So they created the T number which measure how much light is hitting the sensor. Saying a lens is an f1.4 is a build description. Many feel the photography industry should adopt the T number instead of or along with the f number. Here is a short video to explain the difference:

https://www.youtube.com/watch?v=B2tkjcoU55Y

If one is looking for a fast lens, lets say thinking of purchasing a f1.4 factor lens then it would be helpful to know the T stop when comparing lenses, wanting the "fastest" lens. So what is faster, the Canon 50 1.4, Sigma 50 1.4, Tamron 50 1.4, etc.

And quality bokeh is not only a function of the f number, lets say f1.4. It is also a function of the build of the lens, I prefer more rounded blades, preferably 9 or more but 7 can bring happiness as well. My Canon mk 1 50mm f1.8 had 5 non-rounded blades and didn't provide as great a bokeh as my Sigma 50 f1.4 9 rounded blades. The Fujifilm 14 2.8 has 7 rounded blades, the 35mm 2 has 9 rounded blades, and the 50 2 has 9 rounded blades to produce decent bokeh.

To the person who pointed out blur is different than bokeh, I understand this and agree. I had a hard time finding an image with decent bokeh with the 14 f2.8 lens as almost no one takes those kinds of shots, almost all are travel or landscape. At least there were almost no bokeh shots on the Flickr Fujifilm 14mm f2.8 page.

The f number is simply the ratio of the focal length of the lens to the area of the aperture  opening.  So given a 35 mm on an APSC is "equivalent" to a 50 on FF, for the same field of view the aperture area on the APSC lens will be less than on the 50.  The amount of light is determined by the absolute area of the aperture opening.  So yes there is a crop factor but only in an indirect way.  It is not only the opening of the aperture that determines the amount of light incident on the sensor.  There is also the number of glass elements and the coatings on the glass elements.  There is reflection at each air/glass boundary.  Coatings mitigate the reflection but they do not eliminate.  If a air/glass boundary transmits 99% of the light and the lens has 8 elements there are 16 air/glass boundaries and only 85% of the light reaches sensor.  If it has 15 elements, like the new Fuji 33 f1.4 only 74% of the light makes it.  If the transmission is 99.9% (much better glass and coatings required), then for an 8 element lens 98% makes it to the sensor and for a 16 element lens 97% makes it.  Most cinema lenses have more elements than photo lenses so the T number is important.  The T number accounts for the reflections and in the lens.  That is a pretty important thing to know for video.

However, the f number is critical in determining the DOF and hyperfocus distance, magnification, etc.  The Bokeh quality is determined by not only the f-number but the number and type of aperture blades.  A lens with a 12 blade aperture will produce better Bokeh than one with 7 blades.  One with rounded edges will produce better Bokeh than one with straight edges. However, there is more to Bokeh than the blades, there is the type of lens design, the number of elements and the type of elements and the quality of the grinding and polishing of the lens elements.

So in general the T number combined with the f number are both useful in cinema/video.  The T number allows calibration for equivalent exposure, the f number determines the geometric properties.  In photography, the meter compensates for the reflections in determining the exposure so the T number is not all that relevant.