I Tried To Fully Understand Lenses

[Jonathon Laing]

... and I failed...:angry2:

First of all, I'm not being lazy I have researched wiki, google and reduser to understand lenses. But still I feel like I'm not grasping it all so here goes.



1/ How and why does the increase in f-stop change depth of field?

2/ So a lower f-number means more light. More light is good yes and everyone seems to go for more light, but I've heard from people on this forum that the sharpest images are caught at an f-stop somewhere around f/5.6? If this is correct then why?

3/ In simple terms what is the difference between f-stop and t-stop I really did try to learn the difference but I couldn't find an easy answer.

4/ "photos taken with a low f-number will tend to have one subject in focus"( info from wiki) in other words depth of field.? So if my statement about sharpness in question 2/ is correct then; in order to get more sharpness you sacrifice depth of field and vis versa?

5/ When you decrease you f-number your opening the iris correct? If so why is "speed" associated with f-numbers if all your doing is opening a hole.

6/ What would be the optimal lens spec to shoot a super close up (focal length) and what would be the best for a very wide shot. Lastly what specification of a lens do account for when you wish to shoot something at a high frame rate.


Apologies, for I'm sure that some of my questions are bound to be idiotic. The problem I find is I read something and think OK I get it now. Then I hear something else which seems to contradict the first thing I read, then i hear that both facts are true and some how seem to correlate! (if that makes any sense)

Anyways thanks in advance to anyone who helps me on this. I hope this thread will clear up questions that others like me want answers to.

 

[Daniel S Williams]

changing the F stop changes the angle at which incoming light rays bend or refract, causing them to either fall inside of or fall outside of the depth of focus, which is directly related to the depth of field.

Check out this video. You'll learn ALOT.

http://www.snodart.com/tutorials.php

 

[Raul Gonzo]

Unless you are simply looking for a technical answer, a good thing to do is go get a manual camera (DSLR or the old school SLRs) and put to practice what you are reading. This will help you get a better grasp on this.

 

[bobaandy]

You might also want to try posting cinematography.com.

 

[Colin Sheldon]

2/ So a lower f-number means more light. More light is good yes and everyone seems to go for more light, but I've heard from people on this forum that the sharpest images are caught at an f-stop somewhere around f/5.6? If this is correct then why?

The amount of light is neither good, nor bad. Having a properly exposed image is typically a good thing, but that is all. Typically you pick the F/T stop that creates the most detail on the sensor/film. The image can be lightened/darkened in post as long as there is sufficient detail.

So why 5.6? Because for a lot of lenses that is their peak operating point. Will they work at other T stops? Absolutely. But defects in the lens might be more apparent.

3/ In simple terms what is the difference between f-stop and t-stop I really did try to learn the difference but I couldn't find an easy answer.

The reason there are stops on a lens is to measure the amount of light that goes through the lens. F-stop is a mathematical approximation of how much light SHOULD go through the lens. T-stop is how much light ACTUALLY goes through the lens.

Why are they different? Because the different elements in the lens will absorb some light, etc.

6/ What would be the optimal lens spec to shoot a super close up (focal length) and what would be the best for a very wide shot. Lastly what specification of a lens do account for when you wish to shoot something at a high frame rate.

Asking the optimal lens spec is like asking what the best color of paint is. It all depends on the situation. Check out cinematography.com for some great discussions on lens choices.

Anyways, those are the ones I had time for....


cheers,


Colin Sheldon

 

[Dan Hudgins]

Answers...

Answers...

1/ How and why does the increase in f-stop change depth of field?

Draw a triangle with the base the width of the iris as seen from the front of the lens, the point of the triangle meets the surface of the subject, the rays that make up the two long sides of the triangle extend past the tip of the triangle to INF, if they meet something behind the subject focal plane (triangle point) they are not a point but the farther back you go the more spaced they are, likewise if you put something in front of the subject it is not at the point of the triangle and so makes a spot on the image that is not a point, the same for objects behind the triangles point they make a spot that is not a point, the greater the distance from the triangles point near or far makes the image of a point larger than a point.

The distance between the lens and the triangles point is the focus adjustment of the lens, all subjects at another distance will not be in focus and will not render their points as points but as spots of size in ratio to their distance from the focus distance.

f/stop is the ratio of the triangles base to the focal length, but there are two triangles, one behind the lens and one in front of the lens, the iris changes the base of both triangles, the smaller the base the smaller the spot points on the subject will be that are outside the focus distance.

If you shoot something that is flat like a test chart, then chainging the iris does not make a large change in depth of field since there is no depth to the subject, depth of field has to do with some subjects being in the plane of focus (whatever distance you set the lens to focus at) and some being outside that plane. For most subjects 90% or more of the subject will be outside the focal plane and therefore its sharpness will depend on the iris opening, small openings make smaller spots for each subject point and larger openings make larger spots for each point on the subject.

2/ So a lower f-number means more light. More light is good yes and everyone seems to go for more light, but I've heard from people on this forum that the sharpest images are caught at an f-stop somewhere around f/5.6? If this is correct then why?

diffraction happens in all lenses and pin holes, it is a fact of the light wavelength. It is less at large iris openings, and more at small iris openings.

aberration happens in all lenses to varying amounts depending on the quality of the lens.

The image is the sum of the aberaration and the diffraction making the image points larger than you would get with just good triangles of rays.

In good lenses diffraction and aberaration are equal about f/2.8 to f/8 with many being around f/5.6.

For a 4K camera you begin to loose resolution about f/8 because of diffraction, so you would get the sharpest results with a good lens around f/5.6. A less good lens might peak at f/8 or f/11. At stops smaller than f/11 the sensor cannot resolve 4K resolution well.

Lenses more open than f/2.8 will resolve small details at lower contrast since the image points are in a fuzzball of off traget rays.

3/ In simple terms what is the difference between f-stop and t-stop I really did try to learn the difference but I couldn't find an easy answer.

T/stop is ALWAYS a larger number than its corresponding f/stop.

T/stop will be 1/4 to 7/8 Stop slower than the f/stop (less light) for most lenses, with 1/4 for simple prime lenses, 1/2 stop for retrofocus wide angle lenses, and 2/3 stop for zoom lenses being a good value to use if your lens is not marked in T/stops.

In other words, if you need T/5.6 you set the lens to f/4.5 etc.

Each surface reflects 5% of the light without coating, and a 6 element prime lens would have 8 surfaces with air so you loose 40% of the light. 40% of the light is about a 3/4 stop light loss. Coated lenses loose about 1% or less if multi-coated so you can cound the reflections and figure the T/stop for any lens.

The glass also acts as a filter since some glass types are green or yellow, so you can count 10% or more loss from absorption.

T/stop stands for Transmission stop or True stop, the f/stop is used to figure the DOF, the T/stop is used for exposure. Using T/stop to figure DOF will give the wrong value since the f/stop to T/stop ratio is not the same for all lenses.

4/ "photos taken with a low f-number will tend to have one subject in focus"( info from wiki) in other words depth of field.? So if my statement about sharpness in question 2/ is correct then; in order to get more sharpness you sacrifice depth of field and vis versa?

In most cases more things in the frame will get sharper as you stop down to f/11, between f/5.6 to f/11 the smallest details in the plane of focus will get a very little softer, but it would be very hard to see the loss from f/5.6 to f/11.

For the most part things will get "sharper" all over the frame as you stop down to f/11, and if you are going to a small format like DVD you can stop down to f/22.

At stops larger than about f/5.6 everything gets softer, just that the background and foreground get more softer than the focal plane.

5/ When you decrease you f-number your opening the iris correct? If so why is "speed" associated with f-numbers if all your doing is opening a hole.

Speed comes from still photography. With wide f/numbers you can shoot at higher shutter speeds, if you stop the lens down it takes longer to get an exposure. With slow glass plates you needed a "speed" lens to take photos of moving subjects.

For the most part in movie work now "fast" or "speed" lenses are those faster than f/2.0. "Slow" lenses are ones with stops smaller than f/2.8. "Super Speed" lenses would be like f/1.2 or f/1.0.

6/ What would be the optimal lens spec to shoot a super close up (focal length) and what would be the best for a very wide shot. Lastly what specification of a lens do account for when you wish to shoot something at a high frame rate.

Close shots of faces and the optical design come out at about 75mm 6 element lens at about f/5.6 (e.g cooke speed panchro 75mm f/2.0 or Baltar 75mm f/2.3). Wider lenses are not as sharp in the corners of the frame and longer lenses are not as sharp in any part of the frame. f/2.0 or f/2.3 lenses may have a higher peak resolution at f/5.6 than faster "super speed" lenses, but it depends on the maker.

Any lens wider than 18mm has issues, longer than 28mm is not wide enough for most shots. I like the look of a 24mm Double-Gauss lens like the cooke speed panchro Series I. The 18mm cooke speed panchro Series II is also nice but the resolution in the corners falls off at full open, as it does in most wide angle lenses. For wide screen the 18mm might give more height than the 24mm. A 9.8mm is more of an effects lens than something you would shoot lots of shots with. A 14mm falls between an effects lens and an everyday lens for many shots. You can shoot a whole feature film with a 28mm lens since it renders many subjects well at many distances.

The focal lengh does not matter for high frame rates unless you are short of light then you would need a "fast" lens to use with the faster frame rate, that is faster than f/3.5 maybe, but the combo of slow motion and shallow DOF will make it hard to keep the subject in focus.

 

[jimhare]

How's this for a non-technical answer - same reason sometimes have to squint your eyes to focus. It reduces the depth of field and everything is sharper!

Open them really wide and it's not as easy to focus and everything is too bright.

 

[Jonathon Laing]

Thanks everybody for your help. Especially Dan and Sheldon, I appreciate it a lot. You pretty much have put me completely on track. Still a little confused with the f stop messing with the DOF, but I'm getting there. Once I have actual lenses to play with I will understand everything alot easier.


Thanks again

 

[Evin Grant]

This may help...
http://en.wikipedia.org/wiki/Circle_of_confusion