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Why would I need Prime Lenses?

HarrieWheatley

Standard Member
Hey!

Kind of dipping my toe into lenses etc. Having watched videos explaining shutter speed, ISO, aperture etc, I don't understand why people don't just use zoom lenses, surely they're more flexible.

What type of lens is the 18-55mm canon? Im guessing it's zoom? But if it was just, 24mm, that's prime right? So couldn't I just set my 18-55mm to 24mm and use it like a prime would?

Thanks
Harrie
 

RobDickinson

Well-known Member
Yep - zooms you can change the focal length.
Primes have a fixed focal length - no zoom.

Primes are usually faster (larger aperture) , have less distortion and are much sharper than zooms.

The 18-55 kit lens is probably about f4 at 24mm. The 24L has a maximum aperture of f1.4.

Thats 3 stops faster. Faster shutter speeds and shallower DOF/better subject isolation.

Also compar the quality of both of them here
 

RobDickinson

Well-known Member
Oh, also, does a lower f-stop allow more light into the lens? Does a lower f-stop result in a grainy image?


Yes a lower f stop means more light into the camera, a larger aperture.

f2.0 lets in twice the light of f2.8. F1.4 lets in 4 times as much light as f2.8.

f2.8 is the fastest aperture for zoom lenses.

No - lower f stops do not make grainy images, higher ISO makes grainy images - the two arnt the same.
 

HarrieWheatley

Standard Member
Got that, just a couple of questions. What is 24L? And so f-stop is related to both aperture & shutter speed. So a lower f-stop results in more contrasting depth of field?

Thanks
 

RobDickinson

Well-known Member
24L is canons '24mm f1.4 L' lens , canon do a number of primes just usually called by their focal length, 14L, 24L,35L,50L, 85L, 100L (macro), 135L etc. These are all faster than their zooms.

large aperture lenses are called fast because it allows you to use faster shutter speeds.

3 stops faster means that I could shoot at 1/500th instead of 1/60th vs the 18-55 kit lens.

larger aperture means shallower depth of field so isolates the subject more.

And despite the rumours you can actually stop down (make the aperture smaller)... :D
 

HarrieWheatley

Standard Member
So, I mostly film action short films, aside from the standard 18-55mm kit lens, do you think it would be worth investing in a wide angle 24L or even 14L. Also, am I right in thinking that 50mm is considered "human view." Also, does a larger lens, such as 100mm, result in a more narrow image?

THANKS GIVEN :D
 

RobDickinson

Well-known Member
The focal length determines the field of view.

50mm is counted as 'normal' but that is on a full frame camera, the camera you have has a smaller sensor than a standard 35mm piece of film, an aps-c sized sensor. This effectively crops the image to a smaller size (so they are called crop sensors). Which means a 50mm lens on your camera acts (in most ways) like an 85mm lens.

To get a 'normal' field of view (which isnt normal anyhow IMO) you need a 30-35mm lens.

Fast primes have a whole bunch of uses, and are great for filming but are tricky to use sometimes and tricky to focus. They should allow you to shoot with a higher shutter speed in lower light.

Have a look at the 24L or 14L prices. They are wonderfull lenses but very expensive! (note the 14L is only f2.8)

If your using video and manual focus there is a nice 3rd party lens set made by samyang (othertimes called rokinon etc), they make a 14/2.8, 24f1.4, 35f1.4, 85/1.4 etc which are pretty good but manual focus.
 

RobDickinson

Well-known Member
No lens is a fixed aperture.

They always list the maximum aperture, you can always stop down.

The 18-55 kit lens has two aperture values because the maximum aperture changes as you zoom.

Aperture is a mathematical function based on the focal length. Some zooms are constant aperture but tend to be expensive (17-55f2.8, 70-200f2.8L etc).

For video I would seriously consider the 650d.
 

shotokan101

Banned
Primes are not fixed aperture - as Rob mentioned above you can stop them down (decrease the aperture - e.g. stop down an F1.8 lens to F8 to get more Depth of Field in focus) :)

Jim
 

shotokan101

Banned
Thanks for both of your answers, wouldn't stopping down decrease light and result in a darker image?

Yes and you would therefore need to either increase the ISO setting (at risk of increased image noise) or decrease the shutter speed (at risk of introducing camera shake if shutter speed is too low to handhold for the lens you are using) to maintain the same exposure - it's always a trade off depending on what you wish to achieve with your image and is dependant on the lighting available too :)
 

shotokan101

Banned
Shutter speed is only relevant for shooting still images - most cameras will shoot video at a fixed frame rate which can be selected from a few available rates - e.g for EOS 600D :-

Canon EOS Rebel T3i Basic Video Specs

1080p (1,920 x 1,080) HD recording at 29.97, 25, or 23.976 fps
720p (1,280 x 720) HD recording at 59.94 or 50 fps
VGA (640 x 480) SD recording at 29.97 or 25 fps

MPEG-4 AVC / H.264 compression, .MOV container
Single-servo contrast detection autofocus is optionally available during recording, albeit with actuation noise levels depending on the lens used
Phase detection or contrast detection AF are available before capture starts
Manual focus also possible
AF point position can be manually controlled
Programmed-auto or manual exposure (no shutter / aperture-priority)
Exposure compensation and lock are available both before and during recording
Picture style selections provide creative options
Monaural audio recording via built-in microphone
Stereo audio recording via external 3.5mm mic jack
Microphone levels can be manually adjusted
Optional wind filter function
Image stabilization during video capture, if offered by lens
3-10x digital zoom (available at 1080p only)
2, 4, or 8-second video snapshot function creates "album" videos in-camera
 

RobDickinson

Well-known Member
Frame rate is how many frames a second you record.

So 24, 25, 50, 60.

At 24 frames a second you can have a maximum of 1/24th as the shutter speed for each frame (any longer and you take more than 1 second to take all 24 shots..)

But you can have much faster shutter speeds. Like say 1/1000th of a second. So you have 24 * 1/1000th of a second shots and 976/1000ths unphotographed.

This is sometimes better for action/sports as you get cleaner movement. its not quite as filmic though.

Sometimes you cant shoot at 1/24th because there is too much light for the aperture you want etc (movies solve this with ND filters).
 

shotokan101

Banned
No its not.

You know fine well what I mean Rob - for all intents and purposes it is as it's fixed whilst shooting video once you have selected your chosen framerate :)
 

RobDickinson

Well-known Member
No its not.

Frame rate is fixed. Shutter speed is not fixed.

You should/will be shooting in manual mode for video and for a chosen aperture you will manage exposure with shutter speed (and ISO).
 

shotokan101

Banned
No its not.

Frame rate is fixed. Shutter speed is not fixed.

You should/will be shooting in manual mode for video and for a chosen aperture you will manage exposure with shutter speed (and ISO).

I'll take your word for it then Rob as I've obviously misunderstood how DSLR's shoot video - though I can now see what you mean - probably why I don't use my DSLRs for video :)
 

Liquid101

Distinguished Member
No its not.

Frame rate is fixed. Shutter speed is not fixed.

You should/will be shooting in manual mode for video and for a chosen aperture you will manage exposure with shutter speed (and ISO).

This is correct.

However - when shooting video on a DSLR you should always follow the 180 Degree rule where you set your shutter speed as close to double your frame rate as possible. So if you're shooting at 24fps you should aim to have your shutter speed at 50fps, 30fps at 1/60th etc.

This will help give you the right amount of motion blur between frames to appear natural. Faster shutter speeds can give you strange looking unnatural video.

In response to the OT - Prime lenses are generally sharper, give better contrast (important for video) and offer considerably more attractive bokeh (out of focus areas) Personally, I only use prime lenses - I gave up on zoom lenses several years ago.
 
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HarrieWheatley

Standard Member
thanks! As I search for samyang lenses on ebay, a couple of questions come to mind:

1) Is the f value shown the maximum or the minimum?
2) Any other indicators which could show if the lens is fish eye or any other type, wouldn't like to buy one on accident.

Thanks given to you all :D
 

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