Crop sensors need cropped lenes

[carlh]

The issue of a cropped 2.8 and equivalence to 4.2 FF is sure to get some folks wound up and may well produce some long posts. ;D

Certainly

A FF lens being used with an APS-C sensor results in a slower system, lens+sensor. Since the APS-C sensor only captures 40% of the light projected by the lens on the image plane, the combination is slower than if the same lens had been used with a FF sensor. The lens is of course the same and doesn't know what sensor you place behind. But a lens on its own will not produce any pictures. You need a sensor as well. Really, I cannot see the controversy.

My view would be that with any lens designed to be used on a crop sensor, most of the photons entering the lens will hit the sensor, subject of course to the rectangular sensor reading a round lens. In contrast only 40% of the photons that would have hit the sensor on a full frame body will be used in a crop body (assuming a 1.6 crop) with a FF lens.

I'm getting more convinced that long lenses specifically designed for crop bodies would have a weight benefit. I still can't see Canon or Nikon making them. But maybe someone like Sigma might just do it to give themselves a USP.

What you are describing is called a speed booster, and it is available today for mirror less cameras.

http://www.metabones.com/buy-speed-booster

If, as you suggest above, you take, say, a 300 mm f/4 lens and project the otherwise wasted FF-photons onto the smaller APS-C sensor, the smaller sensor would still record the same FOV and the same amount of light as the FF sensor. This is because the aperture still has the same absolute size 300/4=75 mm.

However, by doing this you also decrease the focal length of the lens by a factor 1.6. So, while the absolute size of the aperture stays constant, its relative size increases to about f/2.5.

So, there you have it. The same FOV and the same amount of light captured by APS-C and FF. But, you have to decrease the FL with the crop sensor and keep the absolute aperture the same. This of course increases the relative size of the aperture. Hence the f/4 on FF is equivalent to f/2.5 on APS-C.

Unfortunately, if you want to capture a certain amount of light from a distant object the optics need to be of a certain size. You cannot fool the laws of physics just by cropping the sensor. Otherwise, why would e.g. the astronomers keep building larger and larger telescopes if they could just achieve the same thing by cropping their imaging sensors.

This is why its rather pointless to design APS-C only tele lenses. For WA lenses on the other hand its a different story as you can position the lens elements closer to the sensor due to the smaller mirror. Or lack of a mirror.

 

[Skulker]

Well Carl that was a long post ;D

however you seem to have misunderstood my meaning.

I was not describing or promoting a speed booster, although that would, be one way of making use of the photons that are wasted when you put a bigger than needed lens (such as FF) on a smaller sensor (such as APS)

I would redesign the lens so it did not collect light that was not used. So it would be either lighter or brighter. Or maybe a bit of both.

Please don't try to imply that I'm thinking to break, fool, or in any way mess with the laws of physics. 8) cos I ain't. If you think that is what I'm trying to do then you are misunderstanding me.

The main reason that I'm not talking about this or that f stop or focal length is that as an engineer is that to me it is irrelevant what sensor is behind a lens, when it comes to describing its physical properties.

 

[Pi]

I would redesign the lens so it did not collect light that was not used. So it would be either lighter or brighter. Or maybe a bit of both.

The way to do this is to make the front glass element less curved, with all the implications of this on the rest or them. An EF, say, 70-200/2.8 lens has actually a wider FOV than a hypothetical EF-S 70-200/2.8. By doing so, you will get better performance - a good enough reason to design such a lens, IMO. But ... it would not be smaller, really. The front element will be more or less of the same diameter, approximately equal to 200/2.8 The back one would not be smaller but the size of the back is fixed by the mount anyway.

 

[dougkerr]

A FF lens being used with an APS-C sensor results in a slower system, lens+sensor. Since the APS-C sensor only captures 40% of the light projected by the lens on the image plane, the combination is slower than if the same lens had been used with a FF sensor. The lens is of course the same and doesn't know what sensor you place behind. But a lens on its own will not produce any pictures. You need a sensor as well. Really, I cannot see the controversy.

Not so.

The effect of a lens on exposure depends on its f-number. For, say, an f/4.0 lens, the luminance on the sensor for some part of the scene of course does not depend on the sensor overall size.

If we have two cameras, one with a larger sensor that the other, both with sensor systems that we operate at an ISO SOS of ISO 400, then for a given scene, the same exposure result will be obtained with the same shutter speed and f-number.

Indeed, the total amount of light captured across the entire sensor is less for a smaller sensor than for a larger one. But the light per unit area (the property to which the sensor responds, for any given ISO sensitivity) will remain the same.

Best regards,

Doug

My view would be that with any lens designed to be used on a crop sensor, most of the photons entering the lens will hit the sensor, subject of course to the rectangular sensor reading a round lens. In contrast only 40% of the photons that would have hit the sensor on a full frame body will be used in a crop body (assuming a 1.6 crop) with a FF lens.

I'm getting more convinced that long lenses specifically designed for crop bodies would have a weight benefit. I still can't see Canon or Nikon making them. But maybe someone like Sigma might just do it to give themselves a USP.

What you are describing is called a speed booster, and it is available today for mirror less cameras.

http://www.metabones.com/buy-speed-booster

If, as you suggest above, you take, say, a 300 mm f/4 lens and project the otherwise wasted FF-photons onto the smaller APS-C sensor, the smaller sensor would still record the same FOV and the same amount of light as the FF sensor. This is because the aperture still has the same absolute size 300/4=75 mm.

However, by doing this you also decrease the focal length of the lens by a factor 1.6. So, while the absolute size of the aperture stays constant, its relative size increases to about f/2.5.

So, there you have it. The same FOV and the same amount of light captured by APS-C and FF. But, you have to decrease the FL with the crop sensor and keep the absolute aperture the same. This of course increases the relative size of the aperture. Hence the f/4 on FF is equivalent to f/2.5 on APS-C.

Unfortunately, if you want to capture a certain amount of light from a distant object the optics need to be of a certain size. You cannot fool the laws of physics just by cropping the sensor. Otherwise, why would e.g. the astronomers keep building larger and larger telescopes if they could just achieve the same thing by cropping their imaging sensors.

This is why its rather pointless to design APS-C only tele lenses. For WA lenses on the other hand its a different story as you can position the lens elements closer to the sensor due to the smaller mirror. Or lack of a mirror.
[/quote]

 

[jrista]

However, what was I actually using?

1) 70-200mm f/2.8,
2) 535-1528mm f/2.8, or
3) 535-1528mm f/21?

Sounds like I'd say #1, you'd say #3, and Panasonic would print #2 on the side of their lens barrel, like they do on the Lumix FZ200. :

You were shooting (1), which was equivalent to (3); and (2) is wrong any way you look at it.

It would be equivalent to #2. Lenses focus light. Aperture affects the amount of light focused at any given point resolved at the sensor plane. Cropping out the center doesn't change the fact that an f/2.8 lens focuses the same amount of light on the center point of an APS-C sensor as on an FF sensor. Yes, total quantity of light imaged is less, but the amount of light focused anywhere on the sensor is the same. Therefor, an f/2.8 lens is an f/2.8 lens, regardless of the size of the sensor (even the microscopic iPhone sensor will still receive the same amount of light at the center as an APS-C or FF.)

 

[Pi]

Cropping out the center doesn't change the fact that an f/2.8 lens focuses the same amount of light on the center point of an APS-C lens as on an FF lens.

Right, and that amount is zero.

 

[Pi]

If we have two cameras, one with a larger sensor that the other, both with sensor systems that we operate at an ISO SOS of ISO 400, [...]

Why the same ISO?

 

[jrista]

Cropping out the center doesn't change the fact that an f/2.8 lens focuses the same amount of light on the center point of an APS-C lens as on an FF lens.

Right, and that amount is zero.

Why is it zero?

 

[Pi]

Cropping out the center doesn't change the fact that an f/2.8 lens focuses the same amount of light on the center point of an APS-C lens as on an FF lens.

Right, and that amount is zero.

Why is it zero?

You integrate the intensity (modeled by a continuous function) over a set consisting of one point. Another way to say it - a point has area zero, and receives zero light.

 

[jrista]

Cropping out the center doesn't change the fact that an f/2.8 lens focuses the same amount of light on the center point of an APS-C lens as on an FF lens.

Right, and that amount is zero.

Why is it zero?

You integrate the intensity (modeled by a continuous function) over a set consisting of one point. Another way to say it - a point has area zero, and receives zero light.

Alright, now your just playing games. You know what I mean. Lets call it a "spot", if you want to get that technical.

Anyway, assuming two point light sources of equal intensity, they both focus a spot of light on the sensor plane of identical intensity, regardless of whether that spot is focused on an APS-C or FF sensor. In the little diagram below, the red represents a focused point light source that resolves to a spot on both the APS-C and FF sensors. The aperture blocks some light from the right-hand side of the lens, and the amount of light blocked is the same for both FF and APS-C.

The blue is simply for demonstration. It is a point of light focused outside of the range of the APS-C sensor. It only falls on the FF sensor, however that does not change the fact that it is the lens and aperture that determines the amount of light that is actually focused at that spot, same as with the red. Total volume of light passing through the aperture that is actually imaged by the FF sensor is higher, however for any given spot, it is the same, regardless of sensor size. (I could have thrown a tiny little sensor in there for the iPhone...if I located it under the resolved spot of light for the red sample, it would be receiving the exact same amount of light as the APS-C and FF.)

 

[Pi]

Total volume of light passing through the aperture that is actually imaged by the FF sensor is higher, however for any given spot, it is the same, regardless of sensor size.

OK, you are talking about intensity. It is the same, indeed. So what? Why is intensity of light, on sensors of different sizes, a factor we have to worry about?

 

[jrista]

Total volume of light passing through the aperture that is actually imaged by the FF sensor is higher, however for any given spot, it is the same, regardless of sensor size.

OK, you are talking about intensity. It is the same, indeed. So what? Why is intensity of light, on sensors of different sizes, a factor we have to worry about?

Well, if you go back to the original post I replied to, you stated that using a 70-200 f/2.8 on an iPhone was actually like using a 535-1528mm f/21. Given the fact that aperture affects the intensity of any given spot of focused light, it really is like using a 535-1528mm f/2.8. I am just disputing the notion that would become an f/21 lens, and stating that an f/2.8 lens is an f/2.8 lens regardless of the size of the sensor it is used on. Sensor size has no impact on aperture. You may be trying to say something else, but the terms you used in the original post I responded to is rather misleading about the relationship between aperture and sensor size.

For further evidence...the use of a focal reducer changes the optical nature of the lens, which is why on a mirrorless you can focal reduce an f/4 lens to f/2.5 (as noted by dougkerr). However it should be noted that the change in relative aperture has nothing to do with the fact that the sensor is smaller than the image circle was originally designed for...it has to do with the fact that the focal length of the lens was shortened while the entrance pupil remained the same size.

 

[Loswr]

If we have two cameras, one with a larger sensor that the other, both with sensor systems that we operate at an ISO SOS of ISO 400, [...]

Why the same ISO?

Because that gives the same exposure. Of course, since total light gathered is different, image noise at the same ISO is not equal...

 

[jrista]

I used to have a 7D. A camera I took many pictures with and really enjoyed. But I was using it mainly behind L series lenses because I wanted the quality that those lenses provide. This has proved to be a good investment as I now have a 5D3 and a 1Dx and the lenses work with them.

I don't want more megapixels, I've got more than I uses almost always. And before anyone suggests it I certainly don't want more just so I can crop them away.

One of my main problems with all this kit is the weight, carrying 40Kg on a trek to a wild life photo opertunity can be a pain.

When the 7D2 comes out the thing that would get me to buy one would be if light weight lens were available with similar quality to the L series but making use of the reduced diameter needed for the smaller sensor. (While they are at it they can reduce the price as the elements aren't as big. )

I know its not going to happen but it would be nice if it did. ------- Just think a nice quality 200-400 with built in 1.4x at about 1/2 the weight and cost in front of a crop sensor giving equivalent view to a 300 to 900 on a FF. 8)

( BTW There can be little doubt that someone who thinks they know better will ridicule this idea. If they convince me that they are right I will claim I was being sarcastic ;D )

It seems to me that this is pretty much what the micro4/3rds manufacturers have done. They took a smaller sensor, and designed a whole range of small, compact lenses in just the right way to provide optimal performance for that smaller sensor. Those smaller sensors have smaller pixels, and consequently a similar change in performance relative to the big pixels of a FF sensor.

I doubt Canon or Nikon would ever support the micro4/3 sensor, at least not in their DSLR products. I would say that if you don't care about having an OVF, then mirrorless is really what you are looking for. The Canon EOS-M is an APS-C system that should eventually get a whole range of lenses explicitly designed for that particular sensor. Not only can the lenses be smaller, the whole entire package is smaller, and if size and weight is your greatest concern, then you could save yourself a lot of hassle by using an EOS-M setup (albeit with a more limited range of lenses as of today...i.e. if you need 600mm, your just kind of out of luck at the moment in the Canon world.)

 

[Pi]

OK, you are talking about intensity. It is the same, indeed. So what? Why is intensity of light, on sensors of different sizes, a factor we have to worry about?

Well, if you go back to the original post I replied to, you stated that using a 70-200 f/2.8 on an iPhone was actually like using a 535-1528mm f/21. Given the fact that aperture affects the intensity of any given spot of focused light, it really is like using a 535-1528mm f/2.8.

This does not answer my question. What is so sacred about the same intensity? Why is that of such a big concern?

 

[Pi]

If we have two cameras, one with a larger sensor that the other, both with sensor systems that we operate at an ISO SOS of ISO 400, [...]

Why the same ISO?

Because that gives the same exposure. Of course, since total light gathered is different, image noise at the same ISO is not equal...

... which confirms that the APS-C system in this scenario is "slower", and that was the reason for my remark.

 

[jrista]

If we have two cameras, one with a larger sensor that the other, both with sensor systems that we operate at an ISO SOS of ISO 400, [...]

Why the same ISO?

Because that gives the same exposure. Of course, since total light gathered is different, image noise at the same ISO is not equal...

... which confirms that the APS-C system in this scenario is "slower", and that was the reason for my remark.

It isn't slower. The total light over the area of the sensor MAY be more finely divided (there is no reason an APS-C sensor couldn't have the same pixel pitch as an FF, or for that matter, larger pixels), but that does not change how fast the whole area saturates at a given EV. If the APS-C sensor was slower, the exposure value at a given aperture and shutter speed would be different, yet that is not the case. For any given aperture and shutter speed, exposure value is always the same, as it doesn't have anything to do with sensor area.

The noise increases because the pixels are smaller, not because the sensor, or he lens, is "slower." Back to the same old argument, but shoot the same subject with the same lens at the same distance, crop the FF, and scale the APS-C image to the same size as the cropped FF image. Noise and EV will be the same. Frame the two identically with the same lens, noise will be lower on the FF, while EV will still be the same. The only thing that changed when framing identically is pixels on the subject...the speed of the exposure remains the same.

 

[Sporgon]

You'll open a whole can of worms with the 2.8 on APS is equal to 4.2 FF thing.

Told you

 

[jrista]

You'll open a whole can of worms with the 2.8 on APS is equal to 4.2 FF thing.

Told you

Sorry.

 

[Pi]

It isn't slower. The total light over the area of the sensor MAY be more finely divided (there is no reason an APS-C sensor couldn't have the same pixel pitch as an FF, or for that matter, larger pixels), but that does not change how fast the whole area saturates at a given EV. If the APS-C sensor was slower, the exposure value at a given aperture and shutter speed would be different, yet that is not the case. For any given aperture and shutter speed, exposure value is always the same, as it doesn't have anything to do with sensor area.

The noise increases because the pixels are smaller, not because the sensor, or he lens, is "slower." Back to the same old argument, but shoot the same subject with the same lens at the same distance, crop the FF, and scale the APS-C image to the same size as the cropped FF image. Noise and EV will be the same. Frame the two identically with the same lens, noise will be lower on the FF, while EV will still be the same. The only thing that changed when framing identically is pixels on the subject...the speed of the exposure remains the same.

Totally wrong, sorry. The shot noise is dependent on the total light. Intensity if irrelevant.

EDIT: "Exposure Value" is a vague term you just invented. Pixel size is largely irrelevant. I was wrong with that. Consider it withdrawn.
Read this, again:

http://www.josephjamesphotography.com/equivalence/

The source of the (photon) noise is not the pixels, it is the photon nature of light.

http://www.josephjamesphotography.com/equivalence/#shot

Please, read something before you reply.