Understanding sensor size and lenses

I

ianjmatt

Messages
255
Name
Ian
Edit My Images
Yes
Am I understanding this correctly?

I have a Canon 500D. If I put a 28mm f2.8 lens on there I actually have 44.8mm (multiplying it by 1.6). Is the aperture affected at all, and is that the right calculation?

Thanks -and sorry if this is a stupid question!
 
Short answer, yes, in practical terms you effectively have a 44.8mm lens and the aperture isn't effected.

Long answer, no not really. The sensor crops the FoV so it's equivalent to a 44.8mm lens, but the actual focal length remains unchanged, you still have the perspective of a 28mm lens. And the aperture is still the same size, so in terms of DoF capabilities it's more like a 44.8mm f/4.48 lens would be on 35mm.
 
Right. Does that mean to get a decent wide-angle shot I would need to go down to an 18mm or something? How does that affect image distortion etc?

I am sorry - I still don't get what you said about Aperture - I am being thick! :bang:
 
Well 28mm isn't that wide on a crop sensor.
Distortions depend on which lens you're using exactly. Standard 18-50 kind of zooms tend to to distort at the wide end, but something like a canon 10-22mm used around that focal length will produce very little distortion.

With the aperture thing, what I'm saying is that the aperture remains unchanged. Although you've got a field of view equivalent to a 44mm, it will still have the aperture of a 28mm f/2.8, so you can't get the same DoF that an actual 44mm f/2.8 would on 35mm format.
 
Thanks. Adjusting to the digital world one silly question at a time!
 
The focal length and aperture are characteristics of the lens, and have nothing to do with the size of the sensor. What happens with a smaller sensor is that the field of view becomes smaller, i.e. a smaller part of the scene fills the area of the sensor, so you get a greater magnification in the image.
 
Everything in the lens remains the same as if the lens were on a Full Frame camera, aperture, depth of field, lens distortion, etc, only the central part of the projected image is recorded because of the sensor size. The multiplication is to say what equivalent view would be on a Full Frame sensor camera.

Where you can gain is if the lens was designed for a Full Frame camera, the lens quality normally falls off towards the edge of the lens, especially at wider apertures. If the camera is only recording the central portion of the lens, then you should not get the edge of lens problems as much, because the sensor doesn't see them. :shrug:

Here's an image I found on the internet which shows how much of the image is apparently 'cropped'.
full-frame-crop-factor.jpg
 
Aperture remains the same irrespective of sensor size as it is directly related to the focal length of the lens WHICH STAYS THE SAME.

What changes is the field of view (FoV) i.e. what you see when you look through the camera, the pictures above demonstrate but. What also changes is depth of field i.e. how out of focus the background is. So, if you have a blurry background with an f/1.8 lens on a crop body, you only f/2.8 (1.8 x 1.6 = 2.88) on a full frame camera to achieve the same level of blur (or more correctly, depth of field DoF).
 
Think of it like this:

you're shooting with a full frame body and a 50mm 1.8 lens and you have a full length body shot from 12ft away. You put that lens onto a micro 4/3 camera with a 2x crop (or a 1.6 crop camera) and at the same distance to subject you now have just a half body shot, so you move back twice the distance to get the same framing and now because you're further away from your subject you have more depth of field

a 30mm 1.4 lens on a 1.6 camera will give the same framing as a 50mm 1.4 on full frame at the same distance, but the 50mm on full frame will have less depth of field, or you could stop the 50mm lens down to about f2.5 and it will have equivalent depth of field (and the 50mm lens would now be sharper because it is stopped down) That's the real advantage of full frame, the ability to still get great bokeh at f2.8 instead of f1.4


A full frame camera is also a crop camera, just do the cropping in photoshop after you shoot.


The crop vs full frame argument is based mostly around outdated information. When cropped digital first came on the scene it was impossible to get a wide angle lens as 20mm was about the widest you could get, now of course we have 10-22's and even an 8-16. To get the equivalent of a 50mm lens on a cropped digital you had to use a 28mm lens, and because optically a fast 28mm lens isn't as easy to make as a 50mm that means that most 28mm lenses are f2.8 or slower, and faster lenses are expensive (24 f1.4 L), or not very good (28 f1.8). Now we have the sigma 30mm 1.4 which is a great lens.
 
neil_g said:
Don't know if these help, I took these on the 3 types of Canon sensor to demonstrate to the other half what happens to the FOV.. (ignore the rubbish content and the OOF first shot :LOL:)
Click to expand...

Neil those images also demonstrate rather nicely the differing amounts of light captured by each sensor size (assuming the exposure was the same)
 
iancandler said:
Neil those images also demonstrate rather nicely the differing amounts of light captured by each sensor size (assuming the exposure was the same)
Click to expand...

I always thought that a bigger sensor captured more light, but on another forum I was told that the extra light gathered by the increased surface area only served to illuminate the extra pixels, not to actually add anything to the exposure. I am still none the wiser as to what the truth is about sensor size and light gathering (in terms of how it relates to actual EV at a given aperture and ISO)
 
The amount of light falling on the sensor is purely down to aperture and shutter speed.

There is NO multiplication factor, only cropping factor. The laws of physics cannot be changed. A lens will produce a certain sized image of a subject at a given distance no matter what size film/sensor is used, as shown by redhed17's image.
 
Last edited:
mrjames said:
I always thought that a bigger sensor captured more light, but on another forum I was told that the extra light gathered by the increased surface area only served to illuminate the extra pixels, not to actually add anything to the exposure. I am still none the wiser as to what the truth is about sensor size and light gathering (in terms of how it relates to actual EV at a given aperture and ISO)
Click to expand...

I always thought of the larger photosites on a Full Frame sensor as bigger buckets, that gather more photons of light, per bucket, but the same amount of light enters the camera, and because more photons have been gathered, they data doesn't have to be amplified as much. And all sensors amplify the signal recorded from the sensor, that is what upping the ISO is after all. Which is why (newer) Full Frame sensors generally have less noise, particularly at high ISO, than APS-C and definitely compact cameras.

ISO performance is an ever improving feature though, and it may not be long before an ASP-C camera out performs a Nikon D3. :eek: Then again, not if they keep upping the megapixels. ;)


My way of thinking could be wrong, but that's what works for me. ;) :LOL:
 
The sensitivity is dependent on pixel size (among other things) rather than sensor size.

Imagine two sensors of the same physical dimensions, say 30x20mm, with identical lenses producing identical shots. One has 6 megapixels (3 million by 2 milion), the other has twice the number in each direction (6 million by 4 million) giving 24 megapixels. The 24MP sensor has the same amount of light falling on it as the 6MP, but that light is shared out amongst four times as many pixels. Each pixel therefore has only one quarter of the light to produce a signal compared with the 6MP sensor. All other things being equal, the 6MP sensor is two stops more sensitive.

Of course, all other things aren't equal and the 24MP sensor is most likely later technology which is more sensitive and less noisy, so the gain in the amplifiers can be increased to give similar results.
 
Wow that was quite complicated to get my head around but I think I understand now.

My question is, does the crop thing only apply if you have a full frame lens on a cropped body? I.e. Canon EF lens on a 1000D like mine? If so, will the problem of magnification ratios be eradicated if you used an EF-S lens on a 1000D where to my understanding, an 18-55mm lens would stay 18-55??
 
aleung2 said:
Wow that was quite complicated to get my head around but I think I understand now.

My question is, does the crop thing only apply if you have a full frame lens on a cropped body? I.e. Canon EF lens on a 1000D like mine? If so, will the problem of magnification ratios be eradicated if you used an EF-S lens on a 1000D where to my understanding, an 18-55mm lens would stay 18-55??
Click to expand...

No, you always apply the crop factor even if the lens has been designed for the smaller sensor.
 
redhed17 said:
No, you always apply the crop factor even if the lens has been designed for the smaller sensor.
Click to expand...

Oh right. So does that mean the crop factor will be bigger if you use an EF lens as opposed to an EF-S on a crop camera?
 
aleung2 said:
Oh right. So does that mean the crop factor will be bigger if you use an EF lens as opposed to an EF-S on a crop camera?
Click to expand...

Crop factor is how many times small sensor is smaller then FF sensor. Thus it is always the same ~1.6, no matter what lens you use.
 
But lenses designed for crop bodies (dunno what that is in Canon speak) don't work on full frame as they are only designed to make a picture big enough to cover the crop sensor. If you use them on a FF camera you get massive amounts of vignetting.
 
Better to think of it as a 'Field of view' The EFS lenses produce a smaller image circle and therefore far less cropping is involved if you like to think in terms of crop, also the EFS lenses protrude further into the camera body which wouldn't be good for a full frame camera.
 
aleung2 said:
Oh right. So does that mean the crop factor will be bigger if you use an EF lens as opposed to an EF-S on a crop camera?
Click to expand...

no, with all lenses it does what it says on the tin, so an 18-55 is an eighteen to fifty-five. The only difference is that the EF lens produces a bigger image circle to cover full frame, but needs more glass (weight/size) to do this

think of it like a pipe with a 1 inch diameter and 2 foot long- you can drop a 2 pound coin down the tube (EF), another identical hose pipe is also 2 foot long but has a 1/2" diameter, so you can only drop a penny down the tube. You can still drop a penny down the larger tube, but not the other way round
bad example but you get the idea?
 
Nawty said:
But lenses designed for crop bodies (dunno what that is in Canon speak) don't work on full frame as they are only designed to make a picture big enough to cover the crop sensor. If you use them on a FF camera you get massive amounts of vignetting.
Click to expand...

And possibly a broken mirror with EF-S on FF.... (y)
 
Splog said:
Better to think of it as a 'Field of view' The EFS lenses produce a smaller image circle and therefore far less cropping is involved if you like to think in terms of crop, also the EFS lenses protrude further into the camera body which wouldn't be good for a full frame camera.
Click to expand...

I'm pretty sure you're saying the same thing as what I am getting at. An EF-S lens, which is designed for cropped bodies shouldn't have any cropping because the image it produces onto the sensor is smaller and is closer to the sensor.

With a ff lens on a crop sensor I can fully understand why and imagine the crop. I'm just confused about lens for crop sensors on a crop body. Surely because of the smaller image being produced it will have no crop it is the perfect size for the smaller sensor?

Sorry to be a pain.
 
aleung2 said:
With a ff lens on a crop sensor I can fully understand why and imagine the crop. I'm just confused about lens for crop sensors on a crop body. Surely because of the smaller image being produced it will have no crop it is the perfect size for the smaller sensor?

Sorry to be a pain.
Click to expand...

You are not alone, this confuses me slightly as well.
 
You're right in that a lens designed for a crop sensor will produce a correctly sized image for that sensor, but the "crop factor equivalent lens" is for us old timers who grew up with 35mm cameras and lenses. We have it ingrained in our brains that a 50mm lens will produce a field of view similar to what we see with our eyes, the so-called 'standard' lens. 24mm is a wide angle lens, 18mm is a superwide, 200mm is a 'tame' telephoto and so on.

On crop sensors these numbers are different. The 'standard' lens on a crop sensor is about 32mm for instance. Think of the 'crop factor numbers' as '35mm equivalent' instead.
 
Stratman said:
You're right in that a lens designed for a crop sensor will produce a correctly sized image for that sensor, but the "crop factor equivalent lens" is for us old timers who grew up with 35mm cameras and lenses. We have it ingrained in our brains that a 50mm lens will produce a field of view similar to what we see with our eyes, the so-called 'standard' lens. 24mm is a wide angle lens, 18mm is a superwide, 200mm is a 'tame' telephoto and so on.

On crop sensors these numbers are different. The 'standard' lens on a crop sensor is about 32mm for instance. Think of the 'crop factor numbers' as '35mm equivalent' instead.
Click to expand...

I think I get you now. So basically I'm getting confused with the word "crop" and "field of view". Therefore a lens for crop body on crop sensor will be 50mm for cropped body purposes but the "field of view" will be the equivalent to a 80 on a FF body with FF type lens?

I hope someone says yes, it's really buggin me lol.
 
aleung2 said:
I think I get you now. So basically I'm getting confused with the word "crop" and "field of view". Therefore a lens for crop body on crop sensor will be 50mm for cropped body purposes but the "field of view" will be the equivalent to a 80 on a FF body with FF type lens?

I hope someone says yes, it's really buggin me lol.
Click to expand...
 

Rkkmcccfkkcccmkkfx from funeral f
 
aleung2 said:
I think I get you now. So basically I'm getting confused with the word "crop" and "field of view". Therefore a lens for crop body on crop sensor will be 50mm for cropped body purposes but the "field of view" will be the equivalent to a 80 on a FF body with FF type lens?

I hope someone says yes, it's really buggin me lol.
Click to expand...

Let's put it another way.

A 50mm lens on Canon camera with a 1.6 crop factor will give you the same field of view (FOV) as an 80mm lens on a 35mm film camera or a full frame DSLR. It would be 75mm on a Nikon which has a 1.5 crop factor.

Also, bear in mind that we're talking about full frame as the equivalent of a 35mm negative. This was regarded as a miniature format a few generations ago, and there are far larger - and monstrously expensive - digital formats out there!
 
aleung2 said:
I think I get you now. So basically I'm getting confused with the word "crop" and "field of view". Therefore a lens for crop body on crop sensor will be 50mm for cropped body purposes but the "field of view" will be the equivalent to a 80 on a FF body with FF type lens?

I hope someone says yes, it's really buggin me lol.
Click to expand...

Yes (y)

BUT, the format of the lens makes no difference to field of view, only that if you use a DX lens on an FX camera you get a black circle around the image is the lens isn't big enough to project the image over the whole sensor.

edit: so a FX lens will work on FX and DX bodies exactly how you describe but a DX lens won't work on an FX body. The reason for DX lenses is that they are cheaper and lighter.
 
Last edited:
thanks everyone for helping me understand it. I obviously got confused with crop and field of view. Makes perfect sense to me now cheers
 
You must log in or register to reply here.
Back
Top