EF-S (or Digital Only) Long Tele-Zooms - why don't they exist?

pwm2 wrote in post #3391010
.....

But if you are happy with the viewing angle of a 300mm lens (as seen on a ff body), the lens makers can make smaller and cheaper lenses for APS-style cameras. A 188/4 can most definitely be built smaller and cheaper than a 188/2.5 (or a 300/4) lens.

....

Why not just buy the 200/2.8L and be done with it?

Without reading everyone's replies...

They don't make cheap, small, light, EF-S only super tele's because they already make super tele's, and make a mint on them. If they made the same thing but cheaper, who would buy the pricey ones?

BTW, Jon is correct in post #12 here.

The definition of f/stop is 'focal length divided by aperture diameter'. For a 200 mm f/4 lens, it would mean that the entrance pupil has to be large enough to accomodate a 50 mm lens diameter (200/4). You cannot have a smaller entrance pupil than what's dictated by the maximum aperture of the lens. Otherwise, my 400/2.8 would be the size of a 400/5.6.

And it matters not how large or small your sensor is - the same rules apply to APS-C, 35mm, medium format, and tiny P&S cameras.

Oddly, at the wide-angle end of the spectrum, the entrance pupil is generally larger or much larger than the lens aperture would dictate. This is because the angle-of-view requires the ability to collect light way off of the center axis. The 15 mm fisheye is a good example - the entrance pupil to the lens is much larger than 15 mm, yet the f/stop is only f/2.8. Why? Because the lens has to "see" much wider (180-degrees corner to corner on full-frame). Ultrawides are similar, but to a lessor extent.

So why do we have EF-S lenses in the first place? Because of the backfocus distance primarily, and because on wider lenses, we can get away with smaller diameter glass in the back end of the lens. Consider this - the backfocus distance for an EF lens (distance from the back flange of the lens to the sensor/film) is 44 mm. It's the same on an EF-S lens but the EF-S lens allows the lens to protrude into the mirror box a few mm more. What does this mean? Consider that any lens shorter than 44 mm has to have additional correctional lens elements (retrofocus or 'inverted telephoto' design) to create a focal length that is shorter than the distance from the back of the lens to the sensor. And the shorter the lens focal length, the more correction is necessary. By shortening that backfocus distance (as is done on the EF-S lens), the amount of correction necessary is reduced allowing (at least in theory) for somewhat better image quality.

This is not a problem on longer lenses where often times, the rearmost element of the lens is several mm away from the mounting flange.

I guess based on further reading here and elsewhere is that I'll have to live with wasted light.

The Canon 200mm f2.8L is is $660 on B&H.
The Canon 300mm f4L IS is $1150.

Even if they "converted" the F4L to "digital only", I would end up with 188mm f2.5 and gain IS. That's .3 of an f-stop and IS for $500 and less focal length.

But with Sigma and Tokina both making DECENT 50-1XX lenses (not the crappy Sigma 55-200 or tamron 55-200 consumer grade lenses), we are starting to see some digital only tele-zooms. They just aren't as long as things like the 100-300 or 100-400. Who knows, maybe there will be some day.

However, as I stated earlier the Sigma 50-150 f2.8 EX is $680 at B&H where it's equivalent (roughly) for full frame, the 70-200 f2.8 EX is $890. So for the same EFFECTIVE focal length, it seems that the digital only lenses are being produced at a better cost.

I wonder what a 125mm f2.8 EX HSM OS Sigma would cost? That would be the equivalent of a 200mm f2.8L IS Canon for crop bodies

Layston wrote in post #3391782
Even if they "converted" the F4L to "digital only", I would end up with 188mm f2.5 and gain IS. That's .3 of an f-stop and IS for $500 and less focal length.

This assumption is flawed, as it assumes the total cost (from conceptualization to R&D all the way through to marketing and finally getting it into the hands of the public) of producing each 'mm' of the lens is linear. It is not.

Go with the flow. It is much easier than worrying about the 'wasted light'.

If you are not interested in consumer grade lenses, then you really don't have an option.

I don't think any high-end lenses will be made solely for crop-factor cameras, since they would loose a lot of customers. I would not want to buy a lens that locks me into crop-factor bodies. The only exception is that I'm looking at a really wide EF-S lens, so I don't have to switch to an analog body - or my Dimage 7 - to get below the 20x1.6mm I'm currently limited at.

pwm2 wrote in post #3391134
You are missing the fact that a ff lens is projecting more than half the light outside the sensor.

We're not missing that. However you're missing the minor detail that the "more than half the light outside the sensor", if concentrated on the sensor, might give you a brighter center area, but would seriously degrade the picture, since by definition the light rays that were originally directed outside the sensor area contain NO image-producing information that belongs in the sensor area. They all contain information for the part of the image that falls outside the sensor. If you want to see the effect this would have, try throwing your lens out of focus. Or grab and refocus a MagLite so you get the brightest beam, and then so you see the bulb filaments.

I'm not trying to be difficult, I was asking a question and stimulating debate. I really think that there is a market for this type of lens. As far as "locking into crop-factor bodies" goes Canon makes the 17-55 EF-S lens that is definitely not cosumer level, but it certainly is selling. And yes, I realize that getting wide on crop means you need a lens like this, and that the standard zooms fit all cameras and have certain advantages on crop bodies. But do you consider the 30D a consumer level camera? I certainly wouldn't...

The point is that there's absolutely no design advantage to making a telephoto EF-S lens. The only place where there would be an advantage would be with the extreme zooms like the Tamron/Sigma 18-200, and there only because of the wide end. It won't take any less material, it won't take any less precision manufacturing to make an EF-S tele. And Canon does make a lens that's crop-equivalent of the 70-300 family on FF. It's the 55-200 f/4-5.6. Perfect complement to the 18-55.

Layston wrote in post #3392284
I'm not trying to be difficult, I was asking a question and stimulating debate. I really think that there is a market for this type of lens. As far as "locking into crop-factor bodies" goes Canon makes the 17-55 EF-S lens that is definitely not cosumer level, but it certainly is selling. And yes, I realize that getting wide on crop means you need a lens like this, and that the standard zooms fit all cameras and have certain advantages on crop bodies. But do you consider the 30D a consumer level camera? I certainly wouldn't...

There may be a market for a lens such as the Sigma 50-150 lens, but I wouldn't be one to buy into it. I think that missing the range between 55 and 70 mm is much less important than missing the range from 150 to 200 mm on an APS-C camera. I realize that that's a somewhat personal opinion, but I really don't see that as much value sits in the low end of that range as the long end.

Of course, it all boils down to the individual's shooting needs and style.

Jon wrote in post #3392239
We're not missing that. However you're missing the minor detail that the "more than half the light outside the sensor", if concentrated on the sensor, might give you a brighter center area, but would seriously degrade the picture, since by definition the light rays that were originally directed outside the sensor area contain NO image-producing information that belongs in the sensor area. They all contain information for the part of the image that falls outside the sensor. If you want to see the effect this would have, try throwing your lens out of focus. Or grab and refocus a MagLite so you get the brightest beam, and then so you see the bulb filaments.

No, I am not missing the "minor" detail that the image projected outside the sensor isn't part of the image that the sensor should see (for a specific focal length). However, you might miss that that is a part of the image that costs to produce, even if the camera body has no need - or use - for it.

The problem is that most of the light that falls outside the sensor are collected and correctly handled by the lens because the lens is built for a wider field of view than the camera can make use of.

The front element size decides how much light the lens can pick up. However, a modern lens have quite a lot of lens elements. The diameter of each and every lens element directly affects the price and weight of the lens.

A lot of the work with an ff lens is to to be able to project a large image circle with a high-quality image. A crop-factor camera has no use for a high-quality image being projected outside the sensor. Tha fact is still that a ff lens spends a significant amount on glass on projecting ray paths that are of no use for a crop-factor body, hence a lighter and cheaper lens should be possible to build.

If there is a need - or market - for it is a completely different question.

The fact is that it's only true for wide angle lenses that a significant amount of glass to "project a large image circle with a high-quality image. A crop-factor camera has no use for a high-quality image being projected outside the sensor." On telephotos, there's much less aberration due to barrel or pincushion distortion (which are more pronounced toward the edges) and much more to chromatic aberrations due to the long lens (which are more uniformly distributed). And because the AoV of the crop and FF are much closer, those aberrations which are related to the lens' angle of view require much less additional correction for the "exxtra" area.

Furthermore,

pwm2 wrote in post #3387848
By making the image circle of the lens smaller - just lighting up the smaller sensor - a lens of a fixed physical size would project more concentrated light on the sensor. At the same time, the "crop factor" will disapear, so a 300mm lens will cover as much angle as on a ff body.

is totally untrue. A smaller image circle from the same optics will give you no brighter usable image, which is exactly what I, and several others, have been trying to explain to you. The only way you'll get a 300 mm/FF angle of view on a crop camera is with a 200 mm lens. And with the same entry pupil (which isn't, BTW, especially with wide angle lenses, synonymous with the first element diameter) on that lens it'll be about a stop faster.

Okay let's break down some questions to yes/no answers (please correct me if I'm wrong):

If you adjusted the optics of a standard FF lens so that it projected an APS-C compatible image circle:
a) Would that image circle have the same FOV as the original lens?
Yes, that's the point of the optics adjustment described in the question.
b) Would that image circle now have more light incident per unit area than the original lens?
I believe the answer is yes.
c) Would you still call it the same focal length as the original lens?
I would say no. You have to call it less because we don't change focal length for digital only lenses, we still call them FF equivalent. Hence as has been said, a 300mm converted would be roughly a 200mm lens.

So now from my looking at lens prices, it would seem that once we get up into the 300mm range, if I make the assumption (big assumption, but I believe it would be roughly true) that you could convert a 300mm f4 to a 200mm f2.8 APS-C compatible lens for the same price as the 300mm f4, that 200mm f2.8 APS-C lens would be more expensive than the standard 200mm f2.8L that is on the market. This comes from the fact that a 300mm f4L IS is $1150 and a 200mm f2.8 is $680.

I still say that IF they could make APS-C format tele lenses more inexpensively than their film counterparts, then their would be a market for them, but this seems like it's a bit of a pipe dream when it comes to long lenses. However, that doesn't take away one thing. When I buy the existing 200mm f2.8L and put it on my crop body, I've just effectively bought a really cheap 300mm f2.8L in comparison to having to buy a real 300mm f2.8L to put on a ff camera.

Thanks everyone for your input!

Jon wrote in post #3392643
The only way you'll get a 300 mm/FF angle of view on a crop camera is with a 200 mm lens. And with the same entry pupil (which isn't, BTW, especially with wide angle lenses, synonymous with the first element diameter) on that lens it'll be about a stop faster.

Which is exactly what I already wrote about half a thread back:

pwm2 wrote in post #3391010
If you start with a "full-size" 300/4 and modify it to reduce the image circle by a factor of 1.6, you would get a 188/2.5 lens. It would not work on a ff body, because it doesn't project an image on the full sensor. On a 1.6x camera, it will give a "virtual" focal length of 300mm, but since the physical lens is now only 188mm, it will have the DOF of a 188mm lens.

In short - Canon already sell a 200/2.8.

That is one side of the coin.

The other side of the coin is that I claim a lens could be made cheaper/lighter if all the lens elements where not optimized for giving a high-quality image on a ff sensor.

Yes, I'm quite aware that it is easier to make a tele lens with little distorsion than it is to make a wide-angle lens. But I would really like an example why a lens built for a narrower light path would not be possible to build lighter/cheaper.

Plese observe, I have never said it should be possible to make the lens for half the prize, or something like that. I have only claimed that there is waste to have a lens that is built to project an image circle for a ff sensor.

I have also claimed that even if there where such tele lenses available, I would not be interested in one, since that would lock me harder into crop bodies.

Layston wrote in post #3392760
Okay let's break down some questions to yes/no answers (please correct me if I'm wrong):

If you adjusted the optics of a standard FF lens so that it projected an APS-C compatible image circle:
a) Would that image circle have the same FOV as the original lens?
Yes, that's the point of the optics adjustment described in the question.
b) Would that image circle now have more light incident per unit area than the original lens?
I believe the answer is yes.
c) Would you still call it the same focal length as the original lens?
I would say no. You have to call it less because we don't change focal length for digital only lenses, we still call them FF equivalent. Hence as has been said, a 300mm converted would be roughly a 200mm lens.

So now from my looking at lens prices, it would seem that once we get up into the 300mm range, if I make the assumption (big assumption, but I believe it would be roughly true) that you could convert a 300mm f4 to a 200mm f2.8 APS-C compatible lens for the same price as the 300mm f4, that 200mm f2.8 APS-C lens would be more expensive than the standard 200mm f2.8L that is on the market. This comes from the fact that a 300mm f4L IS is $1150 and a 200mm f2.8 is $680.

I still say that IF they could make APS-C format tele lenses more inexpensively than their film counterparts, then their would be a market for them, but this seems like it's a bit of a pipe dream when it comes to long lenses. However, that doesn't take away one thing. When I buy the existing 200mm f2.8L and put it on my crop body, I've just effectively bought a really cheap 300mm f2.8L in comparison to having to buy a real 300mm f2.8L to put on a ff camera.

Thanks everyone for your input!

a) If you rebuild a lens so that it virtually has a 0.625x TC, then you would have a lens with 0.625 times the original focal length, i.e. a completely different lens. Hence, you would not have the same DOF as the original lens.
b) Yes, just as a normal tele lens with a TC would loose one or more stops, such a rebuild of the lens would produce a new lens with shorter focal length, but gaining about a stop of light.
c) No, it would not be the original focal length anymore. The focal length would have decreased with a factor 1.6.